Neurophysiological investigations in patients with primary writing tremor.

The pathophysiology of primary writing tremor (PWT) is still unknown: it has been classified as a focal form of essential tremor and as a tremulous form of writer's cramp. We studied cortical and spinal excitability in patients with PWT and compared the results with published data of patients with essential tremor, and writer's cramp. We used electrical stimulation of median and radial nerve to study reciprocal inhibition of forearm antagonist muscles and paired transcranial magnetic stimulation at short and long interstimulus intervals (ISIs) to assess intracortical excitability. Both studies were conducted on patients with PWT and on control subjects. The early (presynaptic) and late (disynaptic) phases of reciprocal inhibition were normal as was intracortical excitability at short and long ISIs. Our study suggests that the pathophysiology of PWT is different from that of writer's cramp and partially also from that of essential tremor.