A double-blind, controlled study of botulinum toxin A in chronic myofascial pain.

BACKGROUND: Recent studies have reported a potential analgesic effect of botulinum toxin A (BTXA) in musculoskeletal pain. The present double-blind, randomized, placebo-controlled, parallel clinical trial studied the effect of BTXA on pain from muscle trigger points and on EMG activity at rest and during voluntary contraction.

METHODS: Thirty patients with trigger points in the infraspinatus muscles received either 50 units/0.25 mL of BTXA or 0.25 mL of isotonic saline. Baseline measures were determined during a run-in period of 1 week. Outcome measures including local and referred spontaneous pain, pain detection and tolerance thresholds to mechanical pressure, and shoulder movement were assessed at 3 and 28 days after injection. The interference pattern of the EMG during maximal voluntary effort of infraspinatus muscle was recorded and a standardized search for spontaneous electrical motor endplate activity at the trigger points was performed before and 28 days after BTXA or saline injection.

RESULTS: BTXA reduced motor endplate activity and the interference pattern of EMG significantly but had no effect on either pain (spontaneous or referred) or pain thresholds compared with isotonic saline.

CONCLUSIONS: The results do not support a specific antinociceptive and analgesic effect of botulinum toxin A.