Vestibular System

Front 1

3 reflexes controlled by the vestibular system

Back 1

- Maintenence of posture and muscle tone (control anti-gravity muscles)
- Spatial orientation
- Maintain equilibrium and gaze during movement [includes: Vestibulo-ocular reflex (VOR)]

Front 2

Foveation

Back 2

The goal of eye movements (eg, vestibulo-ocular eye movements):

Putting objects of interest on the fovea of the retina for vision.

Your eyes should normally move equal and opposite to your head movement to keep gaze fixated on a static object of interest. (Vestibulo-ocular reflex- VOR)

Front 3

Testing of VOR Cancellation

Back 3

Ask pt to fixate on your nose while you rotate their head side to side and you sway your head left to right with theirs.

Inability to keep eyes still (VOR cancellation) indicates a central problem such as a cerebellar degenerative disorder.

Front 4

VOR Cancellation

Back 4

aka Smooth Pursuit

One of two ways we can voluntarily shift gaze, the other being saccadic eye movements.

Slow tracking movements allowing you to keep a specific moving target or stimulus in your vision.

Interestingly, we are unable to initiate this without a moving visual stimulus. If you try to slowly scan w/o following a target, your eyes actually perform saccadic movements.

Front 5

semicircular canals

Back 5

6 canals
each ear has 3 semicircular canals, each at right angles to each other:
- horizontal (mediates horizontal head movements)
- anterior (move head down and eyes move up)
- posterior (moving head up and eyes move down)

Front 6

semicircular canals

Back 6

6 angular acceleration devices, 3 in each ear. They lie at right angles to each other:

- horizontal (mediates horizontal head movements)
- posterior (move head up, eyes move down)
- anterior (move head down, eyes come up)

Front 7

Utricle and Saccule

Back 7

4 sensors for linear movements, 2 per ear. Oriented at right angles.

fore and aft
side to side

Front 8

circularvection
and
linearvection

Back 8

The visual system is confused between self-movement and outside movement.

Circularvection- ex. Sitting close to a movie screen in an action-packed movie, at times it feels as if you are the one moving in an angular sense.

Linearvection- ex. You are parked in your car and a car in the space next to you backs out. For a moment you fear that your car isn't in park and you are moving.

Front 9

Nystagmus

Back 9

Jerky movements of the eyes. A reflection of conflicting information between the two vestibular systems. Can create vertigo.

Front 10

Vertigo

Back 10

The illusion of self or environmental movement. Mostly rotary, but not always.

May have one perception of motion with their eyes opened, and a different, opposite feeling with their eyes closed.

Conflict between your eyes being visually static and detecting movement in a vestibular sense.

(eg, reading a book while riding in a car; booking the inner cabin on the cruise ship).

The movement messages are transmitting to the tegmentum of the brainstem, which is next door to the emetic center.

Front 11

Otoconia

Back 11

"Stones" made from calcium carbonate crystals imbedded in the otolithic membrane (utricle and saccule).

Function to make the membrane sensitive to gravity.

Front 12

Macula

Back 12

The sensory epithelium of the otolith organs. (The analogue in the semicircular canals are the cristae.)

Here, the hair cell bundles are embedded in a gelatinous layer and topped with the otolithic membrane, which has otoconia embedded within it.

Hair cells are arranged on either side of the striola, oriented in opposite directions.

Front 13

Skew deviation

Back 13

A supranuclear misalignment of the two eyes that can be overcome with testing of the muscles. Pt will have diplopia.

(All of the muscles are normal. The brain is being tricked.)

With a vertical misalignment, pt may compensate by tilting the head to attempt to achieve foveation.

Front 14

Oscillopsia

Back 14

The illusion of movement caused by nystagmus, poor VOR gain (decreased transmission rate from head movement to eye movement) from bilateral defects, or eye muscle weakness during eye movement.

It could be related to vertigo or nystagmus, or it could be related to not having a good VOR gain and so you have to constantly add saccadic eye movements.

Or, having an internuclear ophthalmoparesis, such as an MLF lesion that causes one eye to move normal and fast while the other eye slowly must catch up with it.

Front 15

Duration of Vertigo Implicates Etiology

Back 15

-Seconds: BPPV
-Minutes: Vertebrobasilar insufficiency (VBI)/Migraine/Ictal (ie. Seizure)
-Hours: Meniere's
-Days: Neuritis (Inflammation)/Labyrinthine Infarction

Front 16

Phobic Postural Vertigo

Back 16

A subjective balance disturbance with onset following a recent illness, particularly a vestibular event, that has since resolved. Characterized by fluctuating unsteadiness, anxiety and vegetative sx. Vertigo attacks triggered by sensory-rich environments, bridges, malls, staircases, auditoriums. OCD personality type.

Front 17

Aminoglycoside risks

Back 17

Ototoxicity- usually permanent

- Onset up to 1 week after d/c
- Delayed dx in 32/36 pts
- Genetic predisposition
- No known safe blood level, total dose, or duration to ensure prevention of ototoxicity

Front 18

Drugs and Dizziness

Back 18

- Alcohol changes the specific gravity of cupular endolymph.

- Tranquilizers- depress central integration

- Anti-hypertensives

- Antobiotics/Anti-neoplastic agents/excessive aspirin: Vestibular hair cell loss

Front 19

Bilateral vestibular loss

Back 19

Sx:
- no vertigo
- no nystagmus
- oscillopsia
- imbalance
- impaired dynamic visual acuity and VOR gain (test w/ high acceleration movement)