Quantitative electromyographical changes in cervical dystonia after treatment with botulinum toxin

Injection of botulinum toxin (BT) into affected neck muscles gives symptomatic relief to patients with cervical dystonia by causing a presynaptic block of acetylcholine release. In a retrospective study of 19 patients, we used the turns-amplitude analysis of the EMG interference pattern for the evaluation of electrophysiological changes as a function of time after BT treatment. EMG was performed immediately before and during injection, and muscles showing abnormally increased activity (> 100 turns/s at rest) were given botulinum toxin A (Oculinum (= Botox)) 40-120 units. A second EMG was done 6-30 weeks later. At attempted rest, the sternocleidomastoid muscle contralateral to the involuntary head rotation showed the most pronounced changes, possibly due to relatively large doses of BT, and the EMG changes were related to the time after BT treatment. Six weeks after treatment the muscle showed decreased turns/s, mean amplitude and ratio (turns/amplitude) at rest. At 30 weeks, turns and mean amplitude reached values as before treatment, while ratio was increased to 175% of the pre-treatment value. This pattern may reflect a reversible and random loss of muscles fibres, due to presynaptic denervation. At maximal voluntary contractions, no correlation was seen between time after BT treatment and quantitative EMG.