• Encephalitis An inflammation of the brain parenchyma, presents as diffuse and/or focal neuropsychological dysfunction


  • The etiology of encephalitis is usually infectious, but may be noninfectious, such as the demyelinating process in acute disseminated encephalitis.
  • Herpes simplex virus type 1 (HSV-1) and type 2 (almost exclusively in neonates)), the most common cause of sporadic encephalitis with a 70% mortality rate if left untreated. Transmitted by respiratory and salivary contact.
  • Arboviruses (epidemic viral encephalitis) are the most common causes of episodic encephalitis and require an insect vector (mosquitoes and ticks), which is generally present between June and October.
  • The 2 most common arboviruses are St Louis encephalitis, found throughout the United States but principally in urban areas around the Mississippi River, and the California virus (in particular, the strain that causes LaCross encephalitis [LAC]), which affects children in rural areas in states of the northern Midwest and East.
  • Other arboviruses causing encephalitis, the deadliest and most uncommon, eastern equine encephalitis (EEE), in New England and surrounding areas; the milder western equine encephalitis (WEE) is most common in rural communities west of the Mississippi River. Powassan virus is the only well-documented arbovirus transmitted by ticks.
  • Less common causes of viral encephalitis: varicella-zoster encephalitis has an incidence of 1 in 2000 infected persons. Measles produces 2 forms of encephalitis: postinfectious, and SSPE.
  • Typically, 0-3 unrelated cases of rabies encephalitis are identified yearly.
  • West Nile virus introduced into the United States in 1999; the first cases were reported in New York, but by 2003. Can be transmitted by means of an organ transplant and via blood transfusions.


  • Clinical presentation and course can be markedly variable.
  • The patient may have history of animal bite for which antirabies treatment may not have been obtained.
  • The general viral prodrome is several days long and consists of fever, headache, nausea and vomiting, lethargy, and myalgias.
  • The specific prodrome in measles, and mumps includes rash, lymphadenopathy, hepatosplenomegaly, and parotid enlargement.
  • The classic presentation is encephalopathy with diffuse or focal neurologic symptoms, including the following: Behavioral and personality changes, decreased level of consciousness, Stiff neck, photophobia, and lethargy, Generalized or localized seizures, Acute confusion or amnestic states, Flaccid paralysis (10% with WNE)


  • Look for supporting evidence of viral infection.
  • The signs of encephalitis may be diffuse or focal
  • Altered mental status and/or personality changes (most common)
  • Focal findings, such as hemiparesis, focal seizures, and autonomic dysfunction
  • Movement disorders (St Louis encephalitis, EEE, WEE)
  • Ataxia
  • Cranial nerve defects
  • Dysphagia (Rabies may account for foaming at the mouth and hydrophobia.)
  • Meningismus (less common and less pronounced than in meningitis)
  • Unilateral sensorimotor dysfunction (PIE)


