Measurement of Hearing Quiz 3 - Hearing Threshold and the Audiogram

Assessing the auditory system

-Behavioral pure tone thresholds

• a primary component of the audiologic test battery

• important for many purposes

-For diagnostic (site of lesion) purposes we measure and compare two thresholds obtained via the

• air conduction pathway and bone conduction pathway

-Dysfunction at any point along these pathways results in HL

Air conduction pathway

-Air conduction (AC)

• outer ear —> middle ear —> inner ear and beyond

-Elevated AC thresholds mean dysfunction somewhere along the pathway

• but you dont know where!

-Bone conduction testing can help

Bone conduction pathway

-bypasses outer and middle ear

• directly stimulates both cochlea simultaneously

-comparing AC and BC thresholds helps isolate site of dysfunction

Air conduction audiometry

-signal source is audiometer

-output transducer options

• supra-aural headphones

  • e.g. TDH-39, 49, 50

• insert earphones

  • e.g. ER-3A, ER-5A

• circumaural headphones

  • e.g. Sennheiser HDA-200

• Loudspeaker (sound field thresholds)

Bone conduction audiometry

-signal source is audiometer

-output transducer is a bone oscillator

• e.g. radio ear B-71

-mastoid placement is typical

• forehead placement may be used with kids, post-mastoidectomy patients, automated audiometers

  • correct calibration values are required

AC pure tone right ear symbol

red circle

AC pure tone left ear symbol

blue X

AC pure tone masked right ear

Red triangle

AC pure tone masked left ear

blue square

audiogram symbols: Pure tone bone

-place bone conduction (BC) symbols to the right (for the left ear) or left (for the right ear) of the frequency axis

• unmasked BC mastoid symbol shows side of transducer placement (not the ear responding)

• Or use the ear “unspecified” symbol “^” (placed on the frequency axis)

• or place the oscillator on the forehead and use the forehead symbols

Bone conduction-mastoid unmasked left ear

>

Bone conduction-mastoid unmasked right ear

<

Bone conduction-mastoid masked left ear

]

Bone conduction-mastoid unmasked right ear

[

bone conduction - forehead unmasked

Carrot pointing down

bone conduction - forehead masked

audiogram symbols: no response

-Symbols for “no response” at highest output level

• add a downward directional arrow to the existing symbol

  • to the left (right ear) or right (left ear) of the symbol

• Do not connect with lines

* symbol

often used to suggest a “vibrotactile” response to BC

Symbols for unaided soundfield threshold

S

-reflects the threshold in the better ear

-If no response- do not connect with lines to actual responses

-Aided: often use “A” bust must be defined on the audiogram

classifying HL

-terms reflect the expected (on average) magnitude of speech understanding problems that someone with that pure tone average (PTA) would have

normal limits

-10 to 15

slight HL

16-25

Mild HL

26-40

Moderate HL

41-55

Moderately-severe HL

56-70

Severe HL

71 to 90

profound HL

91+

Types of audiometers

-Described based on their capabilities

• Pure tone generation (type 1, 2, 3, or 4)

  • Type 1-full diagnostic; Type 4- hearing conservation

• Extended high frequency testing (Type HF)

  • 8000-16000

• Speech (type A, B, or C)

  • Input for recorded materials

  • Microphone for MLV

  • Earphone and/or loudspeaker output

-E.g. Type 1HFA

Frequency response

-The need for a flat frequency response depends on your purpose

• Pure tone threshold measures versus complex stimuli (e.g. speech)

Maximum output

-Need varies with purpose

• Screening audiometer versus diagnostic audiometer

-Max output also varies with frequency

-Increasing levels above the maximum creates distortion

Supra-aural

-TDH series (39,49, 50)

-MX 41 AR cushion

-Headband should be tight enough to hold paper between cushions

-Calibrated in a 6cc coupler

Supra-aural Pros

-reliable/durable

-inexpensive

-familiar

supra-aural cons

-poor frequency response

• high frequency roll off (same as insert phones)

-Low frequency “leakage”

