Phenytoin is an anticonvulsant which may be useful in the treatment of status epilepticus of the grand mal type. The primary site of action appears to be the motor cortex where spread of seizure activity is inhibited. Possibly by promoting sodium efflux from neurons, phenytoin tends to stabilize the threshold against hyperexcitability caused by excessive stimulation or environmental changes capable of reducing membrane sodium gradient. This includes the reduction of post-tetanic potentiation at synapses. Loss of post-tetanic potentiation prevents cortical seizure foci from detonating adjacent cortical areas. Phenytoin reduces the maximal activity of brain stem centers responsible for the tonic phase of grand mal seizures.
The plasma half-life in man after intravenous administration ranges from 10 to 15 hours. Optimum control without clinical signs of toxicity occurs most often with serum levels between 10 mcg/mL and 20 mcg/mL.
A fall in plasma levels may occur when patients are changed from oral to intramuscular administration. The drop is caused by slower absorption, as compared to oral administration, due to the poor water solubility of phenytoin. Intravenous administration is the preferred route for producing rapid therapeutic serum levels.
There are occasions when intramuscular administration may be required, i.e., postoperatively, in comatose patients, for GI upsets. During these periods, a sufficient dose must be administered intramuscularly to maintain the plasma level within the therapeutic range. Where oral dosage is resumed following intramuscular usage, the oral dose should be properly adjusted to compensate for the slow, continuing IM absorption to avoid toxic symptoms.
Patients stabilized on a daily oral regimen of phenytoin experience a drop in peak blood levels to 50 to 60 percent of stable levels if crossed over to an equal dose administered intramuscularly. However, the intramuscular depot of poorly soluble material is eventually absorbed, as determined by urinary excretion of 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH), the principal metabolite, as well as the total amount of drug eventually appearing in the blood.
A short-term (one week) study indicates that patients do not experience the expected drop in blood levels when crossed over to the intramuscular route if the phenytoin IM dose is increased by 50 percent over the previously established oral dose. To avoid drug cumulation due to absorption from the muscle depots, it is recommended that for the first week back on oral phenytoin, the dose be reduced to half of the original oral dose (one third of the IM dose). Experience for periods greater than one week is lacking and blood level monitoring is recommended. For administration of phenytoin in patients who cannot take oral medication for periods greater than a week, gastric intubation may be considered.