Hearing Loss

Question 1

How frequent are hearing disorders?

Answer 1

2% of the worlds population have a disabling hearing loss
10% of the population is to some extent hearing impaired
20% suffer from other hearing disorders such as tinnitus (ringing in the ears) 15% and menieres disease (1-2%)

Question 2

How do we hear?

Answer 2

The pinea and external auditory canal transmit sound/pressure waves down to the tymphanic membrane this will transmit their signals to the cochlea through transmitting them first through the malleus followed by the incus followed by the stapes
The cochlea is connected to nerve which will send a signal to the brain

Question 3

What is the cochlea?

Answer 3

This is a peripheral auditory organ which is deeply embedded in the temporal lobe it is well protected by the bony capsule
It is a hollow organ with 3 interconnected fluid-filled compartments which spiral around a central bony part known as medulus like a staircase there are two openings (the round and open window)

Question 4

What are the different cochlear compartments?

Answer 4

There is the scala vestibule which is separated from the scala media by the very thin Reissners membrane
The scala media is separated from the scala lympani by the organ of corti (which contains the sensory cells and their afferent and efferent nerves)
The lateral wall of the cochlea secretes potassium which is the main driving force of sensory transduction
The secretion is mainly performed by cells in the stria vascularis while potassium recycling is performed mainly by cells in the spiral ligament

Question 5

What are the inner ear fluids?

Answer 5

There is perilymph which has high sodium and very low potassium concentration
There is also endolymph which has high potassium concentration and very low sodium concentration
This difference generates the electrical potential between scala media and scala typani which is responsible for hair cell depolarisation

Question 6

What are the 4 types of cells in the organ of Corti?

Answer 6

The inner hair cells, these are true sensory cells and are fewer in frequency
The outer hair cells, we have more of these cells and their main function is to amplify the signal generated by inner hair cells
Commonalities between the cells are cilia on their apical end which are the site of sensory transduction through channels which open due to the friction between the cilia and the TM membrane
Pilar cells which provide organ structure support
Deiters cells which provide metabolic support

Question 7

What are the three types of hearing loss classified based on their location?

Answer 7

Conductive hearing loss where there is damage to the outer ear this may occur with conditions like otitis media
Sensorineural hearing loss is caused by damage to sensory hair cells and nerve cells in the cochlea
Auditory processing disorders are caused by damage to the central auditory pathways

Question 8

What may result in sensorineural hearing loss?

Answer 8

Genetic disorders
Infection/inflammation
Trauma
Intense sound
Ototoxic drugs
Aging

Question 9

How common is congenital hearing loss and what are the consequences of this?

Answer 9

This 2-3 per 1000 children and it results in poor language and behavioural developments, lower literacy and academic achievements
Genetic factors are thought to cause more than 50% of all incidents of congenital hearing loss in children
There are currently 30+ genes and 150+ mutations underpinning deafness
Non-genetic causes of hearing loss include inauterine infections, prematurity, hypoxia, hyperbilirubinemia and maternal drug/alcohol use

Question 10

How does infection/inflammation cause hearing loss?

Answer 10

The spread of a bacterial or viral infection form the middle ear (otitis media)
As a direct consequence of infection or tissue injury to the inner ear or hearing nerve
Meningitis is also a source of inner ear inflammation and results from infection through the cerebrospinal fluid
The inner ear can rapidly mount an inflammatory response which can cause bystander tissue injury, this response can cause permanent hearing loss

Question 11

What are the most common causes of acquired hearing loss?

Answer 11

Presbyacusis (age-related)
Noise trauma
Ototoxic drugs

Question 12

What are the common types of pathology of hair loss?

Answer 12

There is a loss of hairs cells, particularly outer hair cells rather than inner hair cells which are replaced by scars this is a permanent injury
Nerve degeneration may often follow this

Question 13

What is a commonalty between drug associated and age associated hearing loss?

Answer 13

They both start at higher frequencies first (at the basal end of the cochlea)

Question 14

How common is noise induced hearing loss today?

Answer 14

This is the most common occupational disease in both an industrial and military setting with 60% of US war veterens suffering hearing loss
Leisure activities increasingly affect hearing in the young generation
The deafness related cost is more than 50 billion US dollars a year and predictions state that over 35% of people will be hearing impaired by 2050

Question 15

What are the effects of noise on the cochlea?

Answer 15

This can damage basically any region of the cochlea with the highest impact being on the sterio cilia which may become distorted with high altitude movement this may become permanent if the outer hair cells undergo apoptosis or necrosis
Inner hair cells and the inner hair cell synapse are also sensitive to noise where excessive release of glutamate can cause damage to the nerve ending
This is also cause of swelling of spiral ganglion neurons
Pilar cells buckle under pressure when exposed to noise
It may cause swelling followed by loss of stria vascularis and loss of spiral ligament fibrocytes affecting the electrochemical potential preventing proper signal transduction

Question 16

How does glutamate excitotoxicity damage hearing?

