Otoacoustic Emissions Objective tests of Hearing

Introduction zKemp (1979) was the first to discover that cochlea is not a passive receptor of acoustic sounds, but also produces sounds that can be recorded at ear canal. yOtoacoustic emissions (OAEs) zThese OAEs that were produced without any external stimulation were known as spontaneous otoacoustic emissions (SOAE). zOtoacoustic emission is an objective test that assesses the integrity and function of outer hair cells in the inner ear.

Introduction zWhen a very sensitive microphone is placed in the ear canal, sounds can be detected that are caused by traveling waves in the basilar membrane of the inner ear. zThe traveling waves are conducted in ‘reverse’ through the ossicles and, in turn, vibrate the eardrum. zOtoacoustic emissions are believed to be the result of active processes within the cochlea which cause outer hair cells motility or ‘ability to move’.

Spontaneous OAEs

Introduction zSOAEs is seen in about 50% of the population with normal hearing. Hence they are not a reliable clinical tool. zSOAEs are typically seen in the frequency range of Hz and is about +/- 10 dB SPL in intensity.

Otoacoustic Emissions zAnother class of OAEs that are seen either during or immediately following acoustic stimulation are known as evoked otoacoustic emissions. zTwo major evoked otoacoustic emissions are the transient evoked OAEs (TEOAEs) and the Distortion Product OAEs (DPOAEs).

Clinical Applications zAs hearing loss due to a conductive or cochlear pathology increases, the amplitude of the evoked OAE responses decreases until about 40 dB loss, wherein the response disappears. zWhen a normal appearing evoked OAE response is observed, it can be inferred that the auditory pathway up to the cochlea is unimpaired. zThe entire test time takes about 10 minutes.

TEOAEs zImportant – Evoked OAEs do not provide information about structures beyond the cochlea (ie, auditory nerve and other higher auditory centers). zHence cannot be employed to diagnose higher order auditory pathologies. zHowever can be used to differentiate a cochlear pathology from a neural pathology.

Clinical applications zGood hearing screening tool. zObjective test that can be used to assess the hearing status of the infants/neonates and difficult to test populations. zIs a rapid test and unlike the ABR, does not require the placement of any electrodes.

Clinical applications zI dentify functional or non-organic hearing loss. zMonitoring for medications that are toxic to the ear (ototoxic drugs) since it can detect cochlear dysfunction before actual hearing loss is present. zSimilarly can provide early warning signs of cochlear dysfunction due to noise exposure before any hearing loss is evident

Objective tests of Hearing Auditory Evoked Potentials

Introduction zFrom our review of auditory physiology, we understand that acoustic energy is transformed into electrical energy at the level of the auditory hair cells, for them to be conducted to the brain by the nervous system. zSo it is logical to assume that when a person is hearing a sound, there is some kind of change of electrical activity in the brain stem and brain.

Evoked Potentials zMeasuring electrical activity from the brain using surface electrodes that are attached to the scalp. zIf an auditory stimulus is employed to evoke the electrical activity - Auditory evoked potentials. zUse the terms latency, amplitude and morphology to describe our data.

Auditory Evoked Potentials (AEPs) zWhen a brief acoustic stimulus (e.g., a click or short tone burst) is presented to the ear there is a synchronized burst of electrical activity (in the microvolt range) generated in the auditory nerve which spreads up the central auditory pathway. yThe averaged recorded response has a number of peaks and troughs that spread out several milliseconds after the onset of the stimuli.

Auditory Evoked Potentials (AEPs) zThere are many AEPs identified depending upon the latency or time they occur after the onset of the stimuli. zThe most common AEP employed for clinical purposes is the Auditory Brainstem Response or ABR.

Auditory Brainstem Response zThe time period most commonly studied covers the first 10 msec after the stimulus is presented to the ear and represents the electrical activity evoked in neurons in the auditory nerve and brain stem. yThe auditory brainstem response (ABR). yWaveform consists of V or VII components/peaks. yWe usually focus on peaks I, III, and V.

Auditory Brainstem Response zThis technique is very useful in studying hearing loss of central auditory origin, as may be caused by a lesion affecting the brainstem (e.g., acoustic neuroma or multiple sclerosis). zIt is also helpful in documenting the hearing loss in infants who cannot cooperate with a behavioral-based audiometric exam.

Other advantages zABRs are also useful for ydifferentiating conductive from sensory hearing loss, yFor suspected acoustic neuroma not readily diagnosed by conventional audiometric means. yTo assess recovery of brainstem function after a lesion compressing the brainstem has been surgically removed.

ABR applications zABRs are also useful for yTo diagnosis and monitor demyelinating and degenerative diseases affecting the brain stem (e.g., multiple sclerosis) yTo evaluate infants and children. yHearing assessment for compensation and medico-legal cases. yIntraoperative monitoring.

Normal Auditory Brainstem Response

What do the components represent? zWave I: thought to be generated at the level of the VIIIth cranial nerve. zWave III: thought to be generated at the level of the pons part of the brainstem. zWave V: thought to be generated at the level of the midbrain - is the most robust wave component.

Patient State zWith the ABR, the client can be awake or sleeping, it doesn’t affect the testing. yThis makes it a valuable tool for assessing infants and other difficult-to-test populations. zMuscle movement must be minimal, since it will create too much artifact.