Role of mental state in temporomandibular disorders: A review of the literature

Temporomandibular disorders (TMD) constitute a heterogeneous group of disorders concerning temporomandibular joints (TMJ) and the surrounding structures. The etiology is multifactorial, and comprises biological factors (e.g., internal derangements in TMJ), psychological factors (e.g., depression, anxiety and stress) and social factors (e.g., a learned response to pain). In accordance with the biopsychosocial model of health and illness, psychological factors are recognized as highly significant in the development of TMD.


Introduction
'Temporomandibular disorders' (TMD) is an um brella term for a heterogeneous group of disorders concerning temporomandibular joints (TMJ) and the surrounding structures. Temporomandibular disor ders are the second most common cause of pain in the orofacial region after toothache. 1 The prevalence of TMD in the general population is estimated at around 40%. 2 In young adults, the prevalence of TMD symptoms varies from 42.9% up to 60%. [3][4][5][6][7] The most common symptoms are: pain of the masticatory muscle or in the joint area; headache; reduced mobi lity of the mandible, and joint sounds. 8 The etiology of TMD is multifactorial, [9][10][11] and comprises biological factors (e.g., internal derangements in TMJ), psycho logical factors (e.g., depression, anxiety and stress) and social factors (e.g., a learned response to pain). 12 The Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) introduced by Dworkin and LeResche in 1992, 13 later updated to Diagnostic Cri teria for Temporomandibular Disorders (DC/TMD), is an internationally accepted tool for diagnosing TMD based on the biopsychosocial model of health and ill ness. The RDC/TMD and the latest DC/TMD func tion as a twoaxis system. Axis I focuses on diagnosing biological factors based on physical examination, and Axis II refers to psychosocial factors, studied by means of a series of questionnaires. 13 The twoaxis structure proposed in this diagnostic tool brings out the impor tance of psychosocial factors in TMD, which are still often underestimated or unknown by various clini cians working in the field of dentistry.
The aim of this review was to present the role of chosen mental disorders (depression, anxiety) in TMD and their significance for dental practitioners in the light of current knowledge.

Material and methods
The authors searched 3 electronic databases -PubMed, Scopus and Web of Science -for articles describing the re lationship between anxiety or depression and TMD. Only original, Englishlanguage articles were included. Also the bibliography of the obtained studies was searched for relevant articles. Then, the retrieved papers underwent a qualitative analysis. Finally, 22 articles were included in this review.

