What Is Known About the Atlantomastoid Muscle: a Scoping Review

WHAT IS KNOWN ABOUT THE ATLANTOMASTOID MUSCLE: A SCOPING REVIEW

Rebecca Anne Lee

A thesis submitted to the Department of Biomedical and Molecular Sciences

In conformity with the requirements for the

Degree of Master of Science

Queen’s University

Kingston, Ontario, Canada

(January, 2021)

Background. The atlantomastoid is a muscular variant that has been observed in the suboccipital region in humans yet remains underdiscussed and unknown to many anatomists and medical professionals.

Objectives. The aim of this review was to provide a clear and concise summary of the current literature regarding the atlantomastoid muscle in humans and to summarize the muscle variant’s characteristics including origin, insertion, innervation, and rate of frequency in a given population. It was also conducted to identify gaps in the literature, recommend areas of focus for future research, and comment on the effectiveness of using scoping review methodology.

Methods. Three databases were searched with no restrictions on document type, date of release, or language. Two reviewers independently reviewed the results and identified sources that discussed the atlantomastoid muscle either by directly naming it or by describing a muscle variant that matched that of the atlantomastoid. Sources found post hoc using a narrative review methodology were also included.

Results. Fifteen sources were identified as discussing the atlantomastoid muscle, of which seven contained original observations. Over half of the texts were published prior to 1900 and many were identified exclusively using a narrative review-style search strategy. The muscle variant originates from the transverse process of the atlas and inserts on the mastoid process of the temporal bone, with possible accessory slips from nearby muscles. It can vary in shape and composition and is likely innervated by the suboccipital nerve, with reported frequencies of observation between 12.1% to 30%. The atlantomastoid muscle has also been referred to as the rectus lateralis accessorius, atlantico-mastoideus, atloïdo-mastoïdien, and atlanto-mastoideus.

Conclusion. Additional documentations of the atlantomastoid muscle is recommended, as it would provide more recent observations that would supplement the scant amount of information that currently exists regarding the muscle variant.

Acknowledgements

First, I’d like to acknowledge that I have been living, learning, working, and playing on the unceded traditional Anishinaabe and Haudenosaunee territory. I am grateful to have been a guest on this land while studying and working at Queen’s University.

I must first thank the faculty, staff, and students of the Pattern II Anatomical Sciences program. Thank you Drs. Easteal, Graham, MacKenzie, and Pang, for sharing your knowledge, resources, and guidance with us, and to everyone who worked hard to ensure that the program continued as we entered into the COVID-19 pandemic. Dr. Pang, I want to express my gratitude to you in particular for being a kind and encouraging supervisor with clear sincerity in your desire to support your students. Logan Bale and Earl Donaldson, thank you for making the Anatomy

Learning Centre an engaging and welcoming place for students to learn and work. Thank you especially to Logan for committing your time as second reviewer. And to my peers in the PII program, thank you for giving this graduate experience life and character.

To my friends in the Kingston community, thank you for giving me a life beyond campus, especially the 1st Meadowbrook Pathfinder Unit, Kingston theatre groups, and Café Church. You were a cherished constant in my life as I transitioned out of my work and back into graduate studies. Also, a sincere thank you to everyone who is working towards making Queen’s University a more inclusive institution. It’s encouraging to see students, staff, and faculty taking the initiative to ensure that every person is welcomed, respected, and provided the support they need to thrive.

Finally, thank you to my family and friends who supported me when I was low, advised me when I was uncertain, and celebrated with me for the highs. Many people have helped to bring me to this point in my academic journey, and even though I cannot mention everyone by name, rest assured that if you are wondering if I have thought of you – I have.

iii

Table of Contents

Abstract ...... ii

Acknowledgements ...... iii

List of Tables ...... vi

List of Figures ...... vii

List of Abbreviations ...... viii

Chapter 1 Introduction ...... 1

The Relevance of Anatomical Variations in Medical Practice ...... 1

Disseminating Knowledge Regarding Anatomical Variations ...... 3

The Usefulness of Summarizing Current Knowledge ...... 4

Knowledge of All Variations Is Essential, yet Uncommon ...... 5

The Need to Synthesize Information About the Atlantomastoid Muscle ...... 8

Chapter 2 Methods ...... 11

Initial Methods ...... 11

Inclusion Criteria ...... 12

Post Hoc Methods ...... 12

Rationale for Initial Methods and Post Hoc Methods ...... 13

Chapter 3 Results ...... 16

Origin and Insertion ...... 20

Accessory Slips ...... 21

Characteristics ...... 21

Location and Pathway ...... 24

Frequency ...... 24

Innervation ...... 25

Associated Muscles ...... 26

Alternative Names ...... 26

Chapter 4 Discussion ...... 28

Determining Prevalence ...... 28

Clinical Impact of the Muscular Variant ...... 30

Inconsistencies and Required Clarity ...... 31

Few Publications Identifying the Atlantomastoid Muscle ...... 34

Comment on Methods ...... 35

Limitations ...... 38

Chapter 5 Conclusion ...... 41

References ...... 42

Appendix A Search Strategy for Ovid Embase Classic and Embase ...... 49

Appendix B Search Strategy for Web of Science All Databases ...... 50

Appendix C Search Strategy for ProQuest Health and Medicine ...... 51

Appendix D Permissions For Reprinting ...... 52

List of Tables

Table 1 Reasons References Were Removed From Initial Scoping Review Search Results and

Count of Occurrence ...... 16

Table 2 Summary of Sources Discussing the Atlantomastoid Muscle ...... 19

List of Figures

Figure 1 Suboccipital Muscles ...... 2

Figure 2 Location of Vertebral Artery Buldge in Suboccipital Region ...... 8

Figure 3 PRISMA Flow Diagram of the Scoping Review Process and Incorporation of Post Hoc

Narrative Review Search Results ...... 17

Figure 4 Illustration by Gruber (1876) of Atlantomastoid Muscle from Left Lateral and Posterior

Views ...... 22

Figure 5 Illustration by Mori (1964) Identifying the Atlantomastoid Muscle (Δ) in all Observed

Cases ...... 23

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List of Abbreviations

BIOSIS Previews BioSciences Information Service of Biological Abstracts Previews

Embase Excerpta Medica database

KCI-Korean Journal Database Korean Citation Index – Korean Journal Database

MEDLINE Medical Literature Analysis and Retrieval System Online

PRISMA Preferred Reporting Items for Systematic Reviews and Meta-Analysis

PRISMA-ScR PRISMA extension for scoping review

SciELO Citation Index Scientific Electronic Library Online Citation Index

VA bulge Vertebral artery bulge

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ATLANTOMASTOID: A SCOPING REVIEW

Chapter 1

Introduction

The muscles in the posterior cervical region play an important role in head and neck movements and stabilization of the cervical spine in humans. Deep in this region, closely associated with the craniovertebral junction, lie the suboccipital muscles. The suboccipital muscles are a group of four muscles that specifically assist with extension, lateral flexion, and rotation of the head (Hiatt, 2020). The group is composed of the rectus capitis posterior minor, rectus capitis posterior major, obliquus capitis superior, and obliquus capitis inferior muscles (see Figure 1).

A portion of the human population also have a muscle variant known as the atlantomastoid found in the suboccipital region. Despite it being a fairly common variation (Mori, 1964), the atlantomastoid muscle remains widely undiscussed in the scientific literature. Consequently, information regarding the muscle variant is sparse and can be difficult to locate. To this day there remains no clear, concise, and complete publication summarizing the current literature regarding the atlantomastoid muscle.

The Relevance of Anatomical Variations in Medical Practice

The near-absence of any documentation of the atlantomastoid muscle in the literature is concerning, as proper awareness and appreciation of anatomical variations is crucial in a medical professional’s approach to cautious and careful medical practice. Globally, educators and students in the medical field have acknowledged the significance of anatomical variations in medicine, with such variations being perceived as an important component of medical training by anatomy teaching faculty in U.S. medical schools (Goldberg & Royer, 2016). Acknowledgement of anatomical variations was also identified as a theme in South African student reflective writing regarding cadaveric dissection experiences in anatomy practical sessions (Lazarus et al., 2017). The students felt anatomical variations made it easier to understand and appreciate a structure’s function and recognized variations as having clinical importance in future medical practice (Lazarus et al., 2017). Indeed, a lack of awareness regarding variations has been found to 1

ATLANTOMASTOID: A SCOPING REVIEW lead to clinical malpractice and continues to be an issue and concern for medical professionals (Tomos et al., 2019; Willan & Humpherson, 1999; Yammine, 2014). There is a recognized importance of including anatomical variations in anatomy education, but the possibilities in regards to the ways in which an anatomical structure may vary and the frequency of observing them suggest that students may not be exposed to all common variations during practical learning. It is therefore important that these variations are recorded and communicated between researchers, educators, and individuals learning or working in health care.

Figure 1. Suboccipital muscles. 1 = Rectus capitis posterior minor; 2 = rectus capitis posterior major; 3 = obliquus capitis inferior; 4 = obliquus capitis superior. Adapted from “H24” by Queen’s University Department of Biomedical and Molecular Sciences, n.d., Scalable Gross Anatomy and Histology Image Catalogue. Copyright 2020 by Stephen Pang. Reprinted with permission.

ATLANTOMASTOID: A SCOPING REVIEW

Disseminating Knowledge Regarding Anatomical Variations

Dissections are continually reported to be an integral component of anatomy education and can also be an avenue for familiarizing students with anatomical variations (Ghosh, 2017; Granger, 2004;

Thompson & Marshall, 2020). In fact, anatomy teaching faculty at U.S. medical schools identified dissections as being the primary method of exposing students to such variations (Goldberg & Royer, 2016).

