Viral meningitis

Wallgren coined the term "acute aseptic meningitis" in 19251 to describe acute meningeal irritation, benign and self-limiting, with complete recovery and sterile pleocytic cerebrospinal fluid (CSF).

It has become evident that viruses cause at least 70% of such cases. Viral infections of the central nervous system (CNS) are complications of systemic viral infections and the virus gains access to the brain via the bloodstream or, less commonly, by travelling up peripheral nerves.2 Viral meningitis results from haematogenous infection and, to enter the CNS, the virus must cross the endothelial cell junctions of the blood-brain barrier. The ability to do this is dependent upon surface adhesion molecules on the cells, surface charges and cellular receptors of the virus, and the property of entering infected cells.3 Certain viruses preferentially infect the meninges, choroid plexus, and ependyma rather than cerebral parenchyma and cause meningitis; others infect neurons and glia to cause encephalitis. There is considerable overlap and some viruses cause meningoencephalitis, incorporating signs of both.

Most cases of viral meningitis occur in children and young adults. Infections occur throughout the year, with a preponderance in summer and autumn in temperate climates. The annual reported incidence varies from 11 to 27 cases per 100 000 and several thousand cases are reported annually in the United States. The actual number of infections is almost

Box 9.1 Viral causes of cerebral infection

Meningitis

Encephalitis

Enteroviruses

Herpes simplex

Echo

Varicella zoster virus

Polio

Cytomegalovirus

Coxsackie

Epstein-Barr virus

Herpes simplex 2

HIV

Lymphocytic choriomeningitis virus

Mumps

Varicella zoster virus

Measles

Mumps

Rabies

HIV

Arboviruses

certainly several multiples higher because of underreporting. The recent introduction of polymerase chain reaction (PCR) tests for the detection of enteroviral ribonucleic acid (RNA) in CSF has greatly increased the detection of enteroviral CNS infections and it has become apparent that 85-90% of acute viral meningitis is caused by enteroviruses (coxsackie B or echo).4 Much less commonly meningitis is caused by herpes simplex virus (HSV) type 2, varicella zoster virus (VZV), mumps, lymphocytic choriomeningitis, and HIV (Box 9.1).

The clinical onset is usually rapid over hours, with pyrexia, malaise, headache, neck stiffness, photophobia, myalgia, lethargy, and irritability. Most cases do not progress further and the subject can be roused easily and remains coherent. In approximately 3% of infected people the conscious level falls or focal signs or seizures develop, suggesting encephalitis. Resolution begins within a few days and is complete within two weeks in most. A few will have persistent malaise and myalgia for some weeks. The pathogen can seldom be identified clinically; parotitis and orchitis may point to mumps, arthralgia and lymphadenopathy to HIV, myalgia and myocarditis to coxsackie, and rashes to enterovirus. The illness is not as severe and prolonged as bacterial meningitis and the signs are not so florid.

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