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Hearing Thresholds of OAEs
Figure 2. Influence of negative middle ear pressure on evoked oto- acoustic emissions (OAEs) measured in a 42-year-old male patient complaining of a hearing impairment. A: Conventional audiogram showing reasonably normal-hearing thresholds in response to pure tones for the left ear, especially for frequencies <4 kHz (yellow cir- cles). Gray area, extent of normal-hearing limits for adults. ANSI 69, calibration standards established for testing hearing level in deci- bels by the American National Standards Institute in 1969 for clini- cal audiometers. B: Distortion-product OAE (DPOAE) levels as a function of f2 frequencies (DP-gram) ranging from 1 to 8 kHz at 6 frequency points per octave elicited by moderate f1 and f2 primary tones (yellow circles). Note the increased levels of the measurement system’s noise floor (green curve) for f2 frequencies <2 kHz that in- terfere with measuring low-frequency DPOAEs. Also present are re- duced DPOAE levels for frequencies >3 kHz that likely reflect aging factors. The variability in emission levels (±1 standard deviation) in normal-hearing ears is represented by the top 2 dashed lines. A sim- ilar variability of the related noise floor is indicated by the bottom 2 dashed lines. These variabilities are also indicated for the DP-gram plot in Figure 3B. SPL, sound pressure level. C: A diagnostic tym- panogram (TYPM) indicating the presence of a negative pressure of about −110 decapascal (daPa) for the left ear.
impaired sound conduction. The adverse effects of middle ear dysfunction on the level and frequency attributes of OAEs has been demonstrated in patient groups with various disorders of the conduction apparatus such as perforations or anomalies of the tympanic membrane, ossicular chain dislocations, and middle ear diseases such as chronic otitis media (Owens et al., 1992). Middle ear disease in the form of negative intratympanic pressure significantly reduces the levels of both TEOAEs and DPOAEs, particularly for the low- to midfrequencies. It is clear from Figure 2B that elevated noise floors (green curve) for frequencies <2 kHz interfere with measuring the lower frequency DPOAEs in ears with negative middle ear pressure as indicated in Figure 2C. Although in this case involving a 42-year-old male the audiometric hearing thresholds in Figure 2A were normal over the 1.5- to 3.5-kHz range, the related plot in Figure 2B showing DPOAEs as a function of the f2 test stimuli (DP- gram) were approaching the frequency-distribution area representing abnormally low-level emissions as indicated by the −1 SD boundary.
From studies of the adverse influence of middle ear anomalies on OAE measures, it is well established that if emissions are to be used in clinical settings to assess cochlear status, normal middle ear function is essential. Thus, to distinguish between the effects of middle ear pathology and cochlear abnormali- ties on emitted responses, it is necessary to evaluate middle ear function at the time of OAE testing (Kemp et al., 1986).
To the Clinic
As the initial basic studies on emitted responses were on- going, the significant benefits of OAEs as a clinical test be- gan to be appreciated. It became clear that OAEs offer the practitioner several beneficial features as objective measures of the ear’s ability to process acoustic stimuli. Over the past 35 years or so, a great number of studies have demonstrated that evoked OAEs are useful in contributing to the differen- tial diagnosis of a sensorineural hearing loss; screening of cochlear function in infants and other difficult-to-test pa- tients; monitoring OHC healthiness in patients exposed to either ototoxic drugs or excessive sounds or suffering from certain progressive hearing ailments; evaluating the status of the descending cochlear efferent system; and identifying functional or feigned hearing loss.
An example of this latter application is illustrated in Figure 3 for a 42-year-old male factory worker who operated a wood lathe for about 20 years. This individual claimed that he had poor low-frequency hearing along with no hearing above
46 | Acoustics Today | Fall 2017