Spinal cord transmission of impulses during high spinal anesthesia as measured by cortical evoked potentials

Cortical evoked potentials (CEPs) resulting from posterior tibial nerve stimulation could be recorded before, but not during, diagnostic high spinal anesthesia to T2-3 in six patients with chronic pain. In contrast, spinal cord stimulation at T8-9 during high spinal anesthesia resulted in CEPs with reduced peak amplitudes and increased P1 and N2 latencies. Amplitudes of the CEPs evoked by spinal cord stimulation increased with increases in stimulus intensity before and during high spinal anesthesia (N = 3). During spinal anesthesia, higher stimulus intensities were required to evoke CEPs of similar amplitudes as before. Because CEPs evoked by stimulation of the posterior tibial nerve disappeared, but those evoked by spinal cord stimulation did not, it is concluded that spinal anesthesia is a result of blockade of nerve roots, not blockade of neural transmission within the cord. Therefore, diagnostic high spinal anesthesia cannot exclude an origin of pain within the spinal cord.