Encephalitis

• Infection of the brain parenchyma causing clouding of the sensorium
• Some overlap with meningitis

Differential Diagnosis

• Common causes include HSV, VZV, and enterovirus in developed countries
• Consider rabies virus, Japanese encephalitis virus, West Nile virus, tickborne encephalitis virus, and St. Louis encephalitis virus depending on travel history
• Infectious
  • Viruses
    • Herpes simplex virus
    • Varicella-zoster virus
    • Enteroviruses
    • Eastern or Western equine encephalitis virus, Japanese encephalitis virus, Murray Valley encephalitis virus, St. Louis encephalitis virus, West Nile virus
    • Rabies virus
    • Hendra virus
    • Nipah virus
    • Tickborne encephalitis virus, La Crosse virus, Powassan virus
  • Bacteria
    • Mycoplasma pneumoniae
    • Listeria monocytogenes
    • Borrelia burgdorferi (Lyme disease)
    • Coxiella burnetii
    • Bartonella henselae or B. quintana
  • Other
    • Cryptococcus neoformans
    • Toxoplasma gondii
    • Bayliscarias procyonis
    • Prion disease (CJD)
• Non-infectious (including post-infectious)
  • Acute disseminated encephalomyelitis (ADEM)
  • Vasculitis
  • Collagen vascular disorders
  • Paraneoplastic syndromes
• Cryptogenic in 50-70%

Investigations

• Lumbar puncture for viral culture or PCR
• MRI is the best neuroimaging; CT if MRI not available
• EEG is rarely helpful except to rule out non-convulsive status epilepticus

MRI

Finding	Differential Diagnosis
arteritis and infarctions	Varicella-zoster virus, Nipah virus, Rickettsia rickettsii, Treponema pallidum pallidum
calcifications	CMV (cortical), Toxoplasma (periventricular), Taenia solium
cerebellar lesions	VZV, EBV, Mycoplasma pneumoniae
basal ganglia, thalamus, or brainstem lesions	EBV, eastern equine encephalitis virus, Murray Valley encephalitis virus, St. Louis encephalitis virus, Japanese encephalitis virus, West Nile virus, enterovirus A71, influenza virus, prion disease, Tropheryma whipplei, Listeria monocytogenes
hydrocephalus	Mycobacterium tuberculosis, Cryptococcus neoformans, Coccidioides immitis, Histoplasma capsulatum, Balamuthia mandrillaris
space-occupying lesions	Toxoplasma gondii, Balamuthia mandrillaris, Acanthamoeba species, Taenia solium
subependymal enhancement	CMV
temporal or frontal lobe involvement	HSV, VZV, HHV-6, West Nile virus, enterovirus, Treponema pallidum pallidum (medial lobes)
white matter abnormalities	VZV, CMV, EBV, HHV-6, HIV, Nipah virus, JC virus, measles virus, Baylisascaris procyonis, acute disseminated encephalomyelitis

Management

• Empiric acyclovir 10 mg/kg IV q8h for suspected HSV or VZV encephalitis
• Add doxycycline if rickettsiosis or ehrlichiosis is possible