Anatomy/Neuro-Anatomy of the Visual System Medical ppt Medical ppt

Anatomy/Neuro-Anatomy of the Visual System

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Learning Objectives

• Describe the function of major structures of the visual system

• Describe major milestones in development of the visual system

• Describe normal age related changes in vision and their impact on occupational performance

• Describe changes in visual function associated with pathology

Eye as a camera

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Structures of the Eye and Orbit

The anterior visual system

Orbit

• Eyeball• Optic nerve• Extraocular

muscles• Other nerves• Blood vessels• Lacrimal gland• Fat• Connective tissue

Eyelids and Eyelashes

• Protect eye from foreign bodies

• Help limit light into the eye

• Functions as part of the lacrimal system• Blinking squeezes tears from lacrimal

gland• Tears fill in uneven surfaces of cornea• Nourishes and protects cornea

Eyeball has three layers

Outer protective layer• Sclera and cornea

Middle vascular layer• Uveal tract

• Consists of iris, ciliary body and choroid

Inner sensory layer• Retina

Sclera

• Encloses eyeball except for cornea

• Extension of the dura mater of CNS

• Protects inner contents of eye and

• Helps maintain shape of the eye

• Extraocular muscles attach to its surface

Cornea

• Avascular • Transparent• 5 layers• Protects inner

contents of eye• Refracts light

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Aqueous

• Continuously produced & drained away • trabecular meshwork• canal of Schlemm

• Maintains health of lens and cornea

• Maintains shape & pressure within eye

Iris

• Pigmentation protects retina

• Controls pupil aperture• Dilator muscle

sympathetic control

• Spincter muscle

Lens

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• 65% water 35% protein

• Avascular• Refracts light to

focus image onto retina

• Fibers form throughout life

Ciliary Body

• Ciliary muscle• Shapes lens• Controlled by CN III

• Ciliary process• Secretes aqueous

Vitreous

• Maintains transparency and form of eye

• Holds retina in place

Conjunctiva

• Thin transparent membrane covering sclera and inner eyelid

• Provides protection and moisture

• Many blood vessels, few pain fibers

• Conjunctivitis common condition

Choroid

• Vascular supply for eye

• Capillaries and veins

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Retina

• Lines posterior 2/3rd of eye• Distant receptor organ• 5 layers• Inside out arrangement

Direction of light

Rod Receptor Cells

• Concentrated in periphery• Activate in low illumination• Detect general form, not details• Provide background information

Rod Cells

Cone Receptor Cells

• Capture detail and color• Require direct stimulation• Bright light’• Concentrated in fovea

Cone Cell

Retinal Pigment Epithelium (RPE)

• Works with Bruchs membrane and choroid layer

• Maintains health of receptor cells• Breakdown causes build up of cellular debris

RPE Layer

Retinal Processing Pathway

• Impulses converge onto bipolar cells• Converge again onto ganglion cells• Axons of ganglion cells merge and exit

at optic disc

Ganglion cells

Bipolar cells

Axons formOptic nerve

Optic nerve

• CN II• Each nerve contains

1 million plus heavily myelinated ganglion axons

• Macular fibers inside peripheral fibers outside

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Visual Field

• Visual field• 160-180 degrees horizontally• 120 degrees vertically

• Practical field of vision• Head and eye movement• 270 degrees

Hill of Vision Concept

Visual Field Diagram

Extraocular Muscles (EOM)

• Medial rectus• Lateral rectus• Superior rectus• Inferior rectus• Superior oblique• Inferior oblique

Sup. rectus

Sup. oblique

Inf. oblique

Med. rectus Lat. rectus

Inf. rectus

Cranial Nerves Controlling Extraocular Eye Muscles

• CN III Oculomotor

• CN IV Trochlear

• CN VI Abducens

Oculomotor Nerve (3)

• Innervates 5 muscles• Medial, superior,and inferior rectus

muscles, inferior oblique• Levator palpebrae superioris

• Internal musculature of the eye• Ciliary muscle (lens)• Spincter muscle (pupil)

