Iatrogenic Impairment of Hearing During Surgery for Acoustic Neuroma

Abstract

The decision to attempt hearing preservation during acoustic neuroma (AN) surgery has a dramatic impact on the choice of surgical approach. The translabyrinthine route, which has been in use for many years by otoneurosurgeons, has helped to reduce the mortality and morbidity rates. According to many authors, this is the technique of choice in subjects with poor preoperative hearing function, that is, greater than 50 dB or 70 dB in pure-tone average (PTA) and/or less than 70% or 50% in speech discrimination score (SDS).'-5 In patients with serviceable hearing, the retrosigmoid or middle fossa routes are preferred. '-5 Our group is intensely involved in functional otosurgical procedures and particularly in hearing preservation during AN surgery. To this end we selected the retrosigmoid-transmeatal approach, as it presents a series of advantages such as adequate control of bleeding in the cerebellopontine angle (CPA) and the possibility of monitoring hearing function by means of direct recording of cochlear nerve action potentials (CNAPs).5-0 TWhis method of monitoring is particularly suitable for investigating the mechanisms of auditory damage in AN and during AN surgery. The derived potentials are of large amplitude and are easily visualized on an oscilloscope either directly or after averaging a few responses. They provide rapid information on the function of the cochlear nerve (CN) and the cochlea. They also furnish a unique opportunity to investigate auditory physiology and pathophysiology in humans.11-13During surgery of the posterior fossa, CNAPs have proven to be sensitive in detecting damage to the periph-153

Intraoperative neuromonitoring techniques in the surgical management of acoustic neuromas

“…More specifically, vascular changes to the internal auditory artery, such as occlusion, rupture, or vasospasm, are believed to induce postoperative hearing deficits. 17,19,79,84,88,89,122 Strauss et al 135 found that applying medical therapy to preclude such vasospasms produced preservation rates that were more than twice as high when compared with controls.…”

“…Because the stimulus response must be summed and averaged to obtain a wave of sufficiently high amplitude, the tradeoff to this process is a significant time delay that can last up to several seconds to minutes. 1,18,19,49,50,83 Naturally, such a delay can negatively influence the course of surgery, as BAEPs effectively provide data that were applicable several seconds or minutes prior.…”

“…63,64,149 Direct operative trauma is a potential avenue, with the nerve most prone to exposure during maneuvers, such as drilling into the internal auditory canal, operative traction, or subsequent tumor resection. 1,17,19,51,79,96,129 Cranial nerves are inherently more susceptible to trauma because they are ensheathed in central myelin, thus lacking the extra protective layers, such as the perineurium, that are more prevalent in peripheral myelin. 8,17,19,64,79,123 Ischemic damage also presents further risk of injury.…”

“…1,17,19,51,79,96,129 Cranial nerves are inherently more susceptible to trauma because they are ensheathed in central myelin, thus lacking the extra protective layers, such as the perineurium, that are more prevalent in peripheral myelin. 8,17,19,64,79,123 Ischemic damage also presents further risk of injury. More specifically, vascular changes to the internal auditory artery, such as occlusion, rupture, or vasospasm, are believed to induce postoperative hearing deficits.…”

“…42,99 Similar to the BAEP, the stimulating electrode administers click impulses, and multiple responses must be averaged for a distinct wave pattern to emerge. 18,19,113,114,120,129 As its name suggests, in direct CNAPs, the action potential is measured directly from the acoustic nerve itself. 64,81,129 The recording electrode is placed directly on the acoustic nerve, the negative electrode is attached to the mastoid of the contralateral ear, and a reference electrode is placed on the scalp.…”