Viral infection of the CNS

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Viral diseases of the nervous system are seen in four main patterns:

Viral meningitis, characterized by the development of lymphocytic meningitis.

Cytolytic, in which there iscell destruction producing encephalitis or myelitis. This is the most clinically severe pattern of involvement, most commonly seen with herpes simplex.

Latent, in which virus is integrated into the host cells, with the potential for re-activation as a cytolytic infection. This is extremely 
common with herpes zoster, causing shingles.
  Persistent, in which there is slow, smouldering degeneration of neuronal tissues caused by viral infection in the absence of elimination by immuneresponses,e.g. measles virus infection causing subacute sclerosing panencephalitis.

AIDS commonly affects the nervous system

One of the most common causes of viral infection of the nervous system is HIV. Not only does HIV directly affect the brain and spinal cord, AIDS also predisposes to several complications due to immunosuppression.

The AIDS dementia complex (HIV cognitive/motor complex)} is a clinical syndrome that has elements of intellectual impairment, and behavioural and motor changes. Several pathologically defined changes underlie this clinical syndrome.

Lymphocytic meningitis is seen in patients around the time of seroconversion and is defined as occurring in the absence of any demonstrable opportunistic pathogens.

HIV encephalitis is a multifocal process characterized by inflammatory foci including multinucleate giant cells, mainly seen in white matter, basal ganglia and brain stem.

HIV leukoencephalopathy is characterized by myelin loss, gliosis, phagocytic macrophages and scattered multinucleate giant cells in white matter. There is little or no inflammatory infiltration.

Diffuse poliodystrophy is the term applied to neuronal loss, microglial activation and gliosis in CNS grey matter.

Vacuolar myelopathy is the term used to describe vacuolation in myelin sheaths, with myelin loss in spinal cord.

Cerebral vasculitis is seen most prominently in childhood HIV disease of the brain.

Spongiform encephalopathies are caused by an unconventional protein-only agent and can be genetic as well as transmissible

Creutzfeldt-Jakob disease is an uncommon nervous system disorder. It is characterized by accumulation of a modified normal cell-membrane protein, termed 'prion protein' (PrP). The precise way in which the protein is modified is unknown.

The disease is associated with a rapidly progressive dementia in humans, with histological vacuolation in the brain, known as spongiform encephalopathy. It is similar to kuru in humans, scrapie in sheep and bovine spongiform encephalopathy in cattle, which also accumulate PrP. As these diseases were once thought to be caused by a virus with a long incubation, they were formerly termed 'slow-virus diseases'.

Although most human disease is sporadic, cases have resulted from transplantation of tissue from an affected person, as well as from administration of growth hormone derived from cadavers. Identical and transmissible disease results from hereditary mutations in the gene coding for the prion protein in man (Gerstmann-Straussler syndrome, causing familial cases of ataxia and dementia).
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