Quantitative EMG in cervical dystonia

Within the latest years botulinum toxin A (BT) applied locally in affected muscles has gained a superior position in the treatment of cervical dystonia. EMG is often used as a guidance for the injections, which has caused a need for better knowledge about the electromyographic changes in the muscles involved. In the present study we used the turns-amplitude analysis for the quantitative evaluation of the EMG of the sternocleidomastoid muscles and posterior neck muscles in 44 patients with cervical dystonia, not previously treated with BT. Twelve healthy subjects were examined for comparison. At rest 13 patients showed abnormal activity (defined as > 100 turns/s) in the sternocleidomastoid muscle contralateral to the involuntary head rotation (CS) and the ipsi- and contralateral posterior neck muscles (IPN and CPN): 12 patients had abnormal activity in CS and IPN, and seven patients had abnormal activity in all muscles, including the ipsilateral sternocleidomastoid muscle (IS). Other combinations were seen less often. The distribution of muscles with abnormal activity was not always obvious from the clinical examination. CPN and IS, i.e., apparently unaffected muscles, showed reduced EMG activity during attempted maximal voluntary contraction, indicating difficulties in activating all motor units.