What is the recommended dose of Acyclovir (antiviral medication) for pediatric patients with various indications, including chickenpox, herpes zoster, and HSV (Herpes Simplex Virus) encephalitis, considering factors such as age, weight, and renal function?

Acyclovir Pediatric Dosing

Recommended Dosing by Indication

For most pediatric herpes infections, acyclovir dosing is weight-based at 20 mg/kg per dose (maximum 800 mg/dose), with frequency and duration varying by indication.

Herpes Zoster (Shingles)

• Oral acyclovir 20 mg/kg (maximum 800 mg/dose) four times daily for 7-10 days or until no new lesions appear for 48 hours is the CDC-recommended first-line treatment for children with mild to moderate disease and normal immune function 1
• For older children who can receive adult dosing, valacyclovir 1000 mg three times daily for 7 days is an alternative 2, 1
• Treatment should ideally begin within 72 hours of rash onset, with greatest efficacy when started within 48 hours 3

Chickenpox (Varicella)

• Oral acyclovir 20 mg/kg four times daily (up to 800 mg/dose, maximum 3,200 mg/day) for 5 days 3
• Treatment must be initiated within 24 hours of rash onset to be effective 3
• This regimen shortens time to healing, reduces maximum lesion count, and decreases fever, anorexia, and lethargy 3

HSV Encephalitis

• Intravenous acyclovir 10 mg/kg every 8 hours (or 500 mg/m² every 8 hours for children >1 year) is recommended for severe disease 1, 4
• Recent evidence supports high-dose acyclovir at 60 mg/kg/day (20 mg/kg every 8 hours) for children aged 3 months to 12 years, though standard dosing (30 mg/kg/day) shows similar safety profiles 5
• For children with augmented renal clearance (eGFR >250 ml/min/1.73 m²), doses of 15-20 mg/kg every 6 hours may be necessary to maintain therapeutic levels 6

Mucocutaneous HSV Infections

• For adolescents who can receive adult dosing: valacyclovir 1 gram twice daily 2
• For younger children: oral acyclovir 20 mg/kg per dose 2

Acute Retinal Necrosis

• Initial treatment: acyclovir 10 mg/kg IV three times daily for 10-14 days 1, 7
• Followed by oral therapy for 4-6 weeks:
  • Children receiving adult dosing: valacyclovir 1 gram three times daily 1, 7
  • Younger children: acyclovir 20 mg/kg per dose 1

Special Populations and Dosing Adjustments

Immunocompromised Children

• Intravenous acyclovir is recommended for severe immunosuppression 1
• For acyclovir-resistant cases: foscarnet 40-60 mg/kg IV three times daily for 7-10 days 1

Neonates and Young Infants

• For infants <1 month: acyclovir 24 mg/kg three times daily 8
• For infants 1-3 months: acyclovir 24 mg/kg four times daily 8
• The elimination half-life decreases sharply during the first month of life, from 10-15 hours to 2.5 hours, necessitating age-specific dosing 8
• Infants 1-2 months receiving valacyclovir show approximately 60% higher AUC and 30% higher peak concentrations compared to older children, requiring caution 9

Renal Impairment

• Dosage adjustment is mandatory for patients with reduced renal function, as acyclovir clearance is directly dependent on eGFR 3, 6
• The FDA label emphasizes that half-life and total body clearance are dependent on renal function 3
• Monitor serum creatinine before and during treatment, particularly in older children, those with longer treatment duration, and those receiving higher doses 5

Administration Considerations

Oral Formulations

• Acyclovir tablets may be administered with or without food, as food does not affect absorption 3
• Bioavailability of oral acyclovir is 10-20% and decreases with increasing dose 3
• For oral acyclovir, the preferred dosing when feasible is 15 mg/kg every 6 hours rather than 20 mg/kg every 8 hours to maintain more consistent therapeutic levels 6

Intravenous Administration

• Adequate hydration and urine flow must be maintained during IV therapy 4
• Mental status should be monitored during treatment 4
• For children with normal renal function (eGFR ≤250 ml/min/1.73 m²): 10 mg/kg every 6 hours 6

Key Clinical Pitfalls

Timing of Treatment

• Treatment efficacy is highly time-dependent: herpes zoster treatment is most effective within 48 hours of rash onset, and chickenpox treatment must begin within 24 hours 3

Target Concentrations

• Trough concentrations should remain above 0.56 mg/L for HSV and 1.125 mg/L for VZV 6
• Peak concentrations should not exceed 25 mg/L to minimize toxicity risk 6

Renal Monitoring

• Children who develop renal injury tend to be older, have longer hospital stays, and receive longer treatment durations 5
• The incidence of renal injury is similar between standard and high-dose regimens when appropriate monitoring is performed 5