Reciprocal inhibition in forearm muscles in patients affected by ulnar neuropathy

To investigate the relationship between ulnar neuropathy and focal dystonia, the patterns of EMG activity in antagonistic muscles during voluntary, repetitive, flexion-extension movements of the ring and little fingers (tapping task), were compared in 10 normal controls, and in 10 patients affected by entrapment of ulnar nerve at the elbow. Reciprocal inhibition in forearm muscles was also evaluated. In all 10 normal subjects, the tapping task evoked rhythmic alternating bursts in flexor and extensor muscles. On the contrary, nine out of 10 patients with isolated ulnar neuropathy showed a loss of clear silent periods and a loss of well formed bursts. A clear dystonia like co-contraction pattern was therefore present. In six patients it was possible to record a reproducible H-reflex from the flexor carpi radialis muscle and reciprocal inhibition study revealed a statistically significant reduction of the amount of inhibition in both the dysinaptic and presynaptic phases as observed in focal dystonia. However, none of the patients had dystonic signs. Conceivably, an additional factor, yet unknown, triggers the transition from ulnar neuropathy with abnormal bursts patterns to clinically apparent dystonia.