Vestibular Anatomy and Function

Question 1

What does the vestibular system do?

Answer 1

• perception of head motion and orientation
• angular acceleration
• linear acceleration
• position in relation to gravity
• gaze stabilization: control of eye movements during head movements to permit clear vision
• Postural adjustments/maintenance of equilibrium

Question 2

Vestibular system: peripheral

Answer 2

• vestibular apparatus
• vestibular nerve

Question 3

Vestibular system: central

Answer 3

• Vestibular nuclei and pathways
• vestibular cortex
• vestibulocerebellum

Question 4

Bony labyrinth

Answer 4

• cavity in temporal bone of skull
• lined with connective tissue (periosteum)
• filled with perilymph (filled with high Na concentration, similar CSF)

Question 5

membranous labyrinth

Answer 5

• vestibular apparatus inside bony labyrinth
• comprised of sacs and ducts
• filled with endolymph (fluid with a high K concentration)

Question 6

Vestibular apparatus blood supply

Answer 6

• Basilar => AICA => Labyrinthine artery (two sections)
• anterior vestibular artery: anterior and horizontal semicircular canals,utricle
• Common cochlear => posterior vestibular artery: posterior semiciricular canal and saccule

Question 7

Vestibular nerve branches and what they supply

Answer 7

• superior: anterior semicircular canal, horizontal semicircular canal, and utricle
• inferior: posterior canal and saccule

Question 8

Movements detected by semicircular canals

Answer 8

• semiciricular canals detect angular acceleration
• yaw: spinning or shaking head no
• pitch: tumble sets or nodding head yes
• roll: cartwheels or side bending head

Question 9

Inside the semiciricular canals

Answer 9

• each SCC has an ampula an enlarged space which contains a crista
• sensory hair cells in the crista are embbeded in gelatinous cone like structure called the cupula
• dendrites of sensory neurons terminate at base of each hair cell

Question 10

Cupular deflection

Answer 10

• even at rest vestibular nerve always firing at baseline rate
• when head accelerates endolymph drags behind, bumping cupula like a wave
• endolymph moves oppositve of head acceleration

Question 11

Describe horizontal SCC canals and what happens neurologically with head turns

Answer 11

• canal on side the head is moving toward is one that gets excited
• depolarization occurs in SAME direction as head movement
• left head turn produces depolarization in left horizontal canal

Question 12

Otolith organs

describe the structure

Answer 12

• the utricle and saccule each contains a macula
• hair cells are embedded in a gelatinous membrane that contains otoliths
• weight of crystals on gelatinous mass deflects hair cells
• strong deflection = increased signal intensity

Question 13

Orientation of otolith organs

what information is carried and how does it get there

Answer 13

• utricle: sense forward and backward motion
• saccule: sense up and down motion
• sensory neurons pass info about position to brain in vestibular part of CN VIII
• info also goes to cerebellum

Question 14

Coplanar pairs

how do they work together

Answer 14

• SCCs and otolith organs are linked in function pairs along their ommon planes of motion
• as one half of pair is stimulated the other is inhibited

Question 15

What is the push-pull mechanism

Answer 15

• if signals from pair is not reciprocal = postural control abnormalities, abnormal eye movements nausea
• if one side damaged by injury or surgery CNS will still receive info about head veolcity within that plane from other member of pair
• high speed head movements can cause nerve on inhibited side to fire @rate of 0 (sensory overload) BUT CNS still receives input from ecited side

Question 16

Vestibulo-ocular reflex VOR

Answer 16

• maintains visual stability during rapid head movements
• eyes move opposite direction head

Question 17

How does the VOR travel/work

Answer 17

• SCC (angular) and otolith (linear) input is sent to vestibular nuclei
• input travels to ocular motor nuclei (3, 4, 6) for mediation of VOR
• for arousal and conscious awareness of head and body in space info proceeds further to thalamus and cortex
• for maintenance of postural control, peripheral vestibular input is sent distally as medial and lateral vestibular tracts

Question 18

Peripheral vesibular disorders types

Answer 18

• reduced vestibular function: unilateral vestibular hypofunction, bilateral vestibular hypofunction
• distorted vestibular function: BPPV
• fluctuating vestibular function: Meniere’s disease, perilymphatic fistula

Question 19

Describe a vestibular nerve lesion

what is observed in a patient

Answer 19

• lesion in the left vestibular nerve = right side “driving”
• slow horizontal deviation of the eyes to the left and fast snap back to the right = right sided nystagmus
• right sided lesions would be opposite

Question 20

Nystagmus

Answer 20

• rhythmical oscillation of eyeballs
• slow drift of eyes in one direction (pursuit)
• followed by a rapid recovery movement in opposite direction (saccade)
• direction named for fast component: rigth nystagmus = slow movement of eyes to left, followed by fast recovery to right

Question 20

unilateral vestibular hypofunction

Answer 20

• acute vertigo,
• horizontal nystagmus beating toward the unaffected ear,
• impaired/absent VOR lesioned side,
• postural instability,
• nausea/vomiting
• eye skewed lower on lesion side (rare/acute phase only)
• causes include. vestibular neuritis/labyrinthitis, vestibular schwannoma, head trauma, vascular occulsion, surgical procedures

Question 21

Causes of

UVH

Answer 21

• neuritis: dizziness/vertigo, possible LOB and nausea with NO changes in hearing (tends to affect superior vestibular nerve and spares inferior portion)
• labyrinthitis: same symptoms with hearing loss and possible tinnitus
• sudden onset with gradual recovery

Question 22

vestibular schwannoma

Answer 22

• most common intracranial tumor producing vestibular symptoms
• vertigo, disequilibrium, tinnitus, and asymmetric hearing loss due to compression of the vestibularcochlear nerve
• may compress fascial nerve and trigeminal nerve with continued growth

Question 23

Bilateral vestibular hypofunction (BVH)

Answer 23

• common cause is ototoxicity from systemic antibiotics or chemo
• decreased balance, wide BOS, ataxic gait
• oscillopsia
• impaired/absent VOR bilaterally
• severe loss of dynamic visual acuity
• no nystagmus/vertigo (unless loss if unequal)

Question 24

BPPV

Answer 24

• distorted vestibular function
• otoconia belong in utricle and saccule
• displacement via trauma or normal head movement causes affected canal to be inappropriately excited
• symptoms occur when head moved into particular position
• bried episodes of vertigo
• dysequilibrium
• nystagmus

Question 25

Types of BPPV

Answer 25

• canalithiasis: rocks in the canal
• cupulolithiasis: rare; rocks are stuck in cupulo

Question 26

Meniere’s disease

Answer 26

• overproduction of endolymph with decreased resorption
• low frequency unilateral hearing loss + episodic vertigo (lasts 1-2 hours)
• tinnitus
• C/O fullness in ear
• treatment: prevent fluid buildup
• limit salt, surgar, caffeine, alcohol, nicotine
• increase fluid intake

Question 27

Perilympathic fistula

Answer 27

• membrane separating middle and inner ear ruptures
• perilymph leaks into middle ear through oval or round window
• change in fluid pressure can distort utricle, causing vertigo provoked by valsalva maneuver
• can cause hearing loss and permanent hair cell damage
• tx = bed rest and surgery

Question 28

Central vestibular disorders

Answer 28

• vertebrobasilar ischemic stroke/insufficiency
• trauma head injury
• migraine assoicated dizziness
• neurologic conditions of cerebellum/brainstem (MS, cerebellar degeneration, tumors)