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

OBJECTIVE: During surgery for intrinsic midbrain lesions, we intraoperative ly recorded evoked compound muscle action potentials (ECMAPs) from the extr aocular muscles and evaluated how this type of intraoperative electrophysio logical monitoring could minimize postoperative oculomotor nerve palsy (pto sis 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 re peated electrical stimulations whenever tissue of unknown origin was encoun tered intraoperatively, and this information was used to safely guide surgi cal resection of the tumors. RESULTS: Using these monitoring techniques, the response-free areas were re sected and the areas from which ECMAP responses were recorded were avoided. For all 12 patients, ECMAPs were successfully recorded from the extraocula r. muscles. Ten patients did not exhibit any postoperative deterioration of oculomotor nerve function. Two patients exhibited deterioration of oculomo tor nerve function immediately after surgery, which resolved within 1 month . Equally robust ECMAPs could be recorded with Method 2, compared with Meth od 1. CONCLUSION: Intraoperative ECMAP recordings from the extraocular muscles pr ecisely indicated the locations of the oculomotor nuclei and/or intramedull ary oculomotor tracts. Although Method 2 is a more indirect method for reco rding ECMAPs than is Method 1, Method 2 was equally useful in recording ECM APs, which seemed to be the summed potentials from the superior rectus musc le and the levator palpebrae superioris muscle. These monitoring techniques are valuable in guiding surgeons to avoid causing inadvertent harm to the oculomotot nuclei and tracts during midbrain surgery, particularly when the neuroanatomic features are distorted by the presence of tumor.