HSV Encephalitis
Herpes simplex encephalitis (HSE) is the most common cause of sporadic fatal encephalitis worldwide, with an annual incidence of approximately 2–4 per million population. In adults, HSV-1 is responsible for more than 90% of cases, while HSV-2 predominantly causes meningitis in adults and encephalitis in neonates. The hallmark of HSE is its predilection for the medial temporal lobes and orbitofrontal cortex, producing a distinctive syndrome of fever, altered mental status, seizures, and behavioral changes. Without treatment, mortality exceeds 70%, but early initiation of intravenous acyclovir reduces mortality to approximately 20% and significantly improves functional outcomes. The cornerstone of management is the imperative to start acyclovir immediately upon clinical suspicion — before CSF results or imaging are available. Post-infectious autoimmune encephalitis, particularly anti-NMDA receptor encephalitis, is an increasingly recognized complication that can mimic relapse and requires a distinct immunotherapeutic approach.
Bottom Line
- Start acyclovir immediately: Do NOT wait for CSF PCR or MRI results; mortality drops from >70% to ~20% with early treatment; delay of even hours worsens outcome
- Temporal lobe predilection: MRI shows T2/FLAIR hyperintensity in medial temporal lobes, insula, and cingulate; hemorrhagic changes are common and highly suggestive
- CSF HSV PCR: Sensitivity 96%, specificity 99% — the diagnostic standard; may be negative in the first 24–72 hours and should be repeated if clinical suspicion is high
- Treatment duration: IV acyclovir 10 mg/kg every 8 hours for 14–21 days; ensure aggressive hydration to prevent crystalline nephropathy
- Post-HSV autoimmune encephalitis: Anti-NMDAR encephalitis may develop 2–6 weeks after HSE, especially in children; distinct from viral relapse and requires immunotherapy
- Prognosis: Even with appropriate treatment, significant long-term morbidity is common — particularly anterograde amnesia, behavioral changes, and epilepsy; younger patients and those treated earlier have better outcomes
Epidemiology and Pathophysiology
HSE has a bimodal age distribution, with peaks in children aged 6 months to 3 years (representing primary HSV-1 infection) and adults over 50 years (typically representing reactivation). There is no seasonal variation, and cases are sporadic rather than epidemic.
Mechanisms of CNS Infection
Pathophysiology of HSV Encephalitis
- Reactivation (most common in adults): HSV-1 establishes latency in the trigeminal ganglion after primary oropharyngeal infection; reactivation leads to retrograde spread along the trigeminal nerve to the meninges and temporal lobes
- Primary infection (~30% of adult cases): HSV-1 reaches the CNS during the initial viremic phase, typically via the olfactory nerve to the orbitofrontal cortex or trigeminal nerve to the temporal lobes
- Temporal lobe tropism: The predilection for the medial temporal lobes and orbitofrontal cortex is explained by the neuroanatomic pathways of the trigeminal and olfactory nerves, which terminate in these regions
- Necrotizing encephalitis: HSV causes hemorrhagic necrotizing inflammation; both direct viral cytopathic effect and immune-mediated damage contribute to tissue destruction
- Neonatal HSE: Caused by HSV-2 (less commonly HSV-1) acquired during passage through the birth canal; diffuse brain involvement rather than temporal lobe predilection
Clinical Presentation
HSE typically presents as an acute to subacute encephalitis evolving over hours to days. The prodrome is often brief, with 1–7 days of nonspecific symptoms (headache, malaise, low-grade fever) before the onset of focal neurological features.
