✓ The histopathology associated with chronic cerebellar stimulation is described for three human cerebellum and brain-stem specimens obtained at autopsy. The specimens were from three severely epileptic patients who received cerebellar stimulation at 10 Hz for 6½ to 15 months. The electrode arrays were completely encapsulated with loose connective tissue that included a proliferation of capillaries, an infiltration of lymphocytes, and an occasional macrophage. The capsule of one of the specimens was tightly adherent to the underlying cerebellar cortex, which may have been caused by some trauma during the surgical placement of the electrodes. Severe injury of the cerebellar cortex was generally confined to between 1 and 2 mm directly beneath the electrode array, and included thinning of the molecular layer, and loss of most Purkinje cells, interneurons, and associated fibers. Abnormal Purkinje cell dendritic patterns and loss of climbing fibers were evident 3 to 4 mm from the cortical surface. At a depth of 5 to 6 mm, there was no distinction from the nonstimulated areas. However, in all specimens the most severe changes, including a complete loss of the neuropil, were evident at the caudal border of the electrode array underlying the connecting wires. A calculation of the amount of severe and very severe neuronal damage for two specimens revealed that 1.2% and 3.1% of the total cortical surface was injured by this neuroprosthesis. Neuropathological changes were also evident in the dentate nucleus and the brain stem. Areas of the dentate nucleus that were close to regions of very severe cortical damage showed marked degenerative changes. Retrograde degeneration was evident primarily within the caudal portion of the medial accessory olivary nucleus.