Results and discussion
Mental disorders, such as depression or anxiety, are con sidered by some authors as the potential causes of TMD symptoms. 14,15 They play an important role in the onset of TMD and its perpetuation. [16][17][18] Some authors consider psychosocial comorbid conditions as predictors of a less favorable response to classical therapy. 19 The precise mechanisms linking psyche and TMD are unknown. Potentially, stress may alter the threshold of pain perception in the central nervous system, increase the intensity of parafunctional habits as well as mastica tory muscle fatigue and tightness, and initiate the disor der. 14,15 Or the other way round -pain (especially chronic), through the constant input of painful stimuli, induces central sensitization and causes permanent changes in the central nervous system. 20 Pain and psychological distress seem to create a dynamic vicious circle, in which mental disorders intensify the perceived pain and the perceived pain worsens the course of mental disorders. In many cases, it is hard to distinguish the cause from the effect. 21 The mechanism may also be associated with the dys regulation of the hypothalamic-pituitary-adrenal axis, leading to the production of excessive amounts of stress hormones, such as cortisol and catecholamines. The axis hormones are associated with mental disorders, such as depression, and other somatic illnesses, such as diabetes, hypertension or facial pain. [22][23][24] Other studies confirmed that patients diagnosed with TMD presented with statistically significantly higher levels of stress hormones in comparison with healthy controls. 25,26 Depression A depressive episode is characterized by subsequent symptoms: low mood; loss of interest or no sense of plea sure; psychomotor agitation or retardation; fatigue or loss of energy; significant changes in weight; strong belief in own worthlessness or a sense of guilt; impaired concen tration; and hypersomnia or insomnia. [27][28][29] Patients suf fering from depression are at higher risk of committing suicide. 30 In the last 20 years, the prevalence of depression has in creased by 49.86% and it is estimated that during lifetime, 20% of the society will develop this illness. [31][32][33] The most commonly used questionnaires diagnosing depression are: the Hospital Anxiety and Depression Scale (HADS) 1 ; the Patient Health Questionnaire9 (PHQ9) 34 ; the Symptom Checklist90Revised (SCL90R) 35,36 ; and the Beck Depression Inventory (BDI). 37 The usage of those questionnaires as diagnostic tools is very con venient and timesaving from the point of view of the doctor. Still, it has to be marked that questionnaires can only serve as screening tools for certain psycho logical disorders, and indicate patients that need a de finitive diagnosis and treatment provided by the psy chiatrist. 1,38 Patients with TMD present with higher levels of de pression as compared to healthy controls. 29,39,40 Some studies divided TMD patients based on pain, report ing significantly higher levels of depression in patients with painful TMD than in the case of nonpainful TMD. 36,41,42 What is worth noticing in those studies is the fact that the precise localization of pain (diagno sis from Axis I) was not significant. 36,41,42 Others di vided patients based on the duration of the symptoms, reporting significantly higher levels of depression in patients with chronic TMD than those with acute TMD. 20,38 In the abovementioned studies, chronic TMD was defined as lasting at least 3 months. One of the studies focused on changes in the perceived symptoms of TMD at the time of higher and lower levels of depression and anxiety. 43 A significant difference was found only in the maximum mouth opening with out pain and the level of selfestimated mandibular im pairment. The maximum mouth opening without pain was decreased and the level of selfestimated man dibular impairment was increased at the time of a high level of depression. On the other hand, nonsignificant differences were found in the number of tender points or the electromyographic activity of the masticatory muscle, which seemed not to depend on the level of anxiety and depression. 43 Korszun et al. conducted an interesting study, in which patients with chronic facial pain underwent full medical and psychiatric assessment according to the Diagnostic and Statistical Manual of Mental Disorders IV (DSMIV) criteria. 26 The percentage of patients diagnosed with depression or symptoms charac teristic of depression was statistically significantly higher among patients with chronic facial pain than in healthy controls. Interestingly, fatigue and insomnia were the most common symptoms, reported by 92% of patients with a diagnosis of depression and 50% of patients presenting with only some depressive symp toms. By comparison, 38% of controls reported those symptoms. An important issue is that 50% of patients fulfilling the DSMIV criteria for depression received this diagnosis for the first time, despite various pre vious consultations. This fact might indicate that the number of patients with TMD requiring a specialist psychiatric consultation is underestimated by various practitioners. No statistically significant differences in the occurrence of depression between TMD patients and chronic facial pain patients of different origin was found, either. Thus, the significant variable correlated with depression could be pain, not TMD. 26,44 Several studies suggest that depression and chronic pain syn dromes share the same neurological pathways and neurotransmitters (such as norepinephrine, glutamate and serotonin), and involve the same brain structures (such as the anterior cingulate cortex, the prefrontal cortex, the amygdala, and the hippocampus). [45][46][47] Fur thermore, both chronic pain and depression aggravate each other, and cause maladaptive changes in the brain function and structure. 46,47 Chronic pain intensifies depressive symptoms by causing distress. 46 Depression on the other hand may be a result of monoamine neu rotransmitter deficiency, which leads to increased sensitivity to painful stimuli. 45 It is also suggested that depression may be considered as an inflammatory process in the brain due to increased levels of pro inflammatory cytokines, which results in higher pain sensitivity. 46 Therefore, early diagnosis and proper treatment are crucial.
Another interesting finding is that young children who had contact with adults with depression are at increased risk of the development of painful TMD in their early adulthood. 48 Table 1 presents a summary of data regarding depres sion from the most important studies included in the re view.

