What are the dosing guidelines for phenytoin (antiepileptic medication)?

Phenytoin Dosing Guidelines

For status epilepticus, phenytoin should be administered intravenously at a loading dose of 18-20 mg/kg (maximum 1000 mg) infused at a rate not exceeding 50 mg/min in adults or 1-3 mg/kg/min in children, with close cardiac monitoring. 1, 2

Status Epilepticus Dosing

Adults

• Loading dose: 18-20 mg/kg IV (maximum 1000 mg)
• Infusion rate: Not to exceed 50 mg/min (approximately 20 minutes for a 70-kg patient)
• Maintenance: Follow with 100 mg IV or orally every 6-8 hours

Pediatric Patients

• Neonates: 10 mg/kg IV
• Children: 20 mg/kg IV (maximum 1000 mg)
• Infusion rate: Not to exceed 1-3 mg/kg/min or 50 mg/min, whichever is slower
• Maximum total dose: 40 mg/kg (can repeat dose once after 15 minutes if necessary)

Non-Emergency Maintenance Dosing

When transitioning from IV to oral therapy:

• IV phenytoin is 100% bioavailable while oral phenytoin is approximately 90% bioavailable
• When switching from IV to oral, maintain the same total daily dose
• Therapeutic serum levels: 10-20 mcg/mL (total) or 1-2 mcg/mL (unbound)
• Monitor serum levels to guide dosing adjustments

Special Populations

Patients with Renal or Hepatic Impairment

• Monitor unbound phenytoin concentrations rather than total levels
• Dose adjustments may be necessary due to altered protein binding
• Decreased clearance may require lower or less frequent dosing

Geriatric Patients

• Phenytoin clearance is decreased in elderly patients
• Lower or less frequent dosing may be required

Pregnant Patients

• Decreased serum concentrations may occur during pregnancy
• Periodic measurement of serum levels is recommended
• Monitor unbound phenytoin concentrations due to potential changes in protein binding

Administration Considerations

IV Administration

• Dilute in normal saline only (incompatible with glucose-containing solutions)
• Use an in-line filter
• Continuous cardiac monitoring is essential during administration
• Monitor blood pressure and respiratory function
• If heart rate decreases by 10 beats/min or more, reduce infusion rate

Adverse Effects to Monitor

• Cardiovascular: Hypotension, bradycardia, arrhythmias (especially with rapid infusion)
• Neurological: Nystagmus, ataxia, slurred speech, decreased coordination, somnolence
• Local: Extravasation can cause tissue damage (purple glove syndrome)

Important Precautions

• Fosphenytoin is preferred over phenytoin when available due to lower risk of adverse cardiac effects and local tissue injury
• Phenobarbital is preferred over phenytoin in neonates due to decreased protein binding and increased risk of toxicity with phenytoin
• If QRS widening or hypotension occurs during administration, hold the remainder of the dose
• Weight-based dosing is critical; using estimated weights can lead to significant dosing errors and toxicity 3
• Therapeutic drug monitoring is essential to ensure efficacy and minimize toxicity

Alternative Medications for Status Epilepticus

If phenytoin fails or is contraindicated, consider:

• Valproate: 20-30 mg/kg IV (88% success rate) 4
• Levetiracetam: 40-60 mg/kg IV (44-73% success rate) 4
• Phenobarbital: 10-20 mg/kg IV (58% success rate) 4

Remember that accurate weight-based dosing and careful monitoring during administration are essential to minimize the risk of adverse effects while achieving therapeutic serum levels.