ASEPTIC MENINGITIS

Viral infection of the central nervous system involving brain leads to encephalitis, while infection of the surrounding membranes leads to aseptic meningitis. Meningitis caused by viruses is sometimes termed aseptic meningitis since there is much less of a cellular response, and because bacteriological cultures are negative.

Etiologic agents: 
o Primarily neurotropic viruses: Poliovirus, Lymphocytic choriomeningitis virus and arthropod borne encephalitis viruses
o Viruses not primarily neurotropic: Enteroviruses, Mumps virus, Herpes simplex virus, Varicella-zoster virus, Epstein Barr virus, Hepatitis A virus and Measles virus
o Some bacteria like leptospira, chlamydiae, Treponema pallidum and Mycoplasma pneumoniae may also cause aseptic meningitis.

Pathogenesis: 
The virus enters the body at various sites, depending on the species of the organism. Viral replication at these regional sites gives rise to a primary viremia. Further replication in target organ results in a secondary viremia and passage of the virus to the CNS, where it penetrates susceptible cells and replicates. Penetration of the blood-brain barrier may be accomplished by means of virus-laden phagocytes migrating through blood vessels of the meninges or brain or by passage of virus particles through the choroid plexus. There is always some involvement of brain tissue so the disease is really a meningoencephalitis.

Clinical presentation:  
Symptoms are variable, but commonly include sudden onset, intense frontal or retro-orbital headache, fever that never goes above 104° Fahrenheit and skin rash. At the onset of fever or shortly thereafter malaise, drowsiness, myalgia, nausea and vomiting may be seen. Photophobia, tinnitus and vertigo may or may not be there. Nuchal rigidity develops and there is almost always stiffness of the back.

Laboratory diagnosis:  
Specimen collected is CSF. The specimen is subjected to estimation of protein, glucose, cell type and cell count. The CSF is transparent to slightly turbid. The leukocytes count (mainly mononuclear cells) is <500/mm3. Glucose is normal but protein is elevated. Virus may be isolated on appropriate cell lines or animal model. A rise in specific antibodies should be demonstrated serologically. The viral nucleic acid may be demonstrated in CSF by molecular techniques such as PCR.