Brainstem Auditory Evoked Potentials Recorded from Nasopharyngeal Wall

Abstract

Scientific Background: Reliable demonstration of the first wave of brainstem auditory evoked potentials (BAEPs) is required for accurate calculation of the central auditory conduction latencies. Although several methods, such as increasing the stimulus intensity, decreasing the stimulus rate, performing a electrocochleography or using ear canal electrode, are available to enhance its resolution, still clear registration of wave I cannot be achieved in a substantial proportion of the patients (up to 25%). In comparison to ear lobe or mastoid areas, placement of the recording electrode to the posterior wall of the nasopharynx allows becoming anatomically closer to the presumed brainstem generators of the BAEP waveforms, especially to the generators of the wave ll (proximal postion of the vestibulocochlear nerve and ipsilateral anteroventral and posteroventral cochlear nuclei). We hypothesized that in case of resolution of wave ll improves with nasopharyngeal recordings, ll-lll inter-peak latency (IPL) can be substituted to measure conduction in the lower pontine acoustic pathways in lieu of l-lll IPL.
Objectives: The main purpose of the study is to elp!ore improvement of the resolution of the first and second waves of BAEP recordings from the posterior wall of the nasopharynx given anatomical proximity to their generators which may provide higher signal-to-noise ratio in comparison to BAEP recordings with mastoid electrodes. Material and Methods: We studied nasopharyngeal and standard (with mastoid electrode) BAEPs in 30 young healthy volunteers. With monaural and binaural stimulation, BAEPs were recorded at 5 different montages: Ipsilateral mastoid (Mi) ta Cz (MiC; first channel), contralateral mastoid (Mc) ta Cz (McC: second channel), ipsilateral nasopharynx (Pi) to contralateral nasopharynx (Pc) (PiPc: third channel), ipsilateral nasopharynx to ipsilateral mastoid (PiMi: fourth channel) and ipsilateral nasopharynx to Cz (PiC: fifth channel). In addition, BAEPs on PcCz, PcMc ve PiMc positions were calculated. In addition ta analysıs of absolute peak and inter-peak latencies as well as amplitudes of BAEP waveforms, the recognizabi!ity of BAEP components and repeatability of latency and amplitude indices were compared among the montages described. Result: With constant reference at Cz, placing of the recording electrode to ipsilatera! nasopharynx instead of mastoid area resulted in significantly higher amplitudes and better resolution of all of the individua! BAEP waveforms. In the montages of ipsilateral nasopharnyx referred to contralateral nasopharynx and ipsilateral nasopharynx referred to ipsilateral mastoid, only the first three waves were recorded in higher amplitude and better resolution. The amplitude augmentation by nasopharynx electrodes was observed with both monoaural and binaural stimu!ation in almost similar degree.
Conclusions: Our results suggest that nasopharyngeal recording techniques improves the detection of the first and second BAEP waves and can be useful in the subjects with undetectable first wave peak with conventional methods.