Impaired inhibition in writer's cramp during voluntary muscle activation

We used paired transcranial magnetic stimulation (TMS) to evaluate inhibitory mechanisms in eight patients with writer's cramp during rest and isometric wrist extension. Both stimuli were 110% of the motor threshold; the interstimulus intervals (ISIs) were 20 to 200 ms in increments of 10 ms. Surface EMG was recorded from wrist extensors. In the symptomatic hemisphere, there was no significant difference in the amplitude of the test (second) motor evoked-potential (MEP) between patients and age-matched controls at rest. However, with voluntary muscle activation, inhibition of the test MEP by the conditioning MEP was significantly less in writer's cramp patients than in controls (p = 0.02). The difference was most prominent at ISIs of 60 to 80 ms in which inhibition is maximum. In the asymptomatic hemisphere, there was no significant difference between patients and controls in both rest and active conditions. The silent period was shorter in patients than controls on the symptomatic side (p = 0.003) but not on the asymptomatic side. We conclude that the inhibitory effects induced by magnetic stimulation are reduced in patients with writer's cramp, but only on the symptomatic side during muscle activation. This may relate to the overflow of muscle activity that characterizes this condition.