Diagnosis: Herpes simplex encephalitis
Herpes simplex encephalitis in adults is due to HSV1 which causes a fulminant, necrotizing encephalitis which may have hemorrhage in the later stages and is the most common etiology of viral encephalitis. Adult patients with herpes encephalitis typically present with severe systemic symptoms including high fever, headache, seizures, and changes in mental status. The mortality rate is up to 70% CSF studies and serology may be diagnostic however these may not be positive until the second week after the onset of the disease. Definitive diagnosis is by brain biopsy. Treatment is usually empirical on the basis of the combination of clinical signs and symptoms and the imaging findings. Early in the course of the disease, CT may be normal and later on may show low attenuation changes in the cortex and white matter with hemorrhage. On MR, the characteristic findings are increased T2 and decreased T1 signal involving the cortex extending into the white matter involving the temporal lobe, sub-frontal region and insula with variable involvement of the contralateral side. The cingulate gyrus may also be involved since the disease has a predilection for the limbic system. The focus of increased signal seen in the left basal ganglia on T1 ax pre-gad in this case may be a small hemorrhage. If the disease has the characteristic appearance, as it does here, the differential is limited. Other etiologies for encephalitis may be considered such as equine encephalitis. Other thoughts depending on the imaging findings may include astrocytoma and MELAS. This patient was treated empirically with acyclovir. CSF showed elevated protein and white blood cell count. EEG showed left hemispheric PLEDS which were at their maximum in the temporal lobe compatible with HSV encephalitis. The patient rapidly improved after initiation of acylcovir therapy. Related Cases
Soo MS, Tien RD, Gray L, et al. Mesenrhombencephalitis: MR findings in nine patients. AJR, May 1993; 160(5):p1089-93.
