Dermatomal somatosensory-evoked potentials in the evaluation of lumbosacral spinal stenosis.

DSEPs are very useful in determining which patients with degenerative lumbar spine disease have the added component of neurogenic compromise. Based on sound physiologic principles, level-by-level prolongation of DSEP latencies, reduction of amplitude, asymmetry, or a complete absence of response is associated with dysfunction in that particular neurologic pathway. This result does not necessarily correspond to the exact level of stenosis noted on MRI, because the rootlets in the lumbar and sacral regions pass through multiple spinal segments as they course through the spinal canal. Given that LSSS develops typically over time, it is not unlikely that the degree of abnormality will be variable between different levels. The pattern of abnormality will correspond to the physiologic slowing occurring in the multiple rootlets of the cauda equina. These recordings are not easy to perform and interpret; but, when done correctly, they provide the best evidence for the type of neurophysiologic dysfunction in LSSS that responds favorably to surgical decompression.