Aural Rehabilitation & Hearing Aids Flashcards
Patient candidacy for hearing aid
Pts w/ HL who have communication difficulties either objectively or subjectively
- Type of hearing loss (no longer a factor)
- Degree of hearing loss: more appropriate for mod-severe HL (severe-profound may need CI)
- Word recognition: in general, pts w/ good word recognition are more likely to do better (but a low score should not preclude amplification trial)
- Age, physical and mental health of pt
- Pt’s (not family’s) motivation
- Finances
- Cosmetic considerations
- Communication needs
Pt’s subjective needs are more imp than audiogram
Factors that diminish word recognition
- Reduced audibility (the only factor H.A. can address)
- Cochlear distortions producing reduced freq and temporal selectivity and resolution
- Abnl central auditory processing
- Diminished cognitive fnc
Binaural or Monaural amplification?
Unless there is a significant asymmetry in sensitivity, tolerance to loudness, or word recognition ability, or unless a medical condition exists CI the insertion of anything in the EAC, the standard is to at least try binaural amplification
Reasons Binaural amplification is superior
- Eliminates or minimizes head shadow (reduction in signal intensity from the side of the head opposite the signal)
- Improved localization
- A central release from masking (binaural squelch) may result in better hearing in noise
- W/ binaural loudness summation, absolute binaural thresholds are 2-3 dB better than monaural thresholds
- Possibility of tinnitus reduction bc of inc stimulation to more cortical neural substrate
The dynamic range of listening is greater
CROS
Contralateral Routing Of Signal/Sound
- Usable hearing in 1 ear but no hearing, very poor, or unaidably hearing in 2nd ear
- Microphone placed on side of poorer ear
- Signal routed (via electrical cord of FM) to opposite ear and amplified
Transcranial CROS
Placing a hearing aid in the “dead” ear, producing bone conduction stimulation of the “good” ear
Only use if the poorer ear has no residual hearing that might produce recruitment or other distortion factors
BICROS
Bilateral Contralateral Routing Of Signal
essentially a CROS + conventional aid
-Use if the “good” ear is in need of amplification
-Only one ear is aidable
-Microphones are located on both ears but the signal is routed only to the “good” ear
BAHA
Bone anchored hearing aid
Used when the impaired ear is unaidable (in place of CROS)
Also used when HL doesn’t exceed 45 dB HL but traditional HA can’t be used (canal atresia)
-Titanium piece placed into the bone and allowed to integrate w/ skull for 3-6 mo prior to use
Hearing aid styles
- CIC (completely in canal)
- ITC (custom in the canal)
- ITE (custom in the ear)
- BTE (behind the ear)
- Open-fit mini BTE
- CROS
- BiCROS
- Body aids
CIC A&D
Adv
- Barely noticeable (inserted several mm into EAC terminating 5 mm from TM); removed by monofilament lying near tragal notch
- Partial resolution from occlusion effect
- Use w/ phone improved bc of deep canal location
- Wind-noise problem improved
- Gain in high freq improved due to pinna effect
- Secure fit, reduced feedback 2/2 min venting, good sound localization
Disadv:
- Only for mild-mod HL
- Fragile, freq repairs, shell modifications $$$
- More extensive counseling required to teach use
- Deep canal impressions required
- Small batteries, hard for dexterity-impaired
- FEEDBACK w/ JAW MVMT
- Can’t be used if unfavorable EACs
- Hardest to keep clean (wax blocks receiver)
- Needs digital feedback suppression technology or high feedback bc mic is close to receiver
- Fullness feeling to ear
- Distorted perception of own voice from occlusion effect
ITC A&D
Fills the cartilaginous portion of EAC
More visible than CIC
Adv:
- More cosmetic than larger HA
- Increased amplification provided by pinna boosts gain in high freq
- Placement of the microphone improves sound localization
Disadv:
- Only for mild-mod HL
- Fragile
- Hard for dexterity-impaired
- Small size limits number of controls for adjustments
- Limited venting options
ITE A&D
Fills entire concha
Adv:
- More cosmetic than BTE
- Increased amplification by pinna boosts gain in high freq
- Improved sound localization
- Made of only one component
Disadv:
- Amount of gain limited 2/2 acoustic feedback
- Only for mild, mod, mod-severe HL
- Small size limits number of controls on nonprogrammable aids
- Microphone more prone to breakdowns due to close proximity to cerumen in EAC
BTE
- parts
- A&D
2 parts
- Hearing aid hooks onto and rests behind pinna
- Custom earmold attached to tube that secures the aid and directs sound into EAC
Adv:
- Good for any degree of HL bc of power
- Best option for severe-profound HL
- Large enough for multiple controls for electroacoustic properties, allowing adjustment flexibility
- Less feedback bc mic and receiver further apart –> larger venting and more amplification for severe to profound losses
- Larger batteries last longer and are easier to handle
- Ideal for infants and children due to ext ear growth
Disadv:
- In pts w/ severe-profound HL, earpiece must fit tightly in the canal to eliminate feedback problems
