• Infection of brain parenchyma of the temporal lobes and inferior frontal lobe causing distinct neurologic abnormality
  • Pathophysiology
    • Gray matter is predominantly affected (cognitive / psychiatric signs, lethargy, seizure)


Clinical Features

  • New psychiatric symptoms (HSV)[2]
  • Cognitive deficits
  • Seizure
  • Movement disorder (arboviruses)
  • Fever and meningeal signs are almost always present
  • Dysuria and pyuria (St. Louis encephalitis)
  • Extreme lethargy (West Nile encephalitis)

Differential Diagnosis

Altered mental status and fever


Brain MRI showing high signal in the temporal lobes and right inferior frontal gyrus secondary to HSV encephalitis.
  • Imaging identifies abnormalities in medial temporal and inferior frontal gray matter in HSV[3]
    • CT Head - edema and petechial hemorrhage
    • MRI - increased T2 intensity
  • LP
    • Bloody tap consistent with HSV


Often it is unclear which type of encephalitis is present and starting Acyclovir empirically is appropriate. In addition to the pathogens below, possible causes can include West Nile Virus, EBV, HIV, toxoplasmosis, or rabies.

HSV encephalitis

  • Acyclovir 10mg/kg (10-15mg/kg for pediatrics) every 8hrs

HZV encephalitis

CMV encephalitis

Tick Associated (Borrelia burgdorferi, Ehrlichiosis or Rickettsia)

  • Doxycycline 200 mg IV once followed by 100 mg IV twice daily


Admit in all cases

See Also


  1. Somand D, Meurer W. Central Nervous System Infections. EMCNA 2009; 27: 89-100.
  2. Loring KE, Tintinalli JE: Central Nervous System and Spinal Infections, in Tintinalli JE, Kelen GD, Stapczynski JS (eds): Emergency Medicine, A Comprehensive Study Guide, ed 7. New York, McGraw-Hill, 2011, (Ch) 168:p 1175-1176.
  3. Howes DS et al. Encephalitis Workup. Oct 12, 2015. http://emedicine.medscape.com/article/791896-workup#showall