Pallidal neuronal activity: implications for models of dystonia

Dystonia is a neurological syndrome involving sustained contractions of opposing muscles leading to abnormal movements and postures. Recent studies report abnormally low pallidal neuronal activity in patients with generalized dystonia, suggesting hyperkinetic disorders result from underactive basal ganglia output. We examined this hypothesis in 11 patients with segmental and generalized dystonia undergoing microelectrode exploration of the internal globus pallidus (GPi) before pallidotomy or deep brain stimulation (DBS) implantation. The mean firing rates and firing patterns were compared with those in six patients with Parkinson's disease (PD). In seven patients who underwent surgery under local anesthesia, the mean GPi firing rate was 77 Hz, similar to the 74 Hz observed in the PD patients. However, in three dystonic patients under propofol anesthesia, GPi mean firing rate was much reduced (31 Hz), and the firing pattern was distinguished by long pauses in activity, as reported by others. Low-dose propofol in one other dystonia patient also seemed to suppress GPi firing. These results indicate that an abnormally low basal ganglia output is not the sine qua non of dystonia. The widely accepted pathophysiological models of dystonia that propose global decreases in basal ganglia output need to be viewed with caution in light of these findings.