Late cortical disinhibition in focal hand dystonia.

In writer's cramp (WC), a form of focal hand dystonia, cortical GABAergic inhibitory mechanisms are altered and may cause involuntary tonic contractions while writing. The objective of this study was to explore the time course of long-interval intracortical inhibition (LICI) that involves gamma-amino butyric acid (GABA)-B transmission and late cortical disinhibition (LCD) (that combines GABA-A and GABA-B mechanisms) in patients with WC and in control subjects. A double pulse transcranial magnetic stimulation protocol was used to evoke LICI and LCD while the subjects either gripped a cylinder between their thumb and index fingers or relaxed all their upper limb muscles. We measured the ratio between primed and unprimed motor evoked potential in the first dorsal interosseous at interstimulus intervals ranging between 60 and 300 ms. Though the cortical silent period was not different between the groups, LICI lasted longer in patients with WC, that is, LCD was delayed for more than 30 ms and reached a higher level. In addition to the alteration of inhibitory mechanism mediated by GABA-B transmission, LCD which probably involves presynaptic inhibition is also modified in patients with WC with possible consequences on the activity of primary motor cortex inhibitory and excitatory circuits which control the hand muscles.caus.