A survey conducted among surgical and radiological trainees found that they also believed dissections were the best resource for learning about anatomical variations (Raikos & Smith, 2015). But it would be near impossible for any individual to fully understand and appreciate all of the known possibilities through dissection alone, as it would take an unprecedented amount of cadaveric exploration before the majority of anatomical variances were observed. Evidently, the same group of surveyed surgical and radiological trainees reported primarily learning about anatomical variations from consultants instead of through dissection, despite being of the opinion that dissections were the best resource (Raikos & Smith, 2015).

During dissection, each cadaver presents a singular case, and so educators must turn to additional resources if they wish for students to develop the knowledge and full appreciation of variabilities within human body.

It is for this reason that documentation of anatomical variations is necessary and should be circulated and discussed among students during their medical or other health care training.

The vast variety and frequencies of anatomical variations suggest that students are unlikely to be exposed to all variations through first-hand experience in dissection labs. Therefore, student education must be supplemented through other means. This is often accomplished using some form of knowledge translation, which is defined by the Canadian Institutes of Health Research (2016) as a “dynamic and iterative process that includes the synthesis, dissemination, exchange and ethically-sound application of knowledge”. Forms of knowledge translation include conference presentations, peer-reviewed journal articles, interactive information sessions, seminars, and media engagement (Canadian Institutes of Health

Research, 2016). These all represent a didactic method of teaching and learning, whereas dissection is hands-on and therefore fits more within the category of active learning. And while dissection remains an integral resource for anatomical education, knowledge translation and its didactic forms remain among the 3

ATLANTOMASTOID: A SCOPING REVIEW most reliable methods of increasing awareness and understanding of all the different anatomical variations

(Raikos & Smith, 2015; Sanudo et al., 2020).

The Usefulness of Summarizing Current Knowledge

It is hopefully evident that there are numerous ways in which knowledge about anatomical variations can be disseminated. As previously mentioned, guidance and consultation from more experienced educators is one way an individual can learn about such variations. Another way to promote knowledge translation is by documenting findings from cadaveric and surgical studies that identify and/or discuss observed variations. This can be accomplished using previously identified methods, such as writing peer-reviewed journal articles and creating presentation materials, but the growing amount of literature can make reading such a numerous amount of content an overwhelming task. What’s more, the information about an anatomical variance can be dispersed and take time to synthesize (Fink, 2019), making it difficult for individuals to identify good studies and develop a complete understanding of the current knowledge surrounding different variations. Literature reviews are therefore a useful tool as they are used to identify, evaluate, and synthesize existing literature, and can help to clarify current knowledge on a topic and guide clinical practice (Fink, 2019). They also assist in circulating the findings of previous studies and reports, which in turn supports knowledge translation. Given the range and frequency of anatomical variations in humans, literature reviews can make it easier for an individual to develop a more complete understanding of the current knowledge surrounding different variations.

Other formats of collected information can be of similar use to literature reviews, such as catalogs, textbooks, or databases. These can be valuable sources to discover and understand anatomical variations that may not be otherwise discussed during a student’s education. For example, a catalog of observed variations has been found to be educationally valuable among students in a doctor of chiropractic program, with high levels of agreeability regarding its utility and ease of use (Bale et al., 2018). However, the development of comprehensive educational materials such as catalogs remains dependent upon the dissemination of knowledge, particularly when compiling information regarding anatomical variations, 4

ATLANTOMASTOID: A SCOPING REVIEW since one individual or group of researchers are unlikely to have first-hand documentation for all variations.

Instead, multiple sources discussing anatomical variations are likely to be required and compiled in order to produce comprehensive educational materials. Literature reviews are therefore valuable contributions to the literature that can improve curricula and supplementary materials by summarizing findings from cadaveric and surgical reports. Compiling known sources discussing anatomical variations will assist in the dissemination of these sources and consequent application of the information contained therein.

As mentioned, catalogs and databases of anatomical variations are useful tools that can supplement anatomical education. These may often be in a text-based format, but as medical training and anatomical education moves through the 21st century, there is an increase in reliance and use of technology (Zargaran et al., 2020). In regards to the human musculoskeletal system, computed virtual models and biomechanical simulations are becoming more commonplace and relied upon for education and research (Christodoulakis,

2007; Decker, 2010; Smith, 1999; Zargaran et al., 2020). These models play an important role in the study and analysis of human anatomy, and their accuracy is dependent on a thorough and complete understanding of the musculoskeletal system. This includes consideration of variations within the musculoskeletal system and the effects that these variations may have. There is a recognized need for technologies that model the human musculoskeletal system to improve upon their content regarding anatomical variations (Codd &

Choudhury, 2011), and so an awareness and understanding of all variations – particularly those that are lesser-known – is required if these models are to accurately represent the natural variations that may be possibly observed in the human body. Once again, literature reviews can be useful resources for acquiring such information, conveniently summarizing the literature so that it can be used to develop and improve digital models for research and educational use.

Knowledge of All Variations Is Essential, yet Uncommon

Although it is important to increase and maintain awareness of anatomical variations, not all variations are discussed equally in the literature. There already exist recent literature reviews that demonstrate the importance and prevalence of anatomical variations among certain cervical structures such 5

ATLANTOMASTOID: A SCOPING REVIEW as the vertebral artery (Guan et al., 2017), superficial veins in the neck (Dalip et al., 2018), atlas vertebra

(Rajani, 2014), and ligaments of the craniovertebral junction (Tubbs et al., 2011). Individual studies add to this, providing documentation and discussion of other variations within the cervical region. Yet there are extremely few studies that document or discuss the atlantomastoid muscle, and there is no structured literature review that identifies, evaluates, and summarizes the scant amount of knowledge that does exist surrounding it. This arguably leaves the atlantomastoid muscle as an underappreciated and often unknown variation, despite it having potential clinical relevance.

Appropriate consideration of anatomical variants is particularly important for certain medical practices, such as surgery and radiology. A survey sent to directors and chairs of surgical and radiological specialty programs within Canada and Australia found that 80.7% included anatomical variations within their curricula (Raikos & Smith, 2015). Evidence has continually shown that a thorough appreciation and ability to identify possible anatomical variations during surgery can reduce the chance of iatrogenic injury to a patient (Fisahn et al., 2017; Ogeng’o, 2013; Przywara, 2016; Shao et al., 2016). Yet while arterial, neural, and some muscular variants and their significance are well discussed in the literature (Kontur, 2019;

Kumar, 2010; Ogeng’o, 2013; Przywara, 2016; Shao et al., 2016), there remains limited discussion for that of certain muscular variants such as the atlantomastoid. In fact, muscular variations are among the least common class of anatomical variation included in training curricula according to surgical and radiological trainees (Raikos & Smith, 2015). Of the muscular variations that are discussed, it is unlikely that the atlantomastoid is included given how little documentation there is regarding it. Nonetheless, it ought to be considered during certain cervical surgeries since it lies close to important anatomical structures that can be of surgical interest.

The atlantomastoid muscle originates from the transverse process of the atlas and inserts along the mastoid process of the temporal bone (Mori, 1964), which are important bony landmarks in tasks such as identifying the vertebral artery (Meybodi et al., 2016). The vertebral artery’s most prominent point, the

“VA bulge”, lies within the triangular area defined by the transverse process of the atlas, posterior tubercle of the atlas, and the tip of the mastoid process (see Figure 2). Respectively, it is approximately 19.32 mm, 6

ATLANTOMASTOID: A SCOPING REVIEW

31.85 mm, and 27.56 mm from these points (Meybodi et al., 2016). Given these points include the atlantomastoid muscle’s origin and insertion points, the muscle variant would be in close proximity to the

VA bulge when present. It would also be in close proximity to the rectus capitis lateralis muscle, which originates from the transverse process of the atlas and inserts along the jugular process of the occipital bone.

This muscle is an important surgical landmark for identifying neurovascular structures such as the vertebral artery, occipital artery, internal jugular vein, and cranial nerves VII and IX-XII when approaching the jugular foramen (Cohen et al., 2017). If the atlantomastoid muscle is present but not recognized during surgery, it may be mistaken for the rectus capitis lateralis muscle, particularly at its inferior end where both muscles attach to the transverse process of the atlas. The muscle variant’s proximity to various critical neurovascular structures may also impact these structure’s pathways, exemplifying its importance and need for consideration during posterior craniovertebral surgeries.

An understanding of the possible anatomical variations in the cervical musculature is also notably important for manual therapists such as chiropractors, physiotherapists, and massage therapists. The suboccipital region is an area that is often involved in cervical muscular complaints and treatment methods frequently focus on the muscles within this area (Espi-Lopez et al., 2014; Rezaeian et al., 2019). Studies have shown that muscular variations of the four main suboccipital muscles can also be potential causes of pain and other symptoms, since nerve fibers may be impacted or have altered pathways as a result of the anomalous muscles (Fisher, 2019; Gana & Ball, 2010). The suboccipital muscles have continually been shown to be involved in tension-type headaches (Fernandez-de-las-Penas et al., 2006; Fernandez-de-las-

Penas et al., 2010), which is the most common type of headache (Trkanjec & Aleksić-Shihabi, 2008) and remains a frequent complaint of clients presenting to manual therapists (Field, 2014). While studies have evaluated the role that the doubling or absence of the four suboccipital muscles plays in causing and treating such headaches (Fisher, 2019; Gana & Ball, 2010; Nayak et al., 2011), the atlantomastoid has never been considered and documented in the literature pertaining to cervical muscular complaints. Evidently, there remains a need to synthesize and communicate knowledge regarding lesser-known anatomical variants such

ATLANTOMASTOID: A SCOPING REVIEW as the atlantomastoid muscle among many health professionals, as it may provide useful information when making medical decisions and forming health treatment plans.

