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

Acyclovir Dosing for Viral Meningitis

For HSV meningitis in adults, administer intravenous acyclovir 10 mg/kg every 8 hours for 14-21 days, which reduces mortality and ensures adequate viral suppression. 1

Age-Based Dosing Recommendations

Adults and Adolescents (>12 years)

• 10 mg/kg IV every 8 hours for 14-21 days is the standard regimen for HSV meningitis 1
• This dosing achieves therapeutic plasma concentrations while minimizing toxicity 2
• Early initiation (within 4 days of symptom onset) decreases mortality to 8% 1

Pediatric Patients

• Children 3 months-12 years: 500 mg/m² IV every 8 hours 2
• Adolescents >12 years: 10 mg/kg IV every 8 hours 2
• Neonates: 20 mg/kg IV every 8 hours for 21 days with improved outcomes and decreased mortality to 5% 1

Critical Distinction: Meningitis vs. Encephalitis

HSV meningitis and encephalitis require different clinical approaches, though initial empiric dosing may be similar. The guidelines primarily address encephalitis, but the dosing translates to meningitis management 2.

• Meningitis presents with meningeal signs, headache, photophobia, and lymphocytic pleocytosis without parenchymal involvement 1, 3
• Encephalitis involves altered mental status, focal neurological deficits, and parenchymal brain involvement requiring more aggressive treatment 2
• For suspected encephalitis, acyclovir should be started within 6 hours of admission, even if initial CSF or imaging is normal 2

Treatment Duration and Monitoring

Standard Course

• 14-21 days of IV therapy is recommended to ensure adequate viral suppression and prevent relapse 1
• Treatment duration should not be shortened arbitrarily, as relapse rates can reach 5% even after appropriate therapy 1

Response Assessment

• Obtain repeat CSF PCR at end of therapy if clinical response is inadequate 1
• If HSV PCR remains positive, continue antiviral therapy beyond the initial course 1
• Clinical improvement typically occurs within 3-5 days, but complete resolution may take longer 3, 4

Dose Adjustments for Renal Impairment

Acyclovir must be dose-adjusted in patients with impaired renal function, as the drug is 62-91% renally excreted. 5

• Monitor renal function throughout treatment, as acyclovir causes nephrotoxicity in up to 20% of patients 2, 1
• Reduce dose based on creatinine clearance to prevent crystalluria and obstructive nephropathy 2, 5
• Calculate dosing based on ideal body weight, not actual weight, especially in obese patients to avoid excessive dosing and toxicity 6
• Maintain adequate hydration to reduce nephrotoxicity risk 2

Special Populations

Immunocompromised Patients

• May require higher doses or prolonged therapy 1
• For HIV-infected patients with severe HSV disease: 5 mg/kg IV every 8 hours 1
• If acyclovir resistance is suspected (persistent lesions despite therapy), consider foscarnet 40 mg/kg IV every 8 hours as alternative 1, 7

Recurrent HSV-2 Meningitis

• Suppressive valacyclovir 500 mg twice daily does NOT prevent recurrent meningitis episodes based on randomized controlled trial evidence 1, 8
• Acute recurrent episodes should be treated with the same IV acyclovir regimen as primary episodes 1, 3
• One-third of patients experience 1-4 recurrent episodes despite treatment 8

Common Pitfalls to Avoid

Do not use oral acyclovir for acute viral meningitis requiring hospitalization - IV therapy is mandatory for severe cases 2, 1.

• Avoid dosing based on actual body weight in overweight/obese patients, as this leads to nephrotoxicity and neurotoxicity 6
• Do not discontinue therapy prematurely; complete the full 14-21 day course even if symptoms improve earlier 1
• Do not rely on history of genital herpes to guide diagnosis - 77-92% of HSV-2 meningitis patients have no history of genital herpes 3, 4
• Perform genital examination at presentation, as it is often omitted but may reveal active lesions 4

Adverse Effects Monitoring

• Nephrotoxicity manifests after 4 days of IV therapy in up to 20% of patients; monitor creatinine and maintain hydration 2
• Neurotoxicity including increased seizure activity can occur, particularly with excessive dosing 6
• Rare adverse events include hepatitis, bone marrow suppression, and encephalopathy 2
• Rapid recovery typically occurs within 3 days of stopping acyclovir if toxicity develops 6