Histological evaluation of neural damage from electrical stimulation: considerations for the selection of parameters for clinical application.

The relationship of charge density per phase, or QD/ph (expressed in units of microcoulombs per cm2 per phase of the charge-balanced wave form), and total charge (QDt) to neural damage has been investigated by light and electron microscopy after surface stimulation of the parietal cortex in normal cats. QD/ph values ranging from 40 to 400 were achieved by varying several stimulus parameters. The least amount of neural damage in this study was observed at QD/ph 40). The extent of neural injury at stimulated sites increased with the charge density and was evident as disruption of cell membranes, intracytoplasmic vacoulation, an increasing glycogen content, the deposition of intracellular calcium hydroxyapatite, and neuronal and astrocytic degeneration. Although individual factors contributing to neural damage are isolated with difficulty, charge density and total charge seem to be predominant among the contributing parameters. In view of these findings, recommendations have been made for the selection of electrical stimulus parameters to be used in central nervous system prostheses.