ELECTROMYOGRAPHIC SINGLE MOTOR UNIT POTENTIALS AFTER REPEATED BOTULINUM TOXIN TREATMENTS IN CERVICAL DYSTONIA

Electromyographic (EMG) single motor unit potentials (MUPs) of the ste rnomastoid muscles (STM) were made before and after repeated treatment with botulinum type A toxin (Bx) for cervical dystonia. Post-treatmen t examinations were 6-25 weeks after the latest injection, when sympto ms and EMG interference pattern had recurred and signs of denervation were scarce. Concentric needle EMG records of 200 motor unit potential s in 10 patients showed reduced durations and areas after treatment (P < 0.05). Increased polyphasia or satellite potentials were not observ ed. Macro-EMG records of 110 MUPs in 6 patients showed reduced amplitu des and areas in the injected STM when compared to the untreated side (P < 0.05). Fibre density was within the same range (1.0-1.2). The res ults indicate that the pattern of the terminal innervation is mainly r estored even after repeated Bx treatments, but the number or size of a ctive muscle fibres within the motor unit is reduced. The clinical rel apse could be due to recovery of the original nerve terminals, or to n erve sprouts closely imitating the blocked terminal nerve twigs or bot h.