- Require relatively nl pinna
- Easily affected by perspiration
- Less cosmetically appealing
Open fit mini BTE
Combines acoustic benefits of the larger styles with the cosmetic benefits of the smaller styles
Digital hearing aids
Computer-controlled
Digitization (incoming sounds are converted to numbers, which are then analyzed & manipulated via algorithms)
DSP (Digital signal processing - allows instruments to attempt a differentiation of noise from speech based on spectral composition and temporal characteristics)
Digital feedback reduction
Compression
Non-linear amplification
Provide increased amplification for soft intensities and prevent amplified signal from reaching the loudness discomfort level of wearer by decreasing amplification for high input levels
Best if its multichannel compression
Gain
The difference in dB b/w sound entering the mic and sound exiting the receiver
Linear amplification provides constant gain until output reaches a ceiling (saturation level)
Compression circuitry is able to reduce the gain when a predetermined level, referred to as the kneepoint is reached
Multichannel compression
Solves the problem of early compression circuitry where the entire frequency range gain is reduced when the kneepoint is hit
How to increase speech understanding in noisy environments in the hearing impaired
The Signal-to-noise ratio must increase:
- Place mic closer to speaker (FM or infrared systems)
- Directional or dual mic’s
Directional or dual microphones
Help increase speech understanding in noisy environments by using time delays to minimize gain of sounds entering from behind the person
Low benefit in reverberant environments
Minimum of 3mm for mic separation so not available in CIC aids
Multiple programs in the hearing aid
Electroacoustic characteristics of the aid can be instantly changed to better compensate for particular acoustic environment
Frequency compression
For individuals w/ cochlear dead zones (regions of nonfunctioning inner hair cells)
Shifts the amplified signal away from these freq into adjacent range where person can hear
Requires an aclimatization period or additional training for auditory cortex to adapt
Telecoils
A small inductance loop that picks up and amplifies electromagnetic leakage purposefully produced from telephones
When activated, the mic can be (but doesn’t have to be) shut off, eliminating feedback many wearers c/o when using phones
BTE and ITE aids can contain them
Also used to interface with various assistive listening devices
Hearing Aid features
Digitization Compression Directional and dual mics Multiple programs Frequency compression Wireless connectivity Telecoils
How to validate and verify a successful hearing aid fitting
- Assessment of word & sentence recognition in quiet and noise
- assessment of sound quality
- probe mic measures
- subjective scaling
What problem do assisted listening devices address?
The physical distance b/w the hearing aid and the speaker
Intensity decreases 6 dB for every doubling in distance
They maintain a nl signal-to-noise ratio by transferring the sound signal at the original intensity level directly to the listener or HA
Wireless assisted listening devices (ALDs)
- Infrared
- FM
- Inductance loop transmission
Explain the occlusion effect
- Occurs when the body of the HA blocks the EAC
- Causes a muffled sensation due to shift in the peak of the natural resonance of the EAC
- This results in an increase in low-freq amplification
- In pts with nl low-freq hearing, this amplification is not desired
How is occlusion lessened
Use of aids that:
- don’t occlude canal
- contain a vent
- electronic filtering of low freq
The larger the vent, the less often low freq will be heard by the pt
-Not all HA can be vented (digital HA can’t be vented as well as small HA
Main components of a HA
- Microphone
- Amplifier
- Receiver (speaker)
- Volume control
- Battery power source
Saturation sound pressure level
The maximum amount of sound pressure output or power that a hearing aid can produce
Acoustic gain
Input - output
The difference in the output of a HA relative to its input
Can be linear or nonlinear
Output limiting
The maximum intensity of the amplified signal
Frequency gain response
The amount of gain as a function of frequency
Distortion
The inexact reproduction of a signal produced by a HA
How can HA be prevented from amplifying sound past a comfort level?
2 strategies:
- Peak clipping - a traditional strategy for output limitation that creates distortion when hearing aid is in saturation (reached max output)
- Compression - as hearing input increases, the amount of gain is automatically reduced
What different types of compression are available
- Single-band compression (applied across entire freq range)
- Multi-channel compression (applied to specific freq at which the pt experiences recruitment or loudness discomfort)
How does acoustic feedback occur?
When amplified sound leaks from receiver back to the microphone
Result is unpleasant high-pitched squeal
What causes acoustic feedback?
- Short mic-to-receiver distance
- Wax in canal
- Vents
- Poor HA fit
- ITE and ITC HA
