Herpes simplex encephalitis

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Overview
Herpes simplex encephalitis (HSE) is a very serious disorder and one of the most severe viral infections of the human central nervous system. It is estimated to affect at least 1 in 500,000 individuals per year. HSE is thought to be caused by the retrograde transmission of virus from a peripheral site on the face to the brain along a nerve axon following HSV-1 reactivation. Approximately 50% of individuals that develop HSE are over 50 years of age. About 1 in 3 cases of HSE result from primary HSV-1 infection predominantly occurring in individuals under the age of 18. Although 2 in 3 cases occur in seropositive persons, few of these individuals have history of recurrent orofacial herpes. The virus lies dormant in the ganglion of the trigeminal or fifth cranial nerve but the reason for reactivation, and its pathway to gain access to the brain, remains unclear. The olfactory nerve may also be involved in HSE.

Without treatment, HSE results in rapid death in around 70% of cases. Even with the best modern treatment, it is fatal in around 20% of cases treated, and causes serious long-term neurological damage in over half the survivors. For unknown reasons the virus seems to target the temporal lobes of the brain. Only a small population of survivors (2.5%) regain completely normal brain function. Most individuals with HSE show a decrease in their level of consciousness and an altered mental state presenting as confusion and changes in personality. Increased numbers of white blood cells can be found in their cerebrospinal fluid without the presence of pathogenic bacteria and fungi, and they typically have a fever. Some patients with HSE will have seizures. The electrical activity of the brain (detected using EEG, CT, or MRI scans) changes as the disease progresses, first showing abnormalities in one temporal lobe of the brain, which spread to the other temporal lobe 7–10 days later.