What is the pathogenesis of viral meningitis?

Updated: Jul 17, 2018
  • Author: Cordia Wan, MD; Chief Editor: Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM  more...
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Answer

Viral pathogens may gain access to the CNS via either of 2 main routes: hematogenous and neural. The hematogenous route is more common for penetration of most known viral pathogens. Neural penetration refers to spread along nerve roots and is usually limited to herpes viruses (HSV-1, HSV-2, VZV B) and possibly some enteroviruses.

Multiple host defenses prevent viral inoculum from causing clinically significant infection. These include local and systemic immune responses, skin and mucosal barriers, and the blood-brain barrier (BBB).

The virus replicates in the initial organ system (ie, respiratory or gastrointestinal mucosa) and gains access to the bloodstream. Primary viremia introduces the virus to the reticuloendothelial organs (liver, spleen, lymph nodes). If the replication persists despite immunologic defenses, secondary viremia occurs, which is thought to be responsible for seeding of the CNS. Rapid viral replication likely plays a major role in overcoming the host defenses.

The actual mechanism of viral penetration into the CNS is not well understood. The virus may cross the BBB directly at the capillary endothelial level or through natural defects, such as the area postrema and other sites that lack a BBB.

The inflammatory response is seen in the form of pleocytosis; polymorphonuclear leukocytes (PMNs) lead the differential cell count in the first 24-48 hours, followed later by increasing numbers of monocytes and lymphocytes. The CSF lymphocytes have been recognized as T cells, although B cell immunity is also important in defending against some viruses.

Evidence exists that some viruses gain access to the CNS by retrograde transport along nerve roots. For example, the likely pathway for HSV-1 encephalitis is via the olfactory or trigeminal nerve roots, with the virus being transported by the olfactory fibers to the basal frontal and anterior temporal lobes.


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