Neuromuscular effects distant from the site of botulinum neurotoxin injection.

We assessed the severity and temporal profile of distant neuromuscular effects from a single dose (280 units) of botulinum neurotoxin injected into neck muscles for torticollis. We performed single-fiber EMG studies on the biceps brachii of six patients to measure jitter (20 pairs) and fiber density on the initial treatment day and then again, at least once more, after 2 to 12 weeks. No patient developed weakness beyond the neck muscles or decrement of the biceps response to repetitive 3-Hz nerve stimulation. Between the baseline and the last follow-up study, the average of mean MCD increased from 29 microseconds to 38 microseconds (31%). Mean fiber density increased concurrently or earlier from 1.35 to 1.79 (33%). There were no electrophysiologic signs of presynaptic blockade, even at 2 and 4 weeks. The effects we observed are compatible with stimulation of terminal sprouting by the neurotoxin, without significant presynaptic inhibition of acetylcholine release. We therefore believe that higher dosages of the neurotoxin may be used if clinically indicated.