MECHANISMS OF AUDITORY IMPAIRMENT DURING ACOUSTIC NEUROMA SURGERY

Hearing loss during removal of acoustic neuroma (AN) may be due to lab yrinthine and/or neural and/or vascular damage. Surgical maneuvers rel ating to perioperative and postoperative hearing may give rise to mech anisms of auditory impairment. Recording action potentials from the in tracranial portion of the cochlear nerve (CN) has proven particularly useful for identifying the mechanisms of iatrogenic auditory injury. I n this paper intraoperative and postoperative auditory impairments are investigated in relation to surgical steps in a group of 47 subjects with AN (sire ranging from 5 to 25 mm) undergoing removal by a retrosi gmoid-transmeatal approach. Drilling of the infernal auditory canal (I AC), removal of the AN from the IAC fundus, coagulation close to the C N, lateral to medial tumor traction, separation of the CN from the fac ial nerve, and stretching of the CN have proven to be the most critica l surgical steps in hearing preservation. On the other hand, maneuvers such as intracapsular tumor removal, vestibular neurectomy, suction c lose to the AN, and closure of the IAC defect did not correlate with c hanges in auditory potentials. Predisposing factors to postoperative h earing deterioration were IAC enlargement greater than 3 mm, IAC tumor sire greater than 7 mm, extracanalar tumor size greater than 20 mm, l abyrinth medial to the IAC fundus, severe involvement of the CN in the IAC, preoperative abnormal auditory brainstem responses, and normal v estibular reflectivity Age and preoperative hearing did not prove to b e statistically related to postoperative hearing. The variations in mo rphology and latency of CNAPs are discussed in relation to the mechani sms of iatrogenic injury.