Sleep disorders and their association with laboratory pain sensitivity in temporomandibular joint disorder.

TEMPOROMANDIBULAR JOINT DISORDER (TMD) HAS BEEN DESCRIBED AS A PROTOTYPIC IDIOPATHIC PAIN SYNDROME CHARACTERIZED BY POORLY understood, episodic, masticatory muscle and/or joint pain. TMD affects an estimated 12% of the population.1 As in other idiopathic pain disorders such as fibromyalgia and irritable bowel syndrome, patients often present with overlapping signs and symptoms including psychological distress, neuroendocrine abnormalities, and chronic insomnia.2,3 Recent theoretical perspectives have proposed that these “central sensitivity syndromes” share a common central nervous system substrate characterized by heightened processing of noxious input, which contributes to overlapping daytime sequelae among these disorders.4 Several cross-sectional studies have demonstrated that compared to controls, TMD patients exhibit enhanced responsivity to a variety of painful stimuli measured both at facial and extracranial anatomic sites.5–7 Pain sensitivity at “unaffected” (i.e., non-jaw) sites suggests the involvement of central pain processing mechanisms, beyond peripheral contributions. Recent longitudinal work has reported that enhanced laboratory pain sensitivity in pain free individuals is linked to genetic polymorphisms that predict the development of new onset TMD.8 This suggests that central processes associated with pain amplification may be critical to understanding the etiopathophysiology of TMD. Clinical factors that contribute to pain amplification in TMD, however, are poorly understood. Our group has focused on the possibility that sleep disturbance is one such factor that may directly contribute to central sensitization and pain amplification.9 We recently reported, for example, that sleep onset insomnia symptoms predict the development of chronic pain following serious burn injury.10 While it is often assumed that insomnia or sleep loss occurring in the context of chronic pain occurs secondarily to the sleep interrupting effects of pain, we and others have demonstrated that insomnias associated with chronic pain are often phenotypically similar to primary insomnia.11 Shared features include high levels of pre-sleep cognitive rumination and evidence of maladaptive coping strategies that may exist prior to the development of pain and/or independently contribute to insomnia symptoms. It is unknown, however, whether primary insomnia is associated with alterations in laboratory pain sensitivity when it occurs either as a sole condition or as part of a chronic pain disorder such as TMD. Only a handful of investigations have systematically sought to evaluate the sleep quality of TMD patients. These studies have consistently found that the majority ( > 50%) of TMD patients report poor sleep quality, and that subjective ratings of poor sleep are associated with increased clinical pain severity and psychological distress.12–14 Fundamental descriptive data using polysomnography and standard research diagnostic interviews to quantify the range of sleep disorders in TMD and determine their possible associations with laboratory measures of pain sensitivity are lacking. The extant literature has largely focused on possible relationships between sleep bruxism and TMD.15,16 Sleep bruxism, however, has not been found to be associated with either poor sleep quality or polysomnographic measures of sleep continuity or architecture disturbances.17–19 The objective of this study was to address two critical gaps in the literature: (1) characterize the spectrum of sleep disorders in a well-described sample of myofascial TMD patients, using polysomnography and state-of-the art structured diagnostic interviews; and (2) evaluate possible associations between observed sleep disorder indices and laboratory measures of pain threshold. We hypothesized that rates of primary insomnia would be substantive in TMD and that primary insomnia would be associated with reductions in pain threshold at both masseter and extracranial sites.