Trochlear Nerve (4)

• Innervates superior oblique

• Down and out muscle of eye

Abducens Nerve (6)

• Innervates lateral rectus

• Abducts eye

Birds Eye View of Visual Pathways

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Development of the Visual System

Visual system develops from three types of tissue

• Neuroectodermal from brain• Becomes retina, iris and optic nerve

• Surface ectoderm of head• Forms lens

• Mesoderm• Forms vascular supply and sclera

Embryonic Eye Development3-4 weeks gestation

The eye begins asa groove in theneural fold on thecranial end of theembryo

Over the nextweek, the grooveturns inside outand balloonsoutward and creates a hollowbulb projectingfrom each sideof the neural tube

At 4 weeks, the bulb and stalk are fully formed. The lateral surface of the bulbbegins to flatten and the ectoderm thickens to become the lens placode

The placode turns in on itself to forma deep indentation (the lens pit). Theends of the pit come to together to formthe lens vesicle, which then is pinchedoff to become the lens.

At the same time, the optic vesiclebegins to fold in on itself to form a double walled, bowl shaped structure called the optic cup. The two wallsfuse together, the outside wallbecomes the RPE, the inside wall be-comes the sensory retina. The axons of the ganglion cells converge into theoptic stalk to become the optic nerve.

Lens

Optic nerve

Retina Cornea

• Rim of optic cup eventually becomes the ciliary body and muscle, iris, dilator and sphincter muscles

• Mesenchyme cells develop into the choroid and sclera-both are extensions of vascular and fibrous structures within brain• Sclera-continuation of dura mater• Choroid-continuation of pia arachnoid

• Form a sheath around the optic n.

The relationship between these structures explains why an increase in cerebral spinal fluid after brain injury can be diagnosed by observing the optic disc for papilledema

Maturation of Face and Eyes

• As the embryo develops, the eyes migrate from the sides to the front as the face matures• Face is formed by 14 weeks

• During development, structures may fail to fully form or to close completely• Creates many of the congenital eye

conditions observed in children

Maturation of Visual System Pre-natal Post-natal

• Rods and Cones• 25 wks-both begin to

develop

• Optic Tract• 28-38 wks-begins to

myelinate

• Superior Colliculus• Basic structure

develops 16-28 wks

• Rods and Cones• 4 mos-complete with

rods finishing first

• Optic Tract• Rapid myelination

first 2 mos continued for 2 years

• Superior Colliculus• Myelination

completed at 3 mos

Maturation of Visual System Pre-natal Post-natal

• LGN• Matures after birth

• GC Tracts• Myelination begins at

birth

• LGN• Process takes 9 mos• Stereoscopic vision

at 3-4 mos

• GC Tracts• Completed in 4-5

mos

Maturation of Visual SystemPre-natal Post-natal

• Visual cortex• 25-28 wks-starts

dendritic growth, increasing synaptic density, cortical layers develop

• Visual cortex• Doubles in density

first 2 years, adult synaptic density and functional maturity by age 11

Eye Movement

• Able to fixate and make basic eye movements by 2-3 months

• 2 years to obtain good control

• Up to 9 years to obtain complex control

Visual Acuity

• Newborn• 20/200, sees best in 2-75 cm range

• 3 months• 20/60

• 6 months• 20/20

• 2 years• Acute near vision-fine motor skills develop

Normal Age Related Changes in Vision

Reduced Visual Acuity

• Static acuity• Decreases to 20/30-20/40

• Prevalence 40% by age 70

• Dynamic acuity• Decrease may be due to reduced OM

control

Loss of Accommodation

• A.k.a. presbyopia

• Result of compacting of protein fibers in center of lens

• Lens thickens and loses flexibility

• Occurs gradually beginning in 40s

• Creates need for bifocal

Floaters

• Strands of protein which float in vitreous

• Float more easily in old eye because vitreous is more fluid

• More noticeable in bright light

• Generally benign unless accompanied by bright flashes of light or significant increase in number

