Audiogram fine structure and spontaneous otoacoustic emissions in patients with Menière's disease

Spontaneous otoacoustic emissions, threshold microstructure, and psychophysical tuning over a wide frequency range in humans

Hearing thresholds have been shown to exhibit periodic minima and maxima, a pattern known as threshold microstructure. Microstructure has previously been linked to spontaneous otoacoustic emissions (SOAEs) and normal cochlear function. However, SOAEs at high frequencies (>4 kHz) have been associated with hearing loss or cochlear pathology in some reports. Microstructure would not be expected near these high-frequency SOAEs. Psychophysical tuning curves (PTCs), the expression of frequency selectivity, may also be altered by SOAEs. Prior comparisons of tuning between ears with and without SOAEs demonstrated sharper tuning in ears with emissions. Here, threshold microstructure and PTCs were compared at SOAE frequencies ranging between 1.2 and 13.9 kHz using subjects without SOAEs as controls. Results indicate: (1) Threshold microstructure is observable in the vicinity of SOAEs of all frequencies; (2) PTCs are influenced by SOAEs, resulting in shifted tuning curve tips, multiple tips, or inversion. High frequency SOAEs show a greater effect on PTC morphology. The influence of most SOAEs at high frequencies on threshold microstructure and PTCs is consistent with those at lower frequencies, suggesting that high-frequency SOAEs reflect the same cochlear processes that lead to SOAEs at lower frequencies.

Automatic screening and detection of threshold fine structure

Audiograms measured with a high frequency resolution often show quasi-periodic ripples of up to 15 dB in normal-hearing listeners. This fine structure of the threshold in quiet is commonly associated with the active processes in the cochlea. Therefore its absence is discussed in the literature as an indicator of cochlear vulnerability. In order to enable a quick detection and an objective quantification of threshold fine structure, two instruments are introduced and evaluated in this article: (1) a high-resolution tracking method for measuring fine structure ('FINESS'), and (2) an automatic fine-structure detector ('FINESS-detector'). The method is tested on 22 subjects for its reliability, its accuracy, and drifts with frequency by analysing test/retest experiments and by comparing the measured thresholds to results from a reference procedure. The results indicate that FINESS and the FINESS-detector are suitable techniques for the measurement and detection of threshold fine structure that may help to investigate further into whether fine structure is a sensitive tool for the detection of an early hearing loss.

Fine structure of hearing threshold and loudness perception

Hearing thresholds measured with high-frequency resolution show a quasiperiodic change in level called threshold fine structure (or microstructure). The effect of this fine structure on loudness perception over a range of stimulus levels was investigated in 12 subjects. Three different approaches were used. Individual hearing thresholds and equal loudness contours were measured in eight subjects using loudness-matching paradigms. In addition, the loudness growth of sinusoids was observed at frequencies associated with individual minima or maxima in the hearing threshold from five subjects using a loudness-matching paradigm. At low levels, loudness growth depended on the position of the test- or reference-tone frequency within the threshold fine structure. The slope of loudness growth differs by 0.2 dB/dB when an identical test tone is compared with two different reference tones, i.e., a difference in loudness growth of 2 dB per 10-dB change in stimulus. Finally, loudness growth was measured for the same five subjects using categorical loudness scaling as a direct-scaling technique with no reference tone instead of the loudness-matching procedures. Overall, an influence of hearing-threshold fine structure on loudness perception of sinusoids was observable for stimulus levels up to 40 dB SPL--independent of the procedure used. Possible implications of fine structure for loudness measurements and other psychoacoustic experiments, such as different compression within threshold minima and maxima, are discussed.

Quantification of audiogram fine-structure as a function of hearing threshold

We investigated the amount of fine-structure in the audiograms of patients with Menière's disease. Both ears (often an affected and an unaffected ear) were investigated. Data are presented from 39 ears of 21 patients with hearing thresholds varying from about 0 to 70 dB HL. The fine-structure was measured in the frequency range from 500 to 3500 Hz; this agrees with a part of the basilar membrane from 10 to 22.5 mm from the apex. The fine-structure was characterized by means of the number of peaks N(p) and the sum S(p) of the heights of all the peaks in an audiogram. From these quantities, we also determined average peak height H(p). We found a negative correlation between hearing loss and strength of fine-structure, i.e. the higher the thresholds the smaller N(p) and S(p) as well as H(p). Also N(p) and H(p) were correlated, i.e. the more peaks the higher the average peak height. The summated peak height S(p)=N(p)H(p) showed a strong dependence on the hearing loss. In cases of strong fine-structure, S(p) reached values around 200 dB.

Evoked otoacoustic emissions in patients with Ménière's disease

HYPOTHESIS

This study investigated whether otoacoustic emissions (OAEs) in patients with Ménière's disease show abnormal properties.

BACKGROUND

Patients with Ménière's disease experience vertigo, tinnitus, and hearing loss. OAEs are sounds generated in the inner ear, and their presence is associated with normal hearing.

METHODS

Click-evoked OAEs and distortion product OAEs were measured in 100 patients with Ménière's disease.

RESULTS

The incidence of the emissions in affected ears (56%) was lower than in unaffected (i.e., contralateral) ears (85%). The mean emission amplitude in affected ears was also significantly lower (2.6 dB), and the mean amplitude in unaffected ears was lower than in normal-hearing ears (5.3 dB). These differences were likely caused by the hearing loss involved. Further, ears with OAEs clearly showed smaller hearing losses than ears without OAEs (24-dB difference). The average hearing loss showed correlations with the emission amplitudes, although this correlation was not very strong; when plotted against the smallest hearing loss, a certain upper boundary for the emission amplitude was present. Also, the amplitude of click-evoked OAEs showed a considerable correlation with the largest of the three distortion product OAEs.

CONCLUSION

OAEs in patients with Ménière's disease differed from those in normal-hearing ears but did not differ from those in non-Ménière's ears with equivalent hearing loss. This was best observed by comparing emission amplitude with smallest hearing loss instead of mean hearing loss.