Management of vestibular schwannomas (acoustic neuromas): the value of neurophysiology for intraoperative monitoring of auditory function in 200 cases.
OBJECTIVE
The present study investigated the significance of the presence or absence of auditory brain stem response (ABR) Waves I, III, and V as functional representatives of the cochlea, the nucleus cochlearis, and the colliculus inferior, respectively, and attempted to identify the microsurgical maneuvers that were especially likely to cause isolated or combined component losses and subsequent hearing losses.


METHODS
Based on the previously described ABR classification system, 201 patients with preserved Waves I, III, and V or Waves I and V were investigated for the peak latencies and amplitudes of the waves at 15 defined microsurgical stages. Analysis was performed with respect to the presence or absence of ABR components during specific microsurgical actions and the related danger of deafness.


RESULTS
Temporary or permanent losses of Waves V, I, and III occurred with 21, 27, and 29% of surgical actions, respectively, leading to deafness in 65 to 78% of the patients. Wave III disappearance was identified as the earliest and most sensitive sign. Wave V loss was usually preceded by disappearances of Waves I and III. During the most dangerous actions (drilling, pulling downward, medially, or laterally, and direct nerve manipulation), special attention ws paid to deterioration of Wave-III and then Wave I; if impairment was seen, intermittent breaks or changes in the type or site of microsurgical action were used to enable wave recovery. Acute, simultaneous, and permanent loss of all waves occurred in 27.5% of postoperatively deaf patients, whereas stepwise wave deterioration and losses occurred in 72.5%.


CONCLUSIONS
Useful (in-time) recognition of significant waveform changes is possible and enables a change of microsurgical maneuvers to favor ABR recovery.
Matthies
C
C
Department of Neurosurgery, Nordstadt Hospital, Hannover, Germany.
Samii
M
M
D001301:Audiometry, Pure-Tone; D001306:Auditory Pathways / Q000503:physiopathology; D001933:Brain Stem / Q000503:physiopathology; D003056:Cochlear Nerve / Q000503:physiopathology; D017626:Cochlear Nucleus / Q000503:physiopathology; D003638:Deafness / Q000175:diagnosis / Q000503:physiopathology*; D016057:Evoked Potentials, Auditory, Brain Stem / Q000502:physiology*; D006801:Humans; D007245:Inferior Colliculi / Q000503:physiopathology; D007431:Intraoperative Complications / Q000175:diagnosis / Q000503:physiopathology*; D008866:Microsurgery; D016343:Monitoring, Intraoperative; D009464:Neuroma, Acoustic / Q000503:physiopathology / Q000601:surgery*; D011183:Postoperative Complications / Q000175:diagnosis / Q000503:physiopathology; D011930:Reaction Time / Q000502:physiology; D012307:Risk Factors; D013063:Speech Discrimination Tests; D009435:Synaptic Transmission / Q000502:physiology