Loss of reflex inhibition following muscle tendon stimulation in essential tremor.

Electrical stimulation of human upper limb muscle tendons produces a reflex inhibition (I(1)) in the same muscles. This inhibition is reduced in Parkinson's disease (PD), prompting a similar study of essential tremor (ET). In essential tremor, two of eight subjects had no discernible inhibition, even following supramaximal stimulation (< 80 mA) of the tendons from extensor digitorum communis and extensor pollicis brevis. In the remaining six subjects, the mean thresholds for I(1) in these muscles were increased by 270 and 320%, respectively, relative to controls. The maximal amplitude of the inhibition was significantly reduced in the ET group, as was the following excitation (E(1)). The latency and duration of I(1) were not different in ET subjects and controls. The maximal duration of I(1) was correlated with tremor frequency in individuals, and tendon stimulation was effective in initiating ongoing tremor cycles. These results disclose a peripheral reflex abnormality in ET that is mediated by tendon afferents and can be linked to the coexistent tremor. The response in ET was distinguished from that in PD by its different time-course and by failure of the response to appear in the antagonist muscle. The abnormality may prove a useful marker for ET, which currently lacks a definitive pathological or neurophysiological marker to support objective clinical diagnosis.