Figure 2. Location of vertebral artery bulge in suboccipital region. Red dot = mastoid process; cyan dot = tip of the transverse process of the atlas; orange dot = posterior tubercle of the atlas; white X = location of vertebral artery bulge. Adapted from “H24” by Queen’s University Department of Biomedical and Molecular Sciences, n.d., Scalable Gross Anatomy and Histology Image Catalogue. Copyright 2020 by Stephen Pang. Reprinted with permission.

The Need to Synthesize Information About the Atlantomastoid Muscle

To summarize, the atlantomastoid muscle remains widely under-discussed in the literature, which is concerning for a number of reasons. Anatomical variations are frequently cited as causes or factors in clinical malpractice (Tomos et al., 2019; Willan & Humpherson, 1999; Yammine, 2014), demonstrating the need for them to be given greater consideration in medical care. When present, the atlantomastoid

ATLANTOMASTOID: A SCOPING REVIEW muscle is in close proximity to many important structures in the cervical region that are of surgical interest, yet it is likely absent from any medical curricula and was not mentioned in any surgical publications. Yet students and faculty from medical and health professional schools recognize anatomical variations as being an important topic that should be included in student educational experience (Goldberg & Royer, 2016;

Lazarus et al., 2017). Surgeons, radiologists, and manual therapists may all particularly benefit from an increased awareness of cervical musculature variations such as the atlantomastoid, as it may reduce iatrogenic injury to the area and provide deeper understanding of potential causes of patient/client issues

(Fisahn et al., 2017; Fisher, 2019; Gana & Ball, 2010; Ogeng’o, 2013; Przywara, 2016; Shao et al., 2016).

It is the responsibility of researchers, educators, and all those involved in knowledge translation to support the documentation, dissemination, and discussion of anatomical variations such as the atlantomastoid muscle so that they may contribute to the improvement of health care provision.

Dissections are frequently cited as an irreplaceable component of anatomical education (Ghosh,

2017; Goldberg & Royer, 2016; Granger, 2004; Thompson & Marshall, 2020) which also provide the opportunity to discover variations since each cadaver presents a unique case. However, the frequency and amount of variations that can be observed in the human body make it impossible for a student to learn about all of the variations through dissection alone. Knowledge translation through discussion with consultants and various forms of documentation therefore has an important role in educating individuals on unique or rare observations made by others (Bale et al., 2018; Raikos & Smith, 2015; Sanudo et al., 2020). Literature reviews can be particularly useful since they identify, evaluate, and summarize available information regarding anatomical variations which may be limited or dispersed (Fink, 2019). The atlantomastoid muscle is one variation in which there is extremely limited content available within the literature, and no review has been conducted despite its substantial frequency of occurrence. There remains a need to first compile all of the current literature regarding the atlantomastoid muscle and then identify what must still be further documented and discussed. In doing so, this will support the proper representation of the atlantomastoid muscle within the literature and increase awareness regarding its anatomical presence and clinical importance. The objective of this paper is to accomplish these tasks. 9

ATLANTOMASTOID: A SCOPING REVIEW

There are numerous benefits in having a synthesized collection on anatomical variations, and the scoping review presented here provides a small contribution to that greater collection. This review synthesized knowledge regarding the atlantomastoid muscle so that it could be disseminated to increase awareness and deepen understanding regarding the muscular variant. It can be used to enhance educational materials addressing anatomical variants in health professional programs, as well as improve biomechanical, computerized, and other simulator models. This scoping review sought to identify gaps within the literature that must be addressed if we are to have a more complete understanding of the cervical muscles and possible variants. Collecting and summarizing the current literature regarding the atlantomastoid muscle will assist in increasing awareness of this muscle and perhaps even generate a greater appreciation for muscular variations in human anatomy among readers.

ATLANTOMASTOID: A SCOPING REVIEW

Chapter 2

Methods

Initial Methods

A scoping review was conducted with guidance from the Arksey and O’Malley framework (Arksey

& O'Malley, 2005), adaptations recommended by Levac et al. (2010), and the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) extension for scoping reviews (PRISMA-ScR)

Checklist (Tricco et al., 2018). Relevant literature was identified by searching through multiple databases.

Specifically, Excerpta Medica database (Embase), Web of Science, and ProQuest were searched during

May 2020 with publication date parameters of the oldest available date through to May 2020. The full electronic search strategies for each database can be found in Appendices A-C.

For the primary analysis, titles and abstracts of all initial search results were independently screened by two reviewers to determine if the literature was suspected to be relevant and meet the inclusion criteria as outlined below. For literature that did not have abstracts or were a part of a larger text (e.g. conference proceedings), the entire text was screened. After this primary analysis, all sources identified as relevant by either reviewer underwent full analysis and were read in their entirety to determine inclusion in the final synthesis of results. Those considered to have met the inclusion criteria by both reviewers were included.

Sources identified as relevant by only one reviewer were subsequently discussed until there was consensus.

Reference lists of all sources considered to have met the inclusion criteria were then independently searched by both reviewers to identify any potential documents that were missed. Reviewers then independently read any newly identified sources to determine inclusion. Any discrepancies regarding inclusion were discussed by the reviewers until decisions were unanimous.

All information identified in the literature and deemed relevant to the atlantomastoid was recorded and is outlined in the results section. This includes the muscle variant’s origin, insertion, and innervation, as well as any additional author commentary that supported or uniquely contributed to the body of

ATLANTOMASTOID: A SCOPING REVIEW knowledge. The format and details of each source was also recorded, including which sources contained original observations and which authors were more frequently cited.

Inclusion Criteria

To be included in the final synthesis of results, the literature had to mention the atlantomastoid muscle either by name or by its features, in that it described a muscle variant passing from the transverse process of the atlas to the mastoid process. Published and unpublished (“grey”) literature were both included in the results of the scoping review search. The literature also had to discuss the atlantomastoid muscle observed in humans, as opposed to in other animals, and had to be written in the English language.

Quantitative and qualitative research, case or cadaveric studies, review articles, conference abstracts or proceedings, dissertations, and theses were all eligible for inclusion.

Post Hoc Methods

A post hoc search of the literature using a narrative review-style search method was conducted to determine if there were additional sources that discussed the atlantomastoid muscle but were not included in the search results of the scoping review. Search terms provided in Appendices A-C were used for the narrative review, with terms being refined, edited, or added based upon search results. All narrative review searches were conducted in Google Scholar. The literature search and review of results were conducted by a single reviewer to expedite the process and accommodate availabilities of both reviewers. Any sources that fit the inclusion criteria were included in the final synthesis of results.

A second post hoc decision was then made to include sources that met the inclusion criteria but were written in non-English languages. Therefore, all non-English sources identified during the scoping and narrative review searches were re-reviewed for inclusion by the lead reviewer. Languages were either known by the lead reviewer or translated using Google Translate with subsequent editing by a professional translation service. Those which were not in a known language by the lead reviewer were noted.

ATLANTOMASTOID: A SCOPING REVIEW

Rationale for Initial Methods and Post Hoc Methods

Knowledge synthesis is a key component of knowledge translation. It involves the collection, integration, and contextualization of a wide range of information related to a specific topic (Canadian

Institutes of Health Research, 2016), and it plays a necessary role in the improvement and advancement of human health services and research. There are multiple approaches to knowledge synthesis, each with varying degrees of consistency regarding definition and methodology (Arksey & O'Malley, 2005). Scoping reviews have been more recently defined, with updated iterations of the methodological framework (Levac et al., 2010; Tricco et al., 2018). The first methodological framework was published in 2005 by Arksey and

O’Malley, who encouraged others to continue refining the aim and methodology of scoping reviews

(Arksey & O'Malley, 2005). Five years later, Levac et al. (2010) proposed enhancements to the framework that focused on clarifying the research purpose, determining the breadth and depth of the research, using an iterative approach to study selection and data extraction, and clarifying the reporting of results. Most recently, the PRISMA-ScR Checklist was developed to further improve reporting quality and included a checklist to promote uptake (Tricco et al., 2018).

Scoping reviews vary in purpose; they can be used to review topics that may have limited literature or multiple applicable study designs (Canadian Institutes of Health Research, 2010; Levac et al., 2010;

Tricco et al., 2018). They are an increasingly popular choice for reviewing health research evidence (Levac et al., 2010; Tricco et al., 2018), perhaps as a result of their vast utility. The literature regarding the atlantomastoid muscle is limited, with few studies providing primary sources of data. Secondary sources are also scarce and vary in regards to their reporting format and publication requirements. With limited and variable sources, a scoping review was chosen as the most appropriate form of knowledge synthesis for the topic.

Electronic databases were used in the search methodology because they allow for a rapid search of an extensive collection of articles and sources. While Medical Literature Analysis and Retrieval System

Online (MEDLINE) is frequently used in knowledge synthesis search strategies, Embase was chosen in preference over MEDLINE as it includes all of MEDLINE content plus additional literature, including 13

ATLANTOMASTOID: A SCOPING REVIEW conference abstracts (McGill University Health Centre Libraries, 2019; Women and Newborn Health

Service). Additionally, Web of Science was used to search all subscribed databases, including MEDLINE,

Web of Science Core Collection, BioSciences Information Service of Biological Abstracts (BIOSIS)

Previews, Korean Citation Index (KCI) – Korean Journal Database, Russian Science Citation Index, and

Scientific Electronic Library Online (SciELO) Citation Index. Search strategies, including selection of databases and keyword terms, were developed in consultation with Queen’s University librarians who are specialized in health-related library searches.

