gms | German Medical Science

Although monitoring facial nerve function intraoperatively has already been effectively developed, such efficacious monitoring of auditory function remains to be established. Electrophysiological recordings using electrocochleography, auditory brainstem response (ABR), and compound action potential of the auditory nerve and/or evoked potentials from cochlear nucleus provide considerable intraoperative information about the status of the auditory system, though any hearing damage can only be detected after some delay. This delay in recognizing a surgically induced trauma during the course of an operation can lead to the irreversible damage of the hearing pathway including cochlear function.

One audiological method that permits recording cochlear activity directly involves the measurement of distortion product otoacoustic emissions (DPOAEs), defined as an acoustic energy recorded from the ear canal and generated in the cochlea in response to acoustic stimulation. It is thought that DPOAEs reflect the function of the cochlea that is associated with electromotility of the outer hair cells (OHCs). It has long been known that OHCs are extremely sensitive to such harmful factors as excessive noise, ototoxic drugs, and hypoxic or anoxic conditions. The aim of this study was to investigate the utility of DPOAEs for intraoperative monitoring of the cochlear function in humans during removal of cerebello-pontine angle tumors (CPATs).

Continuous intraoperative monitoring of DPOAEs was performed in patients undergoing surgical removal of CPATs. All patients, who underwent the retrosigmoid approach, had DPOAEs present pre-operatively. In all of them the following test results were obtained before surgery: computed tomography, magnetic resonance imaging, tonal and impedance audiometry, ABR, and DPOAEs. The results of tonal audiometry were expressed as pure-tone average (PTA) calculated for 0.5, 1.0 and 2.0 kHz. Pre- and post-operative tonal audiometry, as well as other measures, were carried out 7 to 10 days before surgery and in most cases 2 to 3 weeks after surgery. In 25 patients, DPOAEs recorded before surgery were present on a level making possible intraoperative monitoring by DPOAEs. DPOAE amplitudes in these qualified cases were higher than 10 dB SPL for f2 ranging from 2.0 kHz to 3.0 kHz, and equal or higher than 5 dB SPL for f2 higher than 3 kHz. On the basis histopathological tests among successfully monitored patients the following diagnoses were made: in 2 cases dermoid tumor, in 1 case meningioma, and in remaining cases schwannoma. All patients were operated on by retro-sigmoidal approach. DPOAE measures were made using ILO 92 Otodynamics System (London, Great Britain). Usually L1 & L2 ranged between 55 and 70 dB SPL, and f2/f1 ratio was equal to 1.22–1.24. DPOAE amplitudes were computed based on the average of 16 (high frequencies) or 32 (low frequencies) emission samples. For all monitored cases by DPOAE in the operating room cochlear activity was recorded for f2 ranging from 1.0 kHz to 6.0 kHz. In most cases an appropriate intraoperative monitoring paradigm was identified in the 3.2–6.4 kHz frequency band (as recorded for f2) measured from 16 points or sporadically in the 2.0–4.0 kHz frequency band.

All intraoperative measurements were performed using a probe modified by Otodynamics Ltd to monitor DPOAEs intraoperatively. This probe made possible otoacoustic emission measures and delivery of acoustic signals from independent external speakers, i.e., for simultaneous measures of ABR.

The results of this study showed that monitoring auditory function during CPAT surgery using DPOAEs was possible in the inherently noisy environment of the operating room. Data points could be renewed every 0.5–2 sec. As expected from previous reports (9), one of the factors that significantly limited utility of DPOAEs as an effective tool for intra-operative monitoring of the cochlear function was found to be operating-room noise during surgery reaching up to 15–20 dB SPL for the frequency band below 2.0 kHz. The simultaneous measured noise for the higher frequency band was stabilized usually below 0 dB SPL. Results of this study showed that relative contribution of the noise to the frequency spectrum of the response below 2 kHz obscured DPOAEs in this frequency band usually making intraoperative monitoring impossible. Therefore picking a higher frequency, more robust DPOAE from pre-operative evaluations allowed rapid and repeatable measurements.

Maintenance of pre-operative hearing levels in these patients was related to intra-operative preservation of DPOAEs. In patients operated for CPATs, various patterns of DPOAE amplitude reductions and recoveries were observed. DPOAEs recorded from basal part of the cochlea (i.e., high frequencies) changed earlier and more profoundly than those from the middle and apical sections (i.e., lower frequencies). In some cases cochlear function was affected irreversibly as reflected by loss of DPOAEs. Bipolar cautery of small vessels, tumor debulking, and compression or stretch of the internal auditory canal contents were found to be the most dangerous procedures relatively often affecting DPOAEs. The status of DPOAEs at the end of tumor dissection correlated with post-operative hearing levels.

Monitoring of cochlea function using the Spectral-History Option for measuring DPOAE at f2=3.2 kHz is shown in Figure 1 A and B [Fig. 1]. Using this option, it could be seen that DPOAE levels decreased rapidly to the background noise levels during various manipulations within the CPA. As demonstrated in our previous papers, compression and stretching of the eighth nerve complex during tumor debulking, micro-bleeding and applying bipolar cautery, and drilling of the posterior wall of the internal auditory canal (IAC) were particularly important surgical maneuvers for inducing both reversible and irreversible DPOAE amplitude level reductions. Figure 1A [Fig. 1] demonstrates example of reversible DPOAEs reduction recorded during tumor removal while Figure 1B [Fig. 1] is an example of a series of DPOAE fluctuation with the final, irreversible DPOAE decrease observed following microbleeding and for this reason used bipolar cautery. DPOAE decrease correlated perfectly with dangerous manipulation of the surgeon in the CPA region.

• The effect of partial deficit in the cochlear blood supply on DPOAE was different for various frequencies; high frequency DPOAEs were found to change faster than middle and low frequency DPOAEs.
• Stretching the eighth nerve structure, drilling the IAC posterior wall, and application of bipolar cautery near the internal auditory canal contents were found to be the most dangerous moment affecting DPOAEs during tumor removal procedure.
• DPOAEs can be used to monitor in the real time domain auditory function during CPAT removal surgeries and the final status of DPOAEs at the end of the operations was related to the post-operative hearing.