Differential involvement of A-delta and A-beta fibres in neuropathic pain related to carpal tunnel syndrome.

Carpal tunnel syndrome (CTS), a common entrapment neuropathy involving the median nerve at the wrist, frequently manifests with neuropathic pain. We sought information on pain mechanisms in CTS. We studied 70 patients with a diagnosis of CTS (117 CTS hands). We used the DN4 questionnaire to select patients with neuropathic pain, and the Neuropathic Pain Symptom Inventory (NPSI) to assess the intensity of the various qualities of neuropathic pain. All patients underwent a standard nerve conduction study (NCS) to assess the function of non-nociceptive A?-fibres, and the cutaneous silent period (CSP) after stimulation of the IIIrd and Vth digits, to assess the function of nociceptive A?-fibres. In 40 patients (75 CTS hands) we also recorded laser-evoked potentials (LEPs) in response to stimuli delivered to the median nerve territory and mediated by nociceptive A?-fibres. We sought possible correlations between neurophysiological data and the various qualities of neuropathic pain as assessed by the NPSI. We found that the median nerve sensory conduction velocity correlated with paroxysmal pain and abnormal sensations, whereas LEP amplitude correlated with spontaneous constant pain. Our findings suggest that whereas paroxysmal pain and abnormal sensations reflect demyelination of non-nociceptive A?-fibres, spontaneous constant pain arises from damage to nociceptive A?-fibres.