gms | German Medical Science

Surgery of the cerebello-pontine angle tumor (CPAT) is associated with a high risk of damaging such important structures as facial nerve, vestibulocochlear nerve, internal auditory artery, anterior inferior cerebellar artery, and inner ear function, if they escaped earlier damage by the invading tumor. Although the attempts to monitor intraoperatively function of the above structures has been still tested, an efficacious monitoring of the inner ear and the eight nerve function remains to be established. Distortion product otoacoustic emissions (DPOAE) or electrophysiological recordings using either electrocochleography (EcoG) involving compound action potentials of the VIII nerve (CAP) and cochlear microphonics (CM), or auditory brainstem responses (ABR), as well as evoked potentials from the cochlear nucleus provide considerable information about the status of the early stages of the auditory system. On the basis of the above tests, any surgically induced trauma whether directly damaging inner ear structures or indirectly harming neurepithelial or neural parts of the inner ear through the compromise of the blood vessels supplying the cochlea, 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 cochlear function and permanent deafness for the ear on the operated side. Audiological tests that permits recording cochlear activity fast and more directly involves DPOAEs and ECoG.

To evaluate and compare the utility of DPOAE and cochlear evoked potentials for monitoring cochlear and eighth nerve functions during reversible cochlear ischemia.

Fifteen albino rabbits were used. The animals were anesthetized routinely with an intramuscular injection of ketamine hydrochloride (50mg/kg) mixed with xylazine hydrochloride (10mg/kg). A head stabilization device was placed according to a standard procedure previously described by Telischi et al [Ref. 1] and Morawski et al [Ref. 2]. The first surgical step was a post-auricular approach to expose the cochlea and round window (RW). Using an operating microscope, the middle ear was entered through the auditory bulla for placement of the laser-Doppler probe to measure CBF. The cerebello-pontine angle was exposed via a sub-occipital posterior craniotomy. With the use of the operating microscope, the porus of the IAC was distinguished. Then, CBF probe tip was positioned at RW niche to measure cochlear blood supply. DPOAE response functions were obtained for each ear using special probe at 4, 8 and 12 kHz GMF by measuring the DPOAE level as a function of the primary tones at 60 dB SPL. Auditory responses were obtained using tone bursts at 4, 8 and 12 kHz and 70 dB SPL intensity. Needle electrodes for EcoG recordings were fixed in the neck muscles (reference), in the back muscles (ground) and as a measuring electrode the otic probe placed into the RW niche was used (Yavuz et al [Ref. 3]). The IAA compression inducing cochlear ischemia was maintained for 3 min. DPOAE-CBF data obtained in the first subgroup of animals and CM/CAP-CBF obtained in the second subgroup were recorded from the onset of ischemia through 20 min after reperfusion.

In all ears following IAA compression CBF was reduced to a background level. The DPOAE phase increase for all frequencies occurred within a few seconds while the DPOAE amplitudes decreased with slightly longer delay. CM/CAP reduction was analogous to DPOAE pattern though in all cases 10–15 sec initial enhancement of CAP was recorded. After reperfusion, in both animal subgroups, DPOAE and CM recovered in the same way for both compressions stabilizing near the baseline. CM measured for 3 min compression for all frequencies was reduced 1–1.5 dB, while DPOAE was reduced only at 12 GMF kHz by 0.5–1 dB. Basically, DPOAE and CM reduction and recovery patterns are very similar while CAP reduction and recovery patterns for test frequencies are different. CM and DPOAE begin dropping in a couple of seconds, and then, during reperfusion begin recovering with relatively short time of delay ranging from 35–45 seconds at all test frequencies. CAP patterns occurring just after IAA compression reveal presence of initial elevation lasting 10–15 sec, and then, during next 20–30 seconds CAP disappears. Following IAA release and subsequent cochlear reperfusion CAP begins returning though differently at all test frequencies. At 4 kHz CAP reaches 50% of its BL value in 34–38 sec and 100% of its BL value in 42–48 sec after IAA release. At 8 kHz 50% and 100% of its BL value CAP reaches respectively in 49–54 sec and 91–100 sec, while at 12 kHz respectively in about 114–125 sec and 215–240 sec. Finally, CAP stabilizes above BL for 4 kHz, while at 8 and 12 kHz around BL values.

In the second part of the result section mutual relationships of DPOAE, CM, CAP and CBF values were analyzed. As presented in Table 1 [Tab. 1], during the first 60 sec of cochlear ischemia all parameters correlated each other on statistically significant levels (R ranged from +0.90 to +0.99; p<0.001; t(Pearson Coefficient) ranged from 10.46 to 32.91).

As presented in Table 1 [Tab. 1] during the first 60 sec of cochlear reperfusion all analyzed parameters also correlated each other on statistically significant levels (R ranged from +0.68 to +0.98; p<0.001; t(Pearson Coefficient) ranged from 4.99 to 26.76) although, in general, R values were lower. The weakest correlation was observed between CBF and 12 kHz DPOAE (R=0.68), CAP and CM at 4 kHz (R=0.70), and CAP and CM at 12 kHz (R=0.75). Such tendencies were results of strong non-linearity observed during 12-kHz DPOAE recovering. Also different patterns of CAP recovery were responsible for slightly weaker correlation between CAP and CM.

• DPOAE effectively mirrored cochlear ischemia confirming earlier data and revealing stronger susceptibility of the basal part of the cochlea to damaging effect of reversible local ischemia;
• CM showed very similar patterns of reduction and recovering during ischemia/reperfusion episode confirming its great usefulness to intraoperative monitoring of the cochlear function;
• CAP reduction and recovering during ischemia/reperfusion episode revealed slightly different characteristics than CM or DPOAE;
• Neural function changes observed during both ischemia onset and reperfusion were non-linear; due to CAP nonlinearity, mostly during reperfusion, its utility for monitoring local ischemic processes is poorer than CM and DPOAE;
• CM/CAP findings recalculated on-line from EcoG in real time domain showed a complex response reflecting the status of the cochlea and the VIII nerve what may be particularly important in the future for intraoperative monitoring in people operated for CPAT.