viral encephalitis
Summary
Viral encephalitis is inflammation of the brain parenchyma caused by direct viral invasion, most commonly HSV-1 in adults/older children. Presents with fever, headache, altered mental status, seizures, and focal neurologic deficits. HSV encephalitis classically involves the temporal lobes and is diagnosed via CSF PCR and MRI; empiric IV acyclovir should be started immediately given high mortality if untreated.
Detail
Viral encephalitis results from direct infection of brain tissue by neurotropic viruses, causing neuronal damage, perivascular lymphocytic cuffing, and microglial nodules. HSV-1 is the most common sporadic cause in the US, with a predilection for the temporal and inferior frontal lobes due to viral latency in the trigeminal ganglion and retrograde spread. Other causes include arboviruses (West Nile, St. Louis encephalitis, Eastern/Western equine encephalitis—transmitted by mosquitoes), enteroviruses, VZV, CMV (in immunocompromised), rabies (with pathognomonic Negri bodies), and JC virus (causing PML in AIDS via oligodendrocyte infection). Clinical presentation includes fever, headache, altered consciousness, seizures, focal neurologic signs, and behavioral changes; HSV encephalitis often causes bizarre behavior, olfactory hallucinations, and aphasia due to temporal lobe involvement. Diagnostic workup includes lumbar puncture showing lymphocytic pleocytosis, elevated protein, normal/mildly low glucose, and RBCs (in HSV due to hemorrhagic necrosis); CSF PCR for HSV DNA is the gold standard. MRI shows temporal lobe hyperintensities on T2/FLAIR in HSV cases; EEG may show periodic lateralized epileptiform discharges (PLEDs). Treatment is empiric IV acyclovir started immediately in suspected cases due to high mortality (~70% untreated) and morbidity, as treatment delay worsens outcomes; therapy should not wait for confirmatory testing. Complications include seizures, cognitive deficits, and death. Rabies encephalitis presents with hydrophobia, hypersalivation, and agitation, and is nearly universally fatal once symptomatic, making pre/post-exposure prophylaxis critical.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Herpes simplex virus encephalitis
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