-Compared to inserts earphones

• Less interaural attenuation ~40dB

• Increased “occlusion” effect

-Can cause collapsing canal

Circumaural

cushions fit around (not on) the pinna

Circumaural pros

-Some have flat frequency response

-may be used with extended HF audiometry

-very comfortable

-better noise isolation than supra-aural

Circumaural cons

-modified calibration methods used

• won’t fit on a 6cc coupler

-big on small heads

-Less interaural attenuation

insert earphones (ER 3A/5A/3C)

-Mimics frequency response of TDH 39 headphones

-calibrated in a 2cc coupler

insert earphone pros

-more hygienic

• disposable foam earplugs

-More comfortable for adults and kids

-inter-aural attenuation ~50-70 dB

-Greater attenuation of ambient noise

-Protect against collapsing canals

-Less variability in threshold due to transducer placement

-Less artifact during electrophys testing (ER 3A/3C)

-Ease in relating behavioral audiometric data to hearing aid measures (both calibrated in a 2cc coupler)

insert headphones cons

-mimics frequency response of TDH 39 earphones

• NOT FLAT frequency response!

-Costs! ~$400.00 pair

• ~$1.00 pair for disposable tips

-Cerumen can block receiver output

-connector is easily lost!

Bone conduction transducers

-Radio Ear B70A, 71, 72, and B81 (new) are common

• Headband tension affects force (levels)

-Calibrated using an “artificial mastoid”

-Compared AC responses frequency responses are peaky and don’t have much low frequency

-Lower output and dynamic range compared to AC transducers

• ~30-50 dB more output via AC

-Newer “B81” model gives more LF output with less distortion than B71

Audiologists describe threshold results in terms of:

-Degree of loss

• normal, mild, … profound

-Configuration of loss - how thresholds vary with frequency

• flat, sloping…

-Type of loss (or site of lesion- outer, middle, or inner ear, neural or higher)

• conductive, sensorineural or mixed

-Symmetry of loss - hearing loss is (or not) similar between ears

• unilateral loss is asymmetric

• bilateral loss can be symmetric or asymmetric

Configuration of loss

-no consensus on how to define configuration

-flat loss

-sloping loss

-rising/reverse slope loss

-trough loss

-peak(ed) loss

Flat loss

-thresholds (in general) ± 20 dB

Sloping loss

-general trend of poorer thresholds as frequency increases

• LF<HF by >20 dB

• May add qualifiers e.g. mild, moderate, or severe based on thresholds

• steeply sloping loss

  • a large change over a narrow frequency range

    • ski slope loss

    • precipitous loss

rising/reverse slope loss

-same as sloping but reverse direction

• HF<LF by >20 dB

trough loss

-worst thresholds in mid-frequencies

• cookie bite loss

Peak(ed) loss

-best thresholds in mid frequencies

• reverse cookie bite loss

• inverted V audiogram

Conductive loss

-bone conduction thresholds are normal and are better than air conduction thresholds

-conductive losses have an air-bone gap (ABG)

• ABG (in dB) =AC threshold-BC threshold

-Conductive loss means the site of lesion is in the outer or middle ear

• an ABG>10 dB is significant even if AC thresholds are normal

Sensory/neural (sensory or neural)

-also sensorineural & SNHL

-with SNHL AC and BC thresholds are similar (ABG<10 dB)

-generally BC<AC (meaning BC is better than AC) thresholds but BC thresholds can be worse

-Sensory (cochlea) or Neural (VIII nerve) or higher

Mixed loss

-Bone conduction thresholds are not normal

-ABG is >10

-Site of lesion is outer/middle ear and cochlea (or higher)

-Any HL that has both a conductive and SN component in the same ear

Why do we care about asymmetry?