Initial search results were independently reviewed by two reviewers to reduce human error and allow for reviewers to discuss study inclusion uncertainties. This is in alignment with current frameworks and scoping studies, which recommend a minimum of two reviewers to strengthen methodology and findings (Levac et al., 2010). In this scoping review, the second reviewer was used to conduct a second independent review of the scoping review search results and identify literature to include in the final synthesis of results.

A narrative review-styled search was conducted post hoc to find any additional sources that may not have been identified through the scoping review search strategy. Rather than follow an explicitly designed methodology, narrative reviews are conducted without pre-defined search terms and are sorted by a reviewer using their experience and knowledge rather than inclusion and exclusion criteria (Dickson et al., 2017). This provides greater flexibility in the search strategy and allows the reviewer to cater their approach to data synthesis to the research at hand (Ryan, 2013). Although narrative reviews are rarely regarded as comprehensive, they can be useful in identifying overlooked content and information gaps

(Dickson et al., 2017). However, this methodology has been critiqued for producing potentially biased results given the lack of structure and clarity in search strategies and data analyses (Baker, 2016). Yet a narrative review-styled approach to the literature search made it possible to identify texts related to the atlantomastoid that would otherwise be missed using the more rigorous methodology of a scoping review.

With the scoping review search strategy producing very few sources, it was beneficial to conduct a post

ATLANTOMASTOID: A SCOPING REVIEW hoc search of the literature using a narrative review search strategy to identify additional sources that were not initially captured.

The inclusion criteria were widely non-specific to limit the likelihood of missing potentially relevant articles. Search strategies used truncated key terms to avoid excluding publications with variations in these terms. Publication date parameters were inclusive of all possible dates to include foundational texts.

The inclusion criteria for the initial scoping review search strategy required all sources to be written in

English because both reviewers spoke the language and because the majority of the scientific literature is published in English and continues to shift towards publishing in this language (Hamel, 2007). Additionally, there is evidence that suggests that restricting literature searches to English-only sources may not have an impact on the results for a majority of cases (Hartling et al., 2017). However, limiting inclusion to English- only sources also risks excluding valuable contributions from non-English studies and creating a language bias. Although the initial intention of this review was to only include English sources, non-English texts were included post hoc in the final synthesis of results given the limited number of sources that met the inclusion criteria.

ATLANTOMASTOID: A SCOPING REVIEW

Chapter 3

Results

Initial searches identified 1360 references. Removal of duplicates resulted in 1325 references for review. During initial screening, 40 references were either inaccessible or not in English and removed from initial consideration for reasons detailed in Table 1. Among the remaining references, nine were identified by reviewers as being potentially relevant. Upon reading the complete text of all nine articles, neither reviewer identified any content relevant to the atlantomastoid muscle. Reference lists from these articles were then reviewed and brought an additional five references to the reviewers’ attention, two of which were deemed relevant and meeting inclusion criteria. A subsequent collection of relevant articles using a narrative review-style search method resulted in identifying seven additional sources that met the inclusion criteria. Collectively, nine English sources were included in the final synthesis of results. Please see Figure

3 for a flowchart of this process and Table 2 section A for a summary of the included English sources.

Table 1

Reasons References Were Removed From Initial Scoping Review Search Results and Count of Occurrence

Reason for non-inclusion Count

Not English 12

Components of graduate work made inaccessible as per the authors’ requests 24

Unavailable through all accessible libraries and electronic databases 4

Total 40

ATLANTOMASTOID: A SCOPING REVIEW

1360 sources identified through scoping review

35 duplicate sources

1325 unique sources identified

28 inaccessible sources 12 non-English sources

1285 sources undergone primary analysis 1276 sources not meeting inclusion criteria after primary analysis 9 sources undergone full analysis 9 sources not meeting inclusion criteria after full analysis

0 sources meeting inclusion criteria; references searched for additional sources

5 sources found in references

2 sources meeting inclusion criteria

7 sources identified through post hoc narrative review and meeting inclusion criteria

9 sources meeting inclusion criteria 6 non-English sources identified in scoping and narrative review results and included post hoc 15 sources included in final synthesis of results

Figure 3. PRISMA flow diagram of the scoping review process and incorporation of post hoc narrative review search results.

ATLANTOMASTOID: A SCOPING REVIEW

The post hoc review of all non-English sources generated during the scoping and narrative review searches resulted in six non-English sources being included in the final synthesis of results: two from the scoping review and four from the narrative review search results. See Table 2 Section B for a summary of the included non-English sources.

All identified texts that discussed the atlantomastoid muscle were published between 1732 and

2016. Less than half of the sources contained original observations. The oldest report was by Winslow, who published a French text in 1732 based upon his dissection findings that included the description of an occasional muscle with the characteristics of the atlantomastoid (Winslow, 1732). This was followed by

Bankart et al. (1869) who provided a summary of observations made between 1866 and 1868 from their dissections of 158 cadavers. Gruber then published a German text in 1876 that contained his observations from 50 cadavers (Gruber, 1876). This was the only text included in the final synthesis of results that was not in a reviewer’s spoken language and was therefore machine translated followed by professional edits.

Knott also provided a report on his dissection observations collected over four years as

Demonstrator of Anatomy in the School of the Royal College of Surgeons and stated that he carefully examined 33 subjects for the atlantomastoid muscle (Knott, 1883). Le Double provided a summary of the texts printed prior to 1897 that discussed the atlantomastoid and added his own observations from 52 cadavers (Le Double, 1897). The two most recent publications of original observations were by Lewis in

1920 and Mori in 1964. Lewis (1920) provided a reconstructed model of the human chondrocranium based upon that of a 21 mm embryo and discussed its characteristics in detail, including muscular attachments.

Mori (1964) recorded observations on the musculature of the Japanese and summarized the atlantomastoid muscle based upon his examination of 54 cadavers for the muscle variant. Three of the texts that are discussed in this review also referenced a document with observations made by an author named Krause, however no document was found that matched these references and therefore could not be considered for inclusion.

ATLANTOMASTOID: A SCOPING REVIEW

Table 2

Summary of Sources Discussing the Atlantomastoid Muscle

Author(s) Year Source Details

Section A: English

Bankart, J., Pye-Smith, 1869 Primary Observations of abnormalities in the dissection P.H., and Phillips, J.J. room during Winter sessions of 1866-7 and 1867-8, examining 158 cadavers

Knott, J.F. 1883 Primary Observations from 4 years of dissection as Demonstrator of Anatomy, examining 33 cadavers

Quain, J. 1892 Secondary Practitioner’s manual

Brockway, F.J. and 1893 Secondary Textbook O’Malley, A.

Lewis, W.H. 1920 Primary Observations of modelled 21 mm embryo, compared with other embryonic specimens

Mori, M. * 1964 Primary Observations from 54 cadavers of Japanese origin

Rickenbacher, J., Landolt, 2013 Secondary Textbook A.M., and Theiler, K.

Bakkum, B.M. and 2016 Secondary Encyclopedia Miller, N.

Bergman, R.A., Afifi, - Secondary Digital encyclopedia A.K. and Miyauchi, R. *

Section B: Non-English

Winslow, J. 1732 Primary Observations from dissections throughout career

Gruber, W. * 1876 Primary Observations from 50 cadavers

Testut, L. 1884 Secondary Book

Testut, L. 1896 Secondary Book

Le Double, A-F. 1897 Primary Book including original observations collected throughout career

Vallois, H.V. * 1926 Secondary Report from societal meetings

Note: * demarks source was identified through scoping review.

ATLANTOMASTOID: A SCOPING REVIEW

In addition to the seven sources containing original observations, eight secondary sources discuss the atlantomastoid muscle, providing a description of the muscle variant and occasionally summarizing characteristics from previous dissection reports. Five of these sources were English textbooks, manuals, or encyclopedias and one was a subsection of a digital anatomical encyclopedia. The other three were German and French texts that were part of textbooks or societal reports. Key findings from all included articles are summarized in this review.

Origin and Insertion

The atlantomastoid muscle arises from the transverse process of the atlas and inserts onto the posterior margin of the mastoid process of the temporal bone (Bakkum & Miller, 2016; Bankart et al., 1869;

Bergman et al., n.d.; Brockway & O’Malley, 1893; Gruber, 1876; Knott, 1883; Le Double, 1897; Lewis,

1920; Mori, 1964; Quain, 1892; Rickenbacher et al., 2013; Testut, 1884; Winslow, 1732), as illustrated in

Figure 4 and Figure 5. According to Gruber (1876) and Le Double (1897), it arises between the obliquus capitis superior and rectus capitis lateralis muscles, while Winslow (1732) stated that its attachment point is near that of the obliquus capitis superior and inferior muscles. Gruber reported that the line of insertion has a length between 3-18 mm and is located either along the posterior border of the mastoid process, the posterior and upper border of the digastric groove, along the mastoid ridge between the digastric groove and occipital artery sulcus in a line towards the mastoid foramen, or a combination of these. He also stated it could be 2-17 mm above the tip of the mastoid process and 8 mm below and 2-17 mm distant from the cranial insertion of the obliquus capitis superior muscle. The summary of information regarding the atlantomastoid muscle by Testut (1884) reiterated all of this information stated by Gruber. Le Double also specified that the size and specific location of the line of insertion may vary, reporting the same insertion points as Gruber but with a length of insertion between 3-20 mm.

