St. Louis encephalitis and the substantia nigra: MR imaging evaluation.

Neuroimaging findings in cases of St. Louis encephalitis (StLE) have yet to be reported despite the relatively high frequency of this entity. An epidemic permitted the documentation of isolated hyperintensity of the substantia nigra on T2-weighted images in two patients with StLE. This distribution of MR imaging abnormality in cases of StLE mirrors the reports presented in the literature that implicate the substantia nigra as peculiarly susceptible to the StLE virus. Isolated lesions of the substantia nigra revealed by T2-weighted imaging should suggest the possibility of StLE.