Predicting vulnerability to acoustic injury with a noninvasive assay of olivocochlear reflex strength

Abstract

Permanent noise-induced damage to the inner ear is a major cause of hearing impairment, arising from exposures occurring during both work- and pleasure-related activities. Vulnerability to noise-induced hearing loss is highly variable: some have tough, whereas others have tender ears. This report documents, in an animal model, the efficacy of a simple nontraumatic assay of normal ear function in predicting vulnerability to acoustic injury. The assay measures the strength of a sound-evoked neuronal feedback pathway to the inner ear, the olivocochlear efferents, by examining otoacoustic emissions created by the normal ear, which can be measured with a microphone in the external ear. Reflex strength was inversely correlated with the degree of hearing loss after subsequent noise exposure. These data suggest that one function of the olivocochlear efferent system is to protect the ear from acoustic injury. This assay, or a simple modification of it, could be applied to human populations to screen for individuals most at risk in noisy environments.

Fig. 1.

Schematic illustration of a cross-section through the sensory epithelium of the inner ear showing one row of inner hair cells (IHCs), three rows of outer hair cells (OHCs), a single auditory nerve afferent contacting an inner hair cell, a representative efferent fiber from the medial olivocochlear (MOC) system, contacting all three rows of outer hair cells, and an efferent fiber from the lateral olivocochlear (LOC) system contacting the peripheral terminal of an auditory nerve fiber. Bold arrows indicate direction of action potential propagation along the neurons.

Fig. 2.

The noninvasive measure of MOC reflex strength is based on the degree of post-onset adaptation of the DPOAE for primary tones f₁ and f₂at 10 and 8.3 kHz, respectively. A–C show data from an animal with a strong reflex; D and E from an animal with a weak reflex. At each test session, post-onset adaptation was measured at each of 176 level combinations off₁ and f₂ (see Materials and Methods). A, B, andD each illustrate raw data, i.e., DPOAE amplitude versus post-onset-time for a single level combination (as indicated in each panel). C and E show the magnitude and sign of the adaptation for all 16 f₂ levels tested with f₁ = 80 dB, including those extracted from A, B, andD, as indicated by arrows. During one complete test session, data such as those in C orE would be obtained at each of 11f₁ levels from 75 to 85 dB SPL (inclusive).

Fig. 3.

Repeatability of the DPOAE-based test of MOC reflex strength over two test sessions separated by a week. As shown, the range of test results can be used to arbitrarily divide these 36 experimental animals into those with “weak”, “intermediate”, or “strong” MOC reflex. Reflex strength was tested in only one ear of each animal. Data from all 176 level combination in one test session are combined into a single metric as described in Materials and Methods.

Fig. 4.

Variability in PTSs in 12 guinea pigs identically exposed to the 2–4 kHz noise band at 109 dB for 4 hr. PTS is computed by subtracting the average CAP thresholds in seven control (unexposed animals) from the CAP thresholds in each of the 12 animals in this group. Threshold shift curves for two of the 12 animals are highlighted: one particularly vulnerable is shown by the open symbols, and one particularly resistant is shown by thefilled symbols. All others are shown ingray.

Fig. 5.

Mean values of noise-induced permanent threshold shift in three sets of animals, when grouped according to the pre-exposure strength of their MOC reflex: animals with the strongest reflexes suffer the least threshold shift. The three panels show results from different sets of animals exposed to different noise bands: 12 animals exposed at 2–4 kHz (A), 12 animals exposed at 4–8 kHz (B), and 12 animals exposed at 8–16 kHz (C). Error bars indicate SEM. CAP data were obtained from both ears of each experimental animal.

Fig. 6.

Correlation between MOC reflex strength and noise-induced PTS is strongest at test frequencies near the peak PTS.A shows the derivation of a correlation coefficient at one CAP test frequency (4.02 kHz) for one group of animals (group A).B shows the correlation coefficient at each test frequency for each of the three exposure groups. Arrowsindicate the test frequency showing peak PTS for each group (from Fig.5).