EMG changes in myofascial trigger point after type A botulinum toxin

Abstract

Neuro-muscular dysfunction with abnormal activity of motor endplates (EP) plays a major role in the origin of a myofascial trigger point (MTP). Intramuscular injection of type A botulinum toxin (BT-A) leads to two effects: direct inhibition of extrafusal muscular fibers and inhibition of muscle spindle activity. Clinical effect is noticeable muscle relaxation and muscle pain relief. We investigated 12 patients with myofascial pain. The control group consisted of 12 healthy volunteers with the same sex and age distribution. From 50 to 100 MU (depending on the size and stage of MTP) of BT-A (Dysport) was injected under EMG control in the MTP of the horizontal part of trapezius muscle. Conventional and single-fiber EMG were done for all patients before BT-A injection, 1 week, 4 months and 1 year after. EP noise in MTP zone was recorded much more frequently than separate spikes. EMG before BT-A injection revealed typical signs of MTP. After 7 days EMG revealed fibrillation potentials, decrease of amplitude and duration of motor unit potentials (MUP) in the injected muscle. There was no change in fiber density. One month after BT-A injection was characterized by normal MUP duration, which is the most sensitive to functional reorganization of motor unit. After 4 months we noticed the tendency for normalization of all MUP parameters. Maximal decrease of fiber density was observed 1 and 4 months after BT-A injection. At 1 year after BT-A injection there was normalization of fiber density.