Role of Psychosocial Factors in the Etiology of Bruxism

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Daniele Manfredini, DDS Aims: To summarize literature data about the role of psychosocial Visiting Professor factors in the etiology of bruxism. Methods: A systematic search in TMD Clinic the National Library of Medicine’s PubMed Database was per- University of Padova formed to identify all peer-reviewed papers in the English litera- Padova, Italy ture dealing with the bruxism-psychosocial factors relationship. All studies assessing the psychosocial traits of bruxers (by using Frank Lobbezoo, DDS, PhD Professor questionnaires, interviews, and instrumental and laboratory Department of Oral Function exams) and reviews discussing the contribution of those factors to Section Oral Kinesiology the etiology of bruxism were included in this review. Results: A Academic Centre for Dentistry total of 45 relevant papers (including eight reviews) were retrieved Amsterdam (ACTA) with a search strategy combining the term “bruxism” with the University of Amsterdam and VU words stress, anxiety, depression, psychosocial and psychological University Amsterdam factors. The majority of data about the association between psy- Amsterdam, The Netherlands chosocial disorders and bruxism came from studies adopting a clinical and/or self-report diagnosis of bruxism. These studies Correspondence to: showed some association of bruxism with anxiety, stress sensitiv- Dr Daniele Manfredini ity, depression and other personological characteristics, apparently Viale XX Settembre 298 54033 Marina di Carrara (MS) in contrast with sleep laboratory investigations. A plausible Italy hypothesis is that clinical studies are more suitable to detect awake Fax: 0039 0585 630964 bruxism (clenching type), while polysomnographic studies focused Email: [email protected] only on sleep bruxism (grinding type). Conclusion: Wake clenching seems to be associated with psychosocial factors and a number of psychopathological symptoms, while there is no evidence to relate sleep bruxism with psychosocial disorders. Future research should be directed toward the achievement of a better distinction between the two forms of bruxism in order to facilitate the design of experimental studies on this topic. J OROFAC PAIN 2009;23:153–166

Key words: anxiety, bruxism, depression, etiology, psychosocial factors, stress

ruxism is a stereotyped oral motor disorder characterized by awake and/or sleep-related grinding and/or clenching of the Bteeth.1 It is considered the most detrimental among all the parafunctional activities of the stomatognathic system, causing tooth wear and representing a major risk factor for temporomandibular disorders (TMD) as well. Nevertheless, despite the importance of its clinical effects, there are still many unsolved issues concerning the etiology of bruxism itself. The etiology of bruxism is a complex and controversial issue. Most authors have suggested a central etiology for bruxism, as pointed out by reviews suggesting a conceptual shift from peripheral (ie, occlusal) to central (ie, stress, emotion, personality)

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Table 1 Medline Search Strategy bruxism-psychosocial factors relationship does exist. Nonetheless, research studies seem to provide Search Word(s) Citations Reviews contrasting suggestions with respect to the clinical Bruxism 2,039 238 setting, being unable to demonstrate such relation- Bruxism and stress 267 43 ships. For these reasons, there is a need to summa- Bruxism and anxiety 70 11 rize data from the literature about the role of psy- Bruxism and depression 49 9 Bruxism and psychological factors 22 5 chosocial factors in the etiology of bruxism, which Bruxism and psychosocial factors 9 0 will be the focus of this systematic review.

Materials and Methods

On January 9, 2008, a systematic search in the regulation.2–6 Psychosocial factors could be National Library of Medicine’s PubMed Database involved in the etiopathogenesis of bruxism, but was performed to identify all peer-reviewed the theory that occlusal interferences represent a papers in the English literature dealing with the neuromuscular stimulus capable of triggering non- bruxism-psychosocial factors relation (Table 1). functional masticatory movements has not been The search strategy used the combination of the completely abandoned.7 Many etiological theories text word “bruxism” (which yielded 2,039 cita- have been proposed over the years, and a multi- tions if used alone), with the words “stress”, factorial model to explain bruxism etiology seems “anxiety”, “depression”, “psychological factors”, to be the most plausible hypothesis, according to or “psychosocial factors”. which psychosocial and pathophysiological fac- Two types of studies were selected and included tors interact with morphological-peripheral for discussion in this review: (1) research studies ones.8–10 addressing the psychosocial traits of bruxers (by The study of the etiology of bruxism is compli- using questionnaires, interviews, and instrumental cated by some diagnostic and taxonomic aspects and laboratory exams), and (2) studies reviewing which have prevented an acceptable standardiza- the available literature on the contribution of these tion of diagnosis to be achieved until recently. A factors to the etiology of bruxism. major concern for researchers approaching this Titles and abstracts obtained from the above phenomenon is the definition of bruxism itself, search were screened according to the type of which is a term grouping different entities.3 For study for possible admittance in the review: all example, “sleep bruxism” and “awake bruxism” studies that appeared to fall within one of the seem as such to recognize a different pathogenesis above-described categories were then retrieved as but are difficult to clinically distinguish between9; complete articles. Abstracts providing unclear data similarly, a clearer distinction between detected were also retrieved as full text articles in order to bruxism and perceived bruxism should be made.11 avoid excluding papers of possible relevance. Unfortunately, bruxism as a pathophysiological The search strategy provided a total of 267 entity can only be detected by means of poly- abstracts, including 43 reviews, for the combined somnographic recordings, the use of which is lim- search words “bruxism and stress.” Screening of ited by the high costs and the low number of ade- the abstracts showed that 40 papers (including quately equipped sleep laboratories.5 Nonetheless, eight reviews) were relevant and satisfied the inclu- even though a clinical approach to the diagnosis of sion criteria. The search words “bruxism and anxi- bruxism still remains incomplete, not allowing a ety” yielded 70 citations, partly overlapping with distinction between the different types of this disor- those identified with the “bruxism and stress” der, it is the easiest and most adopted method to search, and allowed to retrieve two other relevant gather data in large-sample studies. In particular, citations, neither of which was a review. Three fur- studies based on clinical diagnosis of bruxism have ther relevant citations (zero reviews) were obtained provided a detailed description of some tempera- by the words “bruxism and depression”, which mental traits that characterize bruxers (eg, aggres- yielded a total of 49 citations. The combined siveness, hostility, perfectionism, sensitivity to words “bruxism and psychological factors” (22 stress), and also have pointed out a high prevalence citations) and “bruxism and psychosocial factors” of psychosocial disorders in populations of (nine citations) only yielded references that were bruxers.12 These observations seem to strengthen already included within other searches, so that no the widespread opinion among practitioners that a other relevant citations were retrieved by using

