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