DOSE-RESPONSE STUDY OF THE PATHOLOGICAL EFFECTS OF CHRONICALLY APPLIED DIRECT-CURRENT STIMULATION ON THE NORMAL RAT SPINAL-CORD

Electrical stimulation of the mammalian central nervous system (CNS) c an result in extensive destruction of tissue unless applied within spe cific stimulation parameters. Classically, unbalanced or monopolar cur rents have been avoided in order to minimize these harmful effects. Ho wever, direct current (DC) fields have recently been proposed for the treatment of spinal cord injury. Until now, no rigorous analysis has b een made of the safety of these fields in the mammalian CNS. The purpo se of this study was to determine the amount of chronically applied DC current that can be tolerated by the normal rodent spinal cord stimul ated with metal disc electrodes. Thirty-five normal rats underwent imp lantation of DC stimulating devices and were allowed to recover for a period of 2 to 12 weeks. The stimulators delivered constant currents o f 0 to 50 muA through two disc-shaped platinum/iridium electrodes posi tioned extradurally at the C-7 and T-3 levels. Following sacrifice of the animals, serial 8-mum cross sections of the spinal cord at the ele ctrode sites were examined microscopically. Evidence of demyelination presumed due to the physical presence of the rostral electrode was see n in animals from most groups including control animals. Pathological changes directly attributable to the applied fields were seen with cur rent as low as 3 muA. It was concluded that DC's of 3 muA or more are harmful to the mammalian CNS with this method of stimulation. In addit ion, the data suggest that the maximum current density tolerated by th e rodent spinal cord is in the order of 75 muA/sq cm. These findings h ave important implications for the use of chronic DC stimulation in th e mammalian CNS.