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Table 2 Bruxism and Psychosocial Factors: Studies with EMG and/or Polysomnographic Diagnosis of Sleep Bruxism

Psychosocial Authors’ main Authors Sample size Bruxism diagnosis diagnosis Objective* conclusions* Takemura43 17 sleep bruxers; Sleep diagnosis; Rosenzweig Personality and Bruxers are intra- 10 non-bruxers Criteria not specified Picture- behavioral traits aggressive and unable to Frustration test of bruxers and cope with stress masticatory muscle disorder patients Van Selms20 1 myogenous TMD Sleep bruxism; Experienced/ Study of the risk Experienced stress may patient Nocturnal single- anticipated stress factors for be related to daytime channel EMG activity annotation craniomandibular pain clenching Watanabe22 12 sleep bruxers Sleep bruxism; Self-assessment To test whether Bruxism is not Nocturnal intra-oral of daily stress sleep bruxism is strongly related piezoelectric correlated with to daytime activities appliance daily behaviors Pierce21 100 bruxers EMG activity Stress Assessment of the Correlation between EMG (self-reported) relationship between measures and self- stress and bruxism reported stress is significant for 8 out of 100 subjects Clark16 20 bruxers; Nocturnal EMG Stress (urinary Assessment of the Positive relationship 10 control subjects activity catecholamine hypothesis that between increased levels) nocturnal bruxism is epinephrine content and related to periods of high levels of nocturnal increased emotional EMG activity stress Rao and Glaros86 8 diurnal bruxers; EMG activity Responsiveness to Comparison of Bruxers respond more to 8 non-bruxers stress and psycho- diurnal bruxists and stress than normals, logical status normals in while there are no (stressful stimuli) responsiveness to differences in the stress and on psychological status psychological status *Pertinent to bruxism and psychosocial factors. EMG = electromyographic.

these combined search terms. The search was also term to group together both sleep and awake extended to the related articles in the Medline Plus bruxism as well as teeth grinding and clenching, database, but no other papers satisfying the inclu- since the majority of papers did not specify which sion criteria were identified. Thus, a total of 45 type of bruxism was under investigation. When- relevant papers (including eight reviews) were ever possible throughout this review, and in partic- obtained as full-reports and discussed in the review ular in the final section, a distinction between sleep (Tables 2 through 5). bruxism and awake bruxism and grinding and In this review, for the convenience of the reader- clenching was made, in the attempt to provide spe- ship, “psychosocial factors” will be used as an cific information about the relation between TMD umbrella term to group together all those psycho- and the different types of bruxism. logical (eg, stress, anxiety and mood disturbances, The discussion of data from the literature will temperamental traits, and emotions) and social start with an appraisal of the available findings (eg, workplace satisfaction, marital status, cultural about the association between bruxism and stress, and economic conditions, social behaviors, and which for many years was the main etiological expectations) agents that may have an effect on an concept to explain bruxing behavior. Profiles of individual’s health. Actually, most literature on the bruxers, derived mainly from questionnaire studies bruxism-psychosocial factors association is adopting several different psychometric tools, will focused on the study of psychological aspects also be described along with the presentation of rather than social ones, so the former will be the some shortcomings of the current literature and main focus of this review. The word “bruxism”, considerations for future research. when used alone, will be adopted as an umbrella

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Table 3 Bruxism and Psychosocial Factors: Studies with Clinical and/or Self-report Diagnosis of Bruxism

