Tonic vibration reflex and muscle afferent block in writer's cramp

Patients with focal dystonia take advantage of certain cutaneous or proprioceptive sensory inputs to alleviate their symptoms (“sensory trick”). We examined the effects of increasing muscle spindle activity by the tonic vibration reflex maneuver and Decemberreasing it by intramuscular injection of lidocaine. The vibration was applied to the palm or the tendon of forearm muscles in 15 patients with writer's cramp and 15 age‐matched normal subjects. In 11 patients, the vibration induced dystonic postures or movements typical of those seen during writing. Normal subjects showed either no response to the vibration or a gradually developing tonic vibration reflex only in the wrist and finger flexors, which produced visible movements with a significantly longer latency (12.5 ± 6.7 seconds [mean ± standard deviation]) than what was observed in the patients (2.7 ± 2.5 seconds, p < 0.0001). Local injection of lidocaine (0.5%, 5–40 ml/muscle) attenuated the tendon reflex with relatively little effect on the M response. Injection into muscles with increased activity produced Marchked reduction of dystonic movements and significant clinical improvement in 13 patients, whereas injection into the other muscles had no effect. The clinical benefit lasted for 1 to 24 hours after injection. In 13 patients who had additional injections of 10% ethanol, which blocks sodium channels for a longer period than does lidocaine, the duration of action was prolonged to 5 to 21 days. These findings suggest that muscles causing dystonic movements have abnormal sensitivities to vibration at rest and that muscle afferents may play a pivotal role in producing dystonic movements. Local injection of lidocaine or ethanol reduces the effectiveness of muscle spindle afferent and may provide a means of treating patients with writer's cramp without causing unwanted weakness.