Objective: To investigate the value of intraoperative auditory monitoring techniques in acoustic neuroma surgery.
Methods: Ten cases with acoustic neuroma were resected with retrosigmoid approach. Continuous hearing monitoring of auditory brainstem responses (ABR) and transtympanic electrocochleography (ECochG) was performed during operation.
Results: Before surgery, 3 patients had class A hearing, 4 had class B hearing, and 3 had class C hearing. With ABR monitoring, 5 patients had waves I , III and V, 5 had only waves I preoperation. After anesthesia,only 2 cases had waves I, III and V, 6 had wave I (Compound action potential, CAP N1 is equivalent to wave I of ABR) and 2 had no waves. The hearing was preserved in 2 cases, which had class A hearing post operation with tumor size <2 cm. With continuous hearing monitoring, the waves of I , III and V could be evoked in one case. In another case, the waves of I, III could be evoked after the tumor resection with the disappearance of wave V. The hearing was not preserved in 8 cases. The 6 out of 8 cases showed up CAP (waves I ) waveform. The CAP amplitudes decreased significantly in 4 cases and even dropped to zero while dissecting the tumor at the lateral end of the internal auditory canal (IAC) or clamping the internal auditory artery (IAA) during operation. After surgery, the CAP amplitudes were recovered to 50%-60% of normal level or normal. In one case, although the cochlear nerve was cut down, the CAP could still be recorded after the tumor resection. However, the CAP amplitudes was dropped to zero while pressurized and pulled cochlea nerve of brainstem lateral and the wave disappeared post-operation in another cases. The waves had not been recorded in two cases after anesthesia. One of them showed low amplitude of CAP wave when the tumor partially removed. The others had no wave all the time.
Conclusions: In combination with ABR monitoring, ECochG proved to be a useful supplementary tool for hearing preservation in acoustic neurinoma surgery. Drilling of the IAC and tumor removal at the lateral end of the IAC were the most critical steps for achieving hearing preservation. The surgeon's experience are the most significant factors influencing the hearing outcome after removal of acoustic neuroma.