ATLANTOMASTOID: A SCOPING REVIEW

Accessory Slips

Gruber stated that the atlantomastoid has possible accessory slips with the middle scalene, levator scapulae, splenius capitis, and longissimus cervicis (transversalis cervicis) muscles. He specifically noted that the longissimus cervicis muscle may contribute bundles of muscle fibers to the origin of the atlantomastoid muscle (Gruber, 1876). Mori also identified occasional accessory slips, specifically in three cases: one with a slip arising from the transverse process of the atlas to the splenius capitis fasciculus, a second with a slip from the lower atlantomastoid muscle to the transverse process of C2, and a third with a slip from the upper atlantomastoid muscle to the longissimus capitis muscle (Mori, 1964). Neither Gruber nor Mori identify whether these slips are muscular, tendinous, or other in their publications. Aside from those that reiterate Mori’s findings, no other sources mentioned accessory slips.

Characteristics

Gruber (1876), Testut (1884, 1896), and Le Double (1897) all described the atlantomastoid muscle as being cylindrical, band-like, ribbon-like, or fusiform in shape, while Lewis (1920) noted that it is longer and more slender than the occipitomastoid, another muscular variant. According to Gruber, those that are cylindrical or band-like usually widen towards the superior end but may also widen towards the inferior end or both (“bisquitförmig”). Gruber also specified that the muscle variant may be sinewy or fleshy

(“sehnig-fleischig”) with tendinous or aponeurotic attachments of varying lengths and widths that may occupy one sixth to one third of the muscle, particularly at the end of insertion. The same proportion of tendinous component was also reported by Testut. Recorded muscle lengths by Gruber range between 20-

55 mm (reiterated by Testut) with varying widths between 2-24 mm and thickness between 1-4 mm for the muscle belly and 1 mm for the tendon.

ATLANTOMASTOID: A SCOPING REVIEW

Figure 4. Illustration by Gruber (1876) of atlantomastoid muscle from left lateral and posterior views. 1 = mastoid process of temporal bone; 2 = occipital bone; 3 = part of atlas; 4 = part of axis; a, a’ = atlantomastoid muscle, highlighted in yellow; b, b’ = posterior belly of digastric muscle; c, c’ = obliquus capitis superior muscle; d, d’ = obliquus capitis inferior muscle; e, e’ = rectus capitis posterior major muscle; f, f’ = rectus capitis posterior minor muscle; g, g’ = supernumerary rectus capitis posterior minor muscle; h = rectus capitis lateralis muscle; i = middle scalene muscle; k, k’ = levator anguli scapulae muscle; l, l’ = splenius cervicis muscle; m = transversalis cervicis muscle. Reprinted from “Über den musculus atlantico-mastoideus” by W. Gruber, 1867, Archiv für Anatomie, Physiologie und Wissenschaftliche Medicin, pp. 733-738. Licensed under CC-BY-NC 3.0.

ATLANTOMASTOID: A SCOPING REVIEW

Figure 5. Illustration by Mori (1964) identifying the atlantomastoid muscle (Δ) in all observed cases. From “Statistics on the Musculature of the Japanese” by M. Mori, 1964, Okajimas Folia Anatomica Japonica, 40(3), p. 243 (10.2535/ofaj1936.40.3_195). Copyright by Editorial Board of Okajimas Folia Anatomica Japonica. Reprinted with permission.

ATLANTOMASTOID: A SCOPING REVIEW

Location and Pathway

Early documentation by Bankart et al. (1869) described the atlantomastoid muscle as passing between the digastricus and obliquus capitis superior muscles. Gruber (1876) noted that it travels obliquely from the transverse process of the atlas towards the mastoid process, passing through a triangle defined by the posterior border of the mastoid process, the obliquus capitis superior muscle, and a line drawn between the apex of the mastoid process and the transverse process of the atlas. Gruber also stated that it is deep to the sternocleidomastoid, splenius capitis, and longissimus capitis muscles and possibly the aforementioned triangle, while Mori (1964) specified that the muscle variant can be observed by the medial region of the longissimus capitis, deep to the splenius capitis, and is separated by the occipital artery from the obliquus capitis inferior muscle and sometimes from the longissimus capitis. Testut (1884) reiterated the findings by

Gruber and noted that the atlantomastoid muscle jumps over the jugular process of the occipital bone as it passes from origin to insertion. Le Double (1897) confirmed from his observations that the atlantomastoid muscle passed through the triangle as described by Gruber and again by Testut. Bergman et al. (n.d.) as well as Bakkum and Miller (2016) referenced Mori’s observations when discussing the location of the atlantomastoid in relation to other muscles.

Frequency

There remains no consensus regarding the frequency of observation of the atlantomastoid muscle.

Lewis (1920) described it as a rudimentary muscle that is absent in the adult human, whereas Quain (1892) and Bergman et al. (n.d.) said it is often present. Knott (1883) reported observing the muscle variant four times in 33 subjects, although he cautioned readers that the statistics of his findings are “not so valuable as could be desired as an index of frequency” (p. 407). Gruber (1876) provided greater details regarding his findings, noting he examined 50 cadavers (45 male, five female) and observed the atlantomastoid in 11 of them, four presenting the muscle bilaterally and seven with the muscle on the left side only, indicating that the muscle variant was therefore present in 15% of sides. He classified the atlantomastoid muscle as one of

ATLANTOMASTOID: A SCOPING REVIEW the more common abnormal muscles (“zu den häufiger vorkommenden anomalen Muskeln zu zählen”)

(Gruber, 1876, p. 734).

Knott (1883) did not elaborate on laterality of the atlantomastoid muscle but did state that a text by an author named Krause contained reports of a relative frequency of 30%. Le Double (1897) observed the muscle variant in nine of 52 cadavers; more specifically, he observed it in six of 30 males and in three of

22 females. Of the male cases, it was present bilaterally four times and on the left twice. Of the female cases, it was present bilaterally twice and on the right side once. When combining his findings with those of previous publications by Knott, Krause, and Gruber, he estimated the atlantomastoid is present in 25-

26% of cadavers. Prior to Le Double’s publication, Testut (1884) also tallied observations from Gruber,

Krause, and Knott and reported a similar frequency of 45/183 or approximately 25%. Most recently, Mori

(1964) observed the atlantomastoid in 14 of 54 cadavers and noted that the muscle was present bilaterally in five cadavers, on the right side in five cadavers, and on the left in four cadavers, resulting in 17.6% of sides displaying the muscle variant. Bergman et al. (n.d.) as well as Bakkum and Miller (2016) referenced

Mori’s data when discussing frequency.

Innervation

Three of the sources identified in this review discussed innervation of the atlantomastoid muscle.

Mori (1964) stated with certainty that it is innervated by the dorsal branch of the first cervical spinal nerve, also known as the suboccipital nerve, mentioning that this same branch also innervates the superior region of the longissimus capitis and the obliquus capitis superior muscles. The texts by Bergman et al. (n.d.) and

Bakkum and Miler (2016) referenced Mori when stating the atlantomastoid muscle is innervated by the dorsal root of the first cervical spinal nerve but did not provide further evidence to support this argument.

No other text speculated on nerve supply.

ATLANTOMASTOID: A SCOPING REVIEW

Associated Muscles

The atlantomastoid was described as a new muscle found among the suboccipital muscles by

Vallois (1926) in his 1926 publication. However, the earliest observations of the muscle variant were made by Winslow in 1732, and it had also been previously described by Quain (1892) as a variety of the suboccipital muscles. Winslow (1732) classified it as an accessory or supernumerary muscle (“les petits accessoires ou surnumeraires”) (p. 245) that is commonly considered a fifth muscle among the anterior vertebral muscles when found but is more likely an accessory muscle of the obliquus capitis superior. The recent publication by Rickenbacher et al. (2013) also reported it as a part of the obliquus capitis superior muscle. Gruber (1876) compared the atlantomastoid muscle to the rectus capitis lateralis muscle, commenting on how both originate from the transverse process of the atlas and insert onto processes of cranial bones (mastoid process of the temporal bone and jugular process of occipital bone, respectively).

He noted that these processes are homologous to vertebral transverse processes, implying that the atlantomastoid is therefore an intertransverse muscle like the rectus capitis lateralis and not simply a duplication of the latter. Testut (1884) similarly report the atlantomastoid as a long intertransverse muscle.

Alternative Names

Alternative names to the atlantomastoid muscle include: rectus lateralis accessorius, atlantico- mastoideus, atloïdo-mastoïdien, and atlanto-mastoideus. It was originally referred to by Winslow (1732) as an accessory or supernumary muscle (“les petits accessoires ou surnumeraires”) (p. 245) under the subheading of muscles that move the head on the neck (“les muscles qui meuvent particulierement la teste sur le tronc”) (p. 240). Winslow did not provide an alternative name for the muscle variant except for suggesting that it may be called an accessory of the obliquus capitis superior muscle (“Accessoire de l’Oblique Superieur”) (Winslow, 1732, p. 245). Gruber (1876) named the muscle atlantico-mastoideus but acknowledged that it is the same muscle as the one identified by Winslow. Similarly, Le Double (1897) and

Testut (1884) used the term atloïdo-mastoïdien while acknowledging that it was previously called the rectus lateralis accessorius by Winslow, although this term was not found when reading Winslow’s text for this 26

ATLANTOMASTOID: A SCOPING REVIEW review. A later publication by Testut (1896) updated the comparison of synonymous names to note that

Gruber called it the atlantico-mastoïdeus. Around the same time as Testut’s publication, Knott (1883) published his own observations of the muscle variant and called it the atlanto-mastoideus. Mori (1964) used the same term but unhyphenated: atlantomastoideus. Most recently, Rickenbacher et al. (2013) called it the atlantomastoid muscle, as do Bakkum and Miller (2016).

