Viral meningoencephalitis

Inflammation of the brain (encephalitis) and meninges (meningitis) due to viral infection of the central nervous system.

• It's a neurologic emergency with high morbidity and mortality. Herpes simplex encephalitis is often fatal if untreated, so early recognition and treatment (acyclovir) are critical. Distinguishing viral from bacterial meningitis guides proper therapy and frequently appears on exams.

• Usually presents with fever, headache, and altered mental status (confusion, lethargy, even coma). May have seizures or focal neurologic deficits (e.g., aphasia, cranial nerve palsies). Neck stiffness and photophobia can occur (meningeal irritation), but may be mild or absent if encephalitis predominates.
• Context clues: consider season and exposures. Summer/early fall with mosquito exposure → think arboviruses (e.g., West Nile, equine encephalitis); animal bite → consider rabies; vesicular rash (or history of cold sores/shingles) → think HSV or VZV. Neonates with fever and seizures suggest HSV-2 from maternal transmission.
• HSV-1 (the most common sporadic encephalitis) often causes bizarre behavior, personality changes, or olfactory hallucinations (temporal lobe involvement). Enteroviruses (e.g., echovirus) are a leading cause of viral meningitis, especially in children, typically causing milder symptoms (headache, neck stiffness) with normal brain function.

1. Perform a lumbar puncture for CSF analysis as soon as safely possible (after CT head if any signs of ↑ICP). Viral encephalitis classically shows lymphocytic pleocytosis, normal glucose, and moderately ↑ protein. Always send CSF for PCR to detect viruses (especially HSV, enteroviruses).
2. MRI brain is the imaging of choice: it can show characteristic findings (e.g., unilateral temporal lobe edema in HSV; thalamic involvement in some arboviruses) and helps exclude other causes (abscess, stroke). EEG often shows diffuse slowing or focal epileptiform waves (e.g., periodic sharp waves in HSV) and can support the diagnosis.
3. Evaluate for other etiologies in parallel: obtain blood cultures and basic labs to rule out bacterial meningitis and metabolic causes; consider autoimmune encephalitis panel (antibodies) if initial infectious workup is negative.
4. If initial CSF PCR is negative but clinical suspicion for HSV remains high, repeat the lumbar puncture 24–48 hours later. In rare cases, a brain biopsy may be needed for definitive diagnosis if no cause is identified and the patient is deteriorating.

1. IV acyclovir immediately for any suspected HSV encephalitis – do not wait for confirmation. Acyclovir also covers VZV encephalitis (another treatable cause).
2. No definitive antivirals exist for most other viral causes, so care is largely supportive: IV fluids, fever control, and close monitoring. Many patients require ICU support for airway protection (if comatose) and seizure control (IV benzodiazepines or antiseizure meds).
3. Manage complications: for elevated intracranial pressure, use measures like head elevation, IV mannitol, or hyperventilation if needed; treat seizures aggressively with anticonvulsants. In immunocompromised patients, consider specific therapy for less common viruses (e.g., ganciclovir + foscarnet for CMV encephalitis).

• CSF: viral = lymphocytes + normal glucose (vs bacteria = neutrophils + low glucose). If glucose is low, think bacterial or TB/fungal, not routine viral.
• HSV loves the temporal lobe – MRI or EEG showing temporal lobe involvement in a confused patient → strongly suggests HSV-1 encephalitis.
• When in doubt, start acyclovir. Delaying treatment for HSV encephalitis, even for a few hours, can lead to irreversible brain injury – always err on the side of treating while awaiting results.

• Signs of ↑ intracranial pressure (papilledema, focal neurologic deficits, severe obtundation) → obtain head CT before LP to prevent herniation; manage ICP (e.g., elevate head, mannitol) and consider neurology consult.
• Never forget HSV: a patient with encephalitis who isn't promptly treated with acyclovir can suffer permanent neurologic damage. Always initiate empiric acyclovir in suspected encephalitis (especially with altered mental status or focal findings).

1. Suspected CNS infection with altered mental status → stabilize ABCs, assess need for ICU.
2. If no contraindications (no papilledema or focal deficits), perform lumbar puncture immediately. If contraindications exist, do head CT scan first, then LP as soon as safe.
3. Send CSF for cell count, glucose, protein, Gram stain & culture, and PCR panel for viral pathogens (HSV, enterovirus, etc.). Also draw routine labs and blood cultures.
4. Start IV acyclovir empirically (cover HSV/VZV) and appropriate antibiotics for bacterial meningitis until results clarify the cause.
5. Admit for supportive care: monitor in ICU if severe, control fever, give IV fluids, manage pain, and treat seizures. Adjust treatment once pathogen is identified (e.g., continue antivirals for HSV/VZV or supportive care for others).

• Young adult with fever, headache, bizarre behavior, and seizures; MRI shows unilateral temporal lobe lesion; CSF has lymphocytic pleocytosis and RBCs → HSV-1 encephalitis (treat with IV acyclovir).
• Elderly patient in August develops fever, confusion, tremors, and asymmetric flaccid paralysis after mosquito bites → West Nile virus encephalitis (an arboviral meningoencephalitis).
• 2‑week‑old neonate with fever, poor feeding, and seizures; mother had untreated genital lesions at delivery → neonatal HSV-2 meningoencephalitis (requires prompt acyclovir).