2016-12-26T15:09:09

Several studies indicate that females are more sensitive to experimentally induced pain than males. Moreover, it was recently shown that temporal summation of heat pain is greater in females than males, suggesting that central processing of nociceptive input may be upregulated in women. Temporal summation of pain has been examined principally using thermal or electrical stimuli. The purpose of this study was to investigate the temporal summation to noxious mechanical stimulation, and examine gender differences in temporal summation of mechanically evoked pain. A sharp probe was used to apply brief mechanical stimuli on the fingers of ten healthy females and ten healthy males. Trains of ten repetitive stimuli were applied at an intensity of 1.2-1.3 x the individual subject's pain threshold, at interstimulus intervals (ISIs) ranging from 1 to 6 s. The same or different skin sites were stimulated in any single train of stimuli. The pain ratings for the fifth as well as the tenth stimulus were significantly higher than those for the first stimulus. Also, the pain responses for the tenth stimulus were higher than those for the fifth. There was no overall gender difference in pain ratings, however, there was a significant trial # x gender interaction. Males and females provided comparable magnitude estimates for the first stimulus in the train, but females provided higher pain ratings than males for the fifth as well as the tenth stimulus. Temporal summation occurred across all ISIs, but shorter ISIs (1-3 s) elicited significantly greater temporal summation than longer ISIs (4-6 s). Finally, although higher pain ratings were obtained when the ten consecutive stimuli were applied on the same versus different skin areas, the degree of temporal summation was not significantly different. These findings indicate that temporal summation of mechanically evoked pain is higher in females compared to males, is stimulation frequency dependent and is centrally mediated.

Temporomandibular disorder (TMD) patients often exhibit widespread clinical pain, as well as greater sensitivity to experimental pain than pain-free controls, suggesting a role of central pathophysiologic mechanisms in TMD. Moreover, TMD is more prevalent among women, which may be related to the higher sensitivity of women to experimental pain. Women also exhibit greater temporal summation of heat pain compared to men. Temporal summation, the increase in pain intensity upon repetitive noxious stimulation of constant intensity, at a high frequency is centrally mediated. Thus, greater temporal summation in women indicates that their central nociceptive processing is upregulated compared to men. Recent studies in our research center sought further evidence for upregulation of central nociceptive processing in females compared to males and in TMD patients compared to healthy controls, assessing group differences in temporal summation of mechanically evoked pain, and aftersensations following repetitive noxious stimulation. Sixteen series of 10 repetitive, sharp, mechanical stimuli were applied to the fingers of 25 female TMD patients, 25 healthy women, and 25 healthy men, with a computer-controlled small probe. All subjects rated the pain intensity or the unpleasantness evoked by the 1st, 5th and 10th stimulus in the series, and the aftersensations 15 s and 1 min after the last stimulus on visual-analog scales. TMD patients exhibited greater temporal summation of pain and unpleasantness, stronger aftersensations, and more frequent painful aftersensations than controls. Healthy females showed greater temporal summation of pain intensity and unpleasantness, higher intensity and unpleasantness of aftersensations, and more frequent painful aftersensations than males. Greater temporal summation of pain and aftersensations from digital stimulation of TMD patients than controls suggest a generalized hyperexcitability of the central nociceptive system in this patient group. Such hyperexcitability may contribute to the development and/or maintenance of chronic TMD pain. Moreover, greater temporal summation of pain and aftersensations in healthy females than males indicate that their central processing of nociceptive input may be more easily upregulated into pathological hyperexcitability, possibly accounting for the predominance of TMD among women.

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