Answer 16

Glutamate is the principal neurotransmitter used in the inner hair cell auditory nerve synapse with excessive noise causing excessive release of this into the synapse where it will case first swelling and then disruption of hearing or sudden deafness
However due to synaptic plasticity this may recover if the injury is not repetitive or prolonged (as this would cause the neuron to activate its apoptotic program)

Question 17

What is the link between noise damage and inflammation?

Answer 17

After 24 hours of exposure to high levels of noise inflammatory cells can be observed in the cochlea
These mop up cell debris but they may release inflammatory cytokines to cause bystander tissue damage

Question 18

How does oxidative stress provide a mechanism of hearing loss from noise?>

Answer 18

Oxidative stress in the cochlea is the most common factor of hearing loss from noise and can be a result of aminoglycoside antibiotics, ototoxic anticancer drugs and aging
These can help produce free radicals which contribute the loss of function after noise exposure

Question 19

How are free radicals formed as a result of noise?

Answer 19

During noise exposure the electron transport chain of the mitochondria uses large amounts of O2 creating large amounts of free radical by-product
The cells most affected are those with a high metabolic rate like outer hair cells or those of the stria vascularis

Question 20

What is the progression of age-related hearing loss/

Answer 20

The average age of onset is 45-54
44% of the population is affected by age 69
665 of the population is affected by age 79
Over 90% of the population is affected over age 80
Its starts with loss of high frequency and then progresses to lower frequencies

Question 21

What may cause Persbyacusis?

Answer 21

A mixture of acquired auditory stresses, trauma and ontological disease superimposed upon an intrinsic, genetically controlled ageing process

Question 22

What might untreated hearing loss lead to?

Answer 22

It may contribute to social isolation, depression and loss of self esteem

Question 23

What is Persbyacusis characterized by?

Answer 23

Reduced hearing sensitivity and speech understanding in a noisy environment
Slowed central processing of acoustic information
Impaired localisation of sound sources

Question 24

What is the pathology of Persbyacusis?

Answer 24

This affects the same cells as noise induced hearing loss, loss of sensory hair cells, stri vascularis etc

Question 25

How is Persbyacusis classified?

Answer 25

Sensory (outer hair cell loss) this is related to accumulated environmental noise toxicity
Neural (neuronal cell loss)
Metabolic (strial atrophy) this has a high heritability index
Mixed and indeterminate (the later 25% of cases)

Question 26

How is cochlea aging studied?

Answer 26

These typically use animal studies as because the cochlea is deeply embedded in the temporal lobe making it hard to access for study
For example gerbils raised quiet have just as much, or more hearing loss with age than groups of noise-reared animals

Question 27

What is the dead battery theory of Persbyacusis?

Answer 27

Degeneration of the stria vascularis is the most prominent element of this condition where a loss in Na/K ATPase results in reduced potassium secretion and a decline in endocochlear potential

Question 28

What are the mechanisms of Persbyacusis?

Answer 28

There are both genetic and environmental factors including a reduction of vascualrisation in the stria vascularis
Collagen damage
Accumulative noise exposure
Oxidative stress
Mitochondrial damage
Apoptosis

Question 29

What drugs cause ototoxicity?

Answer 29

Permanent hearing loss is caused by two major classes of drugs aminoglycoside antibiotics, Platinum-based chemotherapeutic agents
Both damage the hair cell in the basal turn of the organ of corti, spiral ganglion neurones and the lateral wall tissues resulting in functional deficits

Question 30

How do aminoglycodises get into hair cells and cause toxicity to the ear?

Answer 30

These get into the sensory cells through the cilia where they complex with iron to stimulate the production of free radicals which activate JNK which translocates into the nucleus to activate the intrinsic apoptotic pathway

Question 31

How do cisplatin cause ototoxicity?

Answer 31

These enter the ciliated cells and form monohydrate complexes which will stimulate NOX-3 to produce ROS which will activate the JNK transcription factor to trigger apoptosis
These drugs are associated with tinnitus and bilateral high frequency and sensorineural hearing loss

Question 32

What are the treatment options for hearing loss?

Answer 32

Use of a hearing aid to amplify sound or a cochlear implant in the profoundly deaf
Noise management and hearing loss prevention are still the most effective with noise management and monitoring, use of protection devices, hearing monitoring and education

Question 33

What are the pharmacological interventions which can reduce hearing loss?

Answer 33

Restoring the normal balance of free radicals with antioxidants this can be done with the application of exogenous antioxidant molecules locally or systemically into the body or endogenously by using sound conditioning
Reducing glutamate excitotoxicity with NMDA receptor antagonists
Maintaining adequate cochlear blood flow during and after noise
Suppressing inflammation
Inhibiting pathways to apoptotic cell death to preserve hair cells