CAUSES
1. Endemic
. Herpes simplex virus types 1 and 2
. Epstein-Barr virus
. Varicella-zoster virus
. Adenovirus
. Rabies
. Dengue
. Benign lymphocytic choriomeningitis V
. Infectious mononucleosis
. California encephalitis (CE) virus
. St. Louis encephalitis (SLE) virus
. Russian spring-summer encephalitis
. Murray Valley
. LaCrosse encephalitis
. Powassan encephalitis
2. . Epidemic
. Arboviruses (e.g., St. Louis encephalitis, Japanese encephalitis (JE), eastern equine encephalitis, western equine encephalitis, Venezuelan equine encephalitis, West Nile fever)
. Enteroviruses (most commonly Coxsackie B viruses, but also includes poliovirus, echovirus)
. Nipah virus
. Mumps
. Varicella-zoster
. Infl uenza
. HTLV III (HIV)
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DIFFERENTIAL DIAGNOSIS
. Bacterial infection
. Meningitis
. Brain abscess
. Tuberculosis
. Cat-scratch disease
. Rickettsial infection
. Rocky Mountain spotted fever
. Ehrlichiosis
. Spirochetal infection
. Syphilis
. Lyme disease
. Leptospirosis
. Tick-borne relapsing fever
. Other infectious agents
. Free-living amoeba (Naegleria, Acanthamoeba)
. Toxoplasmosis
. Intracranial hemorrhage
. Intracranial tumor
. Trauma
. Thromboembolism
. Systemic lupus erythematosus
. Toxic ingestion
. Hypoglycemia
* PCR TEST - IS USED FOR INFECTION BY HERPES SIMPLEX ENCEPHALITIS( HSV-I) , RECURRENT LYMPHOCYTIC MENINGITIS ( HSV-II) , ENTEROVIRUSES, CYTOMEGALOVIRUS, EPSTEIN BARR VIRUS & VARICELLA ZOSTER VIRUS INFECTION.
* CSF CULTURE -
. Standard laboratory studies (CBC, serum chemistries) are usually normal or nonspecifically abnormal
. Cerebrospinal fluid examination is essential :
. White cell count usually increased (10-2000 cells/mm3) but may be normal, especially in immunocompromisedhost; neutrophils predominate early, then
see shift to mononuclear cells
. Red cell count usually normal (more likely elevated in herpes simplex infections)
. Protein may be normal or mildly elevated
. Glucose may be normal or mildly decreased
. ELISA detection of IgM helpful retrospectively to determine causative agent
. Polymerase chain reaction (PCR) to amplify viral DNA is the diagnostic choice for HSV (particularly in neonatal HSV), CMV, human herpesvirus 6 (HHV-6),
and enterovirus infections. However, local unavailability of this test limits its application.
SPECIAL TESTS :
β’ Electroencephalographic fi ndings are usually abnormal, with slowing or epileptiform activity present. Temporal lobe abnormalities, particularly periodic lateralized epileptic form discharges (PLEDS), should suggest a diagnosis of HSV encephalitis.
β’ Most viral encephalitis cases have specific diagnosis made by measurement of acute and convalescent (2-3 weeks) serum antibody concentrations for the specific pathogen. A four-fold change in titer is suggestive of the diagnosis.
β’ CSF antibody index can be used to ascertain specific central nervous system production of antibody against the infecting agent: Serum and CSF IgG antibody and serum and CSF albumin concentrations are determined.
Infection may be suggested by a higher specific antibody: albumin ratio in CSF than in serum.
β’ Antigen detection in CSF by polymerase chain reaction has been useful in cases of HSV and enteroviral disease, but is available only in some centers
β’ Enteroviruses may be recovered from CSF viral culture in about 60% of cases, but most other viral agents are present in too low quantities to be detected by these techniques
IMAGING : Imaging studies (CT scan, MRI, brain scan) may be normal early; later, nonspecific abnormalities are seen. Temporal lobe pathology suggests
the diagnosis of HSV. MRI more sensitive than CT in viral encephalidities
DIAGNOSTIC PROCEDURES : Brain biopsy, coupled with Immunohistochemistry, may be useful in certain cases, particularly to identify treatable causes