Empiric Treatment for Encephalitis
Start intravenous aciclovir 10 mg/kg three times daily immediately when encephalitis is suspected based on clinical presentation (altered consciousness, new seizures, or focal neurological signs) and CSF/imaging findings suggestive of viral encephalitis, or within 6 hours of admission if diagnostic results are delayed or the patient is deteriorating. 1
Immediate Empiric Therapy
When to Start Aciclovir
- Initiate aciclovir if initial CSF and/or imaging suggest viral encephalitis, without waiting for HSV PCR confirmation 1
- Start within 6 hours of admission if diagnostic results will not be available promptly or if the patient is very unwell or deteriorating 1
- Continue aciclovir even if first CSF microscopy or imaging is normal but clinical suspicion of HSV or VZV encephalitis remains high 1
- Add empiric antibiotics if bacterial meningitis cannot be excluded, following meningitis guidelines 1
The rationale is compelling: aciclovir reduces mortality in HSV encephalitis from >70% to 20-30%, and delays beyond 48 hours between hospital admission and treatment initiation worsen outcomes significantly 1. HSV is the most commonly identified viral cause of encephalitis in industrialized countries 2, 1.
Dosing and Administration
- Standard dose: 10 mg/kg intravenous three times daily 1
- For VZV encephalitis: 10-15 mg/kg three times daily (VZV is less sensitive to aciclovir than HSV, though most clinicians use 10 mg/kg) 1
- Reduce dose in pre-existing renal impairment 1
- Monitor renal function closely: reversible nephropathy from crystalluria can affect up to 20% of patients, typically manifesting after 4 days of therapy 1
Duration of Treatment
For Confirmed HSV Encephalitis
- Continue for 14-21 days intravenously 1
- Perform repeat lumbar puncture at 14-21 days to confirm CSF is HSV PCR negative 1
- If CSF remains positive, continue aciclovir with weekly PCR testing until negative 1
The original trials used 10 days, but clinical relapses occurred, prompting the current 14-21 day recommendation despite ongoing debate about whether relapses are due to continued viral replication or immune-mediated inflammation 1.
When to Stop Aciclovir in Suspected Cases
Stop aciclovir in immunocompetent patients if: 1
- An alternative diagnosis has been established, OR
- HSV PCR in CSF is negative on two occasions 24-48 hours apart AND MRI is not characteristic for HSV encephalitis, OR
- HSV PCR in CSF is negative once >72 hours after neurological symptom onset WITH unaltered consciousness, normal MRI (performed >72 hours after symptom onset), AND CSF <5 white cells/mcL 1
Critical caveat: Initial CSF PCR can be falsely negative if taken <72 hours after symptom onset or late in illness after viral clearance 1. Do not stop aciclovir based on a single early negative PCR when clinical suspicion remains high 1.
Adjunctive Corticosteroids
Do not use corticosteroids routinely in HSV encephalitis 1. The most recent high-quality evidence from the 2026 DexEnceph trial (94 patients, UK multicenter RCT) definitively showed that dexamethasone plus aciclovir did not improve verbal memory scores at 26 weeks compared to aciclovir alone (adjusted difference 1.77,95% CI -9.57 to 13.12, p=0.76) 3. The safety profile was acceptable, but there was no efficacy benefit 3.
Exception for VZV encephalitis: Consider a short course of corticosteroids (e.g., prednisolone 60-80 mg daily for 3-5 days) given the inflammatory nature of VZV CNS disease, particularly if vasculitic component is present 1.
Specific Etiologies Beyond HSV
VZV Encephalitis
- Aciclovir 10-15 mg/kg IV three times daily for up to 14 days 1
- Consider corticosteroids (stronger indication if vasculitic features) 1
- VZV cerebellitis requires no specific antiviral treatment (self-limiting, immune-mediated) 1
Other Viral Causes
- Most viral encephalitides lack specific treatment (West Nile virus, enteroviruses, arboviruses) 2
- Etiology remains unknown in 32-75% of cases despite extensive evaluation 2
- Focus on supportive care and ruling out treatable causes 2, 4
Common Pitfalls to Avoid
Do not delay lumbar puncture unnecessarily: Unlike meningococcal septicemia where minutes matter, patients with mild encephalopathy can undergo LP before treatment if it can be done promptly 1. However, if strong clinical suspicion exists or patient is deteriorating, start aciclovir first 1
Do not stop aciclovir on a single negative PCR taken early (<72 hours): Repeat CSF examination after 24-48 hours if clinical suspicion persists 1
Avoid empiric aciclovir for all encephalopathy without clinical features of encephalitis: Recent data shows overuse in patients with simple meningitis or delirium, exposing patients to unnecessary nephrotoxicity risk 5, 6
Recognize that altered mental status is the key distinguishing feature: This separates viral encephalitis from meningitis and should guide empiric treatment decisions 5
Consider non-infectious causes: Approximately 10% of initially suspected infectious encephalitis cases ultimately receive non-infectious diagnoses, including autoimmune encephalitis 2, 7