Psychosocial Authors’ main Authors Sample size Bruxism diagnosis diagnosis Objective* conclusions* Winocur45 77 psychiatric Clinical Patients under treat- Assessment of Bruxism and TMD have a patients; 50 healthy ment for different bruxism and TMD higher prevalence in controls psychopathologies prevalence among psychiatric than in psychiatric patients non-psychiatric patients Schneider107 75 sleep bruxers; Clinical (dental German coping Assessment of Deficit of functional coping 38 non-bruxers examination) questionnaire coping strategies in strategies in bruxers bruxers Tahara106 17 healthy subjects Not applicable Not applicable Investigation of the Clenching promotes effect of chewing relaxation in subjects and clenching on under stress stress Lurie105 35 military pilots; Tooth wear Battery of Evaluation of the Coping strategies of 22 military officers psychological potential of work- bruxers are more questionnaires related stress and emotional than psychosocial factors non-bruxers to induce bruxism Marthol104 20 sleep bruxers; Clinical Not applicable Evaluation of Sympathetic cardiac 20 healthy sympathetic cardiac activity of bruxers is volunteers activity in bruxers higher than non-bruxers, suggesting a role of stress Antonio103 2 bruxers (children) Clinical (tooth wear) Life histories Discussion of In both cases, bruxism risk factors seems to be triggered by psychological disturbances Casanova- 506 subjects of Interview Stress, anxiety Identification of risk The effect of stress on Rosado102 general population questionnaires factors for bruxism TMD is related to bruxism and TMD and anxiety presence Camparis and 100 sleep bruxers Clinical RDC/TMD, Evaluation of long- Depression and soma- Siqueira61 EDOF-HC standing sleep tization levels are different bruxism patients between bruxers with and without facial pain Ahlberg101 1,500 employees of Interview Stress and psycho- Identification of risk Perceived bruxism may be a broadcasting (perceived bruxism) social status factors for bruxism a sign of a stressful company questionnaires and association with situation restless leg syndrome Manfredini39 38 bruxers; Clinical MOODS-SR Assessment of the Bruxers’ scores are higher 67 non-bruxers existence of an than non-bruxers for the association between evaluation of both manic bruxism and mood and depressive psychopathology symptoms Manfredini30 34 bruxers; Clinical PAS-SR Investigation for an Clinically diagnosed 64 non-bruxers association between bruxism is associated anxiety and bruxism with panic symptoms and increased stress sensitivity Glaros80 96 TMD patients Self-assessment Questionnaires for Examination of the Parafunctional behaviors mood and stress role of para- and emotional states levels functional and are predictors of jaw pain emotional status on levels TMD symptoms Manfredini29 34 bruxers; Clinical PAS-SR, Investigation for Both mood and anxiety 51 non-bruxers MOODS-SR associations spectra symptoms between clinically diiferentiate bruxers from diagnosed bruxism non-bruxers and psychopathological symptoms Ahlberg25 211 employees of Interview Stress and psycho- Identification of risk Psychosocial factors and a broadcasting (perceived bruxism) social status factors for bruxism stress are interrelated company questionnaires and TMD with bruxism and TMD

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Table 3 continued

Psychosocial Authors’ main Authors Sample size Bruxism diagnosis diagnosis Objective* conclusions*

Ahlberg24 1,500 employees Interview Stress and psycho- Assessment of Dissatisfaction with one’s of a broadcasting (perceived bruxism) social status association between workshift schedule company questionnaires bruxism and work- increases stress and place satisfaction bruxism Melis and 1,014 subjects of Interview Stress Description of Bruxism seems not Abou-Atme28 general population (perceived bruxism) (perceived stress) bruxism habits related to stress Ahlberg23 1,339 employees Interview Stress and psycho- Assessment of Bruxism may reveal of a broadcasting (perceived bruxism) social status association between ongoing stress in normal company questionnaires bruxism and stress work life experience Molina and 100 TMD/bruxers; Clinical Cook-Medley To test whether Support to the relatioship dos Santos42 40 non-TMD/ Inventory, Beck there are differences between moderate to non-bruxers Depression in hostility between severe bruxism and Inventory TMD/bruxism and hostility has been non-TMD/ provided non-bruxism patients Ohayon26 13,057 inhabitants Interview Interview Assessment of risk Anxiety and stress (Odds of UK, Germany, Italy factors for bruxism ratio 1.3:1) are risk fac- in the general tors for bruxism population Vanderas100 314 children Clinical and interview Stress (urinary Assessment of Epinephrine and dopamine diagnosis catecholamine levels) urinary catecholamine levels have a significant levels in children association with bruxism with and without bruxism Da Silva37 45 with tooth wear; Tooth wear Modified and Investigation of Tooth-wear patients have 45 without tooth Perceived association between higher levels of trait wear Stress Scale psychosocial factors anxiety than controls State-Trait Anxiety and tooth wear Inventory Kampe38 29 subjects Clinical KSP personality Comparison of Bruxers are more anxiety inventory personality patterns prone, had higher of bruxers and vulnerability, and were non-bruxers less socialized Fischer and 74 bruxers; Clinical (not specified) Personality tests Assessment of the Chronic bruxers were, O’Toole44 38 non-bruxers (battery) personality traits among others, shy, stiff, of bruxers cautious, apprehensive Harness and Not available Clinical (not specified) MMPI To test whether Bruxism is not associated Peltier51 bruxism is a useful with psychological predictor of psycho- disturbance as measured pathology in a facial by MMPI pain population Hicks and 511 undergraduate Interview Stress (interview) Evaluation of stress Subjects who identified Conti27 students contribution to the themselves as bruxers etiology of bruxism reported more symptoms of stress than non-bruxers Funch and 1 sleep bruxer Clinical Anxiety (battery Evaluation of the role Bruxism resulting from Gale99 of test) of anxiety in bruxing anxiety is not as behavior (ancillary important as anticipatory objective) anxiety resulting in bruxism Heller and 27 bruxers; Clinical and Taylor Manifest Differences between Bruxers differed from non- Forgione36 7 non-bruxers radiographic Anxiety Trait, bruxers and non- bruxers only in anxiety Multiple Affective bruxers in anxiety state Adjective Checklist state and traits Olkinuora41 Not available Interview Battery of Assessment of Bruxers are emotionally questionnaires personality of bruxers out of balance and tend to develop more psychosomatic disorders *Pertinent to bruxism and psychosocial factors. RDC/TMD = Research Diagnostic Criteria for Temporomandibular Disorders, EDOF-HC = University of Sao Paulo orofacial pain clinical questionnaire, MOODS-SR = Mood Spectrum-Self Report, PAS-SR = Panic-Agoraphobic Spectrum-Self Report,, KSP = Karolinska Scales of Personality, MMPI = Minnesota Multiphasic Personality Inventory.

