bb2 vestibular system 2014
TRANSCRIPT
The Vestibular System
Gavin Giovannoni
Barts and The London
Objectives
1. Describe the anatomy and embryology of the vestibular system
2. List the functional roles of the vestibular system
3. Describe how the vestibular system detects angular and linear acceleration
4. Describe the vestibulo-ocular reflex and explain how nystagmus can occur
5. Describe the central pathways of the vestibular system
6. List the clinical signs of vestibular system dysfunction
7. Describe tests for balance disorders
Anatomy & embryology
Vestibular apparatus
Semicircular
Canals
Ampullae
(Crista)
Utricle
(Macula)
Saccule
(Macula)
A
P
H
Vestibular
Ganglion
MRI of the vestibular system
Blood supply
Summary of embryological development
Embryology: formation of the otic vesicles from thickened otic placodes
Embryonic development
of the human inner ear
A, At 28 days.
B, At 33 days.
C, At 38 days.
D, At 41 days.
E, At 50 days.
F, At 56 days, lateral view.
G, At 56 days, medial view.
H, Central reference drawing at 56 days.
(From Carlson B: Patten’s foundations of embryology, ed 6,
New York, 1996, McGraw-Hill.)
Function
of the
Vestibular System
Form & Function
Anatomy & Physiology
A reductionist approach
What is a reflex?
Sensory
Input
Detector
(sense organ)
Peripheral
processing
Relay
(nerve)
Central processing - level 1 Response 1
Response 2
Response 3
Response n
+/- Percept
Central processing - level 2
Central processing - level 3
Central processing - level n
AF
FE
RE
NT
E
FF
ER
EN
T
Vestibular functions
1. Detection & conscious perception of head
position, movement & gravity
2. Compensatory eye movements during head
movement (image stabilization & tracking)
3. Postural reflex adjustments following head
movements
To do this it must connect to the spinal
cord, cerebellum and occulomotor nuclei
Central connections of the vestibular system
Functional connections of the
vestibular apparatus
Cristae
(rotational changes)
Maculae
(linear changes)
S M Vestibular
nuclei
I L
Occulomotor nuclei
Reticular formation
(gaze centres)
Vestibulo-cerebellum
Cervical
Spinal cord LMNs Lumbar
Medial VST
(head stabilization)
Lateral VST
(regulates anti-gravity muscles)
MLF
Thalamus
& Cortex (conscious appreciation of
balance & head position)
Visual
tracking
functions
Balance &
posture
reflex
functions
VST = vestibulospinal tract MLF = medial longitudinal fasciculus
ML = medial lemniscus ICP = inferior cerebellar peduncle
ML
ICP
Motion detection &
vestibular receptors
• Angular (rotational) acceleration
– Semicircular canals
– Vertigo (rotational sensation)
• Linear acceleration/deceleration
– Utricle & saccule
– Dysequilibrium (rocking ship sensation)
Anatomy of semicircular canals
Detect rotational acceleration/deceleration
Cupula
Hair cells (have 1 large
kinocilium and
several small
sterocilia)
} Crista
Ampulla
Endolymph
Deflection of the
stereocilia TOWARD the
kinocilium results in an
INCREASE in the firing
rate of the vestibular fiber
associated with the hair
cell, while deflection
AWAY from the
kinocilium results in a
DECREASE in the firing
rate of the vestibular
fiber.
Function of semicircular canals
Detect rotational acceleration/deceleration
•Canals on either side of head act in a
push-pull rhythm
•Excitation is towards side of rotation
•Push-push rhythm causes nausea &
vertigo
•Nerve damage causes vertigo &
nystagmus due to afferent imbalance.
R
R L
R
L
L
Vestibulo-ocular reflex, VOR (doll’s eye reflex)
MLF - Medial longitudinal fasciculus
Connects vestibular nuclei to ocular motor nuclei (III, IV, VI)
Anatomy of a utricle/saccule
Hair cells of the
maculae are
excited (or
inhibited) by
bending of the
stereocilia
toward (or away
from) the
kinocilium by
opening or
closing
potassium
channels
Macula[ (Sensory epithelium)
hair cell & support cell)
Anatomy of a macula
striola striola
• Utricles - hair cells are polarized (excited) towards the striola which
divides each macula into medial and lateral halves
• Saccules - hair cells are polarized away from the striola (divides each
macula into anterior and posterior halves)
• The hair cells of the utricles and saccule work together to provide for a three
dimensional representation of the direction of linear force.
