Intraoperative electrophysiological monitoring of oculomotor nuclei and their intramedullary tracts during midbrain tumor surgery.

OBJECTIVE

During surgery for intrinsic midbrain lesions, we intraoperatively recorded evoked compound muscle action potentials (ECMAPs) from the extraocular muscles and evaluated how this type of intraoperative electrophysiological monitoring could minimize postoperative oculomotor nerve palsy (ptosis and/or diplopia).

METHODS

The ECMAPs were recorded through a spring electrode applied to the extraocular muscle (Method 1, seven cases) or a needle electrode inserted into the superior intraorbital space (Method 2, five cases). The surgeon repeated electrical stimulations whenever tissue of unknown origin was encountered intraoperatively, and this information was used to safely guide surgical resection of the tumors.

RESULTS

Using these monitoring techniques, the response-free areas were resected and the areas from which ECMAP responses were recorded were avoided. For all 12 patients, ECMAPs were successfully recorded from the extraocular muscles. Ten patients did not exhibit any postoperative deterioration of oculomotor nerve function. Two patients exhibited deterioration of oculomotor nerve function immediately after surgery, which resolved within 1 month. Equally robust ECMAPs could be recorded with Method 2, compared with Method 1.

CONCLUSION

Intraoperative ECMAP recordings from the extraocular muscles precisely indicated the locations of the oculomotor nuclei and/or intramedullary oculomotor tracts. Although Method 2 is a more indirect method for recording ECMAPs than is Method 1, Method 2 was equally useful in recording ECMAPs, which seemed to be the summed potentials from the superior rectus muscle and the levator palpebrae superioris muscle. These monitoring techniques are valuable in guiding surgeons to avoid causing inadvertent harm to the oculomotor nuclei and tracts during midbrain surgery, particularly when the neuroanatomic features are distorted by the presence of tumor.