71 - Vestibular Function

Question 1

Describe the utricle and saccule… What do they make up?

Answer 1

They make up the otolith organs

Question 2

What do otolith organs contain? What is their function?

Answer 2

Otolith organs contain otoliths – Calcium carbonate crystals that are denser than any other tissue and can bend cilia of hair cells

Question 3

What is the result of having otoliths?

Answer 3

• Result – organs sensitive to linear acceleration and position of head with respect to gravity
• Nerves have relatively steady high firing rate when head is upright and changes in response to head tilt

Question 4

What else do the utricle and saccule contain?

Answer 4

Macula (mainly on the floor of the utricle)

• When head is upright, macula of the utricle is oriented almost parallel to the ground
• When head is upright, macula of the saccule is vertical to the ground

Question 5

Semicircular canals

Answer 5

A semicircular canal or a semicircular duct is one of three semicircular, interconnected tubes located inside each ear. They are arranged at right angles to each other. The three canals are:

• the horizontal semicircular canal (also known as the lateral semicircular canal),
• superior semicircular canal (also known as the anterior semicircular canal),
• and the posterior semicircular canal (also known as the inferior semicircular canal).

Question 6

What is the function of the semicircular canals?

Answer 6

• The brain compares firing rate in nerves coming from right and left ears
• The anterior canal output on one side is compared to output from the posterior canal from the other ear
• The brain responds to angular acceleration by changing firing rate from tonic firing level = Phasic (endolymph stops moving)

Question 7

What are vestibular hair cells?

Answer 7

The receptor cells of the vestibular system

They transduce minute displacements into behaviorally relevant receptor potentials, providing the basis for vestibular function

Question 8

Describe the deterioration pattern of vestibular hair cells

Answer 8

system deteriorate w/age

40% decrease by 70 years of age

Question 9

What is the vestibule-ocular reflex (VOR) and how does it function?

Answer 9

• VOR is a reflex that moves eyes in response to small head turns
• Eyes move in the opposite direction of the head turn in order to facilitate fixation on a visual target during head turning
• If you lost this reflex, it would make it impossible to fixate on an object while turning the head

Question 10

Is this automatic or something you initiate?

Answer 10

Automatic - you will have this reflex even if your eyes are closed or in the dark

Although this is automatic, visual input will help to keep it accurate

Question 11

What drives the VOR reflex?

Answer 11

Vestibular input

This is why you don’t need to have your eyes open

Visual input HELPS, but only vestibular input is needed for the reflex to occur

Question 12

Describe the nystagmus seen with vestibular stimulation

“vestibular nystagmus”

Answer 12

There are two phases - slow and fast

It is a type of jerk nystagmus with the slow phase of movement in one direction and the fast phase (jerk of the eyes) in the opposite direction.

The nystagmus seen with vestibular stimulation is normal, a physiologic nystagmus

Question 13

Describe rotatory nystagmus

Answer 13

The slow phase of nystagmus

This phase compensates for turn, so if turning patient to RIGHT slow phase of nystagmus to LEFT

Slow is OPPOSITE of head turn

Question 14

What is post-rotatory nystagmus?

Answer 14

This is vestibular nystagmus that occurs AFTER a person stops turning

Question 15

Why does this occur?

Answer 15

Since endolymph has inertia it continues to move after head stops turning, so even though person is now stationary, signals from semicircular canals make person feel that they are turning in OPPOSITE direction from original movement.

Question 16

Give an example of post-rotatory nystagmus

Answer 16

• Turn patient to the left
• When they are stopped, they have an illusion they are turning to the right
• Once a patient is stopped, the eye movements correlate to their illusion
• So, turn patient left, illusion is right, slow phase of post-rotatory nystagmus is left and fast phase is to the right

Fast phase is always opposite to the direction of the preceding slow phase (helps you get back to midline)

Question 17

Give a summary of the directions of the slow and fast phases in post-rotatory nystagmus

Answer 17

In post - rotatory nystagmus the SLOW phase of nystagmus is in the SAME direction as the previous turning movement. In post - rotatory nystagmus, the fast phase of nystagmus is in the opposite direction to the previous turning movement.

Question 18

Do patient’s in comas demonstrate vestibular nystagmus?

Answer 18

Patients in a coma can have the slow movement of the VOR, but they do not make the fast movement back to the midline.

Question 19

How else can you test nystagmus other than turning the patient?

Answer 19

Caloric testing with cold water

Question 20

Describe how you do caloric testing for vestibular function

Answer 20

Patient’s VOR can be tested without even turning the patient. If a patient is lying in bed with his head elevated about 30 degrees, his horizontal semicircular canal will be almost vertical. Convection movements of the endolymph can then be produced by irrigating one ear with either warm or cold water. Before testing you should make sure that the tympanic membrane is intact.