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Table 4 Bruxism and Psychosocial Factors: Studies on Animal Models

Psychosocial Authors’ main Authors Sample size Bruxism diagnosis diagnosis Objective* conclusions* Rosales48 60 (group 1) + Brux-like activity Emotional and To clarify the relation- Emotional stress induces 36 (group 2) rats in rats fear-like stress ship between brux-like activity in the emotional stress masseter muscle of rats and bruxism Gomez108 45 rats NFMA† Physical stress Expression of The expression of (tail pinch) NFMA during stress parafunctional masticatory and assessment of activity attenuates the its relation with striata effects of stress on dopamine metabolism central catecholaminergic neurotransmission Gomez46 30 rats NFMA† (incisal Biochemical stress Investigation of the Partial support of the role attrition) (injection of role of different risk of central dopaminergic apomorphine) factors for bruxism system in bruxism and suggestion that stress is not important for tooth wear *Pertinent to bruxism and psychosocial factors. †Non-functional masticatory activity.

Table 5 Bruxism and Psychosocial Studies: Reviews

Authors Type of review Main conclusions* Lobbezoo6 Analytic review of bruxism etiology and The body of evidence for a possible causal relationship between effects of bruxism on dental implants bruxism and various psychosocial factors is growing, though not yet conclusive Bracha111 Discussion of the clenching-grinding There is a need for early detection of the clenching-grinding spectrum from a neuropsychiatric/ spectrum in anxiety disorders neuroevolutionary perspective Manfredini7 Review of theories on the etiopathogenesis There is a need to further clarify the role of psychic factors in of oral parafunctions the etiopathogenesis of bruxism Lobbezoo and Naeije3 Review of the etiology of bruxism Bruxism is mainly regulated centrally, not peripherally Bader and Lavigne9 Overview of knowledge on sleep bruxism Attempts to specify the personality traits of bruxers gave controversial results Molin110 Personal view on the role of stress in the A clear line is found from past peripherally based etiologic etiology of oral parafunctions and masticatory theories to psychologic and psychosocial concepts of bruxism muscle pain and masticatory muscle pain etiology Hicks109 Review of the etiologic theories on Psychological variables may play a role in the development of nocturnal bruxism nocturnal bruxism Rugh17 Review of the evidence for the belief that Periods of muscle hyperactivity seem to be correlated with psychological stress is a key factor in the specific daily activities, but there is great variability between etiology of bruxism patients *Pertinent to psychosocial factors.

Bruxism and Stress thus contributing to the confusion on the topic. Therefore, for the readers’ convenience, the term According to recent suggestions, stress can be de- stress will be used in this review to indicate the fined as a real or interpreted threat to the physio- stressful stimuli or events generating the response logical or psychological integrity of an individual (ie, the above-mentioned input). that results in physiological and/or behavioral It must be pointed out that it is a common opin- responses.13 Such a definition implies the existence ion that sleep bruxism, and bruxism in general, is of an input, which may be physiological or psycho- related to stress. This belief is typical of patients, logical, and an output, which may also have a who usually report an increase in their nighttime physiological as well as a psychological component. teeth grinding during stressful life periods, as well In the psychology literature, both input and output as of clinicians, who often attribute a patient’s components are commonly referred to as stress,14 bruxing behavior to an increase in stress. This the-

