Nearly 95% cases of herpes simplex encephalitis (HSE) are caused by herpes simplex virus 1 (HSV1), and it is the most common cause of fatal encephalitis [1,2]. Brain infection is likely a result of direct transmission of the virus from a peripheral site to the trigeminal or olfactory nerve, although other mechanisms include hematogenic transmission.
CT imaging in patients with HSE is frequently non-specific. It may reveal areas of hypodensity in the temporal or frontal lobes, but frank hemorrhage or enhancement is uncommonly seen. MRI should be obtained rapidly in patients suspected of harboring this diagnosis, since MRI can reveal the characteristic neuroanatomical pattern of involvement. Herpes encephalitis is generally unilateral in the initial phase, but there is progressive but asymmetric contralateral involvement in the later stages (Figs. 40.1, 40.2). The medial temporal and inferior frontal lobes are commonly affected, but most characteristically there is involvement of the insula (Fig. 40.1). HSE is frequently hemorrhagic, and enhancement is not a prominent feature in the early disease process. In later stages, patchy enhancement is frequently seen (Fig. 40.2C) . The disease predominantly involves the cortex and subcortical white matter, and there is often extensive involvement of the limbic system.