Presbycusis Flashcards
What are the two most common causes of hearing loss?
Advanced age
Noise exposure
What is presbycusis?
Age related hearing loss
Gradual decline of hearing
Onset and rate of progression is variable
What causes presbycusis?
Progressive loss/degeneration of endocochlear potential, IHCs, and synapses
Changes in the peripheral and central auditory system
Why does the human auditory system decline?
Because it lacks the ability to regenerate
Does presbycusis have a varied and not well-understood etiology?
Yes
Is the auditory system susceptible to wear and tear due to living?
Yes, no one lives in a bubble
Wear starts early and intensively in civilized societies rich in noise
What were the early findings of presbycusis site of lesion?
Primary site was the cochlea and secondary was central involvement due to reduced sensory input
What are the recent findings regarding site of lesion?
Central auditory system undergoes direct morphological and physiological changes independently of peripheral involvement
Does the prevalence of hearing loss increase with age?
Yes
Do men tend to have worse hearing as they grow older than women?
Yes
Do African Americans consistently show lower incidence of hearing loss in the elderly population?
Yes
What is the formula for presbycusis?
Genetics/(ototoxic drugs + noise exposure + age)
Why is there difficulty studying the effects of purely aging?
Environmental noise exposure (lifetime of continuous noise and some high level noise exposure)
Drugs (some are ototoxic, others may have effects after long term use)
Genetics
How did they determine that there is age related hearing loss and not just related to environmental noise?
Animal models and studies
Does extremely high aerobic metabolism take place in the lateral wall of the cochlea?
Yes
Oxygen based metabolism
Anything that effects the blood supply will cause problems with hearing (oxygen in blood)
What do we need high metabolism for?
Maintenance of K+ gradient between endolymph and perilymph
Generation of the endocochlear potential (EP)
Where is mechanical and metabolic damage observed most commonly?
At the basilar end and basal turn of the cochlea
What type of hearing loss is presbycusis?
Slowly progressive
Sloping
High frequency SNHL
What is age-related hearing loss that is not complicated by environmental factors caused by?
Degenerative changes/pathologies of the lateral cochlear wall and not just simply due to a loss of cochlear hair cells
What are the three systems of the cochlea that provide sound sensitivity particularly for high frequencies?
The cochlear amplifier
The power supply
The transduction mechanism
What is the cochlear amplifier?
The active processes located in the OHCs that amplify sound vibrations inside the cochlea
What is the power supply?
The cochlear lateral wall tissue, including stria vascularis, that provides the power the cochlear amplifier needs to function effectively
What is the transduction mechanism?
IHCs and afferent nerve fibers of the auditory nerve that receive the amplified vibrations from the OHCs
They convert the vibrations into neural excitation patterns that are sent to the brain
How much sensitivity loss occurs with the loss of OHCs?
40 to 50 dB HL
What is the amplification provided by the OHCs dependent on?
The potential difference between the scala media and scala tympani (across the OHCs)
This is called the endocochlear potential
Why is the endocochlear potential unique?
It is an extracellular resting potential rather than an intracellular one
It has a positive voltage
It is unusually large (80 to 100 mV)
It is generated from the stria vascularis by the sodium potassium pump
It acts as a battery to drive current through the cochlear hair cells when they move in response to sound stimulation
Does the EP varies from the base to the apex of the basilar membrane?
Yes
Greater that the basal end, where we see the biggest effect
Does the utricle, saccule, and SCC have much smaller resting potentials?
Yes
Is the cochlear amplifier dependent on the EP?
Yes
If the EP drops by 30 mV, the sensitivity to higher frequencies decreases by about 30 dB
What is age-related hearing loss caused from?
Deterioration of the cochlear battery (EP)
What is the cochlear power supply?
Made up of the cochlear lateral wall tissues including the stria vascularis (generates the EP)
What is the power supply dependent on?
Potassium recycling
Pulling it back into the endolymph after the hair cells push it into the perilymph
Generation of the EP in the stria vascularis
How does the recycling pathway work?
This recycling pathway uses a network of cochlear supporting cells along the BM and ion transport fibrocytes along the lateral wall
The supporting cells and the fibrocytes are connected by gap junctions
What is the potential gradient at the top of the OHCs?
About 160 mV
80 mV in the endolymph and -80 mV in the body of the hair cells
Is there a constant flow of current from the scala media into the hair cells due to the potential?
Yes
The potential changes produced by the flow of these electric currents is the cochlear microphonic (CM)
What is the transduction mechanism?
Consists of the IHCs and the afferent auditory nerve fibers
IHCs passively detect BM vibrations and excite the afferent auditory nerve fibers that synapse at the base of the IHCs
In young animal ears raised in quiet, the sensitivity range of afferent nerve fibers covers an intensity of 0 to 90 dB SPL
Are IHCs generally less susceptible to damage than OHCs?
