Diagnosis of the level and severity of cord lesion in cervical spondylotic myelopathy. Spinal evoked potentials.

Experimental and clinical studies were performed to measure the segmental spinal evoked potential (SEP) of the cervical cord after stimulation of the median nerve, to determine the location, severity, and outcome of cord lesion in patients who had cervical spondylotic myelopathy. The SEP in control subjects consisted of two waves, the primary reactive R-wave and the secondary reactive N-wave. The R-wave related to both the dorsal root potential and the funiculus posterior potential; the N-wave related to the post-synaptic potential in the spinal cord. In the early stages of cervical spondylotic myelopathy, the N-wave weakens or disappears at the level of lesion. Then, as damage progresses, the R-wave also weakens or disappears.