Long-latency reflexes of hand muscles in idiopathic focal dystonia and their modification by botulinum toxin

Long-latency reflexes (LLR) in thenar muscles were elicited by electrical median nerve stimulation in 34 patients with idiopathic focal dystonia and 20 healthy control subjects. Twenty-seven patients had cervical dystonia and seven patients had upper limb dystonia. In about one-quarter of all patients the early LLR (LLR 1, occurring at approximately 40 ms) was abnormal with either increased amplitudes or only unilateral occurrence, mostly on the clinically affected side. Later responses (LLR 2, occurring at approximately 50 ms) were obtained bilaterally in all controls but were reduced or absent in some patients, mostly on the clinically affected side. In 12 dystonia patients, LLR studies were also performed after clinically effective injection of botulinum toxin. Following botulinum toxin treatment there was a significant reduction of LLR 2 amplitudes on the clinically affected side. Our findings suggest a differential involvement of LLR generators in idiopathic dystonia with an antagonism between LLR 1 and LLR 2 on the affected sides. We propose that the reduction of the LLR 2 response may arise from overactivity of the supplementary motor area, confirming the current concept that dystonia results from cortical overflow due to disinhibited thalamocortical pathways projecting to the supplementary motor area. In addition, the dystonic motor pattern seems open to afferent modifications induced by peripheral botulinum toxin treatment.