Otoacoustic Emissions

Objective tests of Hearing

Introduction Kemp (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. Otoacoustic emissions (OAEs)

These OAEs that were produced without any external stimulation were known as spontaneous otoacoustic emissions (SOAE).

Otoacoustic emission is an objective test that assesses the integrity and function of outer hair cells in the inner ear.

Introduction

When 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.

The traveling waves are conducted in ‘reverse’ through the ossicles and, in turn, vibrate the eardrum.

Otoacoustic 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

SOAEs is seen in about 50% of the population with normal hearing. Hence they are not a reliable clinical tool.

SOAEs are typically seen in the frequency range of 1000 - 3000 Hz and is about +/- 10 dB SPL in intensity.

Otoacoustic Emissions

Another class of OAEs that are seen either during or immediately following acoustic stimulation are known as evoked otoacoustic emissions.

Two major evoked otoacoustic emissions are the transient evoked OAEs (TEOAEs) and the Distortion Product OAEs (DPOAEs).

Clinical Applications

As 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.

When a normal appearing evoked OAE response is observed, it can be inferred that the auditory pathway up to the cochlea is unimpaired.

The entire test time takes about 10 minutes.

TEOAEs

Important – Evoked OAEs do not provide information about structures beyond the cochlea (ie, auditory nerve and other higher auditory centers).

Hence cannot be employed to diagnose higher order auditory pathologies.

However can be used to differentiate a cochlear pathology from a neural pathology.

Clinical applications

Good hearing screening tool.

Objective test that can be used to assess the hearing status of the infants/neonates and difficult to test populations.

Is a rapid test and unlike the ABR, does not require the placement of any electrodes.

Clinical applications

Identify functional or non-organic hearing loss.Monitoring for medications that are toxic to the

ear (ototoxic drugs) since it can detect cochlear dysfunction before actual hearing loss is present.

Similarly 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

From 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.

So 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

Measuring electrical activity from the brain using surface electrodes that are attached to the scalp.

If an auditory stimulus is employed to evoke the electrical activity - Auditory evoked potentials.

Use the terms latency, amplitude and morphology to describe our data.

Auditory Evoked Potentials (AEPs)

When 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. The 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)

There are many AEPs identified depending upon the latency or time they occur after the onset of the stimuli.

The most common AEP employed for clinical purposes is the Auditory Brainstem Response or ABR.

Auditory Brainstem Response

The 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. The auditory brainstem response (ABR). Waveform consists of V or VII

components/peaks. We usually focus on peaks I, III, and V.

Auditory Brainstem Response

This 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).

It is also helpful in documenting the hearing loss in infants who cannot cooperate with a behavioral-based audiometric exam.

Other advantages

ABRs are also useful for differentiating conductive from sensory

hearing loss, For suspected acoustic neuroma not

readily diagnosed by conventional audiometric means.

To assess recovery of brainstem function after a lesion compressing the brainstem has been surgically removed.

ABR applications

ABRs are also useful for To diagnosis and monitor demyelinating and

degenerative diseases affecting the brain stem (e.g., multiple sclerosis)

To evaluate infants and children. Hearing assessment for compensation and

medico-legal cases. Intraoperative monitoring.

Normal Auditory Brainstem Response

Normal Auditory Brainstem Response

What do the components represent?

Wave I: thought to be generated at the level of the VIIIth cranial nerve.

Wave III: thought to be generated at the level of the pons part of the brainstem.

Wave V: thought to be generated at the level of the midbrain - is the most robust wave component.

Patient State

With the ABR, the client can be awake or sleeping, it doesn’t affect the testing. This makes it a valuable tool for assessing

infants and other difficult-to-test populations.Muscle movement must be minimal, since it

will create too much artifact.