Abnormal premovement gating of somatosensory input in writer's cramp

One characteristic of focal dystonia is the sensory trick, by which sensory input to a certain area of the body can reduce abnormal contractions in mu scles nearby. This suggests that adjusting the link between sensory input a nd movement allows motor commands to be issued more effectively from the br ain. To explore this sensorimotor link, we studied the attenuation (gating) of somatosensory evoked potentials (SEPs) before and during hand movements in patients with writer's cramp. For premovement gating, 10 patients and 1 1 age-matched normal subjects were given a warning sound followed is later by an electric stimulus to the right median nerve at the wrist. The latter served both as a reaction signal to start a finger extension task and as th e input to evoke SEPs over the scalp. Because reaction times always exceede d 70 ms, short-latency SEPs thus obtained were unaffected by the afferents activated by the movement. The amplitudes of frontal N30 components were si gnificantly decreased over the frontal leads compared with SEPs elicited at rest (P < 0.002) in the normal group, whereas significant gating was found not for N30 but for frontal P22 (P = 0.002) in the patient group. For midm ovement gating studies, SEPs to the right median nerve stimulation were rec orded in 16 patients and 12 age-matched normal subjects at rest, and during active and passive finger extension-flexion movements. In contrast to the premovement SEPs, the frontal N30 was equally gated during active and passi ve movements both in the patient (P less than or equal to 0.002) and the no rmal group (P less than or equal to 0.003), These findings indicate that in writer's cramp the sensitivity of sensory input channels from the hand is wrongly set by the central command to move, Perhaps the sensory trick, by s upplying additional input not usually present during unobstructed movement, is a manoeuvre to correct this imbalance. Dystonia may result not only fro m abnormalities in the central motor command but also from disturbed centra l processing of sensory input.