University of Edinburgh
 

Audiology Refresher 3

Presented in February, 2004

Otoacoustic emissions and auditory brainstem response

Introduction

  • Why screen?
  • Basic description of otoacoustic emissions OAE and auditory brainstem response ABR
  • Factors affecting responses
  • What we do in Lothian and some results from screen
  • Video
  • Questions

Why screen?

Normal hearing is prerequisite for development of optimum aural/oral communication.

Profound congenital hearing loss deprives the individual of exposure to the acoustic cues of spoken language.

Congenital mild and moderate hearing loss can also have varying effects on aural/oral language.

Early identification of hearing impairment and appropriate intervention is the main objective of newborn hearing screening.

Why use OAE and ABR?

  • Behavioural techniques for hearing screening of newborns cannot give accurate indications of hearing thresholds.
  • Moderate/high intensity sounds are required to elicit response, even with normal hearing.
  • Responses often unclear and not repeatable.
  • Interpretation very subjective.

ABR and OAE can be used for both screening and diagnostic purposes.

Three main indications for the use of auditory potentials:

  • As a means of estimating hearing threshold;
  • As a means of revealing the cause of an audiological or neurological defect;
  • As a means of monitoring the effect of drugs, surgery, or some other influence on the auditory pathway.

The strength of this type of test is its ability to test young children and babies, even when asleep, sedated or under GA.

There are two widely used types of OAE measurement;

  • Transient evoked OAE
  • Dirstortion product OAE

DPOAEs are better suited to clinical investigations on adult patients.

TEOAEs are the type that are used in newborn hearing screening programmes.

TEOAEs are more sensitive to changes in the cochlea.

What are OAE’s?

OAE’s are sounds found in the ear cana, which originate in the cochlea and can be controlled by externally applied sound. When sound enters the ear, a normal functioning cochlea creates and emits an ‘echo’ (David Kemp Dec 2002).

If the ear canal sound CANNOT be influenced by the application of an external sound, it is very unlikely to be the product of the auditory system.

What is an ABR?

Sound evokes a series of electrical potentials along the entire chain of the auditory pathway, and it is possible following acoustic stimulation to record these potentials.

Sound reaches the cochlea and is converted into electrical signals and travels to the brain.

Electrodes placed on the scalp will pick up these electrical signals at the brainstem.

Factors affecting OAE results

  • Too much noise
  • background noise 40dBA or below
  • restless baby
  • noisy breathing
  • Age of baby (eg; 24 hrs)
  • Not enough stimulus
  • Poor probe fit
  • Blocked ear canal
  • Pathology of middle ear
  • Pathology of inner ear

Factors affecting ABR

Automated ABR (aABR) based on a pre-set algorithm (simple pass/refer)

  • Background noise
  • Restless baby
  • Probe fit
  • Sensory and earmuffs not sticking
  • Floppy ear canals
  • Blocked ear canals
  • Pathology of middle ear
  • Pathology of inner ear

Factors affecting ABR

Click/diagnostic ABR

  • Again, restless baby, best if asleep or sitting quietly.
  • Time, fully diagnostic often need several appointments. Tests can take up to 1 hour or more. Need to do 2 averaged waveform runs at each intensity to check for reliability.
  • May need sedation or GA.
  • Click ABR – not frequency specific, mainly gives information about hearing in 2-4kHz range, however spread of energy from click can be from 1-8kHz. Therefore possible to miss ski slope loss if hearing at 1kHz is good.

Tone Pip

  • More frequency specific than click.
  • More frequency specific near threshold.
  • Take longer and are often more difficult to interpret especially 500 Hz in newborns.
  • Often only able to test 1kHz and 4kHz.

ABR waveforms are also affected by

Conductive comporents – all waves are shifted to right, all waves are delayed, but interwave latencies remain within normal limits.

Age – maturation effects, more so in pre-term babies
Prolonged latencies especially in first few months of life.

Accuracy of results

With good recording conditions most results should be within +/- 10dB of behavioural thresholds when performed, however can be as much as +/- 20sB. Problems when clear responses obtained at 50dB … is it 30, 50 or 70dB? Big difference in management!

Accuracy of interpretation

Wave V most robust of waveform and one we follow for threshold estimation, however, the point at which wave V is no longer recognisable varies between testers according to their experience at pattern recognition.

These electrophysiological tests are not completely objective, as they transfer the subjectivity of the test away from the subject to the clinican!

Figures from March-September 2003
Number of babies Number tested % tested In hospital In comm. % sent for diag.
4489 4410 98% 4201 (95.4%) 209 (4.6%) 3.3%