Postural tremor of Parkinson's disease.

Previous studies have reported the resting tremor (RT) of Parkinson's disease to occur at frequencies between 3-7 Hz and to be characterised by an alternating pattern of electromyographic (EMG) bursting activity between opposing muscles. A postural tremor (PT), of higher frequency (> 6 Hz) and with a synchronous pattern of EMG activity, has also been previously described in Parkinson's disease. We investigated the electrophysiological and pharmacological properties of both the RT and PT of 11 patients with Parkinson's disease and 10 patients with essential tremor in a double-blind, placebo-controlled study of L-Dopa/benserazide and propranolol. Tremor amplitude and frequency were assessed via bidirectional accelerometry, and the pattern of activation of the antagonist muscles of the forearm was determined with use of surface EMG. In the Parkinson's disease group studied, the frequency, EMG pattern of bursts, and response to L-Dopa were similar for the two tremors (median improvement of RT by 70% and PT by 61%). Despite some overlap between the Parkinson's disease and essential tremor groups in the electrophysiology of the tremor, there was no such dramatic pharmacological response in the latter group. These results suggest that the RT and PT of Parkinson's disease share a common pathophysiology and are distinct from essential tremor.