ATLANTOMASTOID: A SCOPING REVIEW

Chapter 4

Discussion

Determining Prevalence

Despite the atlantomastoid muscle being present in a considerable proportion of examined specimens, there remains no consensus regarding the prevalence of it among humans. The most recent and thorough investigation into the atlantomastoid muscle’s prevalence in a given population was conducted by

Mori in 1964. He reported observing the muscle in 25.9% of cadavers or 17.6% of sides with near-equal counts observed on the left side, right side, and bilaterally (Mori, 1964). Perhaps due to the level of detail in which he described the atlantomastoid, his findings were referred to by two of the three other sources in this review that were published after 1964 (Bakkum & Miller, 2016; Bergman et al., n.d.).

Prior to Mori’s work, the only English publication found to document the muscle’s frequency of observation was written by Knott in 1883. Although Lewis (1920), Quain (1892), and Rickenbacher et al.

(2013) provided other early English publications acknowledging the atlantomastoid muscle, Lewis only acknowledged it in embryonic stages, Quain did not speculate on the muscle’s frequency of observation beyond it being “often present” (p. 319), and Rickenbacher et al. only referenced Gruber with an approximate occurrence of 20% but speculated that this was probably too high. Knott (1883) reported observing the muscle in four of 33 subjects (12.1%) and compared his findings to those in the unidentified text by Krause, stating that the prevalence reported in his own findings was much lower than the 30% documented by Krause, but acknowledged that they “…were not so valuable as could be desired as an index of frequency” (p. 407). Mori (1964) reported observing the atlantomastoid muscle more often among cadavers (25.9%), albeit not as frequently as 30%.

Among the Non-English sources, three publications contained information regarding the atlantomastoid muscle’s frequency of observation. Gruber (1876) was the first to publish such findings, reporting that the muscle was observed in 11 of 50 cadavers (22%) and specifically 15 of 100 sides (15%). 28

ATLANTOMASTOID: A SCOPING REVIEW

Testut (1884) then estimated a frequency of approximately 25% by combining reported data from Gruber,

Krause, and Knott. LeDouble’s (1897) publication was the most recent non-English source to discuss frequency and combined his findings with Gruber, Krause, and Knott, reporting an estimated rate of prevalence of 25-26%. Independently, LeDouble observed the atlantomastoid muscle in 17.3% of cadavers and more often in males (20%) than females (13.6%). LeDouble is also the only author in addition to Mori to report laterality. While Mori (1964) observed a near equal likelihood of presence on either or both sides,

LeDouble’s findings suggest that bilateral presence may be more common than unilateral.

Collectively, reported frequencies of observation range between 15% to 17.6% of sides and 12.1% to 25.9% of cadavers. If the references to Krause’s data are considered, then the upper limit of prevalence in cadavers increases to 30%. Combining all observations reported in all publications, frequencies of observation are 15.7% (49/312) of sides based upon the observations by Mori (1964), Gruber (1876), and

LeDouble (1897), and 20.1% (38/189) of cadavers based upon observations by Mori, Gruber, LeDouble, and Knott (1883). Although these four authors reported specific counts of the muscle’s presence, only Mori provided information regarding nationality or ethnicity, as he specified that his observations were regarding a Japanese population. Additionally, although Gruber stated the total number of male and female cadavers examined, only LeDouble provided categorized observations according to sex.

The discrepancies in the limited findings regarding the prevalence of the atlantomastoid muscle may be attributed in part to unreported demographics. The work of Mori (1964) was conducted using cadavers from the Japanese population but did not specify the donors’ sexes nor any major medical conditions that may be of relevance. Gruber (1876) and LeDouble (1897) specified the sexes of the donors, although LeDouble provided a slightly greater amount of detail by categorizing observations according to sex and reporting laterality of the muscle variant. Knott (1883) provided the least amount of information among the four publications that discussed frequencies by giving no description of the demographics of the donors. All other texts that discuss frequency are secondary sources. More information regarding donor demographics of cadavers presenting with the atlantomastoid muscle variant may assist in determining patterns of prevalence between various demographic groups. 29

ATLANTOMASTOID: A SCOPING REVIEW

Clinical Impact of the Muscular Variant

The atlantomastoid muscle’s origin point, pathway, and insertion point puts it in close proximity to certain posterior cervical structures, depending in part on the normal variations of the atlantomastoid muscle location. The muscle variant originates on the transverse process of the atlas near the obliquus capitis superior muscle’s origin and passes deep to the splenius capitis muscle (Gruber, 1876; Testut, 1884) and between the digastricus and obliquus capitis superior muscles (Bankart et al., 1869) as it heads towards its insertion point. The line of insertion of the atlantomastoid muscle varies in regards to its length and position.

Measurements of the line of insertion varied between 2-20 mm and spanned from the tip of mastoid process to nearly the mastoid foramen (Gruber, 1876; Le Double, 1897; Testut, 1884). Occasionally, the atlantomastoid muscle has also been observed with accessory slips, with reports of slips originating from muscles as inferior as the middle scalene (Gruber, 1876) and as superior as the longissimus capitis (Mori,

1964). Collectively, this means that the muscle variant as well as its accessory slips may impact the location or pathway of important neurovascular structures such as the occipital artery, vertebral artery, and dorsal branch of the first cervical spinal nerve, as these structures have either been observed adjacent to the muscle variant or lie within close proximity to the space in which it would occupy (Gruber, 1876; Le Double, 1897;

Meybodi et al., 2016; Mori, 1964).

Muscular variants within the suboccipital region have been associated with compression and variations in the pathways of the vertebral artery (George & Carpentier, 2001) and greater occipital and third occipital nerves (Fisher, 2019), illustrating the potential impact that the atlantomastoid muscle could have on similar structures. Compression, in particular, may lead to adverse effects. Compression or entrapment of the occipital nerves is often considered to play an etiological role in occipital neuralgia, a condition characterized by paroxysmal suboccipital pain (Adamov & Fusco, 2017; Janjua et al., 2020).

Surgical decompression of the greater occipital nerve has resulted in considerable relief from migraine headaches for patients, suggesting that compression of the nerve may be related to the pathogenesis of migraine headaches (Junewicz et al., 2013). Signs and symptoms of vertebral artery compression are 30

ATLANTOMASTOID: A SCOPING REVIEW intermittent and can vary, including dizziness, changes in gait, blurred vision, nystagmus, or slight ataxia

(George & Carpentier, 2001). It has also been demonstrated that anomalous courses of the vertebral artery have been present in cases of iatrogenic injury during surgery (Guan et al., 2017; Osorio et al., 2014), suggesting that iatrogenic injury may arise if variability in arterial courses is not properly considered during preoperative and intraoperative checks. Similarly, variations in the occipital artery running pattern are also common and surgeons who are looking to use it as a donor vessel have been encouraged to consider these variations when harvesting it (Hatano et al., 2019). If the atlantomastoid muscle or its accessory slips cause aberrant courses in the occipital nerves, vertebral artery, or occipital artery, they may contribute to neuropathic conditions such as occipital neuralgia, vascular compression and associated symptoms, and surgical iatrogenic injury related to limited consideration of anatomical variations.

Inconsistencies and Required Clarity

As suggested by its name, the atlantomastoid muscle originates from the transverse process of the atlas and inserts along the mastoid process of the temporal bone. Gruber (1876) and Le Double (1897) claimed its origin is between the obliquus capitis superior and rectus capitis lateralis muscles while Winslow

(1732) stated that it is near that of the obliquus capitis superior and inferior muscles. It is possible that

Winslow was implying that the atlantomastoid muscle originated between these two muscles, however this would then be in contrast to the comments made by Gruber and Le Double. Winslow’s comment could also support their observations if it is interpreted as an estimation of the muscle variant’s origin, as an origin point between the obliquus capitis superior and rectus capitis lateralis muscles could arguably be considered near the obliquus capitis superior and inferior muscles. Yet in Gruber’s sketches, it appears that there were cases of the atlantomastoid muscle originating both anteriorly and posteriorly to the origin of the rectus capitis lateralis muscle (see Figure 4), suggesting that the muscle variant may arise between different muscular attachments along the superior portion of the atlas’ transverse process, depending on the cadaver being observed. Considering the variation in Winslow’s, Gruber’s, and Le Double’s comments, this would

ATLANTOMASTOID: A SCOPING REVIEW appear to be the most likely case, although additional and more detailed documentation regarding the muscle’s origin would provide greater clarity.

There was more agreement in regards to the atlantomastoid muscle’s insertion, as most authors did not specify beyond identifying the mastoid process as the location of insertion and the others (of which there were only two with original observations) had nearly identical descriptions of the specific insertion location. Gruber (1876) and Le Double (1897) both noted that the muscle variant inserted anywhere along the posterior edge of the mastoid process, digastric groove, and mastoid ridge between the digastric and occipital artery grooves, with varying lengths in the line of insertion. The two authors only differed in their range of measurements of insertion length, with Le Double having a larger range by 2 mm.

