Electrophysiological analysis of the mode of action of muscle relaxants in spasticity.

Variations in four electrophysiological tests (H/M, T/M, vibratory inhibition, and recovery curve of Hoffmann's reflex following stimulation at the ankle) were studied following a single administration of four myorelaxant drugs: diazepam (10 mg intramuscularly), baclofen (20 mg intramuscularly), tizanidine (4 mg orally), and idrocilamide (60 mg intramuscularly). Fifty-one spastic patients, divided into four groups, were tested. All four drugs reduced the H/M and T/M ratios very slightly. Only diazepam and tizanidine reinforced vibratory inhibition. Diazepam and tizanidine did not modify the abnormal recovery curves, however, whereas baclofen and idrocilamide did. Reinforcement of vibratory inhibition suggests an increase in presynaptic inhibition mediated by gamma-aminobutyric acid; changes in recovery curves are likely due to modifications of interneuronal reactivity. Matching myorelaxants to the predominant pathophysiological abnormality detected by electrophysiological exploration may lead to better treatment of spasticity.