Question 21

How will a normal patient react to the cold water?

Answer 21

If you test with cold water, a conscious patient will have slow phase of nystagmus towards irrigated ear and fast phase away from irrigated ear.

Question 22

How can you remember this?

Answer 22

The fast phase is more apparent to an observer so the mnemonic to remember direction is COWS (Cold water - fast phase to Opposite ear; Warm water - fast phase towards Same side). If the brainstem and vestibular systems are intact, both eyes will move.

Question 23

What type of injury can cause absent vestibular function?

Answer 23

Brainstem damage at the level of the eighth nerve can cause the VOR to be absent

Question 24

What is doll’s eye maneuver?

Answer 24

It is a way of evaluating brainstem function in an unconscious patient

Question 25

How do you perform the doll's eye maneuver?

Answer 25

- With the patient lying face upwards the horizontal canals are almost in the vertical plane and thus are stimulated when the head is turned from side to side. - You only see the “slow phase of nystagmus” which is opposite to the direction of head turn since a comatose patient does not have the fast phase. - As the patient’s head is turned from side to side the eyes move opposite to the direction of head turn and seem to look up at the examiner.

Question 26

What is positional vertigo? What is its full name?

Answer 26

Benign paroxysmal positional vertigo (BPPV) It is one of the most common types of vertigo and is called "benign" because it is not caused by intra-cranial neoplasm, but by free-floating particles within the semicircular canal

Question 27

What causes positional vertigo?

Answer 27

• Can be caused when part of an otoconium becomes detached from the otoconial membrane of the utricle then lodged in or near an ampulla (usually of the posterior canal). This makes this canal sensitive to gravity so in certain positions there are abnormal vestibular sensations Detachment of otoconia can result from head trauma or viral labyrinthitis.

Question 28

How long to episodes of vertigo last?

Answer 28

* Each episode of vertigo usually lasts for 40 seconds or less * Vertigo will recur until debris leaves canal

Question 29

Is there hearing loss with vertigo?

Answer 29

No - Since this is purely a vestibular problem it does not cause a hearing loss

Question 30

What is Meniere syndrome?

Answer 30

- Meniere patients typically develop a sensation of ear fullness and pressure with transient decreased hearing (sensorineural) and tinnitus in an ear. - This is accompanied by severe, acute vertigo. The vertigo can cause nausea, and vomiting.

Question 31

What causes Meniere syndrome?

Answer 31

Probably due to imbalance between production and reabsorption of endolymph – the fluid in the membranous labyrinth

Question 32

Describe the course of Meniere syndrome

Answer 32

Disease has variable course. Attacks can last minutes to hours and reoccur many times over several years. Some patients have spontaneous recovery, many have permanent damage.

Question 33

What is the difference between dizziness and vertigo?

Answer 33

When you're dizzy, you may feel lightheaded or lose your balance. If you feel that the room is spinning, you have vertigo.

Question 34

What are the four outputs of the vestibular nuclei?

Answer 34

1 - Medial longitudinal fasciculus 2 - Lateral vestibulospinal tract 3 - Medial vestibulospinal tract 4 - Higher centers

Question 35

What is the function of the vestibular nuclei projections in the medial longitudinal fasciculus?

Answer 35

- These fibers travel to the motor nuclei of the extraocular eye muscles - This projection functions in the vestibulo-ocular reflexes

Question 36

What is the function of the vestibular nuclei projections in the lateral vestibulospinal tract?

Answer 36

- This tract participates in control of motor neurons of limb and trunk for maintenance of balance and posture.

Question 37

What is the function of the vestibular nuclei projections in the medial vestibulospinal tract?

Answer 37

- This tract helps control neck muscles and thus head position.

Question 38

What is the function of the vestibular nuclei projections to higher centers?

Answer 38

- Vestibular output via thalamus to cerebral cortex might provide for conscious awareness of body orientation and motion. - Vestibular information integrated with visual and proprioceptive information.

Question 39

What are the main inputs to the vestibular nuclei?

Answer 39

- Neck proprioceptors | - Neck motor commands (i.e. bend the neck)

Question 40

What is the function of the input into the vestibular nuclei (motor and proprioception)?

Answer 40

- This information modulates discharge of vestibular neurons projecting down medial and lateral vestibulospinal tracts. - When a person voluntarily bends their neck, it requires a different postural response than that necessary if the head is tilted (but the neck is straight) because a person is falling forward. - This information is vital for the vestibular system to respond correctly.

Question 41

What are the three major roles of the vestibular system?

Answer 41

1. Subjective awareness of body position and movement of body in space. 2. Postural tone and equilibrium 3. Stabilization of the eyes in space during head movements

Question 42

Which sensory systems help the vestibular system to fulfill these roles?

Answer 42

- Visual system | - Proprioceptive system