What it's for (Indications)
- Phenytoin (e.
- g.
- , Epigran) is an antiepileptic drug indicated for the control of tonic-clonic (grand mal) and complex partial (psychomotor or temporal lobe) seizures.
- It is also effective in the prevention and treatment of seizures occurring during or following neurosurgery.
- Furthermore, intravenous phenytoin is a cornerstone in the management of status epilepticus, providing rapid control of ongoing seizure activity.
- It exerts its therapeutic effects by stabilizing hyperexcitable neuronal membranes, thereby reducing the spread of seizure activity from a focus.
- The precise selection of phenytoin for a patient is often guided by seizure type, tolerability, and potential drug interactions, making careful clinical assessment paramount for optimal outcomes.
- Its role in the treatment of other seizure types, such as absence seizures, is limited and generally not recommended due to potential exacerbation.
Dosage Information
| Type | Guideline |
|---|---|
| Standard | Dosage of phenytoin must be individualized based on patient response, serum concentrations, and tolerability. For adults, a common oral loading dose is 1 gram divided into three doses (e.g., 400 mg, 300 mg, 300 mg) given 2 hours apart, followed by a maintenance dose of 300 mg/day, which can be adjusted in increments of 50-100 mg to achieve therapeutic serum levels, typically 10-20 mcg/mL. Intravenous administration for status epilepticus involves a loading dose of 15-20 mg/kg infused at a rate not exceeding 50 mg/minute in adults, followed by maintenance doses. Pediatric dosing is weight-based and requires careful monitoring. Due to its narrow therapeutic index and non-linear (Michaelis-Menten) pharmacokinetics, small dosage adjustments can lead to disproportionate changes in serum levels. Therapeutic drug monitoring is essential to ensure efficacy and minimize toxicity, especially given inter-individual variations in metabolism. |
Safety & Warnings
Common Side Effects
- Phenytoin is associated with a wide range of side effects, varying from common and dose-related to rare but severe.
- Dose-related neurological effects include nystagmus, ataxia, slurred speech, decreased coordination, and mental confusion, which typically correlate with elevated serum levels.
- Chronic use can lead to gingival hyperplasia, hirsutism, coarsening of facial features, and osteomalacia due to altered vitamin D metabolism.
- Hematologic adverse effects, though rare, include megaloblastic anemia (responsive to folic acid), leukopenia, thrombocytopenia, and aplastic anemia.
- Dermatological reactions range from benign morbilliform rash to severe cutaneous adverse reactions (SCARs) like Stevens-Johnson Syndrome (SJS) and toxic epidermal necrolysis (TEN), which can be fatal.
- Other serious adverse effects include hepatotoxicity, lymphadenopathy, and rarely, lupus erythematosus-like syndrome.
- Cardiovascular effects, particularly with rapid intravenous administration, can include hypotension and cardiac arrhythmias.
Serious Warnings
- Black Box Warning: WARNINGS: RATE OF INTRAVENOUS ADMINISTRATION AND SEVERE CUTANEOUS ADVERSE REACTIONS **Rate of Intravenous Administration:** Serious cardiovascular reactions, including atrial and ventricular conduction depression and ventricular fibrillation, have been reported with intravenous phenytoin administration, especially with rapid infusion. These reactions have resulted in fatalities. Therefore, intravenous phenytoin should be administered slowly, not exceeding 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients. Close cardiac monitoring (ECG, blood pressure) is essential during and after intravenous administration. Reduction in infusion rate or discontinuation may be necessary if adverse cardiovascular reactions occur. **Severe Cutaneous Adverse Reactions (SCARs):** Phenytoin can cause severe cutaneous adverse reactions (SCARs), including Stevens-Johnson Syndrome (SJS), toxic epidermal necrolysis (TEN), Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS), and acute generalized exanthematous pustulosis (AGEP), which may be fatal. These reactions can occur without warning. Genetic testing for HLA-B*1502 allele may be considered for patients of Asian ancestry, as this allele is strongly associated with an increased risk of SJS/TEN when treated with carbamazepine, and this risk may extend to phenytoin. Discontinue phenytoin at the first sign of rash, unless the rash is clearly not drug-related, and do not resume if SCARs are suspected.
- Phenytoin requires careful administration and monitoring due to several serious warnings.
- Withdrawal of antiepileptic drugs, including phenytoin, should be done gradually to minimize the risk of increased seizure frequency and status epilepticus.
- Intravenous phenytoin should be administered slowly, not exceeding 50 mg/minute in adults, due to the risk of severe cardiovascular reactions, including hypotension and arrhythmias.
- Patients with hepatic impairment require careful dose adjustments and monitoring, as phenytoin is extensively metabolized by the liver.
- Suicidal ideation and behavior have been reported in patients treated with antiepileptic drugs, necessitating close monitoring for new or worsening depression, suicidal thoughts, or unusual changes in mood or behavior.
- Hypersensitivity reactions, including Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS), can occur, requiring immediate discontinuation.
- Alcohol consumption can alter phenytoin metabolism and serum levels, requiring caution.
- Concomitant use with other highly protein-bound drugs or cytochrome P450 enzyme inhibitors/inducers can significantly impact phenytoin's efficacy and toxicity profile.
How it Works (Mechanism of Action)
Phenytoin exerts its antiepileptic effects primarily by stabilizing hyperexcitable neuronal membranes. Its principal mechanism involves modulating voltage-gated sodium channels in neurons. By binding preferentially to the inactivated state of these channels, phenytoin prolongs their refractory period, thereby preventing the rapid, repetitive firing of action potentials that characterize epileptic seizures. This action limits the spread of seizure activity from a focal point and reduces the propagation of abnormal electrical discharges throughout the brain. Unlike some other antiepileptics, phenytoin does not significantly depress the entire central nervous system but selectively acts on hyperexcitable neurons. It also has minor effects on calcium channels and potassium conductance, contributing to its overall membrane-stabilizing properties. This selective action makes it effective against generalized tonic-clonic and complex partial seizures without causing profound sedation in therapeutic concentrations.