BC Puretone Testing (6/8/26-6/10/26)

When pressed against the skull, vibrations from the stem of a tuning fork or bone oscillator result in…

The perception of sound!

What are the 4 theories of bone conduction?

1. Osseous mechanism

2. Inertial mechanism

3. Osseo-tympanic mechanism

4. Non-osseous mechanism

   • Each of these ideas work together to give us a total response from the patient — you can’t just believe in one.

Osseous mechanism

• Vibration of bones within the skull stimulates the basilar membrane due to compression/distortion of the cochlea and vestibule

• Predominant in the high frequencies

• The compression of the bones of the skull gives rise to a distortion of the inner ear structures → This in turn produces electromechanical activity in the inner ear → The compression of the bony structures within the cochlea also compresses the fluid-filled space of the cochlea and stimulates the basilar membrane → Which then stimulates the hair cells.

Inertial mechanism

• Ossicles are suspended by ligaments in the middle ear space (forms a folded lever)

• Inertial BC derives from the mass of the ossicle and is predominant in the low frequencies

• Skull vibrates side-to-side → causes the vibration of stapes footplate into oval window

• The exact contribution of inertial BC to the total BC response is determined by the impedance characteristics of the ossicular chain, air within the middle ear space, and the external canal to which it is connected by the TM.

The Inertial Mechanism will be influenced by…

• Middle ear fluid

• Hole in the TM

• Ossicular discontinuity (loosey-goosey ossicles)

• Otosclerosis (Stapes footplate fixated in the Oval Window)

Bone conduction is ____ exclusively a test of the inner ear!

NOT

• The middle ear can sometimes influence BC

Osseo-tympanic Mechanism

• The Occlusion effect: the improvement of hearing sensitivity or increased loudness lateralized to the side of the blocked ear canal

  • Must consider this effect when testing BC, it can impact the results we get

• Vibration of the skull also results in radiation of sound energy into the external meatus (due to vibration of the canal walls)

• When cartilaginous portion of canal is vibrating, sound waves created in the canal excited hearing via the AC route

When testing BC, what should you do with inserts?

Take it out of the test ear

• If the bone oscillator is on the right side, take the insert out of the right ear. Keep the left insert in, incase you need to mask.

• However, sometimes if a pt has completely normal hearing and you want to test BC, you can take out both inserts. Sometimes that insert can magnify the bone response, resulting in a difference b/w AC and BC, and now you have to mask.

Non-osseous Mechanism

• Vibrating bone oscillator induces…

  • Audio-frequency pressure waves in cerebral spinal fluid (CSF)

  • This stimulates the cochlear fluids

  • Results in basilar membrane activation (which causes the hair cells to activate → sound is further sent on to be heard)

• Placement of the bone vibrator on the skull not only results in the activation of the classical BC pathways described earlier, but also a fluid pathway is stimulated.

• The living skull also has elaborate soft tissue → skin!

• The skull and all it’s contents must be vibrated sufficiently to effectively stimulate the hearing organ.

What the the primary sites for bone oscillator placement? (2)

1. Mastoid (most frequently used)

2. Forehead

Mastoid placement: Pros

• Closest to site of cochlea

• Easiest to maintain placement of pt

• Yields lower thresholds

Mastoid placement: Cons

May be impacted by individual variables (uncontrollable factors)

• Subcutaneous fat

• Hair

• Variations in size, shape, & thickness of skull

Test-retest variability for BC can often be __ to __ dB

10 to 15 dB

Forehead placement: Pros

• Less variability of thresholds

• Less contribution of from the middle ear mechanism

• Less impact from individual variables (subcutaneous fat, hair, etc)

Forehead placement: Cons

• Approximately 10 dB less efficient overall → reduced dynamic range

• Needs a different set of calibration values

What frequencies do we test with BC? (different from AC)

ASHA recommends: 250, 500, 1000, 4000, 6000 Hz

Typically seen in clinics: 500, 1000, 2000, 4000 Hz

What AC frequencies are absent from BC testing frequencies?

3000, 6000, 8000 Hz

What is a tactile response?

When a pt feels the sound presented in addition to (or in the absence of) hearing the sound

• This can be tricky, especially w lower thresholds

• Note when this occurs (ex. * = VT (vibro-tactile response))

Maximum outputs for BC signals are limited to…

• 250 Hz: 40 dB (tactile responses will be obtained b/w 35-40 dB)

• 500 Hz: 50 dB (tactile responses will be obtained b/w 35-55 dB)

• 1000-4000 Hz: 60-70 dB

  • As you go through the frequencies, you are less likely to have a tactile response occur

What are the 3 basic types of HL?

1. Conductive (CHL)

2. Sensorineural (SNHL)

3. Mixed (MHL)

Conductive HL (CHL)

• Damage or loss typically occurs within the outer or middle ear

• Presents:

  • BC thresholds are better than AC (BC needs to be in the NORMAL hearing range)

  • ABG 15 dB (or more) to be significant

  • The maximum ABG is ~65 dB

• Very common, especially in children (otitis media)

Can you test someone’s hearing via AC if they don’t have an opening to their ear canal(s)?

Yes!

• You would have to use supra-aural headphones

• Headphones, at a lower threshold, are more likely to cause a BC response as you increase in intensity

Sensorineural HL (SNHL)

• Damage within the cochlea, auditory nerve, or both

• Presents:

  • Essentially equal amount of loss for AC & BC

  • Can have up to a 10 dB difference b/w AC & BC

  • 10 dB vs. > 10 dB

What is an Air-Bone Gap (ABG)?

The difference b/w AC & BC thresholds

• 10 dB ABGs are okay, > 10 dB is not

What is considered an ABG?

≥ 15 dB

Mixed HL (MHL)

• Conductive and sensorineural components

• Presents:

  • ABG is present

  • BC thresholds are not within normal limits

  • AC thresholds are not within normal limits

Tuning fork tests

• Not uncommon to be used during screening procedure

• Currently have limited application

  • Due to significant diagnostic equipment available

  • Can be used in some situations (w/ bone vibrator instead of tuning fork)

• Weber test (most useful)— unilateral hearing loss

• Uf using a bone vibrator, set @ 35-40 dB HL

Weber Test: Purpose

Used for patients reporting unilateral hearing loss

Weber Test: Procedure

Low frequency tuning fork is placed midline on the patient’s forehead

Weber Test: Results

• If lateralized to the ear with loss, loss is conductive

• If lateralized to the ear without loss, loss is sensorineural or mixed

• If sound doesn’t lateralize, the pt either has normal hearing or equivalent HL

Rinne Test: Purpose

Compares AC to BC sensitivity

• AC: not touching mastoid

• BC: touching mastoid

Rinne Test: Procedure

Tuning fork stem is held lateral to the external ear, after which the base is placed on the mastoid process

Rinne Test: Results

• If louder when held lateral to the external ear, loss is sensorineural (Rinne positive)

• If louder when stem is placed on the mastoid process, loss is conductive (Rinne negative)

Bing Test

• Low frequency tuning fork

• Mastoid placement

• Based on the occlusion effect

• Used for pts who have either a bilateral conductive or sensorineural HL

Bing Test: Purpose

Assess the presence of conductive hearing loss

Bing Test: Procedure

Tuning fork is placed on the pt’s mastoid, while the ear canal is alternatively opened & closed by depressing the tragus

Bing Test: Results

• If louder when closed, loss is sensorineural (Bing positive)

• If same when canal is open & closed, loss is conductive (Bing negative)