Cardinal Features
| Clinical Feature | Frequency | Clinical Notes |
|---|---|---|
| Fever | >90% | Usually high-grade (>38.5°C); may be absent early or in immunocompromised patients |
| Altered mental status | 85–95% | Ranges from confusion to coma; progressive decline over hours to days |
| Seizures | 60–70% | May be focal (temporal lobe origin) or generalized; status epilepticus in 10–20%; can be the presenting symptom |
| Behavioral/personality changes | 50–70% | Agitation, psychosis, disinhibition, bizarre behavior — reflects orbitofrontal and temporal lobe involvement |
| Aphasia | 40–60% | Dominant temporal lobe involvement; may present as receptive aphasia or anomia |
| Headache | 80% | Often severe; meningeal signs present in 30–40% |
| Focal motor deficits | 30–40% | Hemiparesis from edema and mass effect; may mimic stroke |
| Memory impairment | Variable acutely | Medial temporal/hippocampal damage; anterograde amnesia often prominent in survivors |
| Olfactory hallucinations | 20–30% | Uncinate fits from mesial temporal involvement; when present, highly suggestive |
Critical Action Items
- Treat first, diagnose second: Any patient with acute febrile encephalitis (fever + altered mental status ± focal deficits or seizures) should receive empiric IV acyclovir immediately while workup proceeds
- Do NOT wait for LP results: Lumbar puncture should be performed promptly, but treatment must not be delayed for the procedure or its results
- Do NOT wait for MRI: While MRI is highly sensitive, a normal early MRI does not exclude HSE; treatment should be initiated based on clinical suspicion alone
- Beware atypical presentations: Immunocompromised patients may lack fever or have atypical imaging; elderly patients may present with subacute confusion mimicking delirium
- Consider HSE in psychiatric presentations: Acute-onset psychosis or bizarre behavior with fever should raise suspicion for HSE or autoimmune encephalitis
Diagnosis
Cerebrospinal Fluid Analysis
| CSF Parameter | Typical HSE Finding | Key Points |
|---|---|---|
| Opening pressure | Normal to mildly elevated | Markedly elevated pressure suggests alternative diagnosis or significant edema/mass effect |
| White cell count | 10–500 cells/μL | Lymphocytic predominance; may be normal in first 24 hours or in immunocompromised; <5 cells does not exclude HSE |
| Protein | Mildly to moderately elevated (60–200 mg/dL) | Elevated in most cases; higher values correlate with more extensive tissue necrosis |
| Glucose | Normal (usually) | Normal glucose helps distinguish from bacterial or tuberculous meningitis; low glucose should prompt alternative diagnoses |
| Red blood cells | Present in 40–50% | Reflects hemorrhagic necrosis — xanthochromia may be present; not unique to HSE but supportive in context |
| HSV PCR | Positive (sensitivity 96%, specificity 99%) | Gold standard diagnostic test; may be negative in first 24–72 hours — repeat LP if high suspicion; may also be negative late in disease course (>14 days) |
CSF HSV PCR: Practical Considerations
- Timing matters: Sensitivity is highest between days 2–10 of illness; a negative PCR in the first 24–72 hours does not exclude HSE — repeat in 3–5 days if clinical suspicion remains high
- Late false negatives: PCR sensitivity declines after 2 weeks of treatment; this affects interpretation of end-of-treatment lumbar puncture
- Quantitative PCR: Higher viral loads correlate with worse outcomes and may guide management in some settings
- Do not stop acyclovir based on a single negative PCR: If clinical suspicion is strong and the sample was obtained within 72 hours of onset, continue treatment and repeat the LP
- CSF antibody testing: Intrathecal HSV IgG synthesis (elevated CSF:serum antibody ratio) can confirm past infection but typically becomes positive only after 10–14 days; useful for retrospective diagnosis
Neuroimaging
MRI is the imaging modality of choice and is abnormal in approximately 90% of HSE cases by day 2–3 of illness. CT is significantly less sensitive, particularly in the first 3–5 days, and a normal CT should never delay treatment.
| Modality | Findings | Sensitivity/Timing |
|---|---|---|
| MRI T2/FLAIR | Hyperintensity in medial temporal lobes (unilateral or bilateral), insula, cingulate gyrus, orbitofrontal cortex; often asymmetric | Most sensitive sequence; abnormal within 48 hours in most cases |
| MRI DWI | Restricted diffusion in affected regions; may be the earliest abnormality | Can detect changes within 24 hours; areas of restricted diffusion correlate with tissue necrosis |
| MRI T1 post-contrast | Gyral or leptomeningeal enhancement; may see hemorrhagic component (T1 bright areas) | Enhancement appears later in disease course (days 3–5) |
| MRI GRE/SWI | Hemorrhagic foci within temporal lobes; “blooming” artifacts | Hemorrhagic necrosis is characteristic of HSE and less common in other viral encephalitides |
| CT | Hypodensity in temporal lobes; mass effect; hemorrhage | May be normal in first 3–5 days; sensitivity only 50–60% overall; should not be relied upon to exclude HSE |
Imaging Pearls
- Bilateral temporal lobe involvement: Present in 40–50% of cases; when asymmetric, the contralateral side often becomes involved over subsequent days
- Sparing of the basal ganglia: Temporal and insular involvement with basal ganglia sparing helps distinguish HSE from middle cerebral artery infarction
- Hemorrhagic component: HSE is one of the few encephalitides that causes hemorrhagic necrosis, along with Naegleria and Balamuthia; this feature on imaging is highly suggestive
- Normal MRI does not exclude HSE: Up to 10% of proven cases may have a normal MRI at presentation, particularly if imaged within the first 24 hours; repeat imaging in 48–72 hours if suspicion persists
- Differential on imaging: Autoimmune encephalitis (anti-LGI1 — medial temporal), glioma, status epilepticus, infarction, other viral encephalitides
Electroencephalography (EEG)
EEG is abnormal in more than 80% of HSE cases and may provide early supportive evidence when imaging is equivocal.
