Journal of Oral & Facial Pain and Headache, 03/2000

A review of the literature on gender and clinical pain reveals a disproportionate representation of women receiving treatment for many pain conditions and suggests that women report more severe pain, more frequent pain, and pain of longer duration than do men. Gender differences in pain perception have also been extensively studied in the laboratory, and ratings of experimentally induced pain also show some sex disparity, with females generally reporting lower pain thresholds and tolerance than males. However, there is little consensus on whether these apparent differences reflect the way men and women respond to pain, differing social rules for the expression of pain, or biologic differences in the way noxious stimuli are processed. In this paper, our working hypothesis is that the higher prevalence of chronic orofacial pain in women is a result of sex differences in generic pain mechanisms and of as-yet unidentified factors unique to the craniofacial system. We will review the evidence concerning gender differences in the prevalence of pain conditions, with a focus on orofacial pain conditions. Evidence and hypotheses concerning biologic and psychosocial factors that could influence prevalence rates will also be discussed.

Atypical facial pain, stomatodynia, atypical odontalgia, and some forms of masticatory muscle and temporomandibular joint disorders all seem to belong to the same group of idiopathic orofacial pain illnesses. The many common clinical features they display have been discussed in a preceding paper. Some of their common pathophysiologic mechanisms are reviewed in this article. The role of female hormones is suggested as a risk factor by the strong female prevalence and by the effects of physiologic and therapeutic modification of estrogen levels in patients with these pain conditions. Osteoporosis, which appears with menopause, and neuralgia-inducing cavitational osteonecrosis have been linked to atypical facial pain. Similar clinical features have also prompted a comparison between atypical facial pain and complex regional pain syndrome of the limbs. The presence of psychosocial factors is also a common feature, but it is not known whether these are causal or whether the pain induces the psychosocial problem. Local inflammatory, infectious, or mechanical irritation as well as minor nerve trauma are frequently reported in these conditions and can also be considered as risk factors. However, none of the above factors can currently be considered as the sole etiologic factor, and instead the following hypothesis is proposed: the idiopathic pain entities depend on one or several neuropathic mechanisms, the development of which is triggered or favored by one or several events or risk factors. Different neuropathic mechanisms may be at work: nociceptor sensitization, phenotypic changes and ectopic activity from the nociceptors, central sensitization possibly maintained by ongoing activity from initially damaged peripheral tissues, sympathetic abnormal activity, alteration of segmental inhibitory control, and hyper- or hypoactivity of descending controls. Research directions that are suggested include epidemiologic approaches to improve the clinical definition of these conditions, studies to test for the factors and mechanisms proposed, and definition of mechanism-based diagnostic and treatment strategies.

Aims: Strong jaw muscle exercises such as tooth grinding in sleep bruxism are frequently believed to be a predisposing factor in myogenous types of temporomandibular disorders. However, it is not known whether tooth grinding in sleep bruxism is associated with increased sensitivity to intramuscular stimuli. This study therefore compared the hyperalgesic effects of an intramuscular injection of capsaicin into the right masseter with and without a prior experimental tooth-grinding exercise. Methods: Ten healthy men participated in 2 randomized sessions (exercise, non-exercise session) separated by 1 week. In the exercise session, 0.1 mL capsaicin (100 µg/mL) was injected into the right masseter immediately after 45 minutes of experimental tooth grinding. In the non-exercise session, the exact same paradigm was used, except that the experimental tooth grinding was omitted. The perceived intensity of pain evoked by intramuscular capsaicin was scored on a 100-mm visual analog scale (VAS). Pain detection thresholds (PDTs) to pressure stimuli and maximal voluntary occlusal force (MVOF) were measured before capsaicin injection; 5, 15, and 45 minutes after the injection; and once a day for the following 3 days. Results: Injections of capsaicin into an exercised or non-exercised masseter did not cause significant differences in peak pain intensity on the VAS (57 ± 6 mm in exercised masseter vs. 53 ± 6 mm in non-exercised masseter; P = 0.464). The PDTs in the exercised masseter were significantly decreased for up to 1 day after the capsaicin injection (P = 0.038), whereas PDTs in the non-exercised masseter were decreased for only 5 minutes (P = 0.017). The MVOF on the right side was decreased 5 minutes after the capsaicin injection in both sessions (P 0.010). The MVOF on the left side was significantly reduced for up to 15 minutes after the capsaicin injection in the exercise session only (P 0.019). Conclusion: Increased sensitivity to percutaneous pressure stimuli probably reflects a post-exercise muscle soreness following tooth grinding, whereas intramuscular sensitivity to noxious chemical stimuli immediately following exercise seems to be unchanged.

Aims: To investigate the natural course of symptoms of temporomandibular disorders (TMD) in a non-patient population and to estimate the strength of the relationship between several hypothesized risk factors and precipitation and perpetuation of the symptoms. Methods: A total of 672 randomly selected citizens of Okayama City was requested to answer the same self-administered questionnaire that they had answered 4 years earlier. The mailed questionnaire failed to reach 58 subjects at the second survey, and 367 of the remaining subjects (59.8%) responded. The fluctuation of TMD symptoms was assessed by comparison of 6 pairs of answers for questions regarding temporomandibular joint (TMJ) pain, limitation of mouth opening, TMJ noise, headache, neck pain, and shoulder stiffness. Six factors (age under 40, female, clenching habit, history of extrinsic trauma, sleep disturbance, and family history of TMD) were tested for their relative risk in precipitating and perpetuating each TMD symptom by the use of its confidence interval to define significance. Results: The incidence of TMD symptoms ranged from 6.1% (TMJ pain) to 12.9% (TMJ noise). More than half of the subjects who had reported TMJ and neck pain at the initial survey no longer reported these symptoms at the second survey, whereas TMJ noise and shoulder stiffness remained in more than 70% of the subjects. Individuals under 40 years old had a 3.3:1 increased risk of precipitating TMJ noise (P 0.01), individuals with a history of extrinsic trauma had a 2.85:1 increased risk of precipitating limited mouth opening (P 0.01), and females had a 2.81:1 increased risk of perpetuating TMJ pain (P 0.01). Conclusion: The possible etiologic significance of these factors in TMD should be validated by future research.