Magnetic resonance imaging in herpes simplex encephalitis.

Four patients with clinical and serologically proven Herpes Simplex Encephalitis (HSE) were examined by Magnetic Resonance Imaging (MRI) following Computed Tomographic (CT) scans. MRI proved more sensitive at detecting the temporal lobe abnormalities than CT and invariably showed the lesions to be more extensive than suspected. The findings were consistent with an acute inflammatory process. Haemorrhage was present in only one patient. The sharp transition to normal at the lentiform nucleus previously described on CT was present in only one of the four patients, but was observed in a different patient with cerebral lymphoma and no evidence of HSE and is thus concluded to be less specific than previously thought. A repeat MRI scan at seven months on one of the patients showed persisting increased signal on T2 weighted images after resolution of the mass effect. The pathology and suspected mechanisms for the MRI appearances are discussed. MRI is concluded to be a valuable tool in the early diagnosis of HSE and in the longer term for further evaluation of residual disabilities.