Physiology and Clinical Aspects of Hearing and Balance Flashcards
What is the function of the middle ear?
Transforms acoustic energy from the medium of air to the medium of fluid by acting as a sound amplifier
What allows the middle ear to act as a sound amplifier?
- The area effect of the tympanic membrane (TM) – ratio of TM to stapes footplate 17:1
- Lever action of ossicular chain – ratio to pressure on stapes footplate on pressure on malleus is 3:1
What is the function of the eustacian tube?
To allow equalisation of ear as the tympanic membrane function is optimal when the middle ear pressure is the same as the atmospheric pressure
What movement opens the eustacian tube?
Swallowing and yawning
What conditions cause loss of energy from the TM to the oval window?
- OM with effusion
- Small perforation
- Eroded incus
- Otosclerosis
How does OM with effusion cause loss of energy form TM to oval window?
- Energy lost from middle to inner ear conductive hearing loss
- Fluid increase behind the TM affects movement so sound gets depleated
What cause OM with effusion?
Chronic dysfunction of the eustacian tube leads to a relative negative pressure in the middle ear. This can lead to retraction of the tympanic membrane and sometime formation of middle ear fluid
How does a small perforation cause loss of energy form TM to oval window?
The tympanic membrane helps with the travelling of sound and a small perforation will cause small hearing loss, and a sub-total (large) perforation would cause larger scale hearing loss.
How does an eroded incus cause loss of energy form TM to oval window?
Due to erosion of bone causing conduction to be cut off
How does otosclerosis cause loss of energy form TM to oval window?
New deposition of bone fixing to ossicles hinder their movement and thus unable to carry out conduction
What structures are involved with hearing?
- Two sections of organ: vestibule (balance) and cochlear (hearing)
- Duct of cochlea - closed tube full of fluid
What type of fluid is in the cochlear?
- Endolymph - in the membranous labyrinth (sensory cells)
* Perilymph - surrounds endolymph
What are the steps the sound transmission?
- Sound waves enter through external
auditory canal - TM vibrates and sends sound waves through ossicles
- Sound waves reach cochlea and travel
along one side through perilymph and
around the other, where it travels along
the basilar membrane. - Pressure waves go through and vibrates
the basilar membrane at different
frequencies, and moves the cilia to generate
an electrical signal through the cochlea nerve
What are the names of the three small bones in middle ear?
Malleus, incus and stapes
What is the benefit of the inner hear cells?
Improves flexibility of basilar membrane and fine tunes it
How does the basilar membrane cause hair cells to send action potentials?
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 outer hair cells. This motion depolarises the inner hair cells which turn sets off sets of afferent electrical nerve impulses
Describe the perception of frequency in the ear
Higher frequency sound waves are picked up most lateral (closest to TM) and low tones more medial, but all go through the cochlea nerve
Describe the auditory pathway
Cochlea nerve cochlea ganglion -> crossover of fibres at base (identifies what side of sound is coming from) Medulla -> mid brain -> cortex
Name five tests that are used to assess hearing and middle ear function
- Clinical testing
- Tuning fork test
- Audiometry
- Tympanometry
- Objective testing
Describe the Weber test
A test of lateralisation
• Detects unilateral conductive hearing loss
• Blockage causes sound to travel out of ear, but bounce back to the cochlea of affected side so sound is heard louder in affected ear
What is the Rinne’s test?
A test that compares loudness of perceived air conduction to bone conduction in one ear at a time
What is the purpose of 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 for each ear.
What are four audiometry tests?
- Pure tone audiometry
- Visual reinforcement audiometry
- Play audiometry
- Tympanometry
What tests are used for children?
- 3-6yrs: pure tone audiometry with play conditioning techniques
- 6mnth-3yrs: Visual reinforcement audiometry – child gets a visual reward when responding
- 6mnths-2/3yrs: Distracting testing
- 0-6mnths: Universal newborn hearing screening was introduced in UK in 2005, the usual method is with optoacoustic emission
Name types of objective testing investigations
- Otoacoustic Emissions (OAEs)
- Auditory Brainstem Response (ABR)
- Tympanometry
What are otoacoustic emissions?
Sounds given off by the inner ear when the cochlea is stimulated by a sound. When sound stimulates the cochlea, the outer hair cells vibrate. The vibration produces a nearly inaudible sound that echoes back into the middle ear.
What is otoacoustic emission testing?
The sound given off by the inner ear can be measured with a small probe inserted into the ear canal
How does hearing loss affect otoacoustic emissions?
People with normal hearing produce emissions, and those with loss of ~30Db do not produce these soft sounds
What can otoacoustic emission tests detect?
Blockage in the outer ear, middle ear fluid and damage to the outer hair cells in the cochlea
What is auditory brainstem response (ABR) testing?
Gives information about the inner ear (cochlea) and brain pathways for hearing, by pasting electrode of the head and recording brainwave activity in response to sound
How is ABR used to test children?
Screening test in newborn hearing screening program, but only one intensity or loudness level is checked
How does tympanometry work?
Detects fluid in the middle ear, perforation of the eardrum or wax blocking the ear canal. It pushes air pressure into the ear canal, making the eardrum move back and forth. The test measures the mobility of the eardrum
Name four management strategies of hearing loss
- Surgery of outer and middle ear
- Sound amplification
- Direct stimulus of cochlear nerve cell
- Intracochlear modification (future)
Name five types of hearing aids
- Hearing aid
- Open fir hearing aid – used for high frequency loss usually in the elderly
- Bone anchored hearing aid (BAHA)
- Middle ear implant
- Cochlear implant
What structures of the inner ear are responsible for balance?
- Otolith organs: saccule and utricle – detect movement forwards/backwards, up/down and side to side
- Fluid filled which flows through the canals
Describe the structure of the otolith organs
Sensory epithelium is ciliated with the gel on cilia containing calcium bicarbonate crystals
What excites the hair cells?
They are excited when the stereocilia are displaced in the direction of the arrows:
• Utricle is horizontal: backwards/forwards, side to side
• Saccule is verticle: up and down
Where are the hair cells located?
Ampullae of semi-circular canals (expanded parts)
What are the features of the semi-circular canals?
- Filled with endolymph and surrounded by perilymph
- Movement causes the semi-circular fluid to move which in turn moves the cupula, and the hair cells send this information to the brain
- Movement of head also cause excitation on one side of the head and inhibition of the other for fine tuning of movement sensitivity
What is the vestibulo-occular reflex?
The VOR stabilises gaze by moving eyes in order to compensate for head and body movement. This fixes image in retina for clear sight.
Name three conditions of the inner ear which affects balance
- Benign paroxysmal positional vertigo
- Vestibular neuritis
- Meniere’s disease
Name a common clinical condition that affects balance
migraine
What happens in benign paroxysmal positional vertigo?
Connection between utricle and semi-circular canal, but calcium carbonate can become dislodged and move into the canals and this movement causes dizziness
What happens in vestibular neuritis?
Failure of vestibular nerve - both nerves L + R need to work in harmony, if one doesn’t work it causes dizziness and sickness
What is meniere’s disease?
Severe vertigo, hearing and balance loss and is a possibly autoimmune disease