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ory is based on some early case series that reported of their findings. The most striking limitations of a relationship between stressful daily events and an these studies are represented by the subjectivity of increase in nocturnal masseter muscle activity.15–18 the self-report diagnosis of bruxism and by the In particular, a case report of a young woman lack of information on whether they are studying whose nocturnal electromyographic (EMG) activity sleep or awake bruxism. Several studies have was recorded over a 140-day period in relation to shown that a patient’s self-report of bruxism is not her daily pain and stress levels contributed consid- reliable diagnostically, since it may be influenced erably to strengthen convictions that a stress-brux- by both the clinician’s and patient’s conviction of ism relationship existed.19 This single case report having a bruxing behavior.11,31 Furthermore, showed that nocturnal EMG activity increased cross-study comparisons are hard to be carried immediately after a stressful event (school exam, out, since homogeneity of diagnostic criteria for meeting with parents, fights with partner, etc) and both bruxism and stress levels exist only within that pain levels increased shortly later. At present, studies by the same group of researchers. Thus, studies by Rugh et al are the only papers in which definitive findings on this issue are not likely to be nocturnal EMG activity was reported to increase achieved with self-report and clinical research with stress. Indeed, as described later in this section, alone, which based both bruxism and stress diag- successive case studies and experimental stud- noses on criteria that are hard to standardize in the ies20,21,22 have failed to demonstrate such an associ- clinical setting. ation. At present, polysomnographic recordings in ade- Apart from the early studies by Rugh et al, the quately equipped sleep laboratories represent the bruxism-stress association has been reported by standard of reference for the diagnosis of brux- some investigations adopting a self-reported diag- ism,5,9,32 but it has found less application than nosis of bruxism. Ahlberg et al23 investigated the expected because of obvious logistic and economic association between perceived bruxism and stress problems. Portable EMG devices, which allow the experience in the work environment. In a sample recording of EMG activity of masticatory muscles of 1,784 employees of a broadcasting company, during sleep in the habitual environment, reduce the authors investigated the frequency of bruxism costs and limit patient discomfort, representing an with a self-assessed question, reporting that fre- acceptable instrument in the research setting.22,33 quent bruxers reported more stress. Successive Unfortunately, only a few longitudinal investiga- papers by the same group24,25 reported similar tions have been performed to study the bruxism- findings, such that the authors stated that bruxism stress association.20–22 Two of them used an EMG- may reveal an ongoing stress in normal work life. based diagnosis of bruxism,20,21 while one adopted Another large-scale population study in which self- a telemetric-based system to diagnose bruxism assessment was taken as the criterion to diagnose when forces at or above 10 percent of maximum bruxism reported that a “highly stressful life” may voluntary clenching were applied onto an intraoral be a significant risk factor for bruxism.26 These appliance.22 Taken together, these studies findings supported those of a questionnaire-based accounted for a total of 113 patients, with a mean investigation on 511 undergraduate students, who of 15.8 recordings per patient. In general, the avail- reported more stress in association with bruxism,27 able data do not support findings from the early but they were not in line with those from a simi- report by Rugh and Harlan,18 since no relation larly designed large-sample study reporting no between EMG-detected sleep bruxism and either association between awareness of bruxism and experienced (the day before the recording night) or age, gender, marital status, occupation, and stress anticipatory (the day following the recording night) in general.28 stress was described. As for the clinical diagnosis of bruxism, no Pierce et al,21 in a study on 100 sleep bruxers paper has addressed the specific issue of the brux- over a 15-night recording period, found a lack of ism-stress relationship by the adoption of stan- association between bruxism and stress in 92% of dardized clinical criteria. However, an increase in the study population. Similarly, Watanabe et al,22 stress sensitivity with respect to non-bruxers was found no relation of bruxism to any of the daytime shown in a group of clinically diagnosed bruxers self-monitored activities (among which, stress lev- who completed a battery of tests investigating the els and sleep quality) of the subjects in a 3-week whole anxiety spectrum.29,30 study on 12 sleep bruxers. These findings were also The above-described studies do not allow con- supported by the single case study by Van Selms clusions to be drawn due to a number of factors et al,20 which is noteworthy in the attempt to that affect their design and limit the generalization describe fluctuations in the daytime and nighttime

