Abnormal spinal interactions from hand afferents to forearm muscles in writer’s cramp

Objective Spinal reflexes from hand to wrist muscles were investigated in writer’s cramp. Methods Stimulus-triggered rectified EMG averages after ulnar nerve and cutaneous stimulation, in wrist flexors and extensors during tonic contraction, were compared in 18 controls and 19 patients. Results On the patient dystonic side, ulnar-induced EMG suppression was decreased in wrist extensors, and facilitation in wrist flexors modified dependent on the dystonic wrist posture during writing. No change was found on the patient non-dystonic side. Cutaneous stimulation increased wrist flexor EMG on both sides of the patients with normal wrist posture during writing, but had no effect in controls and patients with abnormal wrist posture. Conclusions Comparison between cutaneous and mixed nerve stimuli suggests that spindle afferents from intrinsic hand muscles may mediate patients’ ulnar-induced EMG modulations. Abnormal proprioceptive control was only observed on dystonic side, while bilateral unusual cutaneous control was found in patients. Changes in spinal transmission were partly related to the dystonic wrist posture, suggesting that systems involved in sensory processing can be differentially altered in writer’s cramp. Significance Changes in spinal transmission, probably related to peripheral and/or cortical inputs, might either take part in primary or adaptive mechanisms underlying writer’s cramp.