Basic structures of the eye

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Basic structures of the eye
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Learning Objectives

• Basic Visual Optics
• Identification of the major anatomical structures
  of the eye
• Functions of structures of the eye
• Blindness ocular morbidity

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PRINCIPLE OF VISION
TRANSDUCTION FUNCTION The retina translates the
light transmission into nerve pulses, which will
be interpreted by the brain. The retina behaves
like a projection screen which receives inverse
images, the cerebral centre redresses the image.
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Optical System of the Eye (60D)

• Cornea 75 of refracting power (45D)
• Lens 25 of refracting power (15D)

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Refractive Errors

Emmetropia
normal
eye
.
Axial length and refractive power of the eye fit
together

Ametropia
eye requiring

vision

correction
.

Axial length and refractive power are not matching
.
e.g. Myopia,
Hyperopia
, Astigmatism
Source of picture http//www.e-sunbear.com/pp_p
age2.html
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Refractive Errors
Hyperopia
Myopia
Emmetropia
Image
Object
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Myopia
- Commonly referred to as nearsightedness -
An image will be focused in front of the retina
rather than directly on the retina - Images
that are close will be unaffected, but images
that are far away will appear blurry
Source of picture http//www.e-sunbear.com/pp_p
age2.html
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How to get Myopia
Eye ball too long is the major cause of myopia
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Correcting Myopia

• Myopia means overall power of the eye is too
  strong
• Minus lens to reduce the power of the optical
• system

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Hyperopia Far sightedness

• Eye too short or cornea too flat
• Total power too weak Light being bent too less
• Images far away will be less affected, but images
  that are close will appear blurry.
• Farsighted individuals are constantly focusing
  their eyes (Accommodating) to see objects at a
  distance

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Correcting Hyperopia

• Hyperopia means total power of the eye being too
  weak
• Plus lens to increase power of
• the optical system

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Focal Point Plus Lens

• Light passing through a convex lens will focus at
  one point called the focal point

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Focal Point Minus Lens

• An imaginary or virtual focal point occurs in the
  front of a minus lens

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Calculating the Power of a Lens

• 1 Diopter 1
  Focal length of 1 meter
• The focusing power of any lens in diopters can be
  calculated with the formula
• D 1
• f
• D power of lens
• f focal length in meters

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Example 1

• If a lens has a focal length of 2 meters
  (f 2m), then
• Power 1 / 2m
• Power 0.5

Example 2

• If lens has a focal length of 250 millimeters (f
  .25m), then
• D 1 / 0.25 m
• 4D

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Spherical Lenses (Sph)

• Have the same curvature and same power in all
  directions
• All light passing though a spherical lens will
  focus at the same point

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Astigmatism
- Occurs when the cornea has an irregular
curvature (the cornea is shaped more like an
American football than a basketball)
different refractive power in the 2 main
meridians - Inability of the eye to focus
light rays to a point image of a point will be
a line - Appears at cornea lens retina
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Astigmatism

• Cornea has non-spherical surface

Spherical
Toroidal
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Astigmatism

• Myopia - blur distant vision
• Hyperopia - blur near vision (visual fatigue)
• Astigamtism - blue at distant and near

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Astigmatic Lens (Cyl)
A lens correcting astigmatism is call a toric
lens (Cylinder)
Example
Rx Pl / 2.00 x 90 Rx Pl / -2.00 x 90
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Presbyopia

• The crystalline lens losses its elasticity with
  age
• Results the eye fails to focus at near
• Onset early 40s in Asians mid 40s in
  Causacians

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Presbyopia

• Usually 1.00D to 2.50D additional power over
  distance Rx (depends on age reading distance)
• Reading Rx required
• Reading glasses
• Bifocals
• Multifocal

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Measuring Visual Acuity

• Measured by Snellen chart
• Measure of eyes ability to resolve detail
  (resolution power)

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Expressing Visual Acuity

• Visual acuity is expressed as a fraction 20/20 or
  6/6 or in decimal point (e.g. 0.8 or 1.0 etc)
• 20/20 means patient can read the 20/20 line on
  chart at a distance of 20 feet
• 6/6 means patient can read the 6/6 line at a
  distance of 6 meters

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Visual Acuity

• 6/18 20/60 0.33
• Meaning the letter should be barely read at 18m
  (60ft) away is now read at 6m (20ft) away only
• Vision is only 33 of a normal person
• What does 6/4.5 represents ?

