Lecture: Vestibulocochlear function and dz

Question 1

Vestibular function

Answer 1

• Maintenance of posture and balance
• Function intimately with the cerebellum
• Peripheral or Central
• Critical neuroanatomic localization
  
  • huge impact on DDX and prognosis

Question 2

CN VIII

Answer 2

• Only CN not to exit skull
• purely sensory
• two portions
  
  • vestibular
  • auditory

Question 3

Conductive Deafness

Answer 3

• Occlusion
  
  • congenital or acquired
  • rupture of tympanic membrane
  • fluid, exudate, foreign body, mass
  • age (ossicles)

Question 4

Sensorineural Deafness

Answer 4

• Alterations of neural structures of auditory pathway
  
  • Congenital
    
    • hair cells of the OoC
  • Acquired
    
    • infection
    • intracranial disease

Question 5

Congenital sensorineural deafness

Answer 5

• Hair cells of the organ of Corti
• American Paint horses
  
  • especially white coat/blue or violet iris
• Lethal White Foal Syndrome
• Dalmatians
• White dogs & cats with blue eyes

Question 6

Vestibular Labyrinth

Answer 6

Question 7

Vestibular labyrinth

Answer 7

• Utricle and saccule
• Located in the large vestibule
• Detect static or kinetic position
  
  • gravity & linear acceleration
• Receptor: macula
  
  • covered by hair cells projecting into gelatinous otolithic membrane
  • otolitic membrane contains otoliths (statoconia)

Question 8

Semicircular Ducts

Answer 8

• 3 in the semicircular canal
• at right angles to each other
• detect angular movements of the head
• Receptor: crista ampullaris
  
  • covered by hair cells projecting cilia into gelatinous cupula
    
    • no otoliths

Question 9

Entry into skull

Answer 9

• Axons from bipolar neurons in petrous temporal bone enter cranial vault through internal acoustic meatus at cerebellomedullary angle
  
  • rostral medulla oblongata
  • This is CN VIII

Question 10

Entry into the brain

Answer 10

• At level of trapezoid body and caudal cerebellar peduncle
• Most axons synapse on CN VIII nuclei
• Few axons bypass CN VIII nuclei to enter cerebellum

Question 11

Answer 11

Question 12

Vestibular Nuclei

Answer 12

• Four on each side of brainstem
  
  • ventrolateral wall of 4th ventricle
  • Huge
• Axonal projections
  
  • spinal cord: vestibulospinal tracts
    
    • limb extensor tone (anti-gravity muscles)
  • Rostrally: Medial longitudinal fasciculus (MLF)
    
    • Ocular and head movements

Question 13

Vestibulospinal tracts

Answer 13

• Lateral vestibulospinal tract (mainly)
  
  • nucleus projects in ipsilateral ventral funiculus
  • synapse on interneurons in spinal cord ventral gray
  • mediate facilitation of extensor muscles and inhibition of flexor muscles
  • ipsilateral side

Question 14

MLF

Answer 14

• Medial longitudinal fasciculus
• Rostral projections
  
  • to nuclei of CN III, IV & VI
  • responsible for oculocephalic reflex
• Caudal projections
  
  • medial vestibulospinal tract
  • maintain body and limb position relative to head

Question 15

Extraocular muscles

Innervation

Answer 15

• CN III: Oculomotor n.
  
  • Dorsal, medial and ventral recti mm
• CN IV: Trochlear n
  
  • Dorsal oblique m.
  • on opposite side
• CN VI: Abducent n.
  
  • Lateral rectus & retractor bulbi mm.

Question 16

Physiologic Nystagmus

Answer 16

• Moves eyes to hold images during head rotation or target motion
• Receptor: ear and CN VIII
• MLF connects VIII to III/IV/VI
• Bilateral, opposite effects on CN II, IV, VI

Question 17

Vomiting Center

Answer 17

• In the reticular formation of the medulla
• Receives afferent input from the vestibular portion of the vestibulocochlear nerve
  
  • motion sickness

Question 18

Cerebral Projections

Answer 18

• Synapses in thalamus
• provides conscious awareness of the body’s position in space

Question 19

Clinical evaluation

Answer 19

• Think about abnormalities in maintenance
• vestibular inputs are bilateral and tonic
• if you excite one side, you get ipsilateral facilitation of extensors and contralateral facilitation of flexors

Question 20

If a lesion prevents activation of one side…

Answer 20

• The ipsilateral nuclei won’t be as excited as other side
  
  • facilitation of extensors on normal side
  • lack of facilitation on affected side
• body is ‘pushed’ towards abnormal side

Question 21

Clinical Signs

Answer 21

• Head tilt
• Circling, leaning, falling towards lesion
• Unilateral or asymmetric ataxia toward lesion
• Abnormal eye position or movements
• Nausea
• +/- postural reaction deficits

Question 22

Nystagmus

Answer 22

• Involuntary movements of the eyes
  
  • Jerk: fast and slow phases of eye movements
  • Defined by direction of fast phase

Question 23

Physiologic nystagmus

Answer 23

• vestibulo-ocular, Doll’s eye, oculocephalic reflex

Question 24

Pathologic nystagmus

Answer 24

• Horizontal, rotary, vertical

Question 25

Disconjugate nystagmus

Answer 25

• one eye goes one way, and one goes the other way
  
  • this is bad

Question 26

Pendular Nystagmus

Answer 26

• Siamese and Himalayan cat things
• not nystagmus

Question 27

Vestibular Localization

Answer 27

• Peripheral
  
  • ear: canal, bulla, CN VIII
• Central
  
  • brainstem or cerebellum
• Isolate the clinical signs that involve only one!

Question 28

Vestibular Lesion

Peripheral

Answer 28

• NO proprioceptive deficits
• Normal mentation
• Head tilt towards lesion
• Only deficits in CN VII or VIII
• Strabismus
• Nystagmus
  
  • any direction

Question 29

Vestibular Lesion

Central

(Brainstem, cerebellum)

Answer 29

• Proprioceptive deficits
• Dullness, stupor
• Head tilt
• Other CN deficits possible
• Strabismus
• Nystagmus
  
  • any direction
  • positional, vertical, dysconjugate MUCH more suggestive

Question 30

Peripheral Vestibular

Answer 30

• Head tilt, ataxia, nystagmus
• CN VII: Facial n.
  
  • Exits the skull right above the tympanic bulla
  • Innocent bystander
• Temporohyoid osteoarthropathy in the horse
• CN VII
• CN VIII
• Unilateral
• Bilateral
• Head tilt & CN VII
• Horner’s syndrom

Question 31

Horner’s syndrome

Answer 31

• Sympathetic dysfunction
• Ptosis (droopy eyelid), miosis, enophthalmos, protruding nictitans
• Sympathetics to eye, course near/in the middle/inner ear
• Innocent bystander

Question 32

Central Vestibular

Answer 32

• CN deficits other than CN VII & VIII
• Mentation changes
  
  • Ascending reticular activating system
• Proprioceptive deficits
  
  • these pathways don’t go through peripheral system
• cerebellar and forebrain signs

Question 33

Cerebellar Syndrome

Answer 33

• Cerebellum is inhibitory, modulatory
• If disinhibited => spastic
  
  • hypermetria, ataxia, intention tremors
• Ipsilateral signs

Question 34

Paradoxical Vestibular Syndrome

Answer 34

Always central: cerebellum/brainstem

• Head tild away from lesion
  
  • fast phase nystagmus may be towards
• Always central
  
  • Flocculonodular lobe, CCP, rostral & middle vestibular nuclei and dorsal roots of C1-C3
• lesion on same side as proprioceptive deicits