  • Brain Abscess
  • Catscratch Disease
  • Herpes Simplex
  • Herpes Simplex Encephalitis
  • Hypoglycemia
  • Leptospirosis in Humans
  • Meningitis
  • Status Epilepticus
  • Subarachnoid Hemorrhage
  • Systemic Lupus Erythematosus
  • Tick-Borne Diseases, Rocky Mountain Spotted Fever
  • Toxoplasmosis
  • Complete blood count (CBC) with differential: Findings are usually within the reference range.
  • Serum electrolytes: These are usually within the reference range. Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) occurs in 25% of patients with St Louis encephalitis.)
  • Serum glucose level: Use this level as a baseline for determining normal CSF glucose values. The result may be low if glycogen stores are depleted or high in infected patients with diabetes mellitus.
  • BUN/creatinine and liver function tests (LFTs): Assess organ function and the need to adjust the antibiotic dose.
  • Platelet test and a coagulation profile: These are indicated in patients with chronic alcohol use, liver disease, or if disseminated intravascular coagulation (DIC) is suspected. The patient may require platelets or fresh-frozen plasma (FFP) before lumbar puncture (LP).
  • Urinary electrolyte test: Perform this assessment if SIADH is suspected.
  • Urine and/or serum toxicology screening: Perform 1 or both of these tests, if indicated.
  • Examination of CSF is essential. If focal signs are present, a computed tomography (CT) scan should be performed first to exclude a mass lesion.
  • The presence of 10 to 1000 mononuclear cells per cubic millimeter (pleocytosis) in CSF is characteristic. CSF pressure may be elevated, whereas the glucose level is characteristically normal or only modestly reduced. The protein content is usually elevated (50 to 100 mg/dL). Elevated immunoglobulin levels and oligoclonal bands may be observed. An increased protein content and pleocytosis may persist for weeks or months after convalescence.
  • CSF polymerase chain reaction (PCR): A PCR for DNA HSV is 100% specific and 75-98% sensitive within the first 25-45 hours. Types 1 and 2 cross-react, but no cross-reactivity with other herpes viruses occurs. Arguably, a series of quantitative PCRs documenting the decline of viral load with acyclovir treatment may clinch diagnosis without brain biopsy.
  • HSV cultures: These are used to test lesions (also Tzanck smear), CSF (rarely positive), and blood.
  • Viral serology: Complement fixation antibodies are useful in identifying arbovirus. Cross-reactivity exists among one subgroup of arboviruses, the flaviviruses (eg, St Louis encephalitis, JE, WNE), and with antibodies raised in persons inoculated with the yellow fever vaccine.
  • Serologic tests for toxoplasmosis: These can be helpful in light of an abnormal CT scan, particularly in the case of single lesions. However, the overlap in titer between previously exposed but presently uninfected and reactivated groups may complicate interpretation.
  • Perform head CT, with and without contrast agent, in virtually all patients with encephalitis before LP to search for evidence of elevated intracerebral pressure (ICP), obstructive hydrocephalus, or mass effect. It is helpful also in differential diagnosis. MRI is more likely to show abnormalities earlier in disease course than head CT.
  • In HSV, an MRI may show several foci of increased T2 signal intensity in medial temporal lobes and inferior frontal gray matter. Head CT may show petechial hemorrhage in the same areas.
  • EEE and tick-borne encephalitis may show similar increased signal intensity in the basal ganglia and thalami.
  • In toxoplasmosis, contrast-enhanced head CT typically reveals several nodular or ring-enhancing lesions. Because lesions may be missed without contrast, MRI should be performed in patients for whom use of contrast material is contraindicated.
  • Electroencephalography: In HV, characteristic paroxysmal lateral epileptiform discharges (PLEDs) often are observed, even before neuroradiographic changes. Eventually, PLEDs are positive in 80% of cases. The presence of PLEDs is not pathognomonic for HSE.
  • Brain biopsy is the criterion standard because of its 96% sensitivity and 100% specificity. HSV encephalitis.


  • With the important exceptions of HSV and varicella-zoster encephalitis, the viral encephalitides are not treatable beyond supportive care.
  • Treatments for T gondii and CMV encephalitis are available.
  • The goal of treatment for acutely ill patients is administration of the first dose or doses acyclovir with or without antibiotics or steroids as quickly as possible.
  • The standard for acute bacterial meningitis is the initiation of treatment within 30 minutes of arrival.
  • Collect laboratory samples and blood cultures before the start of IV therapy. Even in uncomplicated cases of encephalitis,


 SURGICAL MANAGEMENT (when applicable)

  • Not applicable

EMERGENCY MANAGEMENT (when applicable)

  • General measures: Manage fever and pain, control straining and coughing, and avoid seizures and systemic hypotension.
  • In otherwise stable patients, elevating the head and monitoring neurologic status usually are sufficient.
  • When more aggressive maneuvers are indicated, some authorities favor the early use of diuresis (eg, furosemide 20 mg IV, mannitol 1 g/kg IV) provided circulatory volume is protected. Dexamethasone 10 mg IV q6h helps in managing edema surrounding space-occupying lesions. Hyperventilation (PaCO2 30 mm Hg) may cause a disproportional decrease in cerebral blood flow (CBF), but it is used to control increasing ICP on an emergency basis.
  • Intraventricular ICP monitoring is controversial because some authorities believe dangerous focal edema with a pressure gradient between the temporal lobe and the subtentorial space usually is not detected by the monitor, leading to a false sense of security. In fact, monitor placement may potentially aggravate a pressure gradient.
  • Look for and treat systemic complications, particularly in HSE, EEE, JE, such as hypotension or shock, hypoxemia, hyponatremia (SIADH), and exacerbation of chronic diseases.
  • Empiric adult emergency treatment for HSV meningoencephalitis and VZV encephalitis is acyclovir 10 mg/kg (infuse over 1 h) q8h for 14-21 days. Give acyclovir 10-15 mg/kg IV q8h for neonatal HSV; for HSV encephalitis in the pediatric population, give acyclovir 10 mg/kg IV q8h.
  • In HIV-positive patients, consider foscarnet, given increased incidence of acyclovir-resistant HSV and HZV.

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Dr. Zachary Lahlou



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