The effect of cutaneous input on intracortical inhibition in focal task-specific dystonia

In normal subjects short interval intracortical inhibition (SICI) is topographically modulated by cutaneous input, which may be important for focusing muscle activation during tasks. In patients with writer's cramp, a task-specific focal dystonia characterized by inappropriate and excessive muscle activation of the upper limb during certain motor tasks, intracortical inhibition is reduced at rest and lacks the normal topographically-specific modulation during motor tasks. In the present study we investigated whether cutaneous input modulated SICI in a group of patients with writer's cramp and a control group of subjects. Electromyographic recordings were made from the right first dorsal interosseous (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles. Brief electrical stimuli were applied to either digit II or digit V with ring electrodes. SICI was investigated using a paired transcranial magnetic stimulation paradigm employing interstimulus intervals of 1-15 ms. Cutaneous input from both digit II and digit V modulated motor evoked potentials and SICI in a topographically-specific manner in control subjects. In contrast, cutaneous input failed to modulate motor evoked potentials or SICI in the focal hand dystonia patients. These results provide further evidence of abnormal sensorimotor integration in focal hand dystonia.