Dry Eyes

• Lacrimal glands do not make enough or make poor quality tears

• More prevalent in women

• Can be exacerbated by medication

• Causes itchiness, burning, decreased acuity

• Treated with artificial tears or surgery

Increased Need for Light

• Pupil diameter decreases• A.k.a. senile miosis

• Lens thickens becoming more yellow

• Combined-these two conditions reduce the amount of light coming into eye

• 80 yr old person needs 10x as much light as an average 23 year old

Susceptibility to Glare

• Lens and cornea become less smooth

• Lens & vitreous develop protein strands

• Combine to cause light scatter

• Increased discomfort and disability• Lose acuity under glare condition• Also takes longer to recover from glare

Reduced Dark/Light Adaptation

• Takes longer to reform and store pigments

• Never reach same level of dark adaptation as younger person

• More difficult to go from bright to dark than dark to bright

Reduced Contrast Sensitivity

• Caused by changes in color and density of lens and decreased pupil aperture

• 75 year old needs 2x as much contrast as younger person

• 90 year old needs 10x as much contrast

Reduced Color Perception

• Caused by yellowing of lens

• Decrease in sensitivity at violet end of spectrum

• White objects may appear yellow

Reduced Visual Field

• Changes in facial structure

• Nose grows??

• Orbit loses fat and eye sinks in

Reduced Visual Attention

• Decline in ability to• Attend to objects in complex, dynamic

arrays• Simultaneously monitor central and

peripheral visual fields

• Diameter of visual field decreases• 90 yr olds-40% have an attentional field of

less than 20 degrees

Pathology of the Visual System

Anterior visual system has three jobs to do

• Focus the image on the retina

• Capture the image (encode it)

• Transmit the image to the CNS

Sharp focusing of image on retina depends on:

• Sufficient refraction of light rays entering the eye

• Focal point established on the fovea• Transparency of all intervening

structures between outside of eye and retina

• Adequate illumination

Sufficient Refraction of Light Rays entering the Eye

f

Focal point is established on fovea of retina

Emmetropia

Hyperopia

Myopia

Smoothness of Refracting Surfaces

• Astigmatism• Cornea is spoon shaped or dimpled• Light rays are unevenly refracted• Can develop with trauma and age• Corrected for optically with cylinder

• Cataract• Dead cells deposited in lens, calcify• Begins in periphery, progressing to center• Surface becomes pitted• Causes light scatter and veiling glare• Eventually complete opacity

Closeness of Object

• As object comes closer, focal point on retina is pushed back

EF

Closeness of Object

• As object comes closer, focal point on retina is pushed back

EF

Accommodation

• 3 step process• Convergence• Lens thickens• Pupil constricts to

reduce light scatter

• Controlled by CN III• Affected by lens

• Presbyopia• Aphakia

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Control of Light Scatter

• Light rays are refracted more strongly in periphery than center of lens• Causes wild and scattered light rays

• Reduced by blocking peripheral rays with pupillary constriction

• Increases acuity• Pinpoint vision

Transparency of Intervening Structures

• Any opacity in cornea, aqueous, lens, vitreous will prevent image from reaching retina

• Common conditions• Corneal scarring• Cataract• Trauma-vitreous hemorrhage

• Also causes veiling glare

Adequate Illumination

• Retina must be adequately diffused with light to capture an image

• Amount of light is controlled by pupil• Any condition affecting responsiveness of

pupil will affect• Tolerance of light• Ability to rapidly adjust to changes in light

• Opacity in intervening structures also affects amount of light entering eye

Ability of retina to capture image

• Retinal function can be affected by disease, injury or congenital conditions• Macular degeneration, diabetic retinopathy,

retinitis pigmentosa, retinal detachment

• Damaged retina creates blind spot in vision• Known as a scotoma

• Performance limitations depend on area of retina damaged• Peripheral vs. central

