Palmar cold threshold test and median nerve electrophysiology in carpal tunnel compression neuropathy

The diagnosis of median nerve compression neuropathy at the carpal tunnel is usually confirmed by clinical electrophysiology. The classical findings of a significantly slowed median nerve conduction velocity for both sensory and motor fibres, with a prolonged distal motor latency and a reduced amplitude compared to age-related norms are unambiguous, but these criteria are often present only in part. In such cases another quantitative indicator of compression neuropathy would be extremely helpful. The present study aimed to test whether measurement of warm and cold sensory acuity in cases of putative median nerve carpal tunnel compression would aid diagnostic certainty. Warm sensation is mediated by unmyelinated C-afferents, while cold sensation is conveyed by thinly myelinated A delta afferents. Because compression usually blocks larger diameter fibres first, cold perception on the skin of the palm distal to the compression should be more impaired than is warm perception. Standard electrophysiological measurements (median and ulnar motor and sensory nerve conduction velocities) were made, then perceptual thresholds for both warm and cold stimuli were measured on the skin of the wrist above the carpal tunnel and on the palm of the affected hand in 59 subjects. There was a significantly reduced median motor nerve conduction velocity and prolonged distal motor latency compared to normals. Further, although both thermal thresholds at the wrist were normal, those on the palm were elevated, cold being significantly raised (P less than 0.02) compared both to warm and to age-matched controls. Correlation of the nerve conduction velocity findings and thermal sensory acuity did not yield significant covariance of the positive and negative findings. Overall the results suggest that detection of preferentially elevated cold perceptual threshold (ie reduced cold sensory acuity) on the skin of the palm may aid in the diagnosis of putative carpal tunnel compression in patients with minimal or ambiguous electrophysiological data and provide a functional index of recovery after decompression.