Teaching Case

The patient presented with fever and seizures.

The patient presented with fever and seizures but no definite focal neurology. An initial unenhanced CT scan of brain (Fig. 1a) showed an ill-defined hypodense lesion in the right temporal lobe. A subsequent CT scan of brain after iv contrast medium administration (Fig. 1b) showed reduced attenuation in the right temporal lobe with gyral enhancement. T1-weighted (Fig. 2a) and T2-weighted (Fig. 2b) transverse MR sequences of the brain confirmed the diagnosis.

In this case, the initial CT scan findings of a poorly defined, low attenuation area in the right temporal lobe should have prompted the diagnosis of herpes encephalitis. However, the early changes are often subtle and it is not uncommon to miss them on the initial CT scan of brain. In herpes encephalitis post-contrast CT brain images may later show a typical gyral pattern of enhancement (1) as in this case. The hypointensity of the temporal and inferior frontal lobes on T1-weighted MR images and hyperintensity on T2-weighted MR images with abnormal signal extending along the olfactory gyrus and into the cingulate region is very characteristic and it confirms herpes simplex type 1 encephalitis in this patient.

Herpes simplex encephalitis is caused by the virus herpes simplex type 1. It is common in the paediatric age groups. Usually there will be prodromal symptoms, and then altered conscious level, fever, seizures and vomiting may occur. The virus is harboured in the latent state in the gasserian ganglion. After reactivation, the virus spreads through the trigeminal nerve branches that innervate the leptomeninges in the middle and anterior cranial fossa towards the limbic structures in the frontal and temporal lobes. This explains why the temporal and inferior frontal lobe are targeted (2).Pathologically, HSV type 1 encephalitis is a fulminating necrotic meningoencephalitis that usually begins in the temporal lobe.

Imaging modalities are insensitive in the initial course of the disease. The CT scan of brain is often positive only after a few days (3). The typical findings in CT are poorly defined areas of low attenuation in the anteriomedial portion of the temporal lobe and extension into the insular cortex, but the lentiform nucleus tends to be spared. MR imaging of the brain is more sensitive for herpes encephalitis and it is the imaging modality of choice (4). Unilateral involvement is common, but bilateral changes are not rare. Typical appearances show high signal on T2-weighted images and low signal on T1-weighted images in the temporal and inferior frontal lobe, and especially the cingulate gyrus (3). Post-gadolinium MR images may show cortex and leptomeningial enhancement.

Treatment is with iv acyclovir, which should be started at the earliest time of clinical suspicion. In the later stage of the disease the affected area becomes degenerative and atrophies resulting in encephalomalacia.