Cranial Nerve VIII: Vestibular

Vestibular system is…

TRAINABLE

What are the functions of the vestibular system?

coordinate eye movements

maintains equilibrium

maintains head position

gives us a “sense” of head position

What is the key to understanding the vestibular system?

REMEMBER THE HAIR CELL

Hair cells are directionally sensitive, meaning…

swaying towards kinocilium tends to fire the nerve and hyper polarize the system

What do tight junctions do?

join hair cells to one another near the microvillar ends

each hair cell is bathed in…

partly endolymph and partly perilymph

Characteristics of endolymph

can accumulate too much because it is a fluid

projection of hair cells into it

drains into VENOUS SINUSES of dura mater

Disruption in normal production or drainage of endolymph results in what?

swelling of cochlear duct and utricle/saccule - MENIERE’S DISEASE

What is Meniere’s Disease?

also known as endolymphatic hydrops

makes hair cells discharge in absence of sensory input - both auditory and vestibular symptoms

What are the primary and secondary complaints of Meniere’s disease?

hearing loss and ringing in the ears

vertigo

What is in the three semicircular canals of the ear?

crista - hair cells

ampulla - swelling at the base of SCC

What are the three SCC?

anterior

posterior

horizontal - tipped posteriorly 30 degrees

What is the cupula?

gelatinous mass that swings over hair cells

Vestibulo-Ocular Reflex

fluid will swoosh cupola over the hair cells WHEN YOU TURN YOUR HEAD

changes in firing rate transmitted by CN VIII

functional result: ability to keep gaze on a target as you turn your head

What happens as you turn your head to the left?

Contraction of left medial rectus and right lateral rectus results in drifting of eye to right

What is a nystagmus?

stimulated by rotation of head/body in space

begins with VOR pattern (slow eye movement)

followed by a compensatory rapid eye movement (a saccade)

How do you name a nystagmus?

by the FAST phase… right or left “beating” nystagmus

What is a rotatry nystagmus?

occurs DURING rotation

slow eye movement OPPOSITE direction of rotation

rapid eye movement IN direction of rotation

What is a post-rotary nystagmus?

occurs AFTER rotation has ended

slow eye movement IN the direction of rotation

rapid eye movement OPPOSITE the direction of rotation

What are the three principle sources of input the CNS uses for equilibrium and posture?

visual, vestibular, somato/proprioceptive

Macula characteristics

lives in utricle and saccule

covered with gelatinous substance

contain calcium carbonate crystals called otoconia

hair cells respond to gravitational force exerted by otoliths and otolithic membrane

sensitive to LINEAR ACCELERATION - GRAVITY

What is the gold standard for identifying unilateral vestibular loss?

caloric testing

What is a normal response to caloric testing?

COWS: cold opposite, warm same

cold into R ear = LEFT beating nystagmus

warm into R = RIGHT beating nystagmus

What are the signs and symptoms of vestibular dysfunction?

nystagmus

dizziness

vertigo

ataxia and gait disturbance

What are the treatments for vestibular dysfunction?

medical - pharmacologic and surgery

mechanical - maneuvers in BPPV (repositioning otoconia)

habituation

adaptation

substitution