Maculae detect linear acceleration,
deceleration, tilt & effects of gravity
Saccule (vertical)
Maculae detect linear acceleration,
deceleration, tilt & effects of gravity
Utricle (horizontal)
Biochemistry (1)
Perilymph is a typical extracellular fluid (~ plasma or CSF). The compositions of ST perilymph and SV
perilymph are not the same; SV perilymph has higher K+ and lower Na+ levels.
Endolymph is a unique extracellular fluid, with an ion composition unlike that that found anywhere else
in the body.
ST = scala tympani SV = Scala vestibuli
Biochemistry (2)
Nystagmus
• Is an uncontrolled oscillation of the eyes
• Has a slow phase and a fast phase
• Direction of nystagmus is specified by direction of fast phase
• Slow phase is VOR (for image tracking)
• Fast phase due to cortically derived signal (to allow another object to be tracked)
• Optikokinetic (fixation) nystagmus – normal
• Spontaneous nystagmus – abnormal due to damage to vestibular apparatus, brainstem or cerebellum.
Train travel direction
Head-righting reflex
1. Vestibular neurons receive signals of the changing head position (downwards) relative to gravity. These come from both the utricle and the saccule, and signals on forward rotational acceleration from the vertical semicircular canals.
2. The MVST neurons process this information and transmit inhibitory signals to the neck flexor muscles.
3. At the same time, excitatory signals are sent to the neck extensor muscles. The result is a neck movement upward, opposite to the falling motion, to protect the head from impact.
Rotational nystagmus Left rotation of head & body:
Acceleration in inner ear horizontal canals
Eye mov’t in VOR
(slow)
Eye mov’t in VOR
Reset
(fast)
reset
Nystagmus
Fast component
Slow component
During rotation there is left-beating nystagmus
On abrupt cessation there is a post rotational
nystagmus as the SSC endolymph does not stop as
quickly and so nystagmus appears in the opposite
direction which lasts a few seconds (<15s)
Symptoms and Signs
• Vertigo (or dizziness)
• Syncope (light-headedness or woozy-ness)
• Dysequilibrium (rocking ship sensation)
• Oscillopsia (visual jumping or blurring)
• Nystagmus
• Nausea & vomiting
• Ataxia (unsteadiness of gait)
• Associated cochlear symptoms – Hearing loss
– Tinnitus
Causes
• Nerve/inner ear infections
• Tumours
• Vascular insufficiency
• Trauma
• Endolymph fistulae
Clinical examples
1. BPPV
2. Meniere’s disease
3. Drug toxicity (quinine &
aminoglycoside antibiotics)
4. Usher’s syndrome
Clinical manifestations
Tests for vestibular disorders
Caloric Testing
Clinical Examination
Electronystography (ENG)
Posturography Bárany chair
Eye movements
Hallpike Manoeuvre
Vestibular Testing Vestibular Imaging
MRI
Caloric test
Normal
Cold water decreases
ipsilateral ampulla firing;
warm water increases firing
Unconscious
(brainstem intact, cortex non-
functional)
Bilateral MLF lesion
No F phase due to lack of input
from cortical gaze centres Irrigation produces lateral deviation
of eye only on less active side
Water changes temperature between middle and inner ear causing convection currents to occur in SSCs to
elicit the VOR; tests integrity of pons
“COWS”
Hallpike manoeuvre (a test for BPPV)
1. Lower head to the table and turn to one side.
2. Watch eyes for nystagmus.
3. If patient gets dizzy & exhibits nystagmus, the ear pointed to the floor is the affected ear.
4. If nothing happens, repeat test on the other side to check the other ear.
5. The person is then moved back to the upright position.
Benign Paroxysmal Positional Vertigo (BPPV)
Vertigo due to damaged otoconia from the utricle
being displaced into the semicircular canals within
the inner ear.
Otoconia from the saccule are not able to migrate into
the canals.
The utricle can be damaged by head injury, infection, or
other disorder of the inner ear, or may have
degenerated because of advanced age.
Figure 1. Instruction for the modified Epley’s procedure (for benign paroxysmal
positional vertigo of the posterior semicircular canal of the right ear).
Radtke A et al. Neurology 1999;53:1358
Additional Reading
• The nervous system, Chapter 8
• Web resources
– http://www.sumanasinc.com/webcontent/animations/content/vestib
ular.html
– http://php.med.unsw.edu.au/embryology/index.php?title=Hearing_-
_Inner_Ear_Development
– http://oto2.wustl.edu/cochlea/intro1.htm