Viral encephalitis

Acute viral encephalitis is due to direct invasion of brain parenchyma and the clinical manifestations are caused by cell dysfunction and associated inflammatory change. At the bedside this may be indistinguishable from postinfectious encephalitis, the pathology of which is perivenous demyelination caused by allergic or immune reactions triggered after a latent period by viral infection.10,11 Viruses are far and away the most common cause of encephalitis globally, but in certain locations and seasons other organisms such as malaria and other protozoa, rickettsiae, and fungi may induce an encephalitic syndrome. It is therefore of paramount importance to obtain from the patient or a relative a full account of the recent travel history.

As with meningitis, the virus usually reaches the brain by either haematogenous or neuronal routes. The haematogenous route is the most common. Viral entry may be through the skin following an insect bite, as with arbovirus infection, or via the respiratory or gastrointestinal route. Local replication ensues, resulting in transient viraemia and spread to the reticuloendothelial system whence, following further replication, secondary viraemia increases and spread takes place to other sites including the CNS. Here there is inflammation of the capillary and endothelial cortical vessels and, as disease progresses, astrocytosis and gliosis become prominent histopathological findings. Modification of this process by host immune responses may occur and if these are compromised, disease progression may be fulminant. More rarely, virus may ascend neurons centripetally to lodge in brain cells, as with herpes encephalitis and rabies. There is some evidence that the olfactory tract is one route of access of HSV to the brain. Fortunately, encephalitis is a rare complication of common viral infections and most patients with systemic viral infections do not develop neurological signs.

Certain viruses exhibit tropism towards specific cell types -the limbic system in rabies,12 temporal lobe in herpes simplex encephalitis - and this may produce clinical signs that are diagnostically useful.11 In most cases of encephalitis, however, signs and symptoms are common to most pathogens and diagnostic clues must be sought elsewhere. Is there evidence of infection elsewhere, such as the characteristic rashes of varicella and measles or the parotitis of mumps? Has there been travel to an area that harbours known insect vectors? Even aircraft stopovers in "at-risk" areas may be important;

cases of "runway" infection have been recorded where the insect vectors have entered the aircraft. Is there evidence of a current or past insect bite? What is the season of the year? Diagnostic tests may help, yet in perhaps as many as a third of all cases no specific aetiology can be established.

Viral encephalitis is not rare and occurs globally.11 In the United Kingdom and Europe, most cases are sporadic and are caused by herpes simplex and other herpesviruses (Box 9.1). Since the widespread use of childhood measles, mumps, and rubella (MMR) vaccine, encephalitis caused by these viruses is now rare. In the United States, sporadic and epidemic forms are caused by arboviruses (arthropod borne). Japanese B encephalitis causes most epidemic infections elsewhere. As many as 20 000 cases of encephalitis per annum may occur in the United States.13

Patients who develop viral encephalitis often have several days of a prodrome, which includes myalgia, fever, malaise, mild upper respiratory infection, rash, or parotitis. The development of headache, mental change, and drowsiness implies encephalitis, which is usually associated with meningitic features. As the disease progresses, disorientation and disturbance of behaviour and speech worsen and drowsiness merges into coma. Epileptic seizures are common and focal signs may appear appropriate to the area of the brain that suffers the brunt of the infection. For example, hallucinations and memory upset from the temporal lobes, hemiparesis, spasticity, sensory loss and speech upset from the parietal lobes, and coordination problems and dysarthria with cerebellar deficits. There may also be signs of raised intracranial pressure. The very young, the very old, and those with compromised immune systems often have more severe disease. Signs should be sought in other organ systems, which may point to a particular virus. Some forms of encephalitis have specific features that are briefly discussed later.

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