-consistency

-Diagnostic utility

Asymmetric defintion

-Between ear difference of

• >10 dB at 3 or more frequencies

• >15 dB at 2 or more frequencies

• >20 dB at 1 or more frequencies

-These criterion were based on best match to “expert” judges

• optimizing consistency not diagnostic utility

-Pretty conservative criteria resulting in high prevalence of asymmetry (i.e. >50% cases)

Diagnostic utility of asymmetry

-Asymmetric SNHL can be a red flag for vestibular schwannoma (VS)

• but no standardized criterion/definition of asymmetry for detecting such tumors

-Asymmetry alone is not an accurate diagnostic indicator of retrocochlear pathology

-But in conjunction with other test results, the presence of asymmetry can (and should) influence your clinical decision.

How much asymmetry is “too much?”

-Answer varies across clinicians and sites

-Starting out remember Mangham theory: Average difference >20 at 1,2,4 and 8 kHz was one of the most accurate predictors

• But used alone, sensitivity is not great, and false positives will be high! This will be the case with any asymmetry criteria

-Remember, asymmetry alone is not an accurate diagnostic indicator of retrocochlear pathology

-PTA protocols cannot be used for proper screening or diagnosis of vestibular schwannoma despite their good sensitivity, and MRI remains the gold standard for this purpose

Measuring thresholds

-Multiple methods can be used to measure thresholds (e.g. hearing or visual thresholds)

-Current methods are based on early work by Gustav Fechner

• He developed the “method of limits”

  • basis for current clinical method

Method of limits (MOL)

-A series of ascending and descending runs. Each run consists of a series of “yes-no” trials

• After each trial, the subject says “yes” or “no”

-The threshold is the average of “crossover points”

-A valid and reliable method, but it’s time-consuming, and it’s susceptible to subject bias and criterion effects

• Subject bias: anticipation bias, habituation bias

• Subject criterion: strict, lax

Current threshold search procedure (ASHA)

-Current method is a “modified method of limits”

• First pass- Hughson and Westlake

  • modification of MOL but only “ascending” runs contribute to threshold

• Current version- Carhart and Jerger “modify Hughson and Westlake” procedure

  • Keep only ascending responses contribute to threshold criteria

    • suggest 5 dB step size

  • Use 1-2 second stimulus duration with quiet intervals in between

  • Terminate ascending run at first response

    • decrease level and start next ascending run

Threshold defintion

“the lowest hearing level at which responses occur in 1) at least one-half of a series of 2) ascending trials, with a 3) minimum of two responses out of the three required at a single level.” ANSI

-Repeat at 2, 3, 4, 6, and 8 kHz

• recheck 1000 Hz. if thresholds differ by >5 dB use best threshold and recheck threshold at another frequency

• Finish, in order, 500, 250, and then 125 Hz (if needed)

Minimal response level (MRL)

-Infants and young children are not “little adults”

-MRL: lowest level of stimulus that produces the desired response

• actual thresholds may improve over time

stimulus characteristics

-1-2 seconds duration (all types)

• Vary the time intervals between presentations

  • no shorter than 1-2 seconds

-Pulsed tones (~200-250 ms on/off duration) may reduce false positives and be “preferred”

-Warble tones (or narrow band noises) are required for sound field testing but can affect thresholds, especially for steeply sloping losses

Response characteristics

-Multiple options are available based on what the patient can do

• Press a button

• Raise a finger, arm

• verbal response

• any consistent response that is time locked to your stimulus presentation is acceptable

-Your interpretation of a patient response can affect your threshold

• patient response and your interpretation must be consistent

• slow responders, “off” responders are o.k.

  • look for a faster response at higher levels

• watch for false positives or false negatives

  • reinstruct as needed

Dealing with false positives

-False positive: Person says “yes I hear it” when no tone was presented

• Increase signal level to remind them of the tone

• Make sure your timing of presentations is varied

• Reinstruct or vary instruction

  • e.g. respond “ONLY” if you are sure you hear the tone

• Try pulsed or warbled (FM) tones

Dealing with false negatives

-Person doesn’t respond “yes” even though the signal is above threshold

• Could have a conservative criterion

  • reinstruct

    • e.g. make sure you respond even if it is very faint

• Could be giving you a response that you are not seeing

  • watch the person not just the response light

  • Look