Age-related botulinum toxin effects on muscle fiber conduction velocity in non-injected muscles.

Objective: We studied systemic effects of botulinum toxin (BTX) treatment on muscle fiber conduction velocity (MFCV) and possible effects of age.

Methods: MFCV was determined by an invasive EMG method in the biceps brachii muscle. Seventeen BTX treated patients and 58 controls were investigated. BTX injections were applied in the neck region or forearm, depending on the indication for treatment.

Results: We found an increased ratio between fastest and slowest muscle fiber conduction velocity in BTX treated patients. This suggests systemic BTX effects on MFCV distant from the site of injection, probably fiber atrophy secondary to end-plate dysfunction. Furthermore, we found an increased MFCV in part of the patients, suggesting hypertrophy of some of the muscle fibers. No relation was found between the MFCV disturbances and treatment duration or the cumulative dose of BTX.

Conclusions: We found a strong positive correlation between the age and the BTX-induced changes of MFCV in patients, suggesting a BTX related, diminished repair capacity of end-plates or muscle fibers with age.

Significance: Our findings suggest a reduced repair capacity of end-plates or muscle fibers in elderly patients. MFCV is a sensitive method to show changes related to damage and compensation of the neuromuscular system. Our finding suggests a decreasing efficiency of repair mechanisms in aging.