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EMG masseter activity over a long time period. Bruxism and Other Psychosocial Disorders Interestingly, Lobbezoo and Naeije3 and Lobbezoo et al6 suggested that the presence of 8% of subjects Despite controversial aspects that have yet to be who did show a stress-bruxism association in the defined, studies on the bruxism-stress relationship study by Pierce et al21 can be interpreted as the maintain the suggestion that peripheral sensory possibility that certain bruxers are “sensitive” to influences play only a minor role in the etiopatho- stress, while others (that were the large majority) genesis of bruxism,4,34 while central nervous sys- are not sensitive. Such a hypothesis is also in line tem-related factors are given much more impor- with successive clinical works by Manfredini et tance.5,9 Among these, an interesting topic is al,29,30 who showed that stress sensitivity is one of represented by the study of the other psychosocial the domains in the anxiety spectrum that mostly disorders that may be associated with bruxism. As differentiate bruxers from non-bruxers. Thus, it in the case of the bruxism-stress studies, the major- seems that available data from EMG-based ity of the literature is based upon self-reported or research studies do not support the hypothesis of a clinical diagnoses of bruxism. The work by Pierce strict bruxism-stress relationship, thus contrasting et al21 has been the only EMG study in this field, with studies based on clinical and/or self-reported and found no association between sleep bruxism diagnoses of bruxism. and personality variables, such as anxiety, depres- These considerations must be taken with caution sion, and irritability. By contrast, in a controlled since, as in the case of clinical studies, generaliza- polysomnographical study related to vigilance and tion of results from EMG-based investigations is reaction time, an increased level of anxiety was not possible due to the paucity of investigations, found in sleep bruxers.35 patients, and research groups involved. A more Clinically oriented studies have shown that some widespread use of portable devices to achieve symptoms related to anxiety disorders have a sig- home EMG recordings is needed to perform repre- nificantly higher prevalence in bruxers than in sentative-samples studies within the future, and to non-bruxers30,6–38 and that a number of both help overcome problems of small sample sizes depressive symptoms and manic symptoms of the which currently characterize many experimental mood spectrum seem to characterize bruxers.29,39 studies. Their use might also promote a standard- Clinically or self-diagnosed bruxism has also been ization of bruxism diagnosis and a comparison of associated with emotional tension,40 psycho- results between different investigations. The prob- somatic disorders,41 hostility,42 intra-aggressive- lem of standardization affects the definition and ness,43 apprehension and tendency to worrying,44 diagnosis of stress as well. Several different criteria and psychiatric disorders such as schizophrenia.45 have been adopted in the literature, varying from Interestingly, investigations using animal models dichotomic (stressed/not stressed) to ordinal (ie, by Gomez et al,46 Areso et al,47 and Rosales et al48 nominative rating scales) and numerical (ie, visual have raised a possible physiological mechanism analog scale [VAS] ratings) variables. Inter-study that enables psychosocial factors to cause a move- inhomogeneity should also be reduced in future ment disorder such as bruxism. Indeed, chronic research, possibly with the adoption of a VAS stress, due to physical46,47 as well as emotional scale, at least until some other assessment instru- stimuli,48 is able to activate the dopaminergic sys- ment is validated and shown to be superior in both tem,49 which may cause the occurrence of non- the clinical and research settings. functional masticatory movements. In particular, The conflicting findings between clinical and the capacity of stress to induce bruxism seems to EMG-based studies on bruxism-stress may be depend upon the stressors’ influence on dopamin- explained by at least two biologically plausible ergic pathways activation, which in turn depends hypotheses. First, the relation is much more com- upon the type and duration of the stressors. For plex than previously imagined, involving many instance, rats that underwent repeated emotional other complex psychosocial aspects and disorders, fear-like stress in a communication box48 showed such as anxiety, depression, and personality traits. a strongly significant higher number of brux-like Second, a clearer distinction between sleep brux- episodes compared to controls. Moreover, the ism and awake bruxism and between grinding and same study described the effectiveness of diazepam clenching types of bruxism has to be made at the to reduce such brux-like activity depending upon diagnostic level in an attempt to identify the actual emotional stress. This is in contrast with previous pathogenesis underlying these conditions. These findings by Pohto,50 who found that such a drug hypotheses are not mutually exclusive and will be was not effective in reducing brux-like activity discussed below. triggered by a combination of apomorphine and a

