To the best of our knowledge, the present study is the first report to investigate an association between aural symptoms (otalgia, tinnitus and vertigo), headache, depression and TMD in a young adult Japanese population. In previous studies, vertigo and tinnitus have been reported as aural symptoms in functional disturbances of the masticatory system . Several studies have also noted otologic complaints more often in subjects with TMD than in those without TMD [6, 12, 13]. The present epidemiological study in young adult population also confirmed the results.
The reported prevalence of otological complaints in TMD patients varies in the literature. The most relevant article to this work is a comparably controlled study of otic symptoms in TMD by Tuz et al.. In their 155 study patients with TMD who reported having aural symptoms, the frequency of tinnitus was 59%, whereas in our study subjects, the frequency was 39%. The difference might reflect the older population in their study group (mean age, 49.1 years) who may be more likely to have aural symptoms than our younger study group (mean age, 18.6 years). Our findings showed a relatively strong correlation between tinnitus and subjects having pain in the TMJ (group II) and the combination of clicking and pain in the TMJ (group IV) after adjusting for age and gender in the logistic regression analyses. There is evidence for a link between tinnitus and pain in the TMJ . Tinnitus has been associated with pain upon pressure in the masticatory muscles and the TMJ, mandibular overclosure and posterior displacement of the condyle [9, 18]. Tinnitus may have a central component (as opposed to cochlear tinnitus) but can be modified both by voluntary orofacial movements (including tooth clenching) and purely sensory stimuli . Some investigators have hypothesized that eustachian tube dysfunction, masticatory muscle dysfunction or reflex-sympathetic vasospasm of labyrinthine vessels occurs secondary to abnormal stimulation of autonomic nerves of the TMJ . On the other hand, Toller and Juniper  determined no statistical difference in the results of the analysis of audiograms, tympanograms, and eustachian tube function in TMD patients compared with their control patients. A longitudinal study is warranted to verify these considerations.
Our data are consistent with previous findings suggesting there may be a link between TMD and postural imbalance leading to a dizzy. The frequency of vertigo in patients with TMD ranges from 40% to 70% in a study by Ramirez et al., whereas in our study it was 12 to 50%. The reason for this discrepancy might be due to the different types of study subjects. Ramirez et al. studied only a older patient population where vertigo might be more frequent, whereas our study subjects were 1st year healthy university students who might have less tendency to vertigo. It has been suggested that malpositioning of the mandibular condyle as a result of TMD could lead to eustachian tube blockage and symptoms of aural pain and vertigo . Significantly more patients in a TMD group (70%) also reported vertigo than in a control group (31%) in Chole and Parker’s study . A high incidence of vertigo in their subjects with TMD may relate to the possibility that underlying emotional distress may exacerbate vertigo. Parker and Chole  stated that TMD and vertigo are associated with emotional disorders. However, the pathogenesis of the symptom of vertigo in subjects with TMD is still unknown.
Otalgia is often considered to be a referred pain of orofacial origin, but it could be speculated that otalgia and the sensitivity of the ear canal are influenced by chemical mediators of inflammation  associated with the contiguous TMJ. In our study, the prevalence of otalgia in TMD subjects was 34%. The findings of the present study were consistent with other studies [2, 5]. The otalgia may possibly be explained by the proximity of the temporomandibular joint and the structures of the ear. It may be the consequence of a mechanical irritation of the auriculotemporal nerve or of some interference into the petrotympanic fissure region due to an articular inflammatory-degenerative state . The cause of otalgia in patients with TMD without a pathological condition in the ears or nasopharynx is explained as referred pain from the masticatory muscles or temporomandibular joints . Therefore patients without infection should be referred to a dentist with stogmatognathic experience to rule out stogmatognathic causes of aural symptoms.
Patients diagnosed with painful TMDs often report having headaches . The prevalence (21.9%) of headache in our population is within the range of other studies conducted on Asian Chinese (24.2%), Japanese (22.8%), European (21.9%) and northern American populations (13-21%) [23–25]. In a univariate analysis of our data, headache was associated with symptoms of TMD as a whole and this relationship remained significant also after adjustment for age and sex. For headache, we found a significant risk between subjects with and without symptoms of TMD. The high OR for group II (12.1) and IV (13.2) suggests that in the young adult population, the relationship between headache and TMD may be primarily expressed as pain in the TMJ region. Owing to the cross-sectional study design in this project no etiological conclusions can be drawn and caution should be paid because no clinical confirmation of the location of pain was available.
TMD patients report significantly more tender points upon palpation of the shoulder and neck muscles. The TMJ and the cervical spine acts as a single functional entity, which could be one of the reasons for this association . There is also some evidence from a neurophysiological point of view that the extensive convergence of different types of afferent input on the trigeminal nuclei and on neuronal plasticity  is the reason for the association between TMD and shoulder pain. However, to establish the exact association between shoulder pain and TMD pain further research is required.
Depression causes an increase in muscular tension which spreads to the pericranium muscles and might act as a cause for TMD symptoms. Several studies have shown that many TMD patients are depressed [3, 28]. This study confirms and extends previous reports addressing the association between depression and TMD populations.
All data analyzed in our study were collected from written questionnaires. It is recognized that the data relied on memory and self reporting. The authors recognize that there might have been possible incorrect answers to questions. Another limitation of our study is the absence of clinical diagnosis of TMD in the subjects. Since the clinically determined prevalence (point prevalence) might be less than the prevalence of TMD symptoms reported on the questionnaires (period prevalence), we used period prevalence as the diagnostic criterion for TMD. The results of several studies also support the validity of questionnaires for epidemiological studies on TMD symptoms [29, 30]. An additional limitation was medical records were nor analyzed, nor was a standardized questionnaire for assessing depression used.