Median Nerve Somatosensory Evoked Potentials in Cervical Syringomyelia

Median nerve somatosensory evoked potentials (SEPs) were recorded in 30 patients with cervical syringomyelia before and after surgery. The different SEP components were compared with clinical somatosensory findings. The N13 potential (generated in the dorsal horn at C5-C6) was pathological in 85% of the upper extremities, or 90% of the patients, and correlated with pain/temperature as well as vibration/joint position sense; it was of higher sensitivity in syringomyelia than any other clinical symptom or SEP component. P14 (brain stem) and N20 (postcentral cortex) were less often affected and correlated with only vibration/joint position sense. Short-term postoperative clinical or SEP changes were most often seen after syringoendoscopy and less often after syringostomy, resection of cerebellar tonsils, or tumor extirpation. Alterations of SEPs after surgery occurred in more patients (60%) than did changes in clinical condition (approximately 27%); there was, however, no general correlation between these findings. We conclude that median nerve SEP testing with a proper recording technique identifying the different subcortical components is a valuable supplement in the pre- and postoperative diagnostic evaluation of syringomyelia and is of higher sensitivity than clinical somatosensory examination alone.