Yes
Even so, animals raise in quiet still showed significant shrinkage and loss of afferent nerve fibers
Still observed with normal IHCs
Indicates a pure aging effect independent of environmental factors
What are some common manifestations of presbycusis?
Loss of sensitivity for high frequency sounds (> 1000 Hz) resulting in a sloping high frequency SNHL
Difficulties with speech perception especially in noisy and otherwise adverse acoustic environments (e.g., reverberation + noise) becomes more pronounced
Distorted loudness perception, i.e., recruitment
Do speech discrimination abilities become progressively worse as high frequency hearing loss increases beyond 2000 Hz?
Yes
Is the audibility of low energy high frequency consonants attenuated by peripheral hearing loss?
Yes
When you start losing your hearing, it is very hard to understand the meaning of sound (losing those consonants)
Can upward spread of masking also deteriorate speech perception?
Yes
What is the most common configuration of presbycusis?
Sloping SNHL
What other configurations can be seen besides sloping?
Flat configurations
Mid to late stages
Present the least problems with speech perception abilities (less distortion)
When is the poorest performance on WRS observed?
Between the ages of 71 to 90
Due to increasing age and reduced hearing sensitivity beyond 60 dB HL
Cognitive decline could play a role too
Why is there a need to classify different presbycusis?
To improve differential diagnosis
Individualized biomedical intervention (requires the knowledge of the site of lesion)
What is schuknecht’s classification of presbycusis?
4 types of presbycusis based on postmortem histopathological evaluation
What are the types of presbycusis by schuknecht’s classification?
Sensory presbycusis - loss of primary cochlear outer hair cells and supporting cells
Neural presbycusis- loss of afferent cochlear neurons
Metabolic/strial presbycusis- atrophy of the stria vascularis and lateral wall and loss of EP
Mechanical presbycusis (no evidence) - stiffening of BM and organ of corti (may be a severe case of metabolic presbycusis)
What is Killion and Fikret-Pasa classification?
3 types of SNHL on loudness sensations and intelligibility considerations
Goal was to make a better hearing aid
Wanted to look at other considerations other than pure-tones
What was type 1 of presbycusis in the Killion and Fikret-Pasa classification?
Mild to mod SNHL (no worse than 45 to 55 dB HL), normal loudness sensation, consistent with OHC loss only with normal IHC function
What was type 2 of presbycusis in the Killion and Fikret-Pasa classification?
Moderately severe hearing loss (about 60 dB HL) with no region of completely normal loudness sensation
Partial recruitment
OHC and some IHC loss
Less information being transmitted to the brain with fewer available redundant speech cues (work harder to understand)
Deficits in speech intelligibility, especially in noise, even with the best hearing aids
What was type 3 of presbycusis in the Killion and Fikret-Pasa classification?
Severe hearing loss (about 75 dB or greater)
Loudness and intelligibility is considerably affected
Loudness ceases to be a primary concern
IHC loss
Speech understanding range is narrow
Do best when speech is presented close to UCL
Recruitment common
What is NIHL anatomically characterized by?
Loss of hair cells (initially OHCs)
Loss of secondary or supporting cells
Secondary neural degeneration
What is NIHL physiologically characterized by?
Threshold elevations of APs of the VIIIth N, ABR, and evoked potentials
Loss of cochlear nonlinearities (OAEs absent)
Endocochlear potential is generally unaffected in NIHL
Degeneration of stria vascularis not typical
What is presbycusis anatomically characterized by?
Degeneration of the stria vascularis and lateral cochlear wall
Compromised blood supply (due to the degeneration of the stria)
Mostly normal sensory cells except in the most basal and apical turns
Degeneration of the spiral ganglion
What is presbycusis physiologically characterized by?
Reduction of the endocochlear potential
Is presbycusis a vascular, metabolic, and neural disorder?
Yes
What is the gold standard test for hearing?
Audiogram
Measures auditory thresholds and is a sensitive gauge of cochlear hair cell damage
What is the loss of connection between auditory nerve fibers and hair cells?
Synaptopathy
A kind of hidden hearing loss
Does synaptopathy occur before the threshold elevates?
Yes
Is the audiogram a good indicator of damage to auditory nerve fibers?
No
Is synaptopathy an important component of noise induced and age related hearing loss?
Yes, it has become widely accepted now
Even TTS can be an indication of immediate and irreversible damage to auditory nerve fibers and beginnings of hidden hearing loss
Might hidden hearing loss be the reason that elderly individuals can hear but cannot understand what people are saying?
Yes
