Continuing Education Activity

Otoacoustic emissions testing offers another modality of evaluation of the auditory system beyond conventional audiometry. It may be performed for patients with suspected hearing loss who cannot tolerate or complete conventional hearing tests. The testing does not require recording a patient's subjective response to sound. There are several different ways to measure otoacoustic emissions, each with advantages and disadvantages. This activity reviews the evaluation of hearing loss using otoacoustic emission testing and highlights the role of the interprofessional team in evaluating and treating this condition.

Objectives:

• Review the physiology of how otoacoustic emission is produced.
• Outline the different forms of otoacoustic emission testing.
• Explain the advantages and disadvantages of each type of otoacoustic emission testing.
• Illustrate the clinical significance of performing otoacoustic emission.

Introduction

Otoacoustic emissions (OAE) are sounds generated from the cochlea transmitted across the middle ear to the external ear canal, where they can be recorded.[1]  The production of an OAE is a marker for inner ear health and a simple way to screen for hearing loss.[2] There are two types of OAE: spontaneous OAE (SOAE), which occur continuously without external stimuli, and evoked OAE (EOAE), which requires an acoustic stimulus prior to its measurement. 

The production of sound from the cochlea is due to the cochlear amplifier and builds upon the traveling wave theory developed by Georg von Bekesy in the 1940s.[3] In the traveling wave theory, sound stimulates the stapes, which creates a propagating fluid wave into the cochlea and basilar membrane displacement from the base to the apex. The frequency of the stimulus would peak at different sites along the basilar membrane, with higher frequencies causing maximal vibration at the base of the cochlea, whereas lower frequencies would have maximal vibration at the apex of the cochlea.

In 1978, David Kemp recorded sounds from the external ear canal that were entirely cochlear in origin and contained more energy than the initial stimulus.[1] This led to the theory of the cochlear amplifier.[4] In the cochlear amplifier theory, as the traveling wave peaks at its frequency-specific point in the basilar membrane, the outer hair cells (OHC) produce a secondary disruption of the basilar membrane, amplifying the signal to the brain.[5] This also generates a byproduct lower amplitude wave that travels back along the membrane, through the middle ear, and emerges out the external ear canal as an OAE that can be recorded by a microphone in the ear canal. [6]

In humans, the cochlear amplifier allows us to develop sensitive detection and frequency resolution to distinguish the critical nuances of speech. The cochlear amplifier is lost in patients with sensorineural hearing loss, which manifests as poor speech discrimination.[7]

Function

Spontaneous otoacoustic emissions (SOAE) are sounds generated from the ear without an acoustic stimulus and can be measured with microphones placed in the external ear canal. Their frequencies are between 500 Hz to 4,500 Hz. A small percentage, 1 to 9%, of people can perceive their SOAE as tinnitus.[8]

Evoked otoacoustic emissions (EOAE) can be evoked utilizing three different acoustic stimuli: transient evoked, stimulus-frequency, and distortion product. Transient evoked and distortion product otoacoustic emissions are the most commonly used techniques for a newborn hearing screening.[2]

Transient-evoked OAE (TEOAE) are evoked using a click or tone-burst stimuli. A click stimulus has an abrupt onset, short duration, and covers a broad frequency range up to 4 kHz to evoke responses from multiple nerve fibers. [9]  This is in contrast to tone burst stimuli delivered at a narrower frequency range, especially at lower frequencies, to obtain more frequency-specific responses.[10] 

Since the response emission contains the same frequencies as the stimuli, distinguishing the response emission can be challenging. Multiple repeated stimuli are required and averaged to distinguish the response emission from the initial stimulus. Response emissions are recorded at 2 to 23-millisecond latencies corresponding to the frequency of the stimulus. Higher frequencies propagate a shorter distance along the basilar membrane to the base and need a shorter latency, whereas lower frequencies travel further towards the cochlear apex and require a longer latency.[11]

Stimulus-frequency OAE (SFOAE) is evoked by a single pure tone stimulus. However, the response emission occurs at the same frequency as the stimulus and is hard to distinguish from residual stimulus energy. Thus, there is limited clinical use for this technique. [7]

Distortion-product OAE (DPOAE) is evoked using two simultaneous pure tone stimuli (f1 and f2). Unlike TEOAE, which provides an overall view of cochlear function across a broad range of frequencies, DPOAE can be customized to assess frequencies that match the patient’s audiogram and are more sensitive for detecting high-frequency hearing loss.[12][13] 

Studies have shown a stimulus level of 55 to 65 dB SPL intensity, 10 dB SPL difference between the two tones, the frequency range between 2000 Hz and 8000 Hz, and a frequency ratio (f2/f1) of 1.2 provides the best accuracy in separating normal hearing patients from those with hearing loss.[14]

When measuring the DPOAE, the largest response emission should occur at the frequency calculated from the formula: 2f1-f2.[7] Thus, the advantage of DPOAE is that the response emission occurs at a frequency different from the two pure tone stimuli, which makes its measurement easily distinguishable.

