Spinal cord stimulation for amelioration of spasticity: experimental results

Fourteen cats underwent 500-g/cm dorsal impact injuries to the spinal cord and the placement of stimulating electrodes above and below the level of injury at T8. After recovery from the surgical procedure and the development of spasticity, each animal participated in several trials of spinal cord stimulation (SCS). Cord stimulation was provided above or below the level of injury using currents of less than 0.75 mA at 100 Hz. Electromyogram changes in hamstring and quadricep muscles (during spasms induced by dorsiflexion of the paw) were monitored. All animals showed complete paraplegia and, at 3 weeks, severe spasms. Spasticity was aggravated by SCS delivered above the level of injury. Spasms were markedly suppressed by monopolar stimulation delivered below the level of the lesion. Effects were maximal with the negative electrode applied to the cord and were slightly less with reversal of polarity. Muscle excitation was seen before diminution of spasms when bipolar currents were used. All effects lasted only as long as currents were delivered. These animal trials suggest that the effects of SCS are directly related to the current and its type. Beneficial effects were seen only when currents were delivered below the level of injury; this suggests that SCS activates local inhibitory processes or depolarizes local excitatory pathways. The poor results with bipolar stimulation do not support action on a multisynaptic cord system in short term stimulation.