Physiology and Clinical Aspects of Hearing and Balance Flashcards
What does the middle ear do?
- Middle ear transforms acoustic energy from the medium of air to the medium of fluid
- To do this it acts like a sound amplifier
How does the middle ear transfer energy?
- The area effect of the tympanic membrane, TM, ratio of TM space to stapes footplate is 17:1
- Lever action of ossicular chain, ratio of pressure on stapes footplate to pressure on malleus is 1.3:1
When is the tympanic membrane function optimal?
Tympanic membrane is optimal when the middle ear pressure is the same as the atmospheric pressure
How is the pressure in the middle ear equalised?
Swallowing and yawning opens the nasopharyngeal Eustachian tube orifice allowing equalisation of pressure
What can energy loss of the tympanic membrane to oval window occur due to?
- Otitis media with effusion
- Chronic dysfunction leads to a relative negative pressure of the middle ear that can lead to retraction of the tympanic membrane and sometimes formation of middle ear fluid (otitis media with effusion)
- Causes conductive hearing loss
- Perforation
- Effect on hearing variable
- Main indication for repair is recurrent infections
- Subtotal perforation
- If sensorineural function is maintained, maximum hearing loss is 60dB
- Eroded incus
- Otosclerosis
- New bone fixing stapes footplate
- Reduces movement of stapes footplate causing a conductive hearing loss
Describe how the structure of the cochlea allows it to perform its function?
A pressure wave flows up the scala vestibuli from the piston action of the stapes through the helicotrema at the apex and down the scala tympani, the pressure differential deflects the basilar membrane of the scala media
Descibe what happens to the wave inside the corti?
As the wave travels through the cochlea, it causes movement of the basilar membrane which results in a “shearing” motion of the cilia of the inner and out hair cells. This motion depolarises the inner hair cells which in turn sets of afferent electrical nerve impulses
Describe what happens to the wave inside the cochlea?
A pressure wave flows up the scala vestibuli from the piston action of the stapes through the helicotrema at the apex and down the scala tympani, the pressure differential deflects the basilar membrane of the scala media
Descibe the process of the hearing mechanism in the cochlea?
- Footplate of stapes moves in and out of oval window creating a travelling wave in scala vestibuli and scala tympani of the cochlea
- This causes movement of the basilar membrane and movement of the inner and outer hair cells in the Organ of Corti in relation to the tectorial membrane
- The cilia of the hair cells are deflected and ion channels open
- Cations flow from endolymph into the hair cells
- Depolarisation takes place and an impulse in sent up the cohlear nerve
- Inner hair cells activate the afferent nerves
- Outer hair cells modify the response of the inner hair cells
What is tonotopic arrangement?
For every frequency there is a specified place on the basilar membrane where the hair cells are maximally sensitive to that frequency, this is known as tonotopic arrangement
How can assessment of hearing and middle ear function be done?
- Clinical testing
- Tuning fork tests
- Weber (a test of lateralisation)
- Rinne (a test that compares loudness of perceived air conduction to bone conduction in one ear at a time)
- Audiometry
- Pure tone audiometry
- Determines the faintest tones a person can hear at selected frequencies, from low to high
- During the test earphones are worn so that information can be obtained from each ear
- Visual reinforcement audiometry
- Play audiometry
- Tympanometry
- Pure tone audiometry
- Tympanometry
- Objective testing
- Otoacoustic emissions (OAEs)
- These are sounds given off by the inner ear when the cochlea is stimulated by a sound, due to the outer hair cells vibrating which produces an inaudible sound that echoes back into the middle ear. Can be measured by inserting a probe into the ear canal
- People with normal hearing produce emissions, those with hearing loss greater than 25-30dB do not produce these sounds.
- Often part of a new born hearing screening programme
- Can also detect blockage of the outer ear canal, presence of middle ear fluid and damage to the outer hair cells in the cochlea
What are the different tuning fork tests?
- Weber (a test of lateralisation)
- Rinne (a test that compares loudness of perceived air conduction to bone conduction in one ear at a time)
What does Weber and Rinne test?
- Weber (a test of lateralisation)
- Rinne (a test that compares loudness of perceived air conduction to bone conduction in one ear at a time)
What are examples of audiometry tests?
- Pure tone audiometry
- Determines the faintest tones a person can hear at selected frequencies, from low to high
- During the test earphones are worn so that information can be obtained from each ear
- Visual reinforcement audiometry
- Play audiometry
- Tympanometry
What does pure tone audiometry determine?