Issues of Concern

Following Kemp's discovery of the cochlear amplifier and OAE, he discovered that OAE could be measured from patients with a normally functioning cochlea but not from those with hearing impairments with thresholds over 30 dB HL, illustrating the potential of using OAE as a hearing test. Thus, the presence of OAE is indicative of a functional cochlear amplifier, healthy OHCs, and normal hearing. OAE are also very sensitive in detecting mild hearing impairment. Damage to the outer hair cells from noise trauma or ototoxic medications can appear on OAE before presenting on an audiogram. When OAE are absent, there must be some dysfunction in the cochlea, although the degree of hearing loss is uncertain.

Hearing Screening

The ease of performing OAE is permitted now with automated units that include an aural probe that contains a speaker to deliver the acoustic stimulus and a microphone to detect the emissions. These systems provide an easily reproducible, non-invasive method to assess hearing in newborns, young children, and adults who are unable to cooperate for conventional hearing tests. The all-or-nothing response from OAE makes OAE an excellent screening test for hearing loss. The universal newborn hearing screening program (UNHS) has widely been adopted throughout North America, Europe, and most developed countries. UNHS incorporates a combination of OAE and auditory brainstem response (ABR) testing.[15]

Ototoxicity Monitoring

Over the last decade, OAE has been increasingly used to monitor the ototoxicity of medications, particularly aminoglycoside antibiotics and platinum-based chemotherapy agents. Ototoxic drugs affect the outer hair cells and are detectable on OAE before a conventional audiogram. Its quick application and cost efficiency make it a good clinical choice to follow patients during their therapeutic course. DPOAEs are more sensitive to the higher frequencies, which are commonly affected first in ototoxicity. Prior to starting ototoxic medications, patients should undergo baseline OAE testing and again with each dose of ototoxic medication. Changes of 2.4 dB or more are considered a significant decrease and indicate a change in cochlear function.[16]

Auditory Neuropathy Spectrum Disorder (ANSD)

OAE is often used to evaluate retrocochlear pathology within the central auditory system. Since OAE is a measurement of the peripheral auditory system, retrocochlear pathology will present with normal OAE and abnormal audiogram and auditory brainstem response (ABR).[20] Auditory neuropathy spectrum disorder (ANSD) is one of the most common retrocochlear pathologies evaluated by OAE. It often presents with absent or severely abnormal ABR, poor word recognition, and absent stapedial reflexes. However, OAE may be absent if retrocochlear masses impinge on the internal auditory artery and compromise the blood flow to the cochlea. OAE may also disappear over time in patients with ANSD.[17]

Meniere Disease

Meniere disease can elicit a unique OAE clinical presentation. In patients with Meniere disease who have hearing loss thresholds exceeding 30 dB HL, OAE are expected to be absent. However, several studies show intact OAE in these patients. It is hypothesized that the outer hair cells were spared, and the poor hearing thresholds are due to inner hair cell dysfunction or a disruption in the afferent connections between the inner and outer hair cells. Another hypothesis is that this may reflect the various pathophysiological stages of Meniere disease, where the disease has not yet reached the outer hair cells. Furthermore, Van Hufflen et al. 1998 showed that OAE in the contralateral normal hearing ear of patients has a lower OAE amplitude, which may be a harbinger of future bilateral Meniere disease.[18]

Tinnitus

Tinnitus is an abnormal perception of sound in the absence of external stimuli associated with disorders of the auditory system. Studies show that DPOAE amplitudes are consistently reduced in patients with tinnitus than patients without tinnitus, even in those with normal hearing on audiograms. The abnormal OAE in tinnitus suggests that cochlear and outer hair cell dysfunction may play a role in its generation, particularly at higher frequencies from 6 to 8 kHz. The connection between OAE and tinnitus may also play a role in monitoring progress during tinnitus retraining therapy. However, more research is necessary on this topic.[19]

Clinical Significance

The limitations of OAE screening are the lack of specificity. There is a risk of false positives due to contamination from other sounds, either from the test environment or internal sounds such as breathing and swallowing.[2] During analysis, it may also be challenging to distinguish OAE from background noise. Thus, most OAE requires analysis of the reproducible data and the signal-to-noise ratio of the OAE waveform. Since OAE travel through the middle ear, they can also be affected by any middle ear disease, such as middle ear effusion. OAE may not be measurable in children with adhesive otitis even though the OHCs are healthy.[6] Thus, OAE cannot distinguish between conductive hearing loss and sensorineural loss.

Enhancing Healthcare Team Outcomes

Patients diagnosed with hearing loss should be managed by a multidisciplinary team of otolaryngologists, audiologists, speech-language pathologists, pediatricians, and primary care physicians. Children born in the United States must undergo universal hearing screening using a combination of OAE and ABR. Children screening positive for hearing loss should be immediately referred for a formal hearing evaluation, genetic testing, and hearing augmentation with hearing aids, cochlear implants, and/or speech rehabilitation to promote long-term speech and language outcomes.

The pediatrician should perform long-term monitoring of childhood development with regular follow-up with audiology for routine hearing aid and audiogram assessments. Formal hearing loss support groups can aid children and parents in addressing their concerns. The school system should also provide an optimal classroom learning environment for children with hearing impairment.