Eye Optics and Refractive Errors

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Eye Optics and Refractive Errors

• By John J. Beneck MSPA, PA-C

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Case 1

• 14 year old boy comes to primary care office c/o
  inability to see the blackboard in school

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Case 2

• 51 year old man presents c/o difficulty reading
  the news paper My arms are too short!

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Case 3

• 6 year old girl presents with mom who states she
  squints when looking at anything more than 2 feet
  away.

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Objectives

• Understand the optics of the eye
• Understand visual acuity assessment
• Understand common refractive errors
• Understand color perception assessment

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Objectives (Cont.)

• Understand common refractive errors in terms of
• Etiology/pathology
• Clinical presentation
• Course and prognosis (when appropriate)
• Diagnosis
• Interventions/treatments

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Abbreviations

• C/o complaining of or complains of

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Visual Acuity

• Use Snellen chart
• Positioned 20 feet away
• Each Eye Alone, Then Together
• With Corrective Lenses (If indicated)

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Snellen Charts
http//store6.yimg.com/I/sightmart-eye-care-produc
ts_1753_2381891 accessed 9/5/03
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Visual Acuity

• Visual acuity is expressed as two numbers
• The first indicates the distance of the patient
  from the chart
• The second indicates the distance at which a
  normal eye can read the line of letters
• Ex 20/50

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Visual Acuity

• 20/20
• ability to see letters of a given size at 20 feet
• 20/40
• what a normal person can see at 40 feet, this
  person must be at 20 feet to see.
• 20/200
• what a normal person can see at 200 feet, this
  person must be at 20 feet to see.

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Further Acuity Assessment/Diagnosis

• Optometric examination
• Cornea
• Anterior chamber
• Posterior chamber
• Retinal examination and imaging

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Image Reception

• Optics/Refraction
• Air anterior Cornea
• 2/3 of the refractive power of the eye
• Posterior Cornea aqueous humor
• Iris / pupil
• Variable aperture
• Aqueous humor anterior lens
• Posterior lens vitreous humor

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Image Reception

• Convex refraction
• Refractive index
• Convergence
• Image reversal
• Perception
• Blind spot

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Refractive Principles of a Lens

• Convex lens focuses light rays

Figure 49-2 Guyton and Hall
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The Refractive Principles of a Lens
Figure 49-8 Guyton and Hall
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Refractive Principles of a Lens

• Concave lens diverges light rays.

Figure 49-3 Guyton and Hall
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Whats next?

• Emmetropia (normal vision)
• Myopia (near-sighted)
• Hyperopia (far-sighted)
• Inability of the lens to accommodate adequately
  for near vision
• Presbyopia
• Astigmatism

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Myopia (Near-Sighted)

• The patient is able to focus on objects near but
  not far away
• Typical complaint is difficulty focusing on road
  signs or the black board
• The lens is unable to flatten enough to prevent
  conversion of images before reaching the retina
• The image comes into sharp focus in front of the
  retina
• Frequently squinting is compensatory mechanism

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Errors of Refraction
Normal vision
Far sightedness
Near sightedness
Figure 49-12 Guyton and Hall
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Myopia Correction

• Corrective concave lens use
• Glasses
• Contact lenses
• Surgical
• LASIK (greatest range of correction for myopia)
• Laser-Assisted In Situ Keratomileusis
• Epithelial flap cut and lifted
• Laser applied to deep layers of cornea
• Flap repositioned
• Squinting?

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Correction of Myopic Vision
Myopia corrected with concave lens
Figure 49-13 Guyton and Hall
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Depth of Focus
Effect of pupil size on focus in myopic patients
Note the difference in divergence of rays as they
reach the retinal surface
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Hyperopia (Far-Sighted)

• The patient is able to focus on objects far away
  but not close up
• Typical complaint is difficulty reading
• The image comes into sharp focus behind the retina

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Errors of Refraction
Normal vision
Far sightedness
Near sightedness
Figure 49-12 Guyton and Hall
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Hyperopia Correction

• Corrective convex lens use
• Glasses
• Contact lenses
• Surgical
• LASIK
• Laser-Assisted In Situ Keratomileusis
• Epithelial flap cut and lifted
• Laser applied to deep layers of cornea
• Flap repositioned

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Correction of Hyperopic Vision
Hyperopia corrected with convex lens
Figure 49-13 Guyton and Hall
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Presbyopia The Inability to Accommodate

• Caused by progressive denaturation of the
  proteins of the lens.
• Makes the lens less elastic.
• Begins about 40-50 years of age.
• Near point of focus recedes beyond 22 cm (9
  inches).

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Astigmatism

• Unequal focusing of light rays due to an oblong
  shape of the cornea
• Presents with relatively stable blurry vision
• Patient unable to focus on objects near or far
• Near vision is typically better

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Astigmatism

• Vertical focal point different from
  Horizontal focal point
• Cornea lacks discoid continuity
• More curved in one plane than another
• Unable to correct with a single concavity or
  convexity index

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Exaggerated Astigmatic Corneal Shape
Notice the difference in the degree of curve of
the cornea in 2 planes
Cornea face-on
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Astigmatism Correction

• Cylindrical optical refractive correction
• Glasses
• Contact lenses
• Surgery
• LASIK
• Laser-assisted in situ keratomileusis

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Cataracts

• Cataracts
• cloudy or opaque area of the lens
• caused by coagulation of lens proteins
• More to come

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Cataract
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Cataract Correction

• Surgical
• The lens is replaced
• Induces presbyopia
• Frequently dramatically improves far vision

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Pigment Layer of Retina

• Pigment layer of the retina is very important
• Contains the black pigment melanin
• Prevents light reflection in the globe of the eye
• Without the pigment there is diffuse scattering
  of light rather than the normal contrast between
  dark and light.
• This is what happens in albinos
• poor visual acuity because of the scattering of
  light
• Best corrected vision is 20/100-20/200

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Color Vision

• Color vision is the result of activation of
  cones.
• 3 types of cones
• blue cone
• green cone
• red cone
• The pigment portion of the photosensitive
  molecule is the same as in the rods, the protein
  portion is different for the pigment molecule in
  each of the cones.
• Makes each cone receptive to a particular
  wavelength of light

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Each Cone is Receptive to a Particular Wavelength
of Light
Figure 50-7 Guyton Hall
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Color Blindness

• lack of a particular type of cone
• genetic disorder passed along on the X chromosome
• occurs almost exclusively in males
• about 8 of women are color blindness carriers
• most color blindness results from lack of the red
  or green cones
• lack of a red cone, protanope.
• lack of a green cone, deuteranope.

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Eyes Visual PathwaysIshihara Test for Color
Blindness
http//www.toledo-bend.com/colorblind/Ishihara.htm
l, 2001
The individual with normal color vision will see
a 5 revealed in the dot pattern. An individual
with Red/Green (the most common) color blindness
will see a 2 revealed in the dots.
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Color Vision
Colorblind individuals should see the yellow
square. Color normal individuals should see the
yellow square and a "faint" brown circle.
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How about those cases

• Case 1
• 14 year old boy comes to primary care office c/o
  inability to see the blackboard in school
• Case 2
• 51 year old man presents c/o difficulty reading
  the news paper My arms are too short!
• Case 3
• 6 year old girl presents with mom who states she
  squints when looking at anything more than 2 feet
  away.

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Now, Do You See Things More Clearly???