What are the recommended acyclovir dosing regimens for adult HSV central nervous system infection, neonatal HSV infection, uncomplicated herpes zoster, and genital HSV, and how should the doses be adjusted in patients with renal impairment?

Acyclovir Dosing Recommendations

For adult HSV CNS infection, use acyclovir 10 mg/kg IV every 8 hours for 21 days; for neonatal HSV CNS disease, use 20 mg/kg IV every 8 hours for 21 days with mandatory repeat CSF HSV DNA PCR at days 19-21 before discontinuation; for uncomplicated herpes zoster, use 800 mg orally 5 times daily for 7-10 days; and for genital HSV, use 400 mg orally twice daily for initial episodes (7-10 days) or recurrent episodes (5 days). 1, 2, 3, 4

Adult HSV Central Nervous System Infection

Acyclovir 10 mg/kg IV every 8 hours for 21 days is the standard regimen for HSV encephalitis in adults. 2, 3 This dosing achieves adequate CNS penetration and has been shown to reduce mortality to 28% at 18 months, though outcomes remain suboptimal. 3

Critical timing considerations:

• Mortality decreases to 8% when therapy is initiated within 4 days of symptom onset 3
• Delays beyond 2 days from hospital admission independently predict poor outcomes 3
• Predictors of adverse outcomes include age >30 years, Glasgow coma score <6, and symptom duration >4 days before treatment 3

Important monitoring: If clinical response is inadequate, repeat CSF HSV DNA PCR at end of therapy; continue acyclovir if positive. 3 Relapse rates up to 5% have been reported in adults, though this is less well-documented than in neonates. 3

Neonatal HSV Infection

For neonatal CNS or disseminated disease, use acyclovir 20 mg/kg IV every 8 hours for 21 days. 2, 1 This higher dose (compared to the historical 10 mg/kg) has decreased mortality to 5% with approximately 40% of survivors developing normally. 3

For neonatal skin, eye, or mouth disease only: 20 mg/kg IV every 8 hours for 14 days. 1

Mandatory requirement for CNS disease: Do not discontinue acyclovir until repeat CSF HSV DNA PCR performed at days 19-21 is negative. 1, 2 This is critical as approximately 8% of neonates treated with lower doses for shorter durations experienced documented relapse. 3

Toxicity monitoring in neonates:

• Neutropenia (ANC <1,000/mm³) is the major toxicity, occurring in the majority of infants on high-dose therapy 2
• Grade 3 or higher nephrotoxicity observed in 6% 2
• Long-term oral suppressive therapy (300 mg/m² 2-3 times daily) following neonatal treatment is associated with 46% neutropenia rate, though usually self-limited 2

Uncomplicated Herpes Zoster

Acyclovir 800 mg orally 5 times daily for 7-10 days is the recommended regimen. 4 Treatment should be initiated within 72 hours of rash onset for maximum benefit. 4

Comparative efficacy: Meta-analysis demonstrates that 800 mg five times daily is superior to 200 mg five times daily, shortening blister stopping time (MD -1.29 days), pain duration (MD -2.73 days), and scabbing time (MD -2.42 days) without increasing adverse reactions. 5

Alternative agents with better dosing convenience:

• Valacyclovir 900-1,000 mg three times daily shows superior efficacy to 300 mg twice daily, reducing postherpetic neuralgia incidence (RR 0.28) 5
• Famciclovir 250 mg three times daily is equally effective as higher doses 5

Genital HSV

Initial Episode

Acyclovir 400 mg orally three times daily for 7-10 days is the preferred regimen. 6 Alternative: 200 mg five times daily for 7-10 days. 6, 4 Treatment may be extended if healing is incomplete after 10 days. 6

For severe disease requiring hospitalization: Acyclovir 5-10 mg/kg IV every 8 hours for 5-7 days or until clinical resolution. 7

Recurrent Episodes

Acyclovir 400 mg orally twice daily for 5 days when initiated at earliest sign or symptom. 1 Alternative regimens: 200 mg five times daily or 800 mg twice daily for 5 days. 7

In children <45 kg: 20 mg/kg (max 400 mg/dose) orally three times daily for 5-14 days. 1

Chronic Suppressive Therapy

Acyclovir 400 mg orally twice daily for up to 12 months for patients with frequent recurrences (≥6 per year). 4 This reduces recurrence frequency by at least 75%. 7 Alternative: 200 mg three to five times daily. 7

Re-evaluation: After 1 year of continuous suppressive therapy, discontinue to reassess recurrence rate and need for continuation. 7, 4

Renal Impairment Dose Adjustments

Acyclovir is primarily eliminated unchanged by the kidneys via glomerular filtration and tubular secretion, requiring dose adjustment based on creatinine clearance. 2, 4

Dosing Adjustments by Creatinine Clearance:

For 200 mg every 4 hours (5x daily):

• CrCl >10: No adjustment (200 mg every 4 hours)
• CrCl 0-10: 200 mg every 12 hours 4

For 400 mg every 12 hours:

• CrCl >10: No adjustment (400 mg every 12 hours)
• CrCl 0-10: 200 mg every 12 hours 4

For 800 mg every 4 hours (5x daily) - herpes zoster:

• CrCl >25: No adjustment (800 mg every 4 hours)
• CrCl 10-25: 800 mg every 8 hours
• CrCl 0-10: 800 mg every 12 hours 4

Hemodialysis Patients

Administer an additional dose after each dialysis session. 4 Hemodialysis removes approximately 60% of plasma acyclovir over 6 hours, with a mean half-life of 5 hours during dialysis. 4

Critical dosing modification for dialysis patients: Standard dosing (800 mg twice daily) causes neurotoxicity in dialysis-dependent patients. 8 Recommended regimen: 400 mg loading dose, then 200 mg twice daily maintenance, with an additional 400 mg loading dose after dialysis. 8 This maintains therapeutic levels (6.4 ± 0.8 μM) while preventing neurotoxicity. 8

Peritoneal Dialysis

No supplemental dose required after adjusting the dosing interval. 4

Critical Safety Considerations

Nephrotoxicity prevention:

• Maintain adequate hydration throughout therapy 4
• Exercise caution when co-administering potentially nephrotoxic agents 4
• Risk of reversible CNS symptoms increases with renal dysfunction 4

CNS disturbance risk: Acyclovir carries higher risk of CNS disturbance compared to famciclovir, particularly in patients with severe renal dysfunction (2.29% vs 0.60% in dialysis patients). 9 Supra-therapeutic dosing significantly increases acute kidney injury risk (adjusted OR 12.9). 10

Neurotoxicity in renal failure: Inadequate dose adjustment in dialysis patients leads to neurotoxicity. 8 Rapid elimination by single hemodialysis treatment provides both diagnostic and therapeutic intervention when toxicity is suspected. 8

Phlebitis prevention: Administer IV acyclovir slowly over at least 1 hour to minimize local irritation and phlebitis. 2