MACROSCOPIC AND MICROSCOPIC APPEARANCES

The histologic features of PEM are similar to those of viral encephalitis and myelitis. Neuronal loss, astrocytosis, and an increase in activated microglia are accompanied by an inflammatory infiltrate that consists mainly of lymphocytes and plasma cells and is concentrated in perivascular spaces (Fig. 47.1). The extent of the inflammation is variable and correlates poorly with neuronal loss. Inflammation affects gray matter, but secondary degeneration of white matter tracts can result. A leptomeningitis is nearly always present. No macroscopic abnormalities may be discerned.

Regions of the cerebrum, brain stem, cerebellum, spinal cord, and optic nerve may be affected alone or in combination.

Some cases of PEM are combined with a sensory neuronopathy characterized by ganglionitis, loss of ganglion cells, and secondary degeneration of dorsal column fibers (Fig. 47.2).

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Mild cerebellar atrophy is occasionally observed, but there is generally no macroscopic abnormality in PCD. Histologically, a severe loss of Purkinje cells is evident throughout most of the cerebellar cortex, and is accompanied by variable loss of granule cells, activated microglia, and Bergmann gliosis (Fig. 47.3). An infiltrate of mononuclear leukocytes may be seen in the cortex, and there may be perivascular aggregates of lymphocytes and a leptomeningitis. However, inflammation may be sparse or absent, despite a severe loss of Purkinje cells. In a minority of cases of PCD, there is patchy loss of neurons accompanied by inflammation in other regions of the brain, indicating some overlap with PEM.