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Ocular structures

• Part 1 Surrounding ocular structures
• Part 2 Intra-ocular structures

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Surrounding Structures

• Structures around the eye
• Bony Orbit protection
• Extra-ocular muscles / Orbital fat
• Eyelids protection
• Eyelashes protection
• Conjunctiva

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External eye

• Thin mucus membrane covering the sclera
• Acts as a lining for upper and lower lids

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Conjunctiva

• Thin mucus membrane extending from the limbus
  posteriorly covering the anterior part of the
  sclera
• Then the conjunctiva is reflected at the fornix
  to cover the inside of the upper and lower lids
• The folding of conjunctiva between the eye ball
  and eye lid creates a bag like structure called
  the conjunctival sac

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Tearfilm

• Produced by the tear other glands
• A layer of water, oils and nutrients that flow
  over the exposed surfaces of the eye
• Approximately .05ml/1.0ml of tears are produced
  each day
• Virtually no tears are produced during sleep

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Tear film structure function
Layer Dimension Source Function
Lipid (oily) 0.1micron Meibomian glands (eye lid) Prevents evaporation Creates smooth optical surface
Aqueous (water) 7 microns Lacrimal glands Nutrition to cornea
Mucin (mucous) 0.02-0.05 microns Goblet cells (Conjunctiva) Stability of tear film
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Part 2 The Eye
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ANATOMY OF THE EYE
POSTERIOR SEGMENT
ANTERIOR SEGMENT
Visual axis
vitreous
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Cornea

• Structure
• Transparent No blood vessels
• Transition to sclera via the limbus (location of
  incision)
• Endothelium critical to transparency
  (viscoelastics for protection)
• The shape of the cornea directly impacts visual
  acuity
• Function major refractive structure of the eye
  (45D)
• Pathology Loss of transparency, irregularity

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Corneal structure

1. Epithelium
2. Bowmans Membrane
3. Stroma
4. Descemets Membrane
5. Endothelium

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Limbus

• Located at junction of cornea sclera
• 1.5mm wide

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Sclera

• The White of the Eye
• Interwoven dense tissue
• Tendons of extra-ocular muscles blend with it for
  insertion

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Uvea

• The middle layer of eye, consisting of 3
  structures
• Iris (11 13 mm) and Pupil (3 4 mm)
• Ciliary Bodies Zonules
• Choroid

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Iris pupil

• Circular shaped diaphragm containing 2 sets of
  muscles to control the pupil size. (circular
  radial muscles)
• With pigment non-pigmented cells. Many pigments
  'brown eyes' some pigments 'green eyes
  very few pigments 'blue eyes
• The pupil should be equally big round and
  respond both to light accommodation

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Uvea Ciliary Body

• Contraction of ring shape ciliary muscle causes
  the ciliary body to move anteriorly.
• Also reduces the size of the lumen releases the
  tension of the zonules (suspensory ligament
  linking the ciliary body with the lens) the
  crystalline lens becomes thicker.
• This will increase the total refractive power of
  the eye for near vision. This process is called
  accommodation.

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Crystalline Lens

• Parts of the human lens
• Lens capsule
• Sub-capsular epithelium (at the backside of the
  anterior capsule)
• Cortex
• Nucleus

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Lens

• Biconvex transparent body
• Curvature Ant 10mm Post 6 mm (widely
  varies)
• Lens grows with age
• Diameter 6mm at birth to 9 mm in adult
• Thickness 3.5 mm at birth to 5.0 mm at age 80
• Weight 65mg in infant to 270mg at age 80
• Capsule
• Bodys thickest basement membrane (thickest where
  zonules attached)
• Basically collagen fibres in a mucopolyasccharide
  matrix

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Lens

• Subcapular Lens epithelium
• simple cuboidal in appearance, keep multiplying
  elongates as they migrate from central region to
  the peripheral of the lens capsule.
• Lens cortex elongated columnar cells from
  epithelium (the lens is like an onion with
  layers)
• bulk of the substance of lens
• As the newer cells (fibres) are formed, they will
  compress the older cell to the nucleus of the
  lens
• Nucleus
• Central hardened structure
• Increases in size with age

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Zonules

• Zonules fibres pass from the basement membrane of
  the unpigemted ciliary epithelial cell to the
  lens capsule
• Zonules fibres are 1 10 micron thick are
  formed of collagen filaments about 20nm in
  diameter
• There are 2 major groups of zonular fibres
  (anterior posterior capsular fibre sheets)

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Aqueous humor

• fills anterior chamber (0.34 ml)
• Composition not quite but similar to plasma (less
  protein, higher ascorbate, pyruvate, lactate
  than plasma)
• nourishes lens and other tissues
• Produced by ciliary body
• escapes at the corneo-scleral junction at the
  anterior angle of the eye (the angle between the
  iris and cornea) through the trabecular meshwork
  into the small blood vessels
• Removal rate 2 3 uL / min

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Uvea Choroid

• Thin, spongy, highly vascular, dark brown, layer.
  