Although Gruber (1876) and Mori (1964) discussed contributing muscles and accessory slips to the atlantomastoid muscle, neither the compositions nor the frequency of these structures were described. It is likely that some of these accessory slips are at least partially tendinous, as previous observations of the atlantomastoid muscle reported it having tendinous or aponeurotic points of attachment (Gruber, 1876;

Testut, 1884). Gruber also listed numerous muscles as possible contributors to the atlantomastoid muscle and Mori observed three slips out of 14 cadavers, suggesting that accessory slips were likely present albeit not reported in the cadavers observed by Winslow (1732), Bankart et al. (1869), Knott (1883), and Le

Double (1897). Therefore the rate of occurrence of such accessory slips is unknown. Additional information regarding the presence and composition of accessory slips and common contributing muscles would provide a more complete understanding of the atlantomastoid muscle’s associations with nearby structures and possibly provide insight into its embryological origins.

The location of the occipital artery in relation to the atlantomastoid muscle has been briefly discussed but ought to be further evaluated. Mori (1964) reported that the occipital artery separates the muscle variant from the obliquus capitis inferior muscle and occasionally from the longissimus capitis muscle as well. However, given the general course of the occipital artery travelling superior to the atlas and then running along the occipital groove of the temporal bone and towards the external occipital protuberance, the vessel likely separates the atlantomastoid muscle from the obliquus capitis superior 32

ATLANTOMASTOID: A SCOPING REVIEW muscle rather than from the obliquus capitis inferior as stated by Mori. What’s more, the atlantomastoid and obliquus capitis inferior muscles are only in proximity to one another at their attachment points on the atlas and not their muscle bellies, further suggesting that the occipital artery does not separate the aforementioned muscles. Unfortunately, no other publications discussed the position of the occipital artery in relation to the atlantomastoid muscle to provide further evidence. Additional information regarding this relationship would be clinically relevant to concerns regarding occipital arterial blood flow and may be of future research interest.

Depending on the location of the occipital artery in relation to the atlantomastoid muscle, the artery may also act as a potential blood supply to the muscle variant. Although the occipital artery can have a running pattern that is either lateral or medial to the longissimus capitis muscle, the medial pattern is more commonly observed (Hatano et al., 2019) and places the artery in close proximity to the atlantomastoid muscle. This suggests that it may be the primary blood supply for the muscle variant. Additionally, the suboccipital muscles and the rectus capitis lateralis muscle are all supplied by branches of the occipital and vertebral arteries, providing further reason to suspect that the atlantomastoid muscle also receives blood from these arterial branches given its proximity to these muscles. However there remains no discussion nor observation regarding the muscle variant’s blood supply, and further study is required to confirm any sources.

In addition to there being no discussion regarding the atlantomastoid muscle’s blood supply, there remains limited discussion regarding its innervation. Mori (1964) reported that he could distinctly see that the atlantomastoid muscle was supplied by the dorsal ramus of the first cervical spinal nerve, also known as the suboccipital nerve. However aside from two publications which referred to Mori when discussing innervation of the atlantomastoid muscle, no publications had additional original observations. The suboccipital nerve also supplies the nearby suboccipital muscles and so it would be understandable for it to supply the atlantomastoid muscle. Knowing if the muscle variant shared the same anlage with other muscles in the posterior neck region could assist in determining the atlantomastoid muscle’s innervation, however the only publication that discussed the atlantomastoid in an embryological specimen did not include 33

ATLANTOMASTOID: A SCOPING REVIEW comments on its development or relation to other muscles (Lewis, 1920). Thus, there remains only one author who has discussed the innervation of the atlantomastoid muscle and so additional research regarding its nerves supply is encouraged.

Lastly, there is currently no published photographic evidence of the atlantomastoid muscle. The most recent documentation of the muscle variant was in 1964, as part of a larger collection discussing the statistics of the musculature of a Japanese population. It contained simplified sketches of every atlantomastoid muscle observed by Mori (see Figure 5, Mori, 1964). Certain nearby musculoskeletal structures such as the mastoid process and suboccipital muscles appear to have been included, although these were not labelled and were not drawn with great detail. Additionally, neurovascular structures were not included in Mori’s illustrations. The only other publication to contain original sketches of the atlantomastoid muscle was by Gruber (1876), whose sketches were more detailed than Mori’s and contained nearby musculoskeletal structures, although his illustrations did not contain neurovascular structures either

(see Figure 4). All other publications either contained no illustration of the muscle variant or reproductions of those by Mori and Gruber. Given that Mori’s illustrations were lacking detail, Gruber’s are nearly 150 years old, and neither include surrounding neurovasculature, updated images of the muscle variant such as photographs and detailed sketches would be beneficial for reference.

Few Publications Identifying the Atlantomastoid Muscle

The results of this study have shown that there is scant discussion regarding the atlantomastoid muscle in the literature. In total, only 15 texts that discussed the atlantomastoid muscle were published since 1732 according to the results of this review; of them, only seven provided original observations of the muscle variant and characteristics. The most recent text that included original observations was published in 1964. The only other primary source published in the 20th century discussed the atlantomastoid muscle based upon a plaster model of a 21-mm embryo, but claimed that the muscle was rudimentary and not present in adult form (Lewis, 1920).

ATLANTOMASTOID: A SCOPING REVIEW

It remains possible that there are additional texts beyond those identified in this review that discuss the atlantomastoid muscle. One such text is possibly authored by German anatomist Wilhelm Krause. Three publications included in this review referenced a text by Krause when discussing certain characteristics and statistics regarding the muscle variant, yet no such document was found. Unfortunately, the three publications – authored by Knott (1883), Testut (1884), and LeDouble (1897) – did not provide citations for the references to Krause. In fact, Testut only cited Krause indirectly by referring to Knott’s description of Krause’s findings. If the publication by Krause were to be identified and acquired, it would provide an additional reference for information concerning the atlantomastoid muscle. Additionally, Bakkum and

Miller (2016) cited an 1867 publication by Gruber that was not found during the literature search for this review, although it is suspected that it is the same as the 1876 Gruber publication included in this review since they have identical collection titles, page ranges, and nearly identical section titles and publication dates. However, if there exists a text by Gruber published in 1867 that discusses the atlantomastoid muscle and differs from the text included in this review, this would provide an additional reference that may contain supporting information.

Comment on Methods

Scoping reviews are an increasingly popular form of knowledge synthesis that follow a methodological approach to map and evaluate current information on a variety of topics. They allow reviewers to investigate broader research questions, determine the breadth and depth of available evidence, and to frequently identify evidence gaps (Dickson et al., 2017; Tricco et al., 2016). While systematic reviews are often conducted to critically evaluate and synthesize knowledge addressing specific research questions, scoping reviews are recommended when research questions may be broad or when reviewers wish to examine the extent and range of a research topic (Dickson et al., 2017). Given the scarcity of content related to the atlantomastoid in the literature, a scoping review was selected for this review in an attempt to provide a comprehensive summary using a methodological and systematic approach. Yet it is clear from the results of the review that there exists limited literature which is neither easily discoverable nor 35

ATLANTOMASTOID: A SCOPING REVIEW accessible. When the scoping review search was conducted, the resulting list of sources to include in data analysis contained only two references. A post hoc narrative review was subsequently conducted in an attempt to identify any sources that may have been omitted from the results of the initial literature search.

This resulted in identifying an additional seven sources that met the initial inclusion criteria, with less than half of the total texts providing original observations. A second post hoc decision to include non-English sources resulted in an additional six papers being included in the results, with two having been identified through the scoping review methods. In the end, a total of 15 texts were included in the results with the scoping review methods contributing four of those 15.

The recommendations included within scoping review frameworks are useful in reducing potential biases during search strategy development and identifying them when evaluating methods and results. One of these recommendations is to include two or more reviewers (Levac et al., 2010), which helps to reduce data extraction errors (Dickson et al., 2017) and reviewer bias (McDonagh et al., 2013). This review therefore followed this recommendation and included two independent reviewers in the methods.

Additionally, inclusion criteria were specified and information sources and search strategy were defined, in accordance with recommendations (Levac et al., 2010; Tricco et al., 2018). The selected databases are also among the most commonly recommended one for health science research (Dickson et al., 2017; McGill

University Health Centre Libraries, 2019; Women and Newborn Health Service) and the dates of coverage were kept broad to capture all relevant sources. Publication bias is another form of potential bias that is of concern, and including grey literature is commonly seen as a way to reduce it (Hartling et al., 2017; Mahood et al., 2014; McDonagh et al., 2013). Although “grey” literature was searched for this review, no texts were found to meet inclusion criteria and therefore none were included in the final synthesis of results.

In addition to choosing a methodology that reduced reviewer bias and publication bias, non-English sources were included in the final synthesis of results in efforts to reduce language bias. Evaluations of systematic reviews demonstrate that there is conflicting evidence regarding the impact that non-English studies may have on review findings (Hartling et al., 2017; Neimann Rasmussen & Montgomery, 2018), although similar evidence for scoping reviews was not found. The inclusion criteria for this scoping review 36

ATLANTOMASTOID: A SCOPING REVIEW required sources to be in the English language but no language restrictions were used in the search methods for any database. Instead, non-English sources were manually excluded from the search results by reviewers. After conducting the post hoc narrative review, it was decided to include non-English sources since they appeared to be a main source of information regarding the atlantomastoid muscle. Nearly all of the non-English sources that were deemed worthy of inclusion were exclusively found through the narrative review search results and not the scoping review.

