Correlation between spinal cord compression and abnormal patterns of median nerve somatosensory evoked potentials in compressive cervical myelopathy: comparison of surface and epidurally recorded responses.

To investigate the correlation between the level of spinal cord lesion and the abnormal pattern of median nerve somatosensory evoked potentials (SSEPs), evoked spinal cord potentials (ESCPs) were also recorded from the posterior epidural space intraoperatively in 18 patients with compressive cervical myelopathy. Levels of symptomatic spinal cord compression were determined by ESCP findings. Spinal N13 potential of the SSEPs was recorded from the surface of the posterior neck with anterior neck reference. Brainstem P14 and cortical N20 potential were recorded from the parietal scalp contralateral to the stimulated side. Spinal N13, P14, and N20 potentials were all normal when the ESCPs were abnormal at localized segmental region (C4-5 or C5-6 level alone). Spinal N13 potential was significantly attenuated in all of patients with abnormal ESCP findings at widespread segmental area of the median nerve territory. In four of these seven patients, brainstem P14 potential was also prolonged or diminished, but three patients showed normal P14 and N20 potentials. Isolated P14 abnormality with normal spinal N13 potential was characteristic in patients with abnormal ESCP at the C3-4 lesion. Although sensitivity of abnormal ESCP was higher than that of the SSEPs, abnormal patterns of spinal N13, P14 and N20 potentials following median nerve stimulation were useful in detecting not only the pathology (posterior horn and/or posterior column) but also symptomatic spinal compression level in compressive cervical myelopathy.