Description and analysis of the myotonolytic effects of phenytoin in the decerebrate cat: implications for potential utility of phenytoin in spastic disorders.

Phenytoin (DPH) was evaluated for its capacity to reduce several motor manifestations of decerebrate rigidity in the cat. In doses of the order of 40 to 50 mg/kg i.v., DPH diminished the force necessary to collapse the hyperextended limbs; at about half this dose range, the drug reduced gamma-motoneuron discharges; at still lower doses the drug profoundly depressed mechanical and electromyographic responses evoked by stretch from both forelimb and hindlimb extensor muscles. Serum levels of DPH associated with substantial reduction in electrical and mechanical manifestations of the extensor hypertonus were of the same order conventionally encountered when the drug is administered to humans for acute seizure management. The data are supportive of a centrally and peripherally mediated muscle relaxing effect of the drug in states where muscle spindle involvement is a contributing factor, and may help to explain further the utility of DPH in the treatment of spasticity.