OAE and ABR Notes

Physiological Tests

  • Physiological tests are NOT a test of hearing.
  • They often correlate with hearing levels and for some populations they give you the best estimate of hearing.
  • The “gold standard” for both threshold determination and for showing that hearing has occurred is behavioral audiometry.
  • Behavioral audiometry requires a signal to be detected, perceived and acted upon.
  • Ascending and descending pathways are involved in behavioral audiometry.
  • Physiologic tests for the most part don’t imply full perception/understanding.
  • When possible, physiological test results should be combined with behavioral tests.

Otoacoustic Emissions (OAEs)

  • Emissions originate in the outer hair cell (OHC) of the cochlea (in a normal functioning ear).
  • The emissions travel outward through the middle ear bones and into the ear canal where we measure them with a sensitive microphone.
  • OAEs are found in normal functioning cochleas (mainly from the OHCs).
  • OAEs are VERY soft sounds (−10-10 to 20dB20dB SPL), inaudible without special equipment.
  • People with normal hearing produce OAEs. A moderate or worse hearing loss (of any kind) will obliterate OAEs.
  • They are ACOUSTIC (actual sounds); they are NOT neural signals.
  • Non-behavioral (often used as newborn hearing screen).
  • Quick to administer.

OAE Assumptions and Uses

  • When we obtain OAEs, we can make some assumptions:
    • Hearing is likely normal to no worse than mild.
    • Middle ear function is normal (middle ear disorder will OBLITERATE an OAE).
    • OAEs WILL BE ABSENT WITH MIDDLE EAR CONDUCTIVE HEARING LOSS. If a person has a type B tympanogram and fluid, OAEs will be absent even though the cochlea is normal.
  • Why OAE?
    • Physiological tests are the best tools with children 6 months of age and younger.
    • To obtain ear-specific information on a child when all you have is sound field testing.
    • To confirm what you are getting behaviorally (in children and adults).
    • To assess for cochlear “dead regions” when you fit hearing aids.
    • Quick and easy to administer.

OAE Challenges

  • Patient must be still, quiet and the room you are testing in must be relatively quiet.
  • If emissions are absent, it does not tell you the degree of hearing loss.
  • Unable to obtain in those with middle ear disorders.

Auditory Brainstem Response (ABR)

  • Used to assess the neurological function from the VIII cranial nerve through the brainstem.
  • ABR is just one of a family of tests known as “auditory evoked potentials.”
  • Characterized by a series of Waveforms I-V.
  • NOT a test of hearing but highly correlated with hearing levels (when the nerve responds to a sound, we can infer that the person would perceive/hear that sound).
  • Must be quiet: after 3 months of age we usually need to test under sedation until they are old enough to be still for at least 30 minutes.

ABR Anatomy and Physiology

  • The ABR reflects activity of many auditory nerve fibers and of pathways within the auditory brainstem.
  • The summed electrical activity from a sizable group of nerve fibers firing together is picked up with detectors (electrodes) located as far away as the scalp on the forehead and near the ear.
  • Electrical responses to sound in the auditory nerve fibers and in the brain stem must pass through brain tissue and the skull to each the electrodes on the skin.
  • The ABR consists of a series of peaks and valleys which are referred to as waves.
  • Waves represent activity in one or more regions of the auditory system.
  • Major ABR waves are waves I, III, and V.
  • Generators of ABR Waves
    • Wave I arises from activity in auditory nerve fibers, particularly fibers in the distal end of the auditory nerve near the cochlea.
    • Wave III is produced by combined activity from two or three pathways in the pons portion of the brain stem.
    • Wave V, the most prominent wave, is generated in the rostral midbrain portion of the auditory brainstem.

ABR Measurement

  • Stimulation
    • Sounds are usually presented with insert earphones for ABR measurement in infants and young children.
    • Stimuli used most commonly in clinical ABR measurement are very brief or transient.
    • Common stimuli for eliciting the ABR are:
      • Clicking sounds
      • Very brief tones called tone bursts
  • Detection
    • Electrodes on the scalp detect only a tiny amount of electrical activity from the auditory regions of the brain.
    • Amplitude of the brain activity is increased with an amplifier connected to the equipment.
    • Some non-response frequency information in the brain activity is filtered out during the ABR recording process.

ABR Latency Analysis

  • Calculations made most often during ABR analysis are determination of the latency and amplitude of the waves.
  • Latency of a wave is the time interval from the presentation of the stimulus to the occurrence of a wave.
  • An audiologist compares a patient’s latency values to normal values to determine whether the response is normal or abnormal.

