Vestibulocochlear & Vestibular Syndromes Flashcards
What is the function of the vestibular system?
Maintenance of posture and balance; functions intimately with cerebellum
What is unique about CN VIII?
It is the only CN not to exit the skull and is purely sensory
What are the two portions of CN VIII?
- Vestibular - cell body in vestibular ganglion in bony labyrinth of petrous temporal bone
- Auditory - cell body in spiral ganglion
What are the features of vestibulocochlear neurons?
- Dendritic zone synapse with specialized “hair” cells in receptor organs of inner ear
- hair cells = stereocilia that transform mechanical deformation into neural signals
What is the conduction pathway of a neural signal along cochlear projections?
Fibers leave spiral ganglion > enter brainstem at medulla/pons > synapse in cochlear nucleus > ascend brainstem to caudal colliculus > tectospinal tract - reflex visual and auditory movements (mostly contralateral)
What is sensorineural deafness?
Alterations of neural structures of the auditory pathway; there’s a congenital and acquired form (infection, intracranial dz)
What does congenital sensorineural deafness affect and what animals typically get it?
Affects the hair cells of organ of Corti
- American Paint horses
- Lethal White Foal Syndrome
- Dalmatians
- white dogs and cats with blue eyes
What is the test for sensorineural deafness?
Brainstem Auditory Evoked Response - looking at the waveform latency and amplitude
What are the parts of the vestibular labyrinth and how does it work?
Utricle and saccule, receptor = macula (hair cells projecting into gelatinous otolithic membrane), otholiths;
- detects static or kinetic position - gravity and linear acceleration
What is the function of the semicircular ducts?
- At right angles to each other
- detect angular movements of head
- receptor = crista ampullaris (covered by hair cells projecting into gelatinous cupula)
- NO otoliths
How do the CN VIII axons enter the skull?
Axons from bipolar neurons in petrous temporal bone and enter cranial vault through the internal acoustic meatus at the cerebellomedullary angle (rostral medulla)
What CN also enters the skull through the internal acoustic meatus with CN VIII?
CN VII
How does CN VIII enter the brain?
- At the level of the trapezoid body and caudal cerebellar peduncle
- most axons synapse on CN VIII nuclei, few axons bypass and enter the cerebellum
How many vestibular nuclei are there and where are they located?
Four on each side of the brainstem (8 total)
What are the axonal projections of the vestibular nerve?
- Spinal cord - vestibulospinal tracts (limb extensor tone-anti-gravity mm)
- rostral - medial longitudinal fasciculus (ocular and head movements)
Describe the lateral vestibulospinal tract
Lateral vestibular nucleus projects in ipsilateral ventral funiculus > synapses on interneurons in SC ventral gray matter
- facilitates extensor muscles and inhibits flexor muscles on ipsilateral side
Where do the rostral and caudal projections of the medial longitudinal fasciculus go to and what are their functions?
Rostral: to nuclei of CN III, IV, and VI - responsible for oculocephalic reflex
Caudal projections: medial vestibulospinal tract - maintain body and limb position relative to the head
What is physiologic nystagmus and what is the nerve pathway of it?
- Moves the 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 III, IV, VI
What two areas of the brain do the axonal projections reach to other than the vestibular nuclei?
Vomiting center (in reticular formation of medulla) - motion sickness
cerebral projections: synapses in thalamus - provides conscious awareness of body’s position in space
What are things you look for when you’re clinically evaluating vestibular signs?
Checking for abnormalities in maintenance
- vestibular inputs are bilateral and tonic
- if lesion prevents activation of one side, the ipsilateral nuclei won’t be excited as much as other side, so you get relative facilitation of extensor on normal side and lack of facilitation on affected side > patient is ‘pushed’ towards the lesion
What are clinical signs of vestibular disease?
Head tilt
circling
leaning
falling toward lesion
unilateral or asymmetric ataxia toward lesion
abnormal eye position or movements
nausea
+/- CP deficits
How can you tell if it’s a central vs. peripheral lesion?
- If patient has CP deficits, it is definitely central
- if patient does not have CP deficits, cannot rule out central
Describe nystagmus. What is the difference between physiologic and pathologic nystagmus?
Involuntary movements of eyes; jerk = fast and slow phases of eye movements (defined by direction of fast phase) Physiologic = vestibulo-ocular, Doll’s eye, oculocephalic reflex; Pathologic = horizontal, rotary, vertical, dysconjugate
What is pendulum nystagmus?
It is actually NOT nystagmus (no slow/fast phases)- defect in visual pathways seen in Siamese, Himalayan, crosses
How do you localize a vestibular lesion?
Peripheral = ear: canal, bulla, CN VIII
Central = brainstem or cerebellum
What are the signs of a peripheral vestibular lesion?
- NO CP deficits
- normal mentation
- head tilt towards
- only deficits in CN VII or VIII
- strabismus
- nystagmus (any direction)
What are the signs of a central vestibular lesion?
CP deficits
dullness
stupor
head tilt
other CN deficits possible
strabismus
nystagmus (any direction but positional, vertical or dysconjugate MUCH more suggestive)
What condition might result in peripheral vestibular signs and facial nerve deficits?
Otitis media - facial nerve exits skull right above the tympanic bulla
What are the symptoms of Horner’s syndrome? Why might you get Horner’s with peripheral vestibular lesions?
- Sympathetic dysfunction
- ptosis
- miosis
- enophthalmos
- protruding nictitans
sympathetic to eye course near/in the middle/inner ear
Describe idiopathic geriatric peripheral vestibular disease?
Old dog vestibular diseases
- SEVERE head tilt
- rapid nystagmus
- ataxia (can be severe they are non-ambulatory)
What is a condition that could cause peripheral vestibular disease in a horse?
Temporohyoid osteoarthropathy
Describe central vestibular disease
CN deficits other than CN VII and VIII
mentation changes - ARAS
CP deficits - pathways do NOT go through peripheral system
cerebellar and forebrain signs
What are your differentials for central vestibular disease?
- Neoplasia - choroid plexus, meningioma, glioma, skull tumors, ear adenocarcinoma, SCC
- inflammatory - sterile, immune mediated
- Infectious - otogenic intracranial infection
- Trauma
- Vascular
What is cerebellar syndrome? What is the normal function of the cerebellum?
Cerebellum in inhibitory, modulatory, but if you disinhibition, you become spastic - hypermetria, ataxia, intention tremors; ipsilateral signs
What is paradoxical vestibular syndrome?
- The head tilt is AWAY from the lesion - fast phase of nystagmus may also be TOWARD
- this is ALWAYS central > the lesion is on the same side as the CP deficits