• Important to provide nutrition to the inner eye
  (retina)

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Retina

• Inner layer of eye containing the sensory
  receptors required for transmission of light
• The retina can be divided into10 layers
  contains 120 millions photoreceptors over 1
  Million nerve fibres. It is a highly active
  structure and require a lot of nutrient supply
  mainly from the choroid.
• The retina has a average thickness of 200um, 130
  um in the centre of fovea to 550um at the margin
  of the fovea. Total surface a
• The retina is considered the extension of the
  brain.

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Chorio-retinal structures

1. ILM
2. NFL
3. Ganglion cell layer
4. Inner Plexiform layer
5. Inner nuclear layer
6. Outer plexiform layer
7. Outer nuclear layer
8. ELM
9. Photoreceptor (outer segment)
10. Pigment Epithelium
11. Bruchs membrane
12. Choriocapillaris
13. Choroid

Green 10 layers of the retina
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Inter-connections of cells in the retina

1. ILM
2. NFL
3. Ganglion cell layer
4. Inner Plexiform layer
5. Inner nuclear layer
6. Outer plexiform layer
7. Outer nuclear layer
8. ELM
9. Photoreceptor
10. Pigment Epithelium
11. Bruchs membrane
12. Choriocapillaris
13. Choroid

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The retina
Indirect Ophthalmoscopy
Direct Ophthalmoscopy
Ora Serratta
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Examination of the retina

• Direct ophthalmoscopy
• No pupil dilation
• Magnified view
• Can not exam the periphery of the retina
• More use in examination of the macular Optic
  nerve head

• Indirect ophthalmoscopy
• Pupil dilation required
• No / low magnification
• Give a wider view of the retina (upto Ora
  Serrata)
• Provide a stereoscopic (3D) image
• Use in thorough exam of the retina (retinal
  degeneration or detachment)

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Retina

• High myopia is highly associated with retinal
  degeneration
• Elongation of eyeball stretches on the retina
  resulting in thinning of the retina especially in
  the mid-peripheral area resulting in choroidal
  retinal degeneration
• Thinning and degenerative retina may develop into
  retinal holes / tears which allows liquid in the
  vitreous to enter the sub-retinal space and
  resulting in retinal detachment
• High myopic patients should have an annual
  dilated retinal examination by a Eye Care
  Practitioner using an indirect ophthalmoscope

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Retina Macula Area Fovea

• Light entering eye focuses on an indentation in
  retina called the macula lutea
• Contains the greatest density of cones -
  responsible for central vision

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Retina macula fovea

• Macula (macula lutea and fovea centralis).
• At posterior pole of eye
• It is called the Macular Lutea because it
  contains a lot of Lutein a yellow substance
  which is a form of Vitamin A to protect the
  macular from free radical damages due to high
  energy radiation
• Spot of most acute vision.
• Small shallow depression caused by almost
  complete absence of inner retinal layers.
• At center rods absent and only cones.

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The Retina
The retina contains about 120 million rods and
cones - Cones are more concentrated at the
macula - Rods reach their maximum density at
about 20 from the fovea
Source of picture http//www.webvision.med.utah
.edu/photo2.html
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Optic Nerve

• Carries visual impulses from the retina to the
  brain
• Consist of 1 Million Nerve Fibres

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Image on the L side of the visual field (red) is
focused on the R side of the retina in both eyes
(red). All the nerve fibre in the nasal retina
of the L eye (carrying the signal from the L
visual field) cross over to join the temporal
retina fibre of the R eye (also carrying signal
of the L visual field). Then they travel together
to the R visual cortex via the R Optic track
(Left visual field is interpreted by the R side
of the brain). Similarly, image on the R visual
field will be sent to the L side of the brain.
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Optic Nerve Optic papilla (optic nerve, optic
disc)

• site of the exit of optic nerve fibers
• 3 mm medial to the posterior pole of eye
• Neuro-sensory elements lacking so 'blind spot'

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Vitreous Body
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Vitreous Body

• Fills the posterior chamber (4ml)
• Give support shape to the eye ball
• Composition
• Colorless near structure-less, gelatinous mass
• 99 water in a very fine network of collagen
  fibers
• The vitreous fibre condensed on the surface
  forming a 100um thick cortical vitreous
• Degeneration of the vitreous can result in
  floaters (patient seeing black dots or thread
  like shadows floating around) or even traction to
  the retina
• Vitrectomy is a surgical procedure
• to remove the problematic vitreous

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Common Terminology related to anatomy

• Superior/inferior, temporal/nasal, lateral/medial
  Anterior/posterior
• Clock hours reference
• OSleft eyeLE ODright eyeRE OUboth eyes
• Unilateral / Bilateral
• Extra-ocular, intra-ocular, retro-ocular

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End

• For Further Queries Contact
• Ms. Priyanka Singh
• Head Optometry Service
• Email optometry_at_venueyeinstitute.org