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peripheral stimulus, such as the insertion of causal relationship is still lacking.59 Indeed, the occlusal interferences. Therefore, taken together, temporal criterion for a causal relationship is results from clinical studies are supportive of an hardly demonstrable due to the daily fluctuations association between bruxism and a number of psy- of bruxism events2,9,71,72 as well as TMD symp- chosocial disorders, and suggestions from animal toms,73–75 and it has a poor correspondence in the models seem to provide plausible pathophysiologi- epidemiological characteristics of TMD.59 Besides, cal explanations for such an association. Lavigne et al76 found that bruxers with concomi- The above-described observations are in con- tant jaw muscles pain have fewer bruxing episodes trast with the early results of Harness and Peltier51 per hour of sleep than bruxers without myofascial who found that bruxism was not associated with pain, suggesting that bruxism is not the primary psychological disturbance as measured by the cause of jaw muscle pain and emphasizing the Minnesota Multiphasic Personality Inventory need to investigate if the reduced hourly number of (MMPI). This investigation was performed on a bruxing episodes is pain-related or if it is due to an sample of chronic facial pain patients, thus making influence of pain on sleep. In line with such find- an unequivocal comparison with findings from ings, a recent investigation found no significant other studies impossible and preventing conclu- relationship between self-reported bruxism and sions to be drawn about the actual relation TMD pain.77 between bruxism and psychological factors. Thus, an improved understanding of the brux- Indeed, it is well recognized that facial pain is ism-pain relationship should be useful also for the associated with a number of psychiatric and psy- study of the possible association between bruxism chosocial disorders.52–56 Such an association and psychosocial factors. The selection of subjects applies mainly to anxiety disorders in the acute to be included in a population of bruxers has often stage of pain and depressive disorders in the been based on the presence of jaw pain as a crite- chronic phase,57 and does not depend upon pain rion to diagnose bruxism. This preconceived idea location.58 Nonetheless, facial pain may be associ- might lead to a biased selection of bruxers with ated with bruxism as well.59,60 Thus, the study of facial pain as representative of all bruxers. Thus, the bruxism-psychosocial disorder relationship is as discussed below, findings about the psychoso- complicated by the association that both variables cial profiles of bruxers might have been influenced have with facial pain. One example supporting this by the relationship that psychosocial factors have consideration has come from the study by with pain. Camparis and Siqueira61 who reported significant differences in the psychosocial profile of bruxers with and without chronic facial pain. Discussion It is a common belief among general practitioners that bruxism is a cause of TMD pain due to Taken together, the above-described findings illus- empirical clinical observations that pain in the trate the complexity of the problem and empha- muscles of mastication is a frequent symptom in size the need for achieving a better distinction bruxers. Nevertheless, pain is not present in every between the many forms of bruxism, which are bruxer and the existence of a bruxism-related pain probably related at different levels with both psy- is a controversial issue. Data have come mostly chosocial factors and painful symptomatology. from studies assessing the prevalence of muscular Bruxism can occur during both wakefulness and and temporomandibular joint pain in populations sleep. Bruxism during wakefulness is commonly of subjects who were reported to be bruxers and characterized by a clenching-type activity, while from investigations of the prevalence of bruxism in sleep bruxism by a combination of clenching and samples of patients with or without facial pain. grinding-type activity.5,9 Despite being usually Even though some support for the association grouped together and generically referred to as between teeth clenching and facial pain has been “bruxism,” these disorders may involve a different provided,60,62–65 some studies reported no associa- etiology and be influenced by different local and tion between bruxism and muscle sensitivity, find- systemic factors. Also, there is some consensus ing an association between clenching and joint that clenching-type activity during the day is asso- sounds only66,67; thus, the argument of a cause- ciated more with jaw pain than tooth grinding and-effect relationship between bruxism and TMD during sleep, even though experimental studies on pain is still much debated.59 Such suggestions from tooth clenching as well as studies adopting tooth clinical investigations seem to be confirmed by wear levels as an indicator of tooth grinding suffer experimental studies,68–70 but evidence proving a from some methodological shortcomings.78–80

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Thus, considering the uncertainties and the lack of observations suggest that the association between definite conclusions about many issues concerning bruxism and a number of psychosocial disorders bruxism, it appears logical that efforts should be described in many studies may depend upon the made to discriminate between sleep bruxism and criteria used to select patients, which are more suit- awake bruxism at the etiologic, diagnostic, and able to detecting daytime clenchers rather than therapeutic levels. sleep bruxers. On the other hand, the polysomno- The study of the bruxism-psychosocial disorder- graphic diagnosis of bruxism actually detects sleep pain relationship, which is also exposed to several grinders, since increases in EMG activity, to be biases due to the relationship that each variable has clearly identified as bruxism, have to be correlated with the others, may have some benefit from an to the loud “grinding sound.”5 Therefore, data may approach taking into account differences between actually reflect an association of stress and psy- awake bruxism versus sleep bruxism and grinding chosocial disorders with clenching and not with versus clenching. Indeed, given the findings bruxism as a whole. This hypothesis was already described in this review and the role of stress and suggested by Olkinuora,41 who claimed that day- psychosocial factors in the etiology of bruxism, it is time clenching is a response to stress and that day- likely that divergences in opinions and results time clenchers’ scores on psychological tests would depend upon the inhomogeneity of bruxer popula- be higher in emotional disturbances than subjects tions. This may be the reason why the early find- who brux and grind their teeth nocturnally. To ings15–18 have never been confirmed by subsequent support this hypothesis, an EMG-based study by EMG studies of sleep bruxers.20–22 The case reports Rao and Glaros86 suggested that emotional and sit- of Rugh et al described bruxers who were recruited uational factors may be important in the etiology on the basis of the presence of painful symptoms, of awake bruxism. Taken together, these observa- while the subsequent EMG studies included sleep tions point toward the possibility that awake bruxers with or without pain in the facial area. The (clenching) bruxism and sleep (grinding) bruxism selection of a group of pain-referring bruxers is a have to be considered two different disorders. possible source of bias affecting the early reports on Sleep bruxism has been repeatedly demonstrated the bruxism-stress association, since a third vari- to be part of a complex arousal response of the able (ie, pain) was introduced without assessing its central nervous system,1,2,87–89 which occurs during effect size (ie, influence) on the two variables under changes in sleep depth and is accompanied by gross study (ie, bruxism and stress). This consideration body movements, the appearance of K complexes seems to be strengthened by the subsequent in the electroencephalogram, an increased heart research of the same group81 which reported a lack rate, respiratory changes, peripheral vasoconstric- of habituation to experimentally induced stress in tions, and increased muscle activities.90 Unfor- subjects with TMD. The historical importance of tunately, at present there are no definitive findings these studies cannot be underestimated, since they about the possible influence of emotional factors described a stress response by bruxers with facial on such an activation, and a scarce literature exists pain15–18 and, later, placed this response in relation about their influence on other parasomnias as well. to the presence of TMD pain.81 Such findings, Therefore, future studies on the sleep bruxism- which provided support for the hypothesis that stress (and/or psychosocial disorders) association TMD patients and healthy subjects respond differ- might take advantage of parallel investigation on ently to stress in terms of habituation to stressful the other parasomnias, since many considerations stimuli, have been strengthened by a number of expressed for sleep bruxism in this review (ie, poor publications,82–84 thus indicating that the stress- correspondence between self-reported symptoms bruxism relationship described in the early EMG and objective findings; poor correspondence studies was likely dependent upon the inclusion of between findings of experimental and clinical stud- TMD patients only. ies; diagnostic difficulties) can be extended to other Also a selection bias may be at the basis of differ- parasomnias, such as restless leg syndrome.91 ences between findings of studies adopting self- By contrast, bruxism during wakefulness diagnosis of bruxism and those performing home (clenching) is likely to be a result of emotional ten- EMG recordings or polysomnographic recordings sion or psychosocial disorders that force the sub- in sleep laboratories. Self-referral of bruxism is ject to respond with a prolonged contraction of mainly based on patients’ perception of pain in the his/her masticatory muscles. According to this muscles of mastication in the morning, which is viewpoint, recent findings of a possible association related to daytime clenching (a sort of post-exercise with the complex spectrum of mood disorders29,39 soreness85) and not to sleep bruxism.22 These are worthy of further investigation. An awake

