1 Eye Optics and Refractive Errors By: John J. Beneck MSPA, PA-C

2 Case 1 14 year old boy comes to primary care office c/o inability to see the blackboard in school

3 Case 2 51 year old man presents c/o difficulty reading the news paper: “My arms are too short!”

4 Case 3 6 year old girl presents with mom who states she squints when looking at anything more than 2 feet away.

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

6 Objectives (Cont.) Understand common refractive errors in terms of: –Etiology/pathology –Clinical presentation –Course and prognosis (when appropriate) –Diagnosis –Interventions/treatments

7 Abbreviations C/o – complaining of or complains of

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

9 Snellen Charts accessed 9/5/03

10 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.

12 Further Acuity Assessment/Diagnosis Optometric examination –Cornea –Anterior chamber –Posterior chamber Retinal examination and imaging

13 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

14 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

