Acyclovir Dosage for Encephalitis
For adults with suspected or confirmed encephalitis, the recommended acyclovir dosage is 10 mg/kg intravenously every 8 hours for 14-21 days with normal renal function. 1
Dosing Guidelines by Age Group
Adults and Children >12 years
- 10 mg/kg IV every 8 hours for 14-21 days 1
- For VZV encephalitis specifically, a higher dose of 10-15 mg/kg every 8 hours may be preferred due to lower sensitivity to acyclovir 2
Children 3 months to 12 years
- 500 mg/m² IV every 8 hours for 14-21 days 1
Neonates and Infants <3 months
- 20 mg/kg IV every 8 hours for 21 days 1
- This higher dose in neonates has decreased mortality from 28% to 5% 1
Important Clinical Considerations
Treatment Duration
- Standard treatment course is 14-21 days 1, 2
- Shorter courses (10 days) have been associated with relapse rates of up to 8% in neonates 1, 3
- Consider repeating CSF PCR at the end of therapy; if positive, continue treatment 1
Renal Dosage Adjustments
- Acyclovir requires dose adjustment in renal impairment 1, 4
- Renal function should be monitored throughout treatment 2, 4
- Precipitation of acyclovir crystals in renal tubules can occur if maximum solubility is exceeded 4
- Ensure adequate hydration during treatment to prevent nephrotoxicity 4
Timing of Treatment
- Early initiation is critical - mortality decreases to 8% if therapy is started within 4 days of symptom onset 1
- Delay of >2 days between hospital admission and acyclovir initiation is an independent predictor of poor outcome 1
- Treatment should be started within 6 hours of admission if encephalitis is suspected 1
Special Populations
Immunocompromised Patients
- May require prolonged courses of IV acyclovir 2
- For cytomegalovirus encephalitis, combination therapy with ganciclovir and foscarnet is recommended 1
Elderly Patients
- Acyclovir plasma concentrations are higher in geriatric patients 4
- Dosage reduction may be required due to age-related changes in renal function 4
Monitoring and Safety
Adverse Effects
- Renal impairment occurs in approximately 20% of patients after 4 days of IV therapy 1
- Encephalopathic changes (lethargy, confusion, hallucinations, seizures) occur in approximately 1% of patients 4
- Use with caution in patients with underlying neurologic abnormalities or significant renal, hepatic, or electrolyte abnormalities 4
Efficacy Monitoring
- Consider repeat CSF PCR at the end of therapy to confirm clearance of virus 1
- Poor outcomes are associated with:
- Age >30 years
- Low Glasgow Coma Score (<6)
- Delayed treatment initiation
- Simplified Acute Physiology Score >7 1
Recent Research Insights
A 2022 study suggests that patient weight may influence treatment outcomes, with low-weight patients (<79 kg) potentially benefiting from a minimum dosage of 2550 mg/day (850 mg every 8 hours) 5. However, the established guideline-recommended weight-based dosing of 10 mg/kg every 8 hours remains the standard of care 1.