The post hoc narrative review that was conducted following the scoping review used the Google

Scholar search engine instead of a database, offering access to content that may not be indexed in other databases, such as texts that were published beyond the databases’ dates of inclusion. Narrative reviews, while critiqued for lacking transparency and structure (Baker, 2016), allow reviewers to more freely adjust search terms and strategy (Demiris et al., 2019), using an iterative refinement process similar to one proposed in some scoping review recommendations but without requiring the same amount of time to discuss and document each adjustment. This narrative review process added seven new sources to include in the final synthesis of results of this review that would have otherwise remained unknown to the reviewers.

The majority of these documents were printed prior to 1900, which explains their exclusion from the initial search results. However, narrative reviews are not regarded as including comprehensive literature searches

(Dickson et al., 2017; Demiris et al., 2019) and there may be additional relevant texts that remain excluded from this review.

The post hoc narrative review contained novel information regarding the atlantomastoid muscle such as alternative names for it, supplementing the initial findings from the scoping review. The narrative review made it known to reviewers that the atlantomastoid muscle may be referred to as the atlanto- mastoideus, as used by Knott (1883), or as the atloïdo-mastoïdien or atlantico-mastoïdeus as referred to by

Testut (1896). Vallois (1926) also reported observing a muscle named the atlanto-mastoïdien among the four suboccipital muscles which matched the description of the atlantomastoid muscle. Winslow (1732) called the muscle variant the rectus lateralis accessorius, but given Gruber’s (1876) argument of it being an intertransverse muscle that is similar to the rectus capitis lateralis muscle rather than an accessory to it, one 37

ATLANTOMASTOID: A SCOPING REVIEW could argue that the muscle variant’s name should distinguish it from the rectus capitis lateralis. It is worth noting that the encyclopedia that includes Bakkum and Miller (2016) described the rectus lateralis accessorius muscle in a section separate from the content discussing the atlantomastoid, making it seem as though these are two different muscles. However, other sources used these terms interchangeably and the evidence provided regarding these two terms suggest that they describe the same muscle.

Limitations

The scarcity of sources included in the final synthesis of results makes one pause and evaluate the effectiveness of the methods used and consider potential biases associated with scoping and narrative review search strategies. While the search strategies did not necessitate that texts contain the term atlantomastoid, expanding the search strategies to include the terms atlanto-mastoideus, atlanto-mastoïdien, atloïdo-mastoïdien, atlantico-mastoïdeus, and rectus lateralis accessorius may have resulted in additional texts of interest being included in analysis. In this review, one iteration of the scoping review search process was conducted, although a second or third iteration of the search would have allowed for the inclusion of these additional terms to be included, as suggested by Levac et al. (2010). This may have yielded more comprehensive results by finding additional, more recent publications. However, a second iteration was not conducted due to practical limitations in regards to time and reviewer availability. Additionally, the findings from the initial search showed that even the most recent publications were relying upon the older texts for information regarding the atlantomastoid muscle, suggesting that other more recent publications did not exist.

The findings of this review may have also been impacted by the dependence upon translation services for the one German publication by Gruber (1876) that was included in the final synthesis of results.

The text was translated using Google Translate, which has been shown to provide word-for-word translations that don’t always relay the proper meaning of the original text and can fail in recognizing idioms, leading to literal nonsensical translations (Amilia & Yuwono, 2020). This was followed by a post edit review by a professional linguist, however the quality of edits made following machine translation can 38

ATLANTOMASTOID: A SCOPING REVIEW vary and is dependent upon whether items such as spelling, grammar, syntax, style, and logic are reviewed or not. The professional service selected to translate the document by Gruber provided multiple forms of translation services, each with different editing and review focuses and costs. The service selected for this review focused on correcting errors and improving syntax and fluency (Linguist Systems, Inc., 2020).

Although a full human translation of the text with post edits and revisions would have provided a more accurate translation from which information could be extracted, the cost and administration required to obtain the service were barriers during this review. The lack of language resources such as professional translators has been previously reported as a frequent challenge in including non-English texts in reviews

(Neimann Rasmussen & Montgomery, 2018). Fortunately, certain English and French texts identified in this review discussed content presented in Gruber’s publication, providing confirmation of the content presented in the translated text.

The selection of certain databases over others may have also led to the exclusion of some texts, particularly if there were relevant texts that were only available in printed format. No database is truly comprehensive, and those selected for this review indeed had variations in the texts that they has articled.

Although multiple databases were used and selected in a manner to promoted the widest coverage of all potentially relevant texts, there remains the possibility that sources with relevant or useful information were not captured in the search results. For example, a text by Krause was referred to in three different sources, yet the referenced article was not found in any of the databases searched during this review. The farthest reaching database included references cited as early as 1900 (Clarivate, 2020; ProQuest, 2020; Wolters

Kluwer Health, 2020), yet many of texts that contained information regarding the atlantomastoid were published prior to then. These were only discovered upon conducting the post hoc narrative review. It is possible that selecting alternative databases during the scoping review search strategy would have brought these sources and others to the reviewers’ attention, although this is unlikely to have identified all additional relevant sources given that most databases do not index publications from the distant past (Nursing

Education Research Unit, n.d.). The results of this review demonstrate that no singular database should be relied upon for a complete and comprehensive search of the literature, particularly in cases where texts of 39

ATLANTOMASTOID: A SCOPING REVIEW interest may be published prior to the earliest indexed dates. For these older texts, alternative search engines may provide greater awareness and access than traditional databases.

In addition to the impact that database selection may have had on the results of this review, using search strategies that relied upon automated text searches as opposed to manual searching may have also impacted the results. The search strategies (identified in Appendices A-C) for the scoping review were designed to capture all texts that met the initial inclusion criteria. Databases were searched for texts that included the term “atlantomastoid” or any mention of a muscle between the atlas and mastoid, yet some sources included in the final synthesis of results were not identified in the scoping review search despite meeting these search requirements and having publication dates within database timespans. This may be because many of these texts were scanned images of documents and although their text can be extracted using optical character recognition, it is difficult to do so without error (Maekawa et al., 2019). For a database to generate search results that included these historical texts discussing the atlantomastoid, the documents would have to have the text extracted without error (particularly for the words matching the search terms), be printed within the searchable timespan, and indexed in the database collection. This may explain why the scoping review did not identify all relevant publications. Individuals may wish to consider these limitations when developing search strategies for literature reviews in which relevant content may be contained within historical documents.

ATLANTOMASTOID: A SCOPING REVIEW

Chapter 5

Conclusion

The atlantomastoid is a muscle variant found within the suboccipital region of the posterior neck that remains widely undiscussed in the literature despite it being identified in publications as early as 1732.

Gruber even noted in 1876 that the relatively high occurrence of the atlantomastoid muscle made it an important variant to know and understand better (“ist deshalb werth, genauer gekannt zu sein”) (Gruber,

1876, p. 734). Among the texts that do acknowledge the muscle variant, there is agreement that the atlantomastoid muscle arises from the transverse process of the atlas and inserts onto the posterior part of the mastoid process. The range in estimations of prevalence is wide, although the average of the available data suggests approximately 20.1% of individuals possess the atlantomastoid muscle. However, there is little to no evidence to support the muscle variant’s characteristics including composition of accessory slips, blood supply, innervation, and potential clinical impact, and nearly all reports of dissection observations failed to provide details regarding donor demographics such as sex, race, or geographical origin.

Additionally, the majority of the texts that were identified in this review were published over 100 years ago and only 60% were written in the English language. Newer observations and recordings of the muscle variant would provide evidence for more accurate statistics regarding its prevalence and a better understanding of its relations to and potential impact on nearby structures. Consequently, there is a clear need for more current and thorough research to be conducted in regards to the atlantomastoid muscle and for the acquired information to be more widely disseminated.

ATLANTOMASTOID: A SCOPING REVIEW

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ATLANTOMASTOID: A SCOPING REVIEW

Appendix A

Search Strategy for Ovid Embase Classic and Embase

Performed May 8, 2020. Total = 22 results.

Specifications:

- Advanced search

- All years (1947-2020)

- No limits

1. atlantomastoid*.mp.

2. atlanto-mastoid*.mp.

3. varia*.mp.

4. anomal*.mp.

5. abnormal*.mp.

6. exp Neck Muscles/

7. exp Cervical Atlas/

8. exp Mastoid/

9. suboccipital*.mp.

10. 1 or 2

11. 3 or 4 or 5

12. 11 and 6 and 7 and 8

13. 11 and 6 and 9

14. 13 or 12 or 10

ATLANTOMASTOID: A SCOPING REVIEW

Appendix B

Search Strategy for Web of Science All Databases

Performed May 8, 2020. Total = 320 results.

Specifications:

- Advanced search

- All languages

- All document types

- All years (1900-2020)

1. TS=(atlantomastoid OR atlantomastoideus)

2. TS=(atlanto-mastoid OR atlanto-mastoideus)

3. TS=((varia* OR anomal* OR abnormal*) AND musc* AND suboccipital*)

4. TS=(atl* AND mast* AND musc*)

5. #4 OR #3 OR #2 OR #1

ATLANTOMASTOID: A SCOPING REVIEW

Appendix C

Search Strategy for ProQuest Health and Medicine

Performed May 8, 2020. Total = 1018 results.

Specifications:

- Advanced Search

- Source type: all

- Document type: all

- Languages: all

1. (varia* OR anomal* OR abnormal*) AND musc* AND (atlas OR atlanto*) AND mastoid*

2. atlantomastoid* OR atlanto-mastoid*

3. 1 or 2

ATLANTOMASTOID: A SCOPING REVIEW

Appendix D

Permissions For Reprinting

ATLANTOMASTOID: A SCOPING REVIEW