Intra-operative spinal cord neuromonitoring in patients operated on for intramedullary tumors and syringomyelia.

Intra-operative neuromonitoring is of great help to the neurosurgeon and enables him to operate more precisely and with less risk of post-operative neurological deficit. The purpose of the study was to define the changes of the somatosensory evoked potentials (SEP) elements in intramedullary tumors and syringomyelia, and to show the correlation between the changes of the elements and the location of the lesions in the spinal cord. Thirty patients with pain syndromes, intramedullary tumors and syringomyelia were operated on the spinal cord. The methodology of the intra-operative neuromonitoring was based on electrical stimulation of peripheral mixed nerve (tibial and median), and on intra-operative subpial recording of conducted and interneuronic SEP from the dorsal surface of the spinal cord. The recordings in pain syndrome cases were normal, and were compared with recordings obtained in syringomyelia and tumors. The most stable element of the conducted SEP are the initial negative waves that originate in the spinocerebellar tract. Deterioration of the negative high amplitude potentials is the most sensitive indicator of damage to the somatosensory system of the spinal cord, and indicates damage to the dorsal columns. The N11 and N14 waves are the most stable elements of the cervical and lumbosacral interneuronic SEP, and most probably originate in the dorsal root entry zone. Changes of the N13 and N17 elements of interneuronic SEP suggest damage to the spinal cord gray matter, and are most frequently changed in intramedullary lesions. High frequency waves seen on N13 or N17 probably reflect the somatosensory long tracts, and are even better seen in syringomyelia and pure intramedullary tumors. Changes in the elements of interneuronic and conductive SEP enable us to localize the anatomical site of the lesion, which is of great help to the operating neurosurgeon.