WHAT IS KNOWN ABOUT THE ATLANTOMASTOID MUSCLE: A SCOPING REVIEW by 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) Copyright © Rebecca Anne Lee, 2021 Abstract 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. ii 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 iv 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 v 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 .........................................