"Backfiring" in spinal cord monitoring. High thoracic spinal cord stimulation evokes sciatic response by antidromic sensory pathway conduction, not motor tract conduction.

Spinal cord stimulation has been advocated as an alternative to motor cortex stimulation for motor tract activation. To test this theory, evoked responses were recorded from lumbar spinal cord (L2; n = 14), spinal roots (L4-L7; n = 112), peripheral nerves (sciatics; n = 28), and hind limb muscles (n = 28) after epidural stimulation of the T1-T2 segment of the spinal cord in dogs (n = 12), cats (n = 2), and monkeys (n = 2). The spinal response evoked by spinal cord stimulation was resistant to a dorsal hemisectioning (depth, 7-8 mm) of the midthoracic spinal cord. A minimal attenuation of latency and amplitude occurred with dorsal hemisectioning, suggesting signal transmission through descending or ascending pathways in the ventrolateral and ventral quadrants of the spinal cord. The sciatic nerve response was abolished by a dorsal column transection (depth, 3-4 mm) or ipsilateral lumbar dorsal rhizotomy (four dorsal roots). This shows that the evoked response recorded from the sciatic nerve in our animals was not travelling, as we expected, through the ventral roots, but rather was conducted antidromically through sensory fibers in dorsal roots.