Vestibular Foundations - Session 1: Anatomy and Physiology Part 1

1- input head movements cause the bony & membranous

vestibular labyrinth to move

2- Endolymph in the semicircular canals (which are in the vestibular labyrinth) lags behind producing a relative flow of endolymph in a opposite direction to the movement, causing deflection of cupula causing deflection hair cells, in turn causing excitation or inhibition depending on direction of hair cell movement

3- hair cell deflection triggers neural signals via superior and inferior branches to the vestibular nerve and central processing occurs at the vestibular nucleus complex (inside the brainstem) and cerebellum

4- output mechanisms control the way the body responds to this movement, these signals generate compensatory eye movements, this ensures the eyes remain focused

1- vestibular disorders: dysfunction of inner ear or vestibular system

2- neurological condition: issue with brain or nervous system e.g. MS or stroke

3- medication or alcohol: affects the CNS

4- vision impairment: poor vison or abnormal eye structures

-the purpose is so the visual system adapts & stabilises quickly so the eyes can continue tracking the target

-nystagmus occurs temporary in healthy individuals and stabilises once the head is still. BUT ITS NIT NORMAL

• rapid corrective eye movements which help clinicians diagnose the underlying cause of the nystagmus

• these movements quickly shift in one direction followed by slower movements that return them to a resting position helping maintain visual stability

e.g. if you turn your head to the right, after the slow phase, your eyes quickly move back to the right, away from the target for an instant, before they start moving back to regain focus on the target

During the slow phase, when the head is turned, the eyes move slowly in the opposite direction to keep the target in focus.

1- eye movement recordings: directly measure eye movements during head motion to evaluate VOR function (VOR)

2 - video Head Impulse Testing(vHIT): assess VOR by tracking eye movements during quick, small head turns. (VOR)

3 - caloric irrigation: stimulates the vestibular system by introducing temperature changes in the ear to provoke eye movements (VOR)

4 - Vestibular Evoked Myogenic Potentials (VEMP): evaluates the function of the otolith organs by measuring muscle responses to sound or vibration stimuli. (VOR & VCR)

5 - posturography: measures the body’s ability to maintain balance under diff conditions (VSR)

6 - rotational chair: evaluates vestibular function by rotating the patient and measuring eye movements in response (VCR & VSR)

*tests 1-3 test the VOR, tests 4-6 test VOR, VCR and VSR

Stabilizes vision during fast head movements, compensating for movements of up to 550 degrees per second.

The VOR processes information in about 15 milliseconds due to a short neural pathway involving just three neurons from the vestibular organs to the eye muscles.

-there’s 6 semicircular canals (3 in each ear) which are paired with 6 ocular muscles to ensure compensatory eye movements to head movements

-superior (anterior) semi-circular canals <-> superior & inferior Rectus muscles

-inferior (posterior) semicircular canals <-> superior & inferior Oblique muscles

-lateral (horizontal) semicircular muscles <-> Medial & Lateral Rectus muscles

vHIT & VNG provide objective data about vestibular system performance, helping clinicians identify vestibular dysfunction.

Entirely encased in temporal bone behind the ear, making direct assessment impossible; thus, all vestibular testing is indirect.

-eye movements (most common and we use VOR, the eyes are the most direct way of evaluating vestibular peripheral system)

• Postural control
  -myogenic responses

Eye movement recordings, Video Head Impulse Testing, Caloric irrigation, VOR, Vestibular Evoked Myogenic Potentials, Posturography, Rotational chair.

The VOR is responsible for maintaining stable vision by producing eye movements that are equal in velocity and opposite in direction to head movements.