Anxiety
Anxiety is characterized by a sense of worry, diffi cult to control and causing a feeling of restlessness, fatigue, a sense of tension, nervousness, and sleep dis turbance. 27,49 In some studies, in which the Spielberger StateTrait Anxiety Inventory (STAI) was used, 2 types of anxiety were distinguished: stateanxiety, which is associated with the current level of anxiety and is tran sitory; and traitanxiety, which expresses one's persona lity and is more stable during lifetime. 50 Anxiety is recognized as the most common mental disorder in the European Union and it is 3 times more frequently diagnosed in women than men. 51 Studies employ various questionnaires to diagnose anx iety. These are: HADS 1,37 ; STAI 15,37,52,53 ; the Beck Anxiety Inventory (BAI) 37,52 ; the Generalized Anxiety Disorder7 (GAD7) 34 ; and SCL90R. 35,36 The role of anxiety in TMD is still rather controver sial. Many studies suggest the existence of a correlation between TMD and anxiety, 3,37,54-56 but other researchers present contradictory results, 1,20,57-59 seeing anxiety as a less important factor in the case of TMD patients than depression. 1,20 The different results of the presented stu dies may be explained by differences in many important variables, such as the population under study, the study design and the questionnaire used.
Those authors who managed to find a correlation be tween TMD and anxiety reported significantly higher levels of anxiety in patients with TMD as compared to healthy controls. The individuals affected by anxiety were up to 5 times more prone to develop TMD than non TMD patients. 48 A correlation was found in both acute 60 and chronic TMD. 61 Severe anxiety levels increased by twice the probability of chronic pain, disability and de pression, 53 and the level of anxiety correlated with the du ration of the disorder. 20 Regarding one of the subtypes of anxiety, the analyzed articles were consistent. Traitanxiety occurred more often in patients with TMD than healthy controls. 15,53 The odds of TMD correlated with the level of trait anxiety, regardless of age, gender or the level of educa tion. Patients suffering from traitanxiety at a mode rate level were at higher risk of TMD, while severe trait anxiety doubled the risk. 53 In one of the analyzed articles, higher stateanxiety levels correlated with an increased risk of painful TMD 53 whereas in another study, they did not. 15 This inconsis tency can be easily explained, because, as mentioned before, stateanxiety concerns the momentarily perceived anxiety, which can be influenced by other psychosocial factors, not taken into consideration in either of the pre sented articles.
Similarly to depression, anxiety does not show any cor relation with Axis I diagnosis, leading to the conclusion that the localization of the symptoms is insignificant. 35,50,53 There is weak evidence that the level of anxiety tends to decrease significantly after 1 month of treatment, regard less of the type of therapy used (occlusal splint therapy, manual therapy, counseling, or the combination of occlu sal splint therapy and counseling). 52 Table 2 presents a summary of data regarding an xiety from the most important studies included in the review.

Limitations
The usage of questionnaires as the only diagnostic tools for mental disorders is an important limitation of the included studies. The lack of further psychiatric confir mation may lead to misdiagnosis, biasing the presented results. Furthermore, questionnaires screen only for cer tain symptoms and do not provide a precise psychiatric diagnosis.

Conclusions
Evidence supporting a correlation between men tal disorders, such as depression and anxiety, and TMD is convincing and numerous. The derangements  influence the onset of TMD, the course of the dis ease and a response to treatment. However, the pre cise role of each mental disorder still requires further clarification. Screening for mental disorders in TMD patients by means of questionnaires in general dentists' offices is highly recommended.
Depression correlates with TMD; the presence or ab sence of orofacial pain might by a more important factor than a diagnosis of TMD.
A correlation between anxiety and TMD is rather controversial and depends on various factors, such as the study design, the population under study, the con trol group, etc. Future studies should focus on inves tigating the circumstances in which the correlation is significant.
In further research, precise causal relationships should be established between depression, anxiety and TMD, along with defining the prevalence and coexistence of the abovementioned conditions. A diagnosis of mental dis orders should be confirmed by psychiatric examination, not only based on questionnaires.