Intraoperative electromyographic monitoring of extra-ocular motor nerves (Nn. III, VI) in skull base surgery

Background. Extraocular motor nerves (Nn. III, IV, VI) are at risk of damag e during skull base surgery. A new recording technique was employed in 18 p atients suffering from various skull base tumours in order to extend intra- operative EMG monitoring to the extra-ocular muscles. Methods. Selective intra-operative EMG recordings were obtained from extra- ocular muscles by placement of single-shafted bipolar needle electrodes und er the guidance of B-mode ultrasound to visualise the needle tip within the target muscle in the orbital cavity. Findings. Following bipolar electrical stimulation, the oculomotor nerve (N .III) was intra-operatively identified in 5 out of 7 cases, and the abducen s nerve (N.VI) in 12 out of 18 cases. Postoperative (3-6 months) oculomotor nerve function remained unchanged in 5 and improved in 2 patients. No perm anent deterioration was observed. Abducens nerve function deteriorated in t wo patients and improved in one case, but remained unchanged in 15 cases. N o side effects occurred. There was neither any distinct relation of ocular motor nerve function to the kind and extent of SMA ("spontaneous muscle act ivity") patterns, nor could such relationship be detected with concern to n europhysiological parameters (latencies, amplitudes) of electrically evoked CMAP ("compound muscle action potentials"). Interpretation. The EMG technique proposed proved to be mainly effective as a mapping tool for intra-operative localisation and identification of ocul ar motor nerves in skull base surgery. However, the predictive value of con ventional neurophysiological parameters for clinical outcome, seems to be r ather poor. Further studies on a larger number of patients are therefore re quired to develop new quantification techniques which enable an intra-opera tive prediction of ocular motor nerve deficits. Further efforts are also ne cessary to extend this technique to the trochlear nerve.