Download Pure Tone Audiometry

Pure Tone Audiometry
SPA 4302
Summer A, 2004
The Pure-Tone Audiometer
• Electronic device that generates tones for
determining hearing thresholds
• Manufactured to specifications of the American
National Standards Institute (ANSI)
• Air/Bone Conduction
• Testable frequencies (A/C): 125, 250, 500, 750,
1000, 1500, 2000, 3000, 4000, 6000, 8000 Hz
• Testable frequencies (B/C): 250 through 4000 Hz
• Masking control available
Test Environment
• Background noise may affect audiometric results
by elevating thresholds
• Three ways room noise may be attenuated
• Earphone enclosure device
• Insert earphones – foam tipped receivers that are
inserted directly into the ears
• Sound isolated chambers
The Patient’s Role
• Patients must be aware that they are to indicate
when they hear a tone
• Patient response: hand raise, finger raise, signal
button, vocal response, play
• False responses
– False negatives: patient fails to indicate when they’ve
heard a tone - misunderstood or forgotten instructions,
feigning or exaggerating loss
– False positives: patients responds when no tone is
presented – usually occurs when there are long silent
periods in the test
The Clinician’s Role
• Convey task instructions to patient
• Ensure understanding
• Patient position
– Position patient so they cannot observe the clinician’s
movements
Air-Conduction Audiometry
• Specifies hearing sensitivity at various frequencies
• Can’t tell whether deficit is conductive or
sensorineural, or mixed
• Earphone placed with diaphragm aimed directly
over canal opening
• Be careful of canals that collapse due to the
pressure of the earphones – use insert earphones if
this is a potential problem
Air-Conduction Audiometry
• Test the known or suspected better ear first
• Test at the octave points and the mid-octaves (750,
1500, 3000, 6000 Hz) if there is a difference of 20
dB or more between adjacent octaves
• Begin at 1000 Hz – easily heard by most and high
test-retest reliability
Measuring a Threshold
• Start presenting pure tones at 30 dB HL
– No response? Raise the level to 50 dB HL
– Still no response? Raise the level in 10 dB increments
• Whenever person responds, lower the level 10 dB
• Whenever no response, raise the level in 5 dB
• Threshold=the lowest level at which the patient
can correctly identify the tone presentation at
least 50% of the time, with a minimum of 3
responses at a given level.
Air-Conduction Audiometry
• Pure-tone average (PTA)=average of air
conduction thresholds obtained at 500, 1000, and
2000 Hz in one ear
– Useful for predicting threshold for speech
• Percentage of Hearing Impairment
– Ignores audiometric configuration and looks only at
average hearing loss
– Often confusing and misleading to patients
Air-Conduction Audiometry
PTA (dB)
Degree of Communication
Impact
0-15
16-25
26-40
41-55
56-70
71-90
> 91
None
Slight
Mild
Moderate
Moderately Severe
Severe
Profound
Air-Conduction Audiometry
• The Audiogram
– Frequency (in hertz) on the x-axis, Intensity (in dB
HL) on the y-axis
– Moving left to right, frequency increases; moving top
to bottom, intensity increases
– Symbols are placed to correspond to threshold at a
given frequency:
Air conduction Bone conduction Air—Masked Bone—Masked
Right
O
Left
X
<
>
[
]
The
Audiogram
• Thresholds
by frequency
• Hearing by
air and bone
transmission
Severity of Hearing Loss
Bone-Conduction Audiometry
• 3 Mechanisms of Bone Conduction
– Distortional Bone Conduction
– Inertial Bone Conduction
– Osseotympanic Bone Conduction
• Bone Oscillator Placement
Mastoid process,
or,
Forehead
Bone-Conduction Audiometry
• Occlusion Effect
– When the ears of patients with normal hearing or SNHL are
covered or occluded, there is an increase in intensity of sound
delivered via a bone oscillator
– Affects 1000 Hz and below
– Result of increase in SPL in the ear canal when the
outer ear is covered
– Markedly decreased when insert phones are used (as
opposed to supra-aural headphones)
Bone-Conduction Audiometry
• No matter where the oscillator is placed, you can
never be sure which cochlea is being stimulated!
(more on this to come)
• Frequencies usually tested:
– 250, 500, 1000, 2000, and 4000 Hz
• Symbols for bone conduction are only connected
on the audiogram (with dashed lines) when there is
a conductive or mixed loss.
Audiogram Interpretation
Look at:
• hearing sensitivity by AC
• hearing sensitivity by BC
• AC/BC difference (a.k.a. the air-bone gap)
No air-bone gap = normal or SNHL
AC worse than BC = conductive hearing loss
• Watchout: low frequencies at high levels via BC
can be perceived as a tactile signal!
Another Thing to Watch Out For:
• Cross Hearing: sound delivered to one ear but
perceived in the other ear.
• Interaural Attenuation (IA)—How much sound it
takes to reach the other side:
– Air conduction IA = 40 dB
– Bone conduction IA = 0 dB
• Danger for cross-hearing
– For AC—If AC threshold in the test ear, minus IA, is
greater than or equal to the BC threshold of the
opposite ear
– For BC—If Air-bone gap of test ear exceeds 10 dB
Masking
• Masking—keeping the non-test ear “busy” in order to
ensure that it is actually the test ear which is responding
• Noises used to mask:
– White noise—has approximately equal energy per cycle & covers a broad
range of frequencies
– Narrowband noise—made up of frequencies that immediately surround the
pure tone being tested
• Insert earphones recommended because:
– They lessen the occlusion effect
– They provide much more interaural attenuation
Effective Masking:
Calibration of the noise
• dB EM (Effective Masking) describes the level to
which a threshold will shift in the presence of a
given level of noise
• So, 45 dB EM should raise the threshold for a tone
to 45 dB HL in the ear in which both are
presented.
Masking
• Masking for air conduction
–
–
–
–
“Shotgun” Approach
Minimum-noise method
Maximum-noise method
Plateau method
• Masking for bone conduction
– Similar to air conduction
– Beware of occlusion effect, and overmasking
Computerized Audiometry
• Using a device remotely operated by a computer
and data is stored
• Computer can control all aspects of testing and
masking and analyze patient responses
• Used more often for military, industrial, and
educational applications (large number of people
to test)