Seizures •
Tonic-clonic seizures: Mainly used in the prophylactic management of tonic-clonic seizures with complex symptomatology (psychomotor seizures). A period of 5–10 days of dosing may be required to achieve anticonvulsant effects. •
Focal seizures: Mainly used to protect against the development of focal seizures with complex symptomatology (psychomotor and
temporal lobe seizures). Also effective in controlling focal seizures with autonomic symptoms. •
Absence seizures: Not used in treatment of pure absence seizures due to risk for increasing frequency of seizures. However, can be used in combination with other anticonvulsants during combined absence and tonic-clonic seizures. • Seizures during surgery: A 2018 meta-analysis found that early antiepileptic treatment with either phenytoin or phenobarbital reduced the risk of seizure in the first week after neurosurgery for brain tumors. •
Status epilepticus: Considered after failed treatment using a
benzodiazepine due to slow
onset of action. Though phenytoin has been used to treat seizures in infants, as of 2023, its effectiveness in this age group has been evaluated in only one study. Due to the lack of a comparison group, the evidence is inconclusive.
Other •
Abnormal heart rhythms: may be used in the treatment of
ventricular tachycardia and sudden episodes of
atrial tachycardia after other
antiarrhythmic medications or
cardioversion has failed. It is a
class Ib antiarrhythmic. •
Digoxin toxicity: Intravenous phenytoin formulation is a medication of choice for arrhythmias caused by
cardiac glycoside toxicity. •
Trigeminal neuralgia: Second choice drug to
carbamazepine.
Special considerations • Phenytoin has a narrow
therapeutic index. Its therapeutic range for both anticonvulsant and antiarrhythmic effect is 10–20 μg/mL. • Avoid giving intramuscular formulation unless necessary due to skin cell death and local tissue destruction. • Elderly patients may show earlier signs of toxicity. • In the obese, ideal body weight should be used for dosing calculations. • Pregnancy:
Pregnancy category D due to risk of
fetal hydantoin syndrome and fetal bleeding. However, optimal seizure control is very important during pregnancy so drug may be continued if benefits outweigh the risks. Due to decreased drug concentrations as a result of
plasma volume expansion during pregnancy, dose of phenytoin may need to be increased if only option for seizure control. • Breastfeeding: The manufacturer does not recommend breastfeeding since low concentrations of phenytoin are excreted in breast milk. • Liver disease: Do not use oral loading dose. Consider using decreased maintenance dose. • Kidney disease: Do not use oral loading dose. Can begin with standard maintenance dose and adjust as needed. • Intravenous use is contraindicated in patients with
sinus bradycardia,
sinoatrial block, second- or third-degree
atrioventricular block,
Stokes-Adams syndrome, or hypersensitivity to phenytoin, other
hydantoins or any ingredient in the respective formulation. ==Side effects==