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Surgery of intrinsic brainstem lesions is extremely dangerous, consequently the knowledge of anatomy and safe entry zones is extremely important. The technological progress was achieved with the widespread clinical use of intraoperative neurophysiological monitoring (IONM) which has proven to further improve the safety of operating on intrinsic brainstem lesions. The relevance of the study is conditioned by the fact that the tumor alters the normal anatomy and distorts the location of well-known landmarks and structures. In this regard, this paper is directed at identification of the placement of conductive pathways in the thickness of the brain stem and directly in the depth of the lesion (in the tumor or cavern), respectively to the surgical manipulations aimed at injury prevention. The results were achieved by a retrospective review of medical reports. The paper represents the results of surgical treatment of 42 patients with brainstem lesions, reviews the localization of the lesion and neurological status of patients, describes available methods of intraoperative monitoring (IOM) and on-line techniques for the brainstem surgery. The materials of the paper are of practical significance for the development of brainstem surgery with IONM modifications for wider and more comfortable use by neurosurgeons.

Summary. this article presents a review of literature on cerebellopontine angle tumors, anatomy and physiology of the vestibular nerve, intraoperative neurophysiological monitoring and method of registration of auditory evoked potentials. In view of the development of neurosurgical equipment in our time, it becomes possible to perform surgical interventions while maintaining functionally significant structures and thus improving the patient's quality of life in the postoperative period. The aim of the study -to demonstrate the effectiveness of the use and the method of intraoperative neurophysiological monitoring in the modality of the auditory evoked potentials aimed at preserving the function of cochlear nerve during surgery on the cerebellopontine angle tumors. the object of the study is the function of the cochlear nerve, and the subject of the study is the auditory evoked potentials. Materials and Methods. the auditory evoked potentials reflect the entire sensitive path of the vestibulocochlear nerve. using medtronic xomed nIm-eclipse intraoperative neuromonitoring system with 32-channel neurophysiological unit was performed. the impulses move through the conductive path of the auditory analyzer: the first auditory neuron -bipolar cells -spiral ganglion, second neuron -ventral and dorsal nuclei at the lateral angle of the rhomboid fossa, third auditory neuron -the lower lobes of the corpora quadrigemina, fourth auditory neuron is -the medial geniculate bodies -heschl gyrus. on the monitor screen, we get seven waves designated by the roman numerals I, II, III, Iv, v, vI, vII. the evaluation of the data of the amplitude and latency of all the waves makes it possible to analyze the functional integrity of the leading hearing path at all its levels. with this method, the neurosurgeon gets the information on the functional status of the nerve. Results and Discussion. In total, 35 (100 %) patients with cerebellopontine angle tumors underwent surgical operations in the uzhhorod regional Clinical Center of neurosurgery and neurology during the period dating from February 2016 till august 2017. among them, 30 (85.7 %) patients lost the hearing at the preoperative stage; 5 (14.2 %) patients had subnormal hearing preserved on the affected side. the auditory evoked potentials were not registered before the beginning of the main stage in the group of patients, who lost the hearing at the preoperative stage, and the predominant diameter of the tumor was more than 25 mm. as for the patients with preserved hearing, the diameter of the tumor was less than 25 mm. Conclusions. the correlation between the clinical signs at the preoperative stage (preserved hearing) and tumor size was determined. the use of this method for treating cerebellopontine angle tumors up to 25 mm in diameter is effective.