Hearing & Balance

Question 1

What is the function of the middle ear?

Answer 1

Amplifying and converting sound from acoustic energy in an air medium to a liquid medium

Question 2

How does the middle ear amplify sound?

Answer 2

= 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

Question 3

List some common pathologies of the middle ear

Answer 3

• 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

Question 4

How does the inner ear contribute to hearing? (in detail)

Answer 4

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

Question 5

what happens if there’s damage to the cochlear hair cells?

Answer 5

Damage to them (particularly the inner ones) will lead to hearing loss and distortion of sounds.
This is known as sensorineural hearing loss

Question 6

What types of tests do we have for assessing hearing?

Answer 6

• Clinical tests e.g. Whispered hearing test
• Tuning Fork Tests E.g. Rinne & Weber
• Audiometry
• Objective Testing E.g. Otoacoustic Emissions (OAEs)

Question 7

How does the Rinne test work?

Answer 7

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

Question 8

How does the Weber test work?

Answer 8

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.

Question 9

What are the types of audiometry?

Answer 9

Pure Tone Audiometry:
- Determines the faintest sound someone can hear at various frequencies

Visual Reinforcement Audiometry

Tympanometry:

• Measures ear drum mobility by pushing air into the canal to bounce it back & forth.
• Shows if the ear is stiff, loose or perforated.

Play Audiometry

Question 10

What is an Otoacoustic Emission?

Answer 10

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

Question 11

How are Otoacoustic Emissions used as an objective test for hearing loss?

Answer 11

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

Question 12

Describe noise induced hearing loss

Answer 12

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

Question 13

Describe Presbycusis?

Answer 13

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

Question 14

How does conductive hearing loss appear on a graph of audiometry?

Answer 14

Hearing via bone conduction would be around normal

But Hearing by air conduction would be lost across the frequencies.

Question 15

How do we manage hearing loss?

Answer 15

Surgery

Sound amplification:

• Hearing aids
• Middle ear implants

Cochlear implants to directly stimulate cochlear nerve cells

In the future –> Intracochlear Modification

Question 16

What part of the inner ear is covered in cilia just like the cochlea?

Answer 16

The vestibular Apparatus!

Question 17

Describe the structure of the vestibular apparatus?

Answer 17

Contains an ant., lateral and post. semi-circular canal.

Also two otolith organs, the Utricle and saccule

Question 18

What is the function of the semi-circular canals in the vestibular apparatus of the inner ear?

Answer 18

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.

Question 19

What is the vestibulo-ocular reflex?

Answer 19

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

Question 20

What systems are involed in regulating balance?

Answer 20

Input:

• Visual
• Vestibular (rotation & gravity)
• Proprioceptive (pressure)

Signals sent to the CNS (specifically the cerebral cortex, brainstem & Cerebellum)

Output:

• Vestibulo-ocular reflex
• Postural Control

Question 21

List some inner ear pathologies that can affect balance?

Answer 21

• Benign Paroxysmal Positional Vertigo
• Vestibular Neuritis (Commonly reactivation of a previous viral infection)
• Meniere’s Disease

Question 22

What is meniere’s disease?

Answer 22

Excess endolymph in the ear canals results in excessive stimulation on movement of the head.
It can also effect hearing

Question 23

What non-inner ear pathologies commonly affect balance?

Answer 23

MIGRAINES

These can be paroxysmal or visually evoked.