- Audiometry
- Tympanometry
- Adv Audiology
- Vestibular
- Allergy Testing
- Tinnitus
- Hyperacusis
- Misc. Audiology
- Clinical Photography
Advanced Audiology encompasses a range of investigations that are
objective in nature. This means that any data recorded is obtained from
physiological responses to sound rather than requiring the cooperation
of the subject.
These types of investigations can be used on all age groups for varying
reasons, from neonatal screening, infant, pre-school, learning
difficulties and non-organic loss amongst other things. These types of
audiological investigations are objective, in that no behavioural
response is required, so the procedure is not affected by a patient’s
attention, motivation or cognitive state.
Otoacoustic
Emissions
Otoacoustic emissions (OAE) are sounds produced by the inner ear when
the normally functioning cochlea is stimulated by sound (or they can
occur spontaneously (SOAE)).
When sound stimulates the cochlea, the outer hair cells (located in the
cochlear) vibrate. The outer hair cells serve as an amplifier
contributing to the normal passage of sound waves to the
electrophysiological signal that enters the hearing systems within the
brain. The vibration produces a nearly inaudible sound that also echoes
back into the middle ear, this normally in the 0-15dB range.
A probe placed in the ear canal can produce sound waves then detect and
record these vibrations. People with normal hearing produce emissions
but those with hearing loss greater than 25–30 decibels (dB) do not
usually produce these very soft sounds. The presence of OAE is evidence
of normal cochlear function while an absence of OAE can be consistent
with abnormal cochlear function or the presence of an outer/middle ear
problem but can also be a normal variant.
OAE are technically simple to perform, non-invasive and usually only
requires a few minutes to complete a screening test. A soft disposable
tip is inserted into the ear canal, a moderate intensity of sound is
played through a miniature speaker/s and a microphone records any
response.
There are three types of OAE:
Transient evoked OAE (TOAE or TEOAE)
Distortion product OAE (DPOAE)
Stimulus Frequency OAE (SFOAE)
Transient OAE is evoked by very brief sounds usually made of tone
bursts or multiple frequency clicks, they are usually presented at an
intensity of 80dB SPL. The cochlear echoes that these elicit are
usually in the 500-4,000Hz range.
Distortion Product OAE is evoked with sets of two pure tone frequencies
(f2 & f1) that are closely spaced and simultaneously presented,
again at moderate intensities (such as 55 and 65dB). The cochlear
echoes from DPOAE can be across a frequency range of 500 to 8,000Hz.
Stimulus Frequency OAE applies a pure tone stimulus and records any
echo. These are measured by detecting the vectorial difference between
the waveform of the stimulus and the waveform that is recorded.
The status of the middle ear can interfere with the recording of OAE,
if there is a middle ear effusion (fluid) or Eustachian tube
dysfunction resulting in a negative middle ear pressure then OAE may
not be recorded so Tympanometry should be
performed before any OAE investigation is attempted.
It is important to remember that OAE is not a test of 'hearing' but a
test of cochlear function. OAE may be absent in normally hearing
subjects with middle ear disorders and conversely OAE may be present in
inner hair cell disorders or retro-cochlear pathology (auditory nerve
and beyond) such as auditory neuropathy. This would be diagnosed by
comparing the OAE result (normal) with the Auditory Brainstem Response
(usually abnormal).
Auditory
Evoked Responses
Another
objective investigation concerning audiology is one that assesses the
central auditory system. It is known by a few different names but are
essentially the same test:
Auditory
Brainstem Response (ABR)
Brainstem
Auditory Evoked Potential (BAEP)
Brainstem
Auditory Evoked Response (BAER)
Two
other investigations that are similar to ABR but have some differences
are discussed later:
Evoked
Response Audiometry (ERA)
Auditory
Steady State Responses (ASSR)
ABR
The ABR
is an electrophysiological response that is evoked by auditory stimulus
produced by headphones, aural inserts or bone conductors. The stimuli
can be either frequency-specific tones, click stimuli or tone pips.
Surface electrodes attached to the scalp record the ABR after signal
averaging has been performed on the background EEG.
The
signal averaged ABR generates seven waveforms, however the ABR is only
concerned with the first five waves of the auditory pathway. The first
two waves represent actual action potentials; Wave I from the Auditory
Nerve (Cranial Nerve VIII) and Wave II is from the Cochlear Nucleus.
The remaining three waves of interest are post-synaptic activity from
major brainstem auditory centres; Wave III is the Superior Olivary
Complex, Wave IV from Lateral Lemniscus and Wave V from Inferior
Colliculus (For interest Wave VI is the Medial Geniculate Body and Wave
VII is the Auditory Radiation (Cortex)).
ABR's
are usually conducted using the traditional click stimulus that evokes
responses in the frequency range from 100-8000 Hz and is insufficient
for estimating hearing sensitivity. However, click stimulus is useful
in the examination of neural integrity by stimulating at a set
intensity (loudness) above a person's threshold and then recording and
measuring the Waveform's amplitude and latency, the inter-peak
differences and the contra-lateral (opposite side) measurements and
therefore remains a crucial tool in diagnostic audiology.
ERA
ERA is
similar to the ABR but threshold information is obtainable by using
specific stimuli and reducing the intensity and recording the ABR until
the Waveforms are no longer seen.
Threshold
information can be recorded at 500, 1000, 2000, and 4000Hz and there is
a good agreement (+/- 10 - 15Hz) between frequency specific ABR and
behavioural audiograms in infants and young children.
ASSR
Again,
ASSR is similar in principal to ABR, but is another investigation to
assess objective hearing thresholds. It is able to potentially evaluate
four frequencies (500, 1000, 2000 & 4000Hz) in both ears
simultaneously by using an amplitude/frequency modulated stimulus
presented rapidly to stimulate the auditory system in both ears
simultaneously. ASSR results are dependent on the statistical analysis
of the probability of a response, usually at a 95% confidence level.