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bruxism-depression association was quite unpre- Conclusions dictable, and it was detected almost casually with the administration of a complete battery of tests The role of psychosocial factors in the etiology of for the evaluation of two psychopathological spec- bruxism is probably one of the most debated issues tra, which aimed primarily at evaluating the brux- concerning this disorder. Although the first studies ism-anxiety association.24 The hypothesis that on this argument date back to more than 30 years wake clenching is strictly related to depression, or ago, definite conclusions cannot yet be drawn. may be an expression of a depressed mood, is fas- Factors such as the concurrent presence of pain, cinating and has found some support in the psy- which may be strongly linked to psychopathology, chiatric literature suggesting that bipolar patients act as confounding variables that complicate the are characterized by disturbances in the central study of the bruxism-psychosocial factors relation. neurotransmitter system which may also be Indeed, the selection of heterogeneously diagnosed involved in the etiology of bruxism.92–95 Future samples of bruxers is the main reason for the differ- research should try to describe common neurologi- ent findings that have been reported in the literature. cal deficits or pathogenetic pathways between Even though most data on the etiology and manic and depressive disorders and bruxism, if characteristics of bruxism have come from sleep existing. In this sense, there is a need to clarify the laboratory studies, there is a paucity of literature role of some neurotransmitters, and dopamine in on the role of stress and psychosocial disorders in particular, which are seen as key factors in the polysomnographically monitored bruxers. These etiopathogenesis of both bipolar disorders and few studies have failed to demonstrate an associa- bruxism. tion with any of the investigated psychosocial fac- Similarly, the actual link between anxiety and tors, so dismantling the early hypothesis of a strict stress and awake bruxism has to be better defined. bruxism-stress relation. By contrast, the majority Indeed, awake bruxism may be the result of a tran- of data about the association between psychosocial sitory anxious reaction to stressful daily events disorders and bruxism have come from studies (state anxiety) or a phenomenon related to a more adopting a clinical and/or self-report diagnosis of complex psychopathological disorder (trait anxi- bruxism. In general, this kind of study has shown ety). Data based on the use of questionnaires have some sort of association of bruxism with anxiety, linked bruxism to both types of anxiety,29,37,38,86 stress sensitivity, depression, and other personality even though most data have come from studies characteristics, apparently in contrast with sleep adopting psychiatric instruments that are mainly laboratory investigations. A plausible hypothesis is suitable to detect trait anxiety. These considera- that clinical studies are more suitable to detecting tions seem to suggest the existence of an awake awake bruxism (clenching type), while the classical bruxism personality profile, strictly related to the polysomnographic studies focused only on sleep sphere of mood and anxiety disorders, even bruxism (grinding type). though it has not yet been defined. In conclusion, awake clenching may be mainly Unfortunately, some issues complicate the inves- associated with psychosocial factors and a number tigation of the above-discussed aspects. Indeed, the of psychopathological symptoms, while there difficulties to objectively diagnose awake seems to be no evidence to relate sleep bruxism bruxism,11 the difficulties to standardize the psy- with psychosocial disorders. Future research chosocial diagnoses outside the psychiatric should be directed toward the achievement of a setting,96,97 and the complexity of the relation that better distinction between the two forms of brux- both bruxism and psychosocial disorders may ism in order to facilitate the design of studies on have with pain,59,98 make standardization and this topic. comparison of results from different studies difficult and represent an obstacle to the design of unbiased investigations. Nevertheless, although definitive proofs are still lacking, there are several indications that the importance of emotional and psychosocial factors is different in awake bruxism and in sleep bruxism, thus suggesting that efforts have to be made toward a better definition of these disorders at both the etiologic and diagnostic levels.

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