Intraoperative recordings of evoked extraocular muscle activities to monitor ocular motor nerve function

During 22 operations in 18 patients, we stimulated the ocular motor nerves electrically, intracranially, and recorded compound muscle action potentials (CMAP) directly from the extraocular muscles with a ring electrode that we developed. Recording electrodes were applied in 52 instances to the superior rectus, medial rectus, superior oblique, or lateral rectus muscle and to the levator palpebrae superioris in 2 instances; CMAP were recorded successfully from 22 muscles. Evoked CMAP were not recorded in 2 instances because of problems with recording equipment; in the remaining 30 instances, no evoked CMAP were recorded because 1) the oculomotor or abducens nerve was not exposed during the operation; or 2) the recording electrode on the superior oblique muscle had not been properly placed to record trochlear nerve CMAP. Placement of this electrode is difficult. Ocular motor nerve function was analyzed preoperatively and postoperatively to evaluate the usefulness of this intraoperative electrophysiological monitoring method in preventing damage to ocular motor nerves. The results of this study showed that monitoring enables surgeons to locate precisely ocular motor nerves that would otherwise have been overlooked and thus possibly injured during surgery. Monitoring results also confirmed the surgeons' visual findings, thus helping the surgeons operate with greater confidence. Further, intraoperative monitoring provided us with some insights into the pathophysiology of ocular motor nerve dysfunction caused by skull base lesions; we documented electrophysiologically the occurrence of the slowing of conduction in the ocular motor nerves. We conclude that monitoring ocular motor nerve CMAP can reduce the incidence of surgical complications such as functional blindness due to inadvertent sectioning of one of these nerves and that it would be worthwhile to conduct studies of this technique in many more cases to validate our findings.