Increased doses of phenytoin for infants.

A case report describing the problems of using conventional phenytoin dosages to achieve control of myoclonic seizures in a 6-month-old infant weighing less than 10 kg is presented. The physiologic explanations for the necessary phenytoin dosage adjustment in infants weighing less than 20 kg are discussed. It is suggested that more rapid metabolism, a larger volume of distribution and impaired gastrointestinal absorption may be responsible for the higher phenytoin dosages needed in such patients. Methods of determining appropriate dosage regimens, based on Michaelis-Menten pharmacokinetic principles or on body surface area, are discussed. Monitoring of plasma phenytoin concentrations is necessary to provide therapeutic levels of phenytoin and to prevent toxicity.