ABR Clinical Applications: Early Identification and Diagnosis of Hearing Loss in Infants

  • The importance of detecting and defining hearing loss in early childhood was recognized in the earliest years of audiology.
  • Research clearly confirms that early intervention means the initiation of appropriate management within the first six months after a child with hearing impairment is born.
  • Before the 1970s, audiologists were faced with a major clinical dilemma.
  • There was a demand to identify, diagnose, and manage hearing loss in young children in order to maximize speech and language development.
  • No accurate or clinically feasible technique was available.

ABR and Hearing Levels

  • Not a test of hearing
  • Hearing levels can, however, be estimated from ABR results
  • Since it does not include the cerebral cortex, it is not affected by sleep or sedation
  • Since the patient must be still, ABR is done under sedation with certain populations.
  • Repeat: the ABR reflects neural synchrony from the auditory nerve (CNVIII) through the brainstem. ABR does not involve the cerebral cortex
  • You can be unconscious and not fully “hear” and have a normal ABR

ABR Clinical Applications: Estimating Auditory Thresholds with ABR

  • Currently, evidence based test protocols permit accurate estimating hearing thresholds for different audiometric test frequencies even in infants within days after birth.
  • To objectively estimate auditory thresholds in infants and young children, the ABR is repeatedly recorded at progressively lower stimulus intensity levels for click stimuli and for tone burst stimuli at different frequencies.
  • Three major changes in the waveforms as stimulus intensity level is decreased:
    • Latency of the waves increases as stimulus intensity level decreases
    • Amplitude of the waves decreases as stimulus intensity level decreases
    • Only wave V remains at the lowest intensity level at which an ABR is still present
  • A patient’s auditory threshold corresponds to the intensity level where ABR wave V disappears.
  • Hearing threshold assesses by behavioral testing is about 10dB10 dB lower than the minimum intensity level producing an ABR wave V.
  • ABR threshold is measured for tone burst stimulation at different frequencies such as 500Hz,1000Hz,2000Hz500 Hz, 1000 Hz, 2000 Hz, and/or 4000Hz4000 Hz.

ABR: Not a Test of Hearing

  • ABR does not reflect activity of all types of neurons in the auditory system.
  • ABR primarily reflects activity of the nerve fibers, not the many collections of nerve cell bodies within the auditory system.
  • With Wave I as an exception, there is not a one-to-one relation between a single auditory structure and a single ABR wave.
  • ABR provides no information on auditory structures above the level of the brainstem.
  • Even though it is a valuable clinical tool, the ABR is not a test of hearing.

Why is ABR important in Hearing Assessment of Children?

  • ABR is essential for describing both the degree and type of hearing loss in infants and young children.
  • ABR findings lead directly to early intervention for hearing loss.
  • Early intervention of hearing loss in children (before 6 months after birth) is critical to increase the chances of normal development of speech, language, and effective communication.

Why ABR?

  • For babies, young children and non-cooperative patients to identify hearing loss or confirm suspected hearing loss (can be a screening for newborns performed by a technician or a diagnostic evaluation to diagnose and quantify hearing loss performed by an audiologist)
  • Automated ABR is a screening. Sedated ABR is NOT a screening
  • Threshold ABR is a diagnostic test to estimate the degree of hearing loss
  • ABR can be done by air conduction and by bone conduction
  • ABR can estimate thresholds for normal hearing, CHL, SNHL and mixed HL. Even with an absent outer ear you can do a bone conduction ABR to estimate cochlear sensitivity.
  • Neurologic ABR when a disorder of the VIII nerve is suspected (typically acoustic schwannoma tumor) (usually adults)
  • Evidence of demyelization (MS)

ABR Advantages and Challenges

Advantages

  • Unlike OAE, can get auditory information on all different hearing levels (not just normal and mild) to diagnose hearing loss
  • Often can get enough information to move forward with treatment plan (HA, CI, etc.)
    Challenges
  • Patient must be asleep or restful (natural vs. artificial)
  • Can have interference from electrical noise (surgical suites!)
  • Can be time consuming
  • Not as sensitive to finding tumors as MRI

ASSR – Auditory Steady State Response

  • Evoked potential test similar to ABR but can get estimates of a 4-frequency audiogram
  • Time consuming
  • More difficult to elicit than ABR
  • Usually under sedation
  • Provides better estimates of audiogram for hearing aid fitting
  • Note: OAE is pre-neural and both ABR and ASSR are pre-cortical (before the level of the cerebral cortex) so OAE, ABR and ASSR are not affected by sedation.