PRIMARY WRITING TREMOR

Primary writing tremor (PWT) is considered to be a type of task-specif ic tremor in which tremor predominantly occurs and interferes with han dwriting. We describe the clinical and neurophysiological features of 21 patients (20 male and one female) with PWT. Mean age at tremor onse t was 50.1 years. A family history of PWT was obtained from seven pati ents. Ten patients obtained benefit from drug treatment (mainly propra nalol or primidone) and seven responded to alcohol. The writing speeds of the patients (mean +/- SEM: 73.1 +/- 6.6 letters per minute) when using their preferred hand were significantly reduced (Student's t tes t: P < 0.001) compared with those of healthy control subjects (mean +/ - SEM: 127.7 +/- 6.4). Surface polymyography performed during writing showed 4.1-7.3 Hz rhythmic activity predominantly in the intrinsic han d and forearm muscles. Alternating, extensor activation alone, skippin g from alternating to extensor activation and co-contracting EMG paste rns were recorded from the flexor and extensor muscles of the forearm. There was no evidence for excessive 'overflow' of this rhythmic EMG a ctivity as similar activity was detected in comparable muscle groups o f healthy control subjects. Accelerometry confirmed that the frequency of PWT ranged from 4.1-7.3 Hz (median 5.5 Hz) and that normal subject s wrote with a 4.0-7.7 Hz oscillation (median 4.6 Hz). Forearm recipro cal inhibition was normal in PWT (n = 13), and thus patients with PWT cart be distinguished from those with writer's cramp in whom decreased presynaptic inhibition has been Sound, Patients were sub-classified a s having either type A (n = 11) or B (n = 10) PWT depending on whether tremor appeared during writing (type A. task induced tremor) or whils t writing and adopting the hand position used in writing (type B: posi tionally sensitive tremor). However the only differences between these two groups were that a co-contracting EMC pattern and tremor induced by tendon taps to the volar aspect of the wrist were present in type B but not type A cases.