Temporal summation of pain from mechanical stimulation of muscle tissue in normal controls and subjects with fibromyalgia syndrome.

Individuals diagnosed with fibromyalgia syndrome (FMS) report chronic pain that is frequently worsened by physical activity and improved by rest. Palpation of muscle and tendinous structures suggests that nociceptors in deep tissues are abnormally sensitive in FMS, but methods of controlled mechanical stimulation of muscles are needed to better characterize the sensitivity of deep tissues. Accordingly, force-controlled mechanical stimulation was applied to the flexor digitorum muscle of the forearm in a series of brief contacts (15 stimuli, each of 1s duration, at 3 or 5s interstimulus intervals). Repetitive stimulation was utilized to determine whether temporal summation of deep muscular pain would occur for normal subjects and would be enhanced for FMS subjects. Moderate temporal summation of deep pain was observed for normal controls (NC), and temporal summation was greatly exaggerated for FMS subjects. Temporal summation for FMS subjects occurred at substantially lower forces and at a lower frequency of stimulation. Furthermore, painful after-sensations were greater in amplitude and more prolonged for FMS subjects. These observations complement a previous demonstration that temporal summation of pain and after-sensations elicited by thermal stimulation of the skin are moderately enhanced for FMS subjects. Abnormal input from muscle nociceptors appears to underlie production of central sensitization in FMS that generalizes to input from cutaneous nociceptors.