Herpes encephalitis

This can give rise to a particularly severe form of amnesic syndrome.(25) The majority of cases are said to be primary infections, although there may be a history of a preceding 'cold sore' on the lip. Characteristically, there is a fairly abrupt onset of acute fever, headache, and nausea. There may be behavioural changes. Seizures can occur. The fully developed clinical picture with neck rigidity, vomiting, and motor and sensory deficits seldom occurs during the first week. (26) Diagnosis is by finding a raised titre of antibodies to the virus in the cerebrospinal fluid, but often this is missed and a presumptive diagnosis is made on the basis of the clinical picture as well as severe signal alteration, haemorrhaging, and atrophy in the temporal lobes on MRI brain imaging.

Neuropathological and neuroimaging studies show that there is extensive bilateral temporal lobe damage, (27> although, occasionally, the changes are surprisingly unilateral. There are often frontal changes, most commonly in the orbitofrontal regions, and there is a variable degree of general cortical atrophy. The medial temporal lobe structures are particularly severely affected, including the hippocampi, amygdalae, entorhinal and perirhinal cortices, and other parahippocampal structures. Evidence from studies of bilateral temporal lobectomy as well as animal lesion studies has indicated that these structures are particularly critical in memory formation.

The chronic memory disorder in herpes encephalitis is often very severe, (25 but it shows many resemblances to that seen in the Korsakoff syndrome, consistent with the fact that there are many neural connections between the thalami, mammillary bodies, and the hippocampi.(28) Encephalitis, like head injury, can also implicate basal forebrain structures which give cholinergic outputs to the hippocampi; since these are thought to modulate hippocampal function, this may further exacerbate the damage. Contrary to what was postulated in the 1980s, there appears to be no difference between patients with the Korsakoff syndrome and those with herpes encephalitis in terms of rates of forgetting or in the relative effect upon recall versus recognition memory. The patients with herpes appear to have better 'insight' into the nature of their disorder and a 'flatter' temporal gradient to their retrograde memory loss (i.e. less sparing of early memories), and they may have a particularly severe deficit in spatial memory when the right hippocampus is involved.(20) However, the similarities in the episodic memory disorder tend to outweigh the differences.

On the other hand, a more extensive involvement of semantic memory is characteristic in herpes encephalitis, and this results from the widespread involvement of the lateral, inferior, and posterior regions of the temporal lobes. Semantic memory refers to a knowledge of facts, concepts, and language (see ChapteĀ£2..5.2). Left temporal lobe pathology in herpes encephalitis commonly gives rise to an impairment in naming, reading (a so-called 'surface dyslexia'), and other aspects of lexicosemantic memory. Right temporal lobe damage may lead to a particularly severe impairment in face recognition memory or knowledge of people.

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