Effect of diazepam and desmethyldiazepam in spasticity and rigidity. A quantitative study of reflexes and plasma concentrations.

Benzodiazepines are known to reduce increased muscle stretch reflexes. To investigate the relationship between the necessary plasma concentrations of diazepam and its major metabolite desmethyldiazepam on the one hand and the phasic and tonic ankle reflex activity on the other, 10 mg diazepam was given intravenously to nine patients, seven with spasticity due to multiple sclerosis and two with parkinsonian rigidity. Diazepam and desmethyldiazepam both had a normalizing effect on the increased phasic ankle reflex seen in spasticity. No effect was observed on the increased tonic reflexes in rigidity. The concentrations of diazepam necessary to reduce spasticity ranged between 300-2,200 mg/l and were so high that drowsiness did occur. However, the study may indicate that desmethyldiazepam has a higher potency and a more long lasting effect on the increased phasic reflexes than diazepam.