What is the recommended dosage of acyclovir (antiviral medication) for the treatment of viral meningitis?

Acyclovir Dosage for Viral Meningitis

For HSV meningitis in adults with normal renal function, administer acyclovir 10 mg/kg intravenously every 8 hours for 14-21 days. 1

Critical Distinction: Meningitis vs. Encephalitis

Before initiating treatment, it is essential to distinguish between viral meningitis and encephalitis, as they represent different disease severities:

• Meningitis presents with fever, headache, meningismus, and photophobia without altered mental status or focal neurological deficits 1, 2
• Encephalitis involves altered mental status, focal neurological deficits, and parenchymal brain involvement requiring more aggressive treatment 1
• The dosing regimen is the same for both conditions (10 mg/kg IV every 8 hours), but the clinical urgency and monitoring differ 3, 1

Standard Dosing by Age Group

Adults

• 10 mg/kg IV every 8 hours for 14-21 days in patients with normal renal function 3, 1
• Early initiation (within 4 days of symptom onset) decreases mortality to 8% 3, 1
• Treatment duration of 14-21 days ensures adequate viral suppression and prevents relapse 1

Pediatric Patients

• Children 3 months to 12 years: 500 mg/m² IV every 8 hours 1
• Adolescents >12 years: 10 mg/kg IV every 8 hours 1
• Neonates: Higher-dose acyclovir (20 mg/kg IV every 8 hours for 21 days) has shown improved outcomes with decreased mortality to 5% 3, 1

Renal Dose Adjustments

Acyclovir is 62-91% renally excreted and requires dose adjustment in renal impairment 1, 4:

• CrCl >80 mL/min: Standard dosing (10 mg/kg every 8 hours) 4
• CrCl 50-80 mL/min: Reduce dose or extend interval 4
• CrCl 15-50 mL/min: Further dose reduction required 4
• Anuric patients: Significant dose reduction necessary (half-life increases from 2.5 to 19.5 hours) 4

Critical pitfall: In obese patients, calculate the dose based on ideal body weight for height rather than actual weight to avoid excessive dosage and toxicity 5

Route of Administration

• Intravenous therapy is mandatory for acute viral meningitis requiring hospitalization 1
• Do not use oral acyclovir for severe cases requiring hospitalization 1
• Oral valacyclovir (1 g three times daily for 1 week) may be used for initial treatment in less severe cases, but IV is preferred for hospitalized patients 6

Treatment Response Monitoring

CSF PCR Follow-up

• Consider obtaining a repeat CSF specimen for HSV PCR at the end of therapy in patients who have not had appropriate clinical response 3, 1
• If PCR remains positive for HSV at the end of treatment, continue antiviral therapy 3, 1
• A negative CSF PCR result at the end of therapy is associated with better outcomes 3

Renal Function Monitoring

• Monitor renal function throughout treatment, as acyclovir can cause nephrotoxicity 1, 5
• Nephrotoxicity manifests after 4 days of IV therapy in up to 20% of patients 1
• Maintain adequate hydration to reduce nephrotoxicity risk 1
• Rapid recovery typically occurs within 3 days of stopping acyclovir if toxicity develops 5

Recurrent HSV-2 Meningitis (Mollaret's Syndrome)

For patients with recurrent episodes:

• Acute episodes: Acyclovir 10 mg/kg IV every 8 hours for 14-21 days 1
• Suppressive therapy: Valacyclovir 500 mg twice daily was NOT shown to prevent recurrent meningitis episodes in a randomized controlled trial 7, 6
• The majority of recurrent lymphocytic meningitis cases are caused by HSV-2 7
• Recurrence is not universal; only one-third of patients experience 1-4 meningitis episodes over 2 years 2, 6

Important caveat: A double-blind randomized controlled trial demonstrated that valacyclovir suppression at 500 mg twice daily did not prevent recurrent HSV-2 meningitis and may even increase recurrence risk after cessation (hazard ratio 3.29), possibly representing a rebound phenomenon 6

Common Pitfalls to Avoid

• Dosing in obesity: Always use ideal body weight for height, not actual weight, even in non-obese overweight patients 5
• Overuse in suspected meningitis: Do not routinely prescribe IV acyclovir for suspected meningitis without features of encephalitis (altered mental status, focal deficits) 8
• Inadequate hydration: Maintain adequate hydration throughout treatment to prevent crystalluria and obstructive nephropathy 1
• Premature discontinuation: Complete the full 14-21 day course to prevent relapse, which occurs in up to 5% of cases 3, 1

Adverse Effects

• Nephrotoxicity: Most common, occurring in up to 20% of patients after 4 days of IV therapy 1
• Neurotoxicity: Rare but can include increased seizure activity and encephalopathy 1, 5
• Other rare effects: Hepatitis and bone marrow suppression 1

Special Populations

Immunocompromised Patients

• Standard dosing of 10 mg/kg IV every 8 hours is appropriate 1
• If acyclovir resistance is suspected (persistent lesions despite therapy), consider foscarnet 40 mg/kg IV every 8 hours 1

Geriatric Patients

• Higher plasma concentrations occur due to age-related renal function decline 4
• Dose reduction may be required based on creatinine clearance 4
• Greater risk of CNS adverse events compared to younger patients 4