Macular Scotoma

• Area of reduced light sensitivity within central 20 degrees of the visual field

Macular Scotoma

• Occurs with retinal diseases• Affects ability to

• See small details• Discriminate contrast• Discriminate color

• Primary pathology dealt with in patients with low vision• 83% of patients referred for low vision services

found to have dense macular scotomas regardless of disease

Scanning Laser Ophthalmoscope

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Central Scotoma

• Scotoma impinges on and involves the fovea

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Para-central Scotoma

• Within the central 20 degrees of the field but not involving the fovea

Ring Scotoma

• Surrounds the fovea on 4 sides

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Scotomas may vary in density

• Dense• No response to light

• Relative/Threshold• Responds to light if

bright enough

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Adaptation to Scotoma

• Scotoma creates a hole in visual field• Deprives CNS of vision needed to identify

objects

• CNS adapts using various mechanisms• Perceptual completion• Metamorphopsia• Development of PRL

Perceptual Completion

• Perception in which objects or a visual scene appears complete despite missing visual input

• Example of top down “cognitive processing” where we see something because we expect to see it

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• Scotomas less than 5 degrees• CNS can perceptually complete

• Example: own blind spot• Person unaware of presence of scotoma

• Scotomas greater than 5 degrees• CNS will attempt to perceptually complete but may

not be successful• Person does not perceive black hole but instead a

blurriness or inability to bring object into focus

Metamorphopsia

• Scotoma is too big to complete perceptually

• Objects appear warped or misaligned

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Preferred Retinal Locus (PRL)

• If scotoma covers fovea, CNS adopts an eccentric retinal area to act as a pseudo-fovea for visual tasks previously completed by the fovea

• Develops within 24 hours of loss of fovea

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40% place PRL above the scotoma on the retina(leaves lower portion of field clear)

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35% place PRL to the right (leaves left side of page clear)

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20% place PRL to left

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7% place PRL below

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• Person may develop more than one PRL and use a different one depending on task and lighting conditions

• Ability to use PRL to direct eye movements is more highly correlated to reading ability than other visual function

• Although person develops PRL, he/she may not be aware of it

• Important to assess ability to use PRL

Peripheral Visual Field Deficit

• Person also exercises perceptual completion• May be completely unaware of deficit

• Will not interfere with perception of visual details (acuity)

• But will affect mobility• Reduces detection of motion and form

Ability of optic nerve to transmit visual input

• Can be damaged by disease, trauma and congenital conditions• Glaucoma• Optic neuritis• Head injury

• Can lose all or part of field• Depending on location, extent of damage

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Deficits in Posterior Visual System

Visual Field Deficits

• Lesions along geniculocalcarine tracts or in occipital lobe

• Most common cause in adult is stroke• Posterior cerebral artery (PCA)

• Pure visual stroke (sometimes affects language)

• Middle cerebral artery (MCA)• Mix of motor, sensory, visual, cognitive

• Lesion behind LGN will always cause homonymous loss

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Homonymous Hemianopsia

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Hemianopsia with Macular Sparring

Cortical Blindness

• Also called cortical visual impairment• CVI

• Damage is so significant in occipital lobe, CNS is not able to complete any cortical processing of vision

Person loses:

Person loses:

• Object identification through visual system

• Visual orientation to space

• Cognitive application of vision

Person retains

Person retains

• Subcortical processing of vision• Navigational vision• Vision for safety

• Other sensory processing• Haptic discrimination• Auditory discrimination

Alteration of Visual Attention

• Difficulty arousing attention• Difficulty attending globally• Difficulty attending to details• Difficulty sustaining attention• Difficulty dividing/shifting attention• Asymmetrical attention

• Unilateral spatial neglect

Diminishment of Attention

• Has pervasive effect on cognition• Person takes in information in incomplete

disorganized fashion

• CNS cannot properly analyze incoming information

• Decision making is based in incomplete and/or incorrect information• Garbage in - garbage out

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