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 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
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