- Determines the faintest tones a person can hear at selected frequencies, from low to high
- During the test earphones are worn so that information can be obtained from each ear
What is objective testing?
- Objective testing
- Otoacoustic emissions (OAEs)
- These are sounds given off by the inner ear when the cochlea is stimulated by a sound, due to the outer hair cells vibrating which produces an inaudible sound that echoes back into the middle ear. Can be measured by inserting a probe into the ear canal
- People with normal hearing produce emissions, those with hearing loss greater than 25-30dB do not produce these sounds.
- Often part of a new born hearing screening programme
- Can also detect blockage of the outer ear canal, presence of middle ear fluid and damage to the outer hair cells in the cochlea
When do people not produce otacoustic emissions?
- People with normal hearing produce emissions, those with hearing loss greater than 25-30dB do not produce these sounds.
How is childrens hearing assessed?
- Pure tone audiometry with play conditioning techniques
- Ages 3 to 6 years
- Visual reinforcement audiometry
- The child gets a visual reward when responding
- Age 6 months to 3 years
- Distraction testing
- Age 6 month to 2/3 years
- Hearing screening using otoacoustic emissions
- Age 0 to 6 months
What does ABR test stand for?
Auditory brainstem response (ABR) test
What does ABR test provide?
- Gives information about the inner ear (cochlea) and brain pathways for hearing
- Sometimes referred to as auditory evoked potential (AEP)
- Test can be used with children or others who have a difficult time with conventional behaviour methods of hearing screening
- Performed by passing electrodes on the head and recording brain wave activity in response to sound, person being tested rests quietly or sleeps whilst the test is being performed as no response is necessary
- Can also be used as a screening test in new born hearing screening programs, when used as a screening test only one intensity or loudness level is checked and the baby either passes or fails
What does tympanometry assess?
- Can assist in the detection of fluid in the middle ear, perforation of the eardrum or wax blockage of the ear canal
- Pushes air pressure into the ear canal making the eardrum move back and forth, the test measures mobility of the eardrum
- Graphs are created called tympanograms
Describe the management of hearing loss?
- Surgery of outer and middle ear
- Sound amplification
- Direct stimulus of cochlear nerve cells
- Intracochlear modification
What are some different kinds of hearing aids?
- Normal hearing aid
- Open fit hearing aid
- Bone anchored hearing aid (BAHA)
- Middle ear implant
- Cochlear implant
What organs are used for balance?
-
Otolith organs
- Hair cells are excited when the stereocilia are displaced in a direction
- Ampullae of semi-circular canals
-
Vestibulo-occular reflex
- Stabilises gaze by moving eyes in order to compensate for head and body movement, fixing image on retina for clear sight
What does the vestibulo-occular reflex do?
- Stabilises gaze by moving eyes in order to compensate for head and body movement, fixing image on retina for clear sight
What systems regulate body balance?
- Input
- Visual
- Vestibular (rotation and gravity)
- Proprioception (pressure)
- CNS
- Cerebral cortex
- Brainstem
- Cerebellum
- Output
- Occular reflex
- Postural control
What are some examples of clinical conditions that affects balance?
- Benign paroxysmal positional vertigo
- Vestibular neuritis
- Meniere’s disease
- Migraine
Benign paroxysmal positional vertigo - pathology
- Loose otoconia move out of utricle into semi-circular canals most commonly the posterior canal
- When moving the head the otocania moves in the canal, stimulating the cristae and provoke vertigo and nystagmus
What is the test for posterior canal involvement of benign paroxysmal positional vertigo?
- Test for posterior canal is the Dix-Hallpike manoeuvre
Benign paroxysmal positional vertigo - treatment
- Treatment is the Epley manoeuvre
Vestibular neuritis - presentation
- Initial clinical presentation
- Acute onset of vertigo, nausea and vomiting
- Rarely hearing loss (if present then called labyrinthitis
- Nystagmus present if seen early enough
Vestibular neuritis - aetiology
- Definitive aetiology rarely proven
- Histological evidence points to viral damage of vestibular nerve rather than sensory cells of labyrinth (similar to Bell’s palsy)
- If hearing loss is present then mumps, measles or infectious mononucleosis may be responsible
Meniere’s disease - epidemiology
(how common)
- Rare condition
Meniere’s disease - presentation
- Classic triad
- Vertigo
- Hearing loss (unilateral)
- Tinnitus
Meniere’s disease?
- Endolymph produced by stria vascularis
- Hydrops due to malabsorption of endolymph in endolymphatic duct and sac
