Phenytoin (Eptoin) Dosing for Head Injury
For seizure prophylaxis in head injury, administer a loading dose of 15-20 mg/kg IV in pediatric patients or 10-15 mg/kg IV in adults, infused at a rate not exceeding 1-3 mg/kg/min (or 50 mg/min, whichever is slower), followed by maintenance doses of 100 mg every 6-8 hours. 1
Loading Dose Administration
Pediatric Patients
- Loading dose: 15-20 mg/kg IV 2, 1
- Infusion rate: Maximum 1-3 mg/kg/min or 50 mg/min, whichever is slower 2, 1
- This dosing achieves therapeutic serum concentrations between 10-20 mcg/mL 1
Adult Patients
- Loading dose: 10-15 mg/kg IV 1
- Infusion rate: Maximum 50 mg/min (approximately 20 minutes for a 70-kg patient) 1
- Alternative dosing up to 18-20 mg/kg may be used, with 97% of patients achieving therapeutic levels immediately after infusion 3, 4
Maintenance Dosing
- 100 mg IV or oral every 6-8 hours following the loading dose 1
- For oral maintenance in adults: typically 300-400 mg/day (4-6 mg/kg/day) divided into 1-3 doses 4
- Target therapeutic serum total concentrations: 10-20 mcg/mL (unbound: 1-2 mcg/mL) 1
Critical Safety Monitoring
Continuous cardiac and respiratory monitoring is mandatory during administration 2, 1:
- Monitor ECG continuously for bradycardia, arrhythmias, and heart block 2, 1
- Monitor blood pressure for hypotension 2, 1
- Observe for respiratory depression 1
- Reduce infusion rate if heart rate decreases by 10 beats/min 2
Administration Technique
- Administer through large-gauge catheter into large peripheral or central vein 1
- Flush with sterile saline before and after each dose to prevent local venous irritation 1
- Dilute in normal saline only (never dextrose-containing solutions due to precipitation) 2, 1
- If diluted for infusion, final concentration should be ≥5 mg/mL 1
Important Clinical Context
Efficacy Evidence for Head Injury
The evidence for phenytoin prophylaxis in head injury shows effectiveness only for early seizures (first 7 days) 5:
- Phenytoin reduces seizures in the first week after severe head injury (3.6% vs 14.2% with placebo, p<0.001) 5
- No benefit beyond the first week: 21.5% phenytoin group vs 15.7% placebo group had seizures between day 8 and year 1 5
- In pediatric head injury, one study found no significant benefit (7% phenytoin vs 5% placebo experienced seizures within 48 hours) 6
However, conflicting evidence exists, with one older study showing failure to prevent early posttraumatic seizures 7, while another suggested reduced incidence with proper dosing regimen 8.
Common Pitfalls to Avoid
- Never use intramuscular route for acute seizure management due to erratic absorption (peak levels may require up to 24 hours) 1
- Never exceed recommended infusion rates - rapid administration causes hypotension, bradyarrhythmias, and cardiac arrest 2, 4
- Never mix with dextrose solutions - causes precipitation 2, 1
- Neonates have increased toxicity risk due to decreased protein binding; phenobarbital is preferred in this population 2
Special Populations
Patients with renal/hepatic disease or hypoalbuminemia 1:
- Monitor unbound (free) phenytoin concentrations rather than total levels
- These patients have increased fraction of unbound phenytoin