Early axonal lesion and preserved microvasculature in epilepsy-induced hypermetabolic necrosis of the substantia nigra.

The time course of structural change in epilepsy-induced necrosis of the substantia nigra was studied by light and electron microscopy, and was correlated with previous metabolic studies. By light microscopy, tinctorial pallor appeared early, followed by pan-necrosis and macrophage infiltration. Mild lesions showed neuropil vacuolation but sparing of neurons, rather than a selective neuronal vulnerability. Electron microscopy of the evolving necrosis revealed an orderly sequence of structural damage involving first axons, then dendrites, neurons, and glia. No necrotic endothelial cells could be found, even in areas of apparent pan-necrosis by light microscopy. Pericytes near the vascular lumen were spared, whereas those in outer locations were necrotic. Edema, measured densitometrically, was absent. Previous metabolic studies of this lesion have demonstrated a pronounced focal lactic acidosis due to anaerobic hypermetabolism. Although the lesions resemble infarcts, hypermia rather than ischemia has been shown to accompany their development. Structural preservation of endothelial cells and inner pericytes likely stems from proximity to the moving blood stream, away from the site of lactic acid production in the neuropil. The findings indicate that the perfusion of necrotic tissue occurs via a persisting, intact microcirculation. The relative neuronal sparing and the early axonal rather than dendritic lesion show a clear distinction from excitotoxic pathology.