71 - Vestibular Function Flashcards
Describe the utricle and saccule… What do they make up?
They make up the otolith organs
What do otolith organs contain? What is their function?
Otolith organs contain otoliths – Calcium carbonate crystals that are denser than any other tissue and can bend cilia of hair cells
What is the result of having otoliths?
- 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
What else do the utricle and saccule contain?
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
Semicircular canals
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).
What is the function of the semicircular canals?
- 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)
What are vestibular hair cells?
The receptor cells of the vestibular system
They transduce minute displacements into behaviorally relevant receptor potentials, providing the basis for vestibular function
Describe the deterioration pattern of vestibular hair cells
system deteriorate w/age
40% decrease by 70 years of age
What is the vestibule-ocular reflex (VOR) and how does it function?
- 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
Is this automatic or something you initiate?
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
What drives the VOR reflex?
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
Describe the nystagmus seen with vestibular stimulation
“vestibular nystagmus”
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
Describe rotatory nystagmus
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
What is post-rotatory nystagmus?
This is vestibular nystagmus that occurs AFTER a person stops turning
Why does this occur?
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.
Give an example of post-rotatory nystagmus
- 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)
Give a summary of the directions of the slow and fast phases in post-rotatory nystagmus
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.
Do patient’s in comas demonstrate vestibular nystagmus?
Patients in a coma can have the slow movement of the VOR, but they do not make the fast movement back to the midline.
How else can you test nystagmus other than turning the patient?
Caloric testing with cold water
Describe how you do caloric testing for vestibular function
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.
How will a normal patient react to the cold water?
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.
How can you remember this?
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.
What type of injury can cause absent vestibular function?
Brainstem damage at the level of the eighth nerve can cause the VOR to be absent
What is doll’s eye maneuver?
It is a way of evaluating brainstem function in an unconscious patient
How do you perform the doll’s eye maneuver?
- 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.
What is positional vertigo? What is its full name?
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
What causes positional vertigo?
• 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.
How long to episodes of vertigo last?
- Each episode of vertigo usually lasts for 40 seconds or less
- Vertigo will recur until debris leaves canal
Is there hearing loss with vertigo?
No - Since this is purely a vestibular problem it does not cause a hearing loss
What is Meniere syndrome?
- 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.
What causes Meniere syndrome?
Probably due to imbalance between production and reabsorption of endolymph – the fluid in the membranous labyrinth
Describe the course of Meniere syndrome
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.
What is the difference between dizziness and vertigo?
When you’re dizzy, you may feel lightheaded or lose your balance. If you feel that the room is spinning, you have vertigo.
What are the four outputs of the vestibular nuclei?
1 - Medial longitudinal fasciculus
2 - Lateral vestibulospinal tract
3 - Medial vestibulospinal tract
4 - Higher centers
What is the function of the vestibular nuclei projections in the medial longitudinal fasciculus?
- These fibers travel to the motor nuclei of the extraocular eye muscles
- This projection functions in the vestibulo-ocular reflexes
What is the function of the vestibular nuclei projections in the lateral vestibulospinal tract?
- This tract participates in control of motor neurons of limb and trunk for maintenance of balance and posture.
What is the function of the vestibular nuclei projections in the medial vestibulospinal tract?
- This tract helps control neck muscles and thus head position.
What is the function of the vestibular nuclei projections to higher centers?
- 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.
What are the main inputs to the vestibular nuclei?
- Neck proprioceptors
- Neck motor commands (i.e. bend the neck)
What is the function of the input into the vestibular nuclei (motor and proprioception)?
- 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.
What are the three major roles of the vestibular system?
- Subjective awareness of body position and movement of body in space.
- Postural tone and equilibrium
- Stabilization of the eyes in space during head movements
Which sensory systems help the vestibular system to fulfill these roles?
- Visual system
- Proprioceptive system
