Phenytoin Dosing After Subdural Hematoma Evacuation
Administer a loading dose of 15-20 mg/kg IV phenytoin (or fosphenytoin equivalents) at a maximum infusion rate of 50 mg/min in adults, followed by maintenance dosing of 300-400 mg/day (4-6 mg/kg/day), with mandatory continuous cardiac monitoring during administration. 1, 2
Loading Dose Protocol
The standard loading dose is 15-20 mg/kg administered intravenously. 1, 2 This dosing achieves therapeutic serum levels (10-20 mcg/mL) within minutes after completion of the infusion. 3
Critical Infusion Rate Limits
- Never exceed 50 mg/min in adults to prevent life-threatening cardiovascular complications including hypotension, bradyarrhythmias, and cardiac arrest. 3, 1
- In pediatric patients, the maximum rate is 1-3 mg/kg/min or 50 mg/min, whichever is slower. 3, 1
Administration Requirements
- Dilute only in normal saline (never dextrose-containing solutions, which cause precipitation). 3, 1
- Maintain final concentration ≥5 mg/mL. 1
- Continuous ECG monitoring is mandatory throughout the infusion. 1
- Monitor blood pressure continuously for hypotension. 1
- Reduce infusion rate immediately if heart rate decreases by 10 beats/min. 3, 1
Maintenance Dosing
Begin maintenance therapy 6-24 hours after the loading dose at 300-400 mg/day (4-6 mg/kg/day) divided into 1-3 doses. 2 For extended-release formulations in adults with controlled seizures, once-daily dosing of 300 mg may be considered. 2
Critical Dosing Considerations in Post-SDH Patients
Higher loading doses (15 mg/kg minimum, preferably closer to 20 mg/kg) are essential in critically ill trauma patients. 4 A study of ICU patients found that 49% had suboptimal free phenytoin concentrations, with lower per-kilogram doses (12.8 vs 16.3 mg/kg) strongly associated with inadequate levels. 4
Factors Affecting Dosing in Neurosurgical Patients
- Heavier patients require careful attention to weight-based dosing as they are at higher risk for suboptimal levels. 4
- Hypoalbuminemia (common post-trauma) increases free phenytoin fraction but may result in misleadingly low total levels. 4, 5
- Phenytoin clearance may increase during initial therapy in critically ill trauma patients, potentially requiring dose adjustments within the first week. 5
Monitoring Requirements
- Serum phenytoin levels should be checked 24-48 hours after loading to ensure therapeutic range (10-20 mcg/mL total, or 1-2 mg/L free). 4
- In patients with low albumin (<3.5 g/dL), measure free phenytoin levels or use correction formulas. 4
- Steady-state is achieved in 7-10 days, and dosage adjustments should not occur more frequently than this interval. 2
Safety Monitoring
Dose-Related Adverse Effects
Monitor for ataxia, nystagmus, tremor, and somnolence (common with therapeutic levels). 3, 1
Cardiovascular Complications
- Hypotension, bradycardia, and cardiac arrhythmias occur primarily with rapid infusion. 3, 1
- Purple glove syndrome and tissue necrosis can occur with IV phenytoin (less common with fosphenytoin). 3
Fosphenytoin as Alternative
Fosphenytoin may be preferred over phenytoin due to superior safety profile, with significantly lower risk of local tissue reactions, hypotension, and cardiac complications. 3 Dosing is given in phenytoin equivalents (PE): 15-20 PE/kg IV at 1-3 PE/kg/min (maximum 150 PE/min). 3, 1 However, acquisition costs are substantially higher. 3
Common Pitfall
The most critical error is using inadequate loading doses (<15 mg/kg), which results in subtherapeutic levels during the highest-risk period for post-traumatic seizures. 4, 6 A study in neurosurgical patients demonstrated that 10 mg/kg loading doses resulted in 70% of patients falling below therapeutic range within 6 hours, while 15 mg/kg maintained therapeutic levels in all patients. 6