Referred pain from intraneural stimulation of muscle fascicles in the median nerve.

Microelectrode recordings were obtained from 118 cutaneous and 26 muscle fascicles in the intact median nerves of healthy human subjects. The exploring electrodes were also used for painful electrical stimulation of the identified fascicles. Cutaneous pain was accurately projected to fields within the median innervation territory. Deep pain was projected to muscles innervated by the median nerve, but in 7 experiments it was also segmentally referred to muscles in the ipsilateral upper arm, axilla or chest. Reaction time measurements indicated that referred pain was conveyed by afferent group III fibres from muscle, but did not exclude a possible contribution by group IV fibres. Referred pain was influenced by temporal and spatial summation of the afferent inflow. The magnitude of referred pain was positively correlated to the stimulation frequency of deep nociceptive fibres. The results from this study on experimentally induced pain confirm clinical observations of proximal referral of pain in patients with median nerve entrapment, and prompt consideration of possible involvement of nerve fascicles supplying deep structures in the forearm or hand in the differential diagnosis of pain in the chest and upper arm.