The variability in the clinical effect induced by botulinum toxin type A: the role of muscle activity in humans.

When botulinum toxin (BT) is administered for the first time at fixed doses, variable clinical responses can be observed in patients with the same form of dystonic disorder. Many factors may contribute to this phenomenon, including the variability rate of absorption of the drug. Animal experimental models (rat diaphragm preparation) have demonstrated an increased absorption of BT in the terminal nerve endings of the muscle under repetitive electrical stimulation, suggesting that "muscle activity" also may play an important role. The aim of our study was to evaluate in humans the role of the muscle activity on the variability of the effect induced by BT type A. Eleven patients with blepharospasm and idiopathic facial hemispasm were studied by using neurophysiologic techniques. In nine patients, both extensor digitorum brevis (EDB) muscles were injected with low (3 IU), fixed doses of type A BT. For the first 24 h after administration of the drug, periodic electrical stimulation of only one EDB was used. The subsequent percentage changes in compound muscle action potential (CMAP) amplitude was calculated at different intervals over a 30-day period. The percentage in the CMAP for the stimulated EDB was compared with that of the contralateral nonstimulated side. We found that the effect of the induced neuromuscular blockade was significantly greater for the stimulated side. In the other two patients, we injected BT in one EDB and the same volume of normal saline solution in the contralateral muscle to assess the stability of the CMAP in untreated muscle over time. We observed that the CMAP was unchanged in the untreated EDB; therefore we concluded that muscle activity plays an important role in the variability of clinical response often seen.