Abnormal surround inhibition does not affect asymptomatic limbs in people with cervical dystonia

Surround inhibition is a neural mechanism hypothesized to facilitate goal-directed action by disinhibiting agonist muscle activity while simultaneously inhibiting antagonist and other uninvolved muscle activity. The present study was designed to investigate if abnormalities in surround inhibition are found in asymptomatic body parts (the hand) of people with focal cervical dystonia (neck). Participants with (n = 7) and without (n = 17) cervical dystonia completed a protocol in which they abducted their index finger while EMG was recorded from the first dorsal interosseous (agonist) and abductor digiti minimi (uninvolved) muscles. Transcranial magnetic stimulation was delivered over the primary motor cortex at intervals ranging from 0 to 950+ms after the onset of agonist muscle activity. Motor-evoked potential (MEP) amplitudes from both muscles were compared. In control participants, MEPs from the uninvolved muscle were significantly lower than agonist MEPs at intervals from 0 to 480 ms. Similarly, in the hands of participants with cervical dystonia - the asymptomatic body part - MEPs from the uninvolved muscle were significantly lower than agonist MEPs from 0 to 175 ms. These findings suggest that surround inhibition in people with focal dystonia may be intact in asymptomatic hands. In other words, abnormalities in surround inhibition may be restricted to the dystonic limb.