In an 11‑month‑old boy with a single generalized tonic seizure lasting 10 minutes and two undocumented fevers in the preceding 24 hours, should aciclovir be initiated?

Should Aciclovir Be Started in This 11-Month-Old Boy?

Yes, intravenous aciclovir should be started immediately at 500 mg/m² every 8 hours if there is clinical suspicion of encephalitis, but this decision hinges entirely on whether the child has encephalitis versus a simple febrile seizure. 1

Critical Clinical Distinction Required

The management depends on distinguishing between:

• Simple febrile seizure (fever + single brief seizure, normal consciousness between episodes) – aciclovir is not indicated 2
• Viral encephalitis (altered mental status, confusion, behavioral changes, prolonged seizure, focal neurological deficits) – aciclovir is mandatory 1

A 10-minute generalized tonic seizure is concerning and longer than typical febrile seizures (usually <5 minutes), raising suspicion for encephalitis. 1

When to Start Aciclovir Empirically

Start aciclovir within 6 hours of admission if: 1

• The child has altered consciousness, confusion, or behavioral changes beyond post-ictal state
• Focal neurological signs are present
• The seizure was prolonged (>5-10 minutes) or focal in nature
• There will be delays in performing lumbar puncture or obtaining CSF/imaging results
• The child is deteriorating or severely ill

Do not wait for CSF or imaging confirmation – delays beyond 48 hours significantly worsen outcomes, and HSV encephalitis carries 70% mortality untreated versus 20-30% with aciclovir. 1

Dosing for This Patient

For an 11-month-old child (3 months to 12 years): 1

• 500 mg/m² IV every 8 hours (approximately 20 mg/kg/dose)
• Continue for a minimum of 21 days before considering stopping, as children in this age group have relapse rates of 26-29% with shorter courses 1
• Adjust dose if renal impairment is present 1

Diagnostic Workup While Starting Treatment

If encephalitis is suspected, perform immediately: 1

• Lumbar puncture (unless contraindicated by signs of raised intracranial pressure)
• CSF analysis: cell count, protein, glucose, HSV PCR
• Brain imaging (MRI preferred over CT, but CT acceptable if MRI unavailable)
• Blood cultures and bacterial meningitis workup

Key point: Even if aciclovir is started empirically, CSF PCR for HSV remains positive for 7-10 days after treatment initiation, so delayed lumbar puncture can still confirm diagnosis. 1

If This Is a Simple Febrile Seizure

Aciclovir is not indicated if: 2

• The child has normal consciousness between fever episodes
• No altered mental status, confusion, or behavioral changes
• No focal neurological deficits
• The seizure was brief and generalized
• CSF shows normal findings or typical viral meningitis pattern (lymphocytic pleocytosis, normal glucose)

The British Infection Association states there is no evidence supporting aciclovir for viral meningitis (as opposed to encephalitis), and treatment should focus on supportive care with analgesia and fluids. 2

Common Pitfalls to Avoid

Do not delay treatment for imaging – unnecessary CT scans before lumbar puncture are a major cause of treatment delays; only perform CT if there are true contraindications to immediate LP (focal signs, papilledema, severely reduced consciousness). 1, 3

Do not use oral aciclovir – it does not achieve adequate CSF concentrations for CNS infections. 1

Do not stop treatment prematurely – 10-day courses lead to 26-29% relapse rates in children; this age group requires minimum 21 days. 1

Do not forget hydration and renal monitoring – nephrotoxicity occurs in up to 20% of patients after 4 days of IV therapy; maintain adequate hydration and monitor renal function throughout treatment. 1

Safety Monitoring During Treatment

• Monitor renal function (creatinine, urine output) regularly 1
• Ensure adequate IV hydration to prevent crystalluria and obstructive nephropathy 1
• Rare adverse events include hepatitis, bone marrow suppression, and encephalopathy 1

Treatment Duration and Stopping Criteria

Continue IV aciclovir for 14-21 days (minimum 21 days in children 3 months-12 years), then: 1

• Perform repeat lumbar puncture with HSV PCR
• If PCR remains positive, continue aciclovir with weekly PCR testing until negative
• If PCR is negative and an alternative diagnosis is established, aciclovir can be stopped

The decision to start aciclovir in this case depends on clinical assessment for encephalitis features beyond a simple febrile seizure – the 10-minute seizure duration and need to rule out HSV encephalitis (a treatable cause with high mortality if missed) favor empirical treatment while awaiting diagnostic results. 1