| EEG Pattern | Description | Clinical Significance |
|---|---|---|
| Periodic lateralized epileptiform discharges (PLEDs) | Sharply contoured periodic complexes over temporal regions at 1–3 second intervals | Present in 60–80% of HSE; highly suggestive but not pathognomonic (also seen in stroke, tumors) |
| Temporal slow waves | Focal delta slowing over one or both temporal regions | Most common EEG abnormality; present early in disease course |
| Bilateral PLEDs (BiPLEDs) | Independent periodic discharges over both temporal regions | Indicates bilateral involvement; associated with worse prognosis |
| Electrographic seizures | Rhythmic temporal discharges with evolution | Subclinical seizures common; continuous EEG monitoring recommended in obtunded patients |
| Diffuse slowing | Generalized theta-delta activity | Nonspecific; reflects severity of encephalopathy |
Treatment
Antiviral Therapy
| Parameter | Recommendation | Key Notes |
|---|---|---|
| Drug of choice | IV acyclovir | The only proven treatment; reduced mortality from >70% to ~20% in landmark NIAID trial (Whitley et al., 1986) |
| Dose | 10 mg/kg IV every 8 hours | Based on ideal body weight; dose-adjust for renal impairment (CrCl-based adjustments) |
| Duration | 14–21 days | 21 days preferred for severe cases or immunocompromised patients; 14 days minimum for immunocompetent adults |
| Hydration | Aggressive IV hydration | Acyclovir is excreted renally and can precipitate in renal tubules → crystalline nephropathy; maintain urine output >75 mL/hr during and after infusion |
| Monitoring | Renal function every 2–3 days | Hold or dose-adjust if creatinine rises; neurotoxicity (tremor, confusion, myoclonus) can occur with supratherapeutic levels, especially in renal insufficiency |
| End-of-treatment LP | Recommended in immunocompromised patients | Repeat CSF HSV PCR; if still positive, continue treatment for additional 7 days; routine repeat LP in immunocompetent patients is debated |
Acyclovir Adverse Effects and Monitoring
- Crystalline nephropathy (most common serious AE): Risk factors include rapid infusion, dehydration, pre-existing renal disease, and concurrent nephrotoxins; prevent with slow infusion (≥1 hour) and aggressive hydration
- Neurotoxicity: Tremor, myoclonus, confusion, hallucinations, seizures — more common with renal impairment (accumulation of 9-carboxymethoxymethylguanine, the principal metabolite)
- Hematologic: Rare thrombocytopenia, neutropenia; monitor CBC periodically
- Phlebitis: Acyclovir is alkaline (pH 10–11); use large-bore IV in proximal vein; rotate sites
- Oral valacyclovir is NOT a substitute: Oral formulations do not achieve adequate CNS levels for treatment of encephalitis; IV acyclovir is mandatory
Supportive Care
Comprehensive Management Beyond Antivirals
- Seizure management: Empiric antiseizure medication for patients with seizures; levetiracetam or phenytoin/fosphenytoin are first-line; continuous EEG monitoring in obtunded patients to detect subclinical seizures
- Cerebral edema: Temporal lobe edema can cause uncal herniation; osmotic therapy (mannitol, hypertonic saline), head of bed elevation, and in severe cases decompressive craniectomy may be required
- ICU monitoring: All patients with significant altered consciousness should be managed in an ICU setting; monitor for aspiration, hemodynamic instability, and status epilepticus
- Corticosteroids: Role is controversial; theoretical benefit in reducing immune-mediated damage, but no definitive RCT evidence; the German GACHE trial was underpowered; some experts use short courses in patients with significant edema
- Temperature management: Aggressive antipyretics; hyperthermia worsens neuronal injury
- Early rehabilitation: Cognitive, speech, and physical therapy should be initiated early in the recovery phase
Differential Diagnosis
| Diagnosis | Key Distinguishing Features | Diagnostic Test |
|---|---|---|
| Autoimmune encephalitis (anti-NMDAR, anti-LGI1) | Subacute onset; psychiatric symptoms prominent; anti-LGI1 affects medial temporal lobes (mimics HSE on MRI); faciobrachial dystonic seizures (LGI1) | CSF and serum neuronal antibody panels; MRI pattern; poor response to acyclovir |
| Other viral encephalitis (VZV, EBV, HHV-6, enterovirus) | May be clinically indistinguishable; HHV-6 can also affect temporal lobes (transplant patients) | CSF multiplex PCR panel; clinical context (transplant, rash) |
| Temporal lobe glioma | Subacute; mass effect; enhancing lesion; less edema relative to mass size | MRI with perfusion and spectroscopy; biopsy if needed |
| Status epilepticus | Prolonged seizures can cause temporal lobe edema and DWI changes mimicking HSE | EEG; clinical history; improvement with antiseizure treatment |
| CNS vasculitis | Multifocal involvement; vessel irregularity on angiography | Vessel wall imaging; conventional angiography; biopsy |
| Acute disseminated encephalomyelitis (ADEM) | Post-infectious; multifocal white matter lesions; responds to corticosteroids | MRI pattern (multifocal, bilateral); clinical context; MOG antibody |
Post-HSV Autoimmune Encephalitis
An increasingly recognized and clinically important complication of HSE is the development of autoimmune encephalitis weeks after the acute viral infection. This is most commonly mediated by antibodies against the NMDA receptor and occurs in 15–25% of HSE cases, with a higher incidence in children and adolescents.
Post-HSV Anti-NMDAR Encephalitis
- Timing: Typically 2–6 weeks after HSE onset; occurs after initial clinical improvement (the “lucid interval”)
- Proposed mechanism: HSV-mediated neuronal destruction releases NMDAR antigens, triggering an autoimmune response in genetically susceptible individuals
- Clinical features: Choreoathetosis and dyskinesias (prominent, especially in children), psychiatric symptoms (agitation, psychosis, catatonia), autonomic instability, decreased level of consciousness, new-onset seizures
- Distinguishing from viral relapse: Post-HSV autoimmune encephalitis features negative CSF HSV PCR but positive anti-NMDAR antibodies in CSF; new MRI changes may affect areas beyond the original temporal lobe involvement
- Treatment: Immunotherapy (first-line: IV methylprednisolone, IVIG, or plasma exchange; second-line: rituximab); acyclovir is not effective as this is an immune-mediated process, not active viral replication
- Prognosis: Majority improve with immunotherapy, though recovery may be prolonged; relapse can occur if immunotherapy is tapered too quickly
- Screening recommendation: Some experts recommend testing for NMDAR antibodies in any patient who deteriorates after initial improvement from HSE
Viral Relapse vs. Autoimmune Encephalitis After HSE
- True viral relapse: Rare (<5%); positive CSF HSV PCR; treat with IV acyclovir; consider acyclovir resistance in immunocompromised patients (foscarnet as alternative)
- Post-HSV autoimmune encephalitis: More common than relapse; negative CSF HSV PCR; positive neuronal antibodies; treat with immunotherapy, not antivirals
- Clinical clue: Prominent choreoathetosis or dyskinesias in the “relapse” phase strongly suggests autoimmune encephalitis rather than viral relapse
- Send both: In any patient who deteriorates after initial HSE improvement, send CSF for BOTH HSV PCR and neuronal antibody panel simultaneously; start acyclovir empirically while awaiting results
Prognosis and Long-Term Outcomes
| Outcome Measure | With Acyclovir Treatment | Without Treatment |
|---|---|---|
| Mortality | ~20% | >70% |
| Good functional recovery (independent) | 40–50% | <5% |
| Moderate disability | 25–35% | — |
| Severe disability | 10–20% | — |
Predictors of Outcome
| Favorable Prognostic Factors | Unfavorable Prognostic Factors |
|---|---|
| Younger age (particularly <30 years) | Age >60 years |
| Acyclovir started within 24–48 hours of symptom onset | Delayed treatment (>48 hours from symptom onset) |
| GCS >10 at presentation | GCS ≤6 at presentation |
| Limited MRI involvement (unilateral, no hemorrhage) | Bilateral temporal involvement, hemorrhage, extensive edema |
| Lower CSF viral load | High CSF viral load on quantitative PCR |
| Absence of comorbidities | Immunosuppression, significant comorbidities |
Long-Term Sequelae
Common Long-Term Complications in HSE Survivors
- Anterograde amnesia: The most common and debilitating sequela (60–70% of survivors); bilateral hippocampal damage causes severe new memory formation deficits
- Retrograde amnesia: Variable; often temporally graded (remote memories better preserved)
- Behavioral and personality changes: Klüver-Bucy syndrome features (hyperorality, hypersexuality, visual agnosia, placidity) from bilateral temporal damage; frontal lobe dysfunction with disinhibition and apathy
- Epilepsy: Develops in 40–60% of survivors; often medically refractory temporal lobe epilepsy; may require surgical evaluation
- Aphasia: Persistent in 20–30% with dominant temporal lobe involvement
- Anosmia: From orbitofrontal and olfactory cortex damage
- Psychiatric disorders: Depression, anxiety, and PTSD are common in survivors and their caregivers
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