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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-products_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

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

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

Refractive Principles of a Lens

• Concave lens diverges light rays.

Figure 49-3; Guyton and Hall

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What’s 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

Figure 49-12; Guyton and Hall

Normal vision

Far sightedness

Near sightedness

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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

Figure 49-13; Guyton and Hall

Myopia corrected withconcave lens

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Depth of FocusEffect 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

Figure 49-12; Guyton and Hall

Normal vision

Far sightedness

Near sightedness

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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

Figure 49-13; Guyton and Hall

Hyperopia corrected with convex lens

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

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

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

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

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

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.

http://www.toledo-bend.com/colorblind/Ishihara.html, 2001

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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???