Atypical central pain processing in sensory modulation disorder: absence of temporal summation and higher after-sensation.

Sensory over-responsivity (SOR), a subtype of the proposed sensory modulation disorder (SMD), is characterized by over-responsiveness to stimuli in several sensory modalities. SMD individuals demonstrate abnormal responses to naturally occurring stimuli in a manner that interferes with daily life participation. Previous psychophysical testing of the somatosensory system revealed that SOR individuals rated pain sensations higher than controls, demonstrating hyperalgesia that can be centrally mediated. Temporal summation (TS) of second pain and after-sensation are manifestations of central sensitization; therefore, this study explored these measures for better characterization of central pain processing in SOR. Twelve SOR adults and 12 healthy controls participated. TS was produced by a train of fifteen repetitive heat pulses, 0.7 s duration each, and 2 s of inter-stimulus interval, applied to the thenar-eminence, while four pain ratings were obtained. An after-sensation was then measured for 5 min, obtaining six pain ratings. No TS of pain was indicated in the SOR group (SOR: p = 0.36; control: p < 0.001). Further, while controls reported a gradual disappearance of pain after-sensation, individuals with SOR continued to report pain for the duration of the 5 min measured (p = 0.002). These results demonstrate an atypical response pattern, suggesting alteration in pain processing and/or modulation at a central level in individuals with SOR. These possible neural changes may manifest themselves as interference with daily functioning as well as shed light on some of the between-subject variability seen in psychophysical testing in non-painful subjects.