Flashcards in Hearing & Balance Deck (23)
What is the function of the middle ear?
Amplifying and converting sound from acoustic energy in an air medium to a liquid medium
How does the middle ear amplify sound?
= A high area effect ratio from tympanic membrane to stapes footplate (17:1) - So a small vibration of the membrane results in a large vibration of the stapes footplate
= The lever action of the ossicular chain also results in a pressure on the stapes footplate 1.3x that on the malleus
List some common pathologies of the middle ear
- Otitis Media +/- effusion
- Perforation (small, subtotal or total)
- Eroded Incus. Often occurs with chronic middle ear infections or vasculitis
- Otosclerosis. Bone deposition in ligaments preventing movement of the oval window
- Fibrosis of ossicles to middle ear walls preventing movement
How does the inner ear contribute to hearing? (in detail)
1) Stapes pushes pressure waves into perilymph of Scala Vestibuli
2) Pressure waves travel round through the Scala Vestibuli to Scala Tympani
3) Waves stimulate the basilar membrane triggering pressure waves in the endolymph of the scala media
4) Stimulates cochlear hair cells triggering action potential in nerve fibres
what happens if there's damage to the cochlear hair cells?
Damage to them (particularly the inner ones) will lead to hearing loss and distortion of sounds.
This is known as sensorineural hearing loss
What types of tests do we have for assessing hearing?
- Clinical tests e.g. Whispered hearing test
- Tuning Fork Tests E.g. Rinne & Weber
- Objective Testing E.g. Otoacoustic Emissions (OAEs)
How does the Rinne test work?
Tuning Fork placed next to ear then against the mastoid process.
Ask the patient to tell you which is louder, the Air Conduction or the Bone Conduction.
Louder through the bone indicates conductive hearing loss in that ear
How does the Weber test work?
Tuning fork placed on middle of forehead.
- Normal or bilateral hearing loss would show as audible sound even between the two ears.
- The sound would be louder in an ear with conductive hearing loss as the sound is bounced back by the obsturction.
- It would be quieter in an ear with sensorineural hearing loss.
What are the types of audiometry?
Pure Tone Audiometry:
- Determines the faintest sound someone can hear at various frequencies
Visual Reinforcement Audiometry
- Measures ear drum mobility by pushing air into the canal to bounce it back & forth.
- Shows if the ear is stiff, loose or perforated.
What is an Otoacoustic Emission?
The sound given off by the inner ear when the cochlea is stimulated:
Sound stimulates Cochlea
--> Outer hair cells vibrate
--> Produce nearly inaudible sound
--> Echoes back into middle ear
--> Small probe in External ear canal measures it
How are Otoacoustic Emissions used as an objective test for hearing loss?
It detects outer ear blockage, middle ear fluid and outer hair cell damage
- Those with hearing loss >25-30dB dont make these sounds
- Also used in Newborn Hearing Screening Programs because you need a response from the patient
Describe noise induced hearing loss
Loud noises damage the cilia, starting at the high frequency end of the cochlea.
So a louder volume is required to hear sounds at that frequency
Its the most common form of sensorineural hearing loss.
Occurs due to aging of the auditory system
It occurs gradually, starting at the high frequency end of the spectrum
How does conductive hearing loss appear on a graph of audiometry?
Hearing via bone conduction would be around normal
But Hearing by air conduction would be lost across the frequencies.
How do we manage hearing loss?
- Hearing aids
- Middle ear implants
Cochlear implants to directly stimulate cochlear nerve cells
In the future --> Intracochlear Modification
What part of the inner ear is covered in cilia just like the cochlea?
The vestibular Apparatus!
Describe the structure of the vestibular apparatus?
Contains an ant., lateral and post. semi-circular canal.
Also two otolith organs, the Utricle and saccule
What is the function of the semi-circular canals in the vestibular apparatus of the inner ear?
They detect angular acceleration.
When you tilt your head the canal moves and the inertia of the fluid causes it to move round the canal in the opposite direction, this movement stimulates the cilia triggering depolarisation.
What is the vestibulo-ocular reflex?
It stabilizes your gaze allowing you to focus the image on your retina even as your head moves.
Basically the vestibular apparatus tell us about movement of the head and the eyes compensate by moving in the opposite direction at the same rate as the head turns, so it feels like your eyes arnt moving at all
What systems are involed in regulating balance?
- Vestibular (rotation & gravity)
- Proprioceptive (pressure)
Signals sent to the CNS (specifically the cerebral cortex, brainstem & Cerebellum)
- Vestibulo-ocular reflex
- Postural Control
List some inner ear pathologies that can affect balance?
- Benign Paroxysmal Positional Vertigo
- Vestibular Neuritis (Commonly reactivation of a previous viral infection)
- Meniere's Disease
What is meniere's disease?
Excess endolymph in the ear canals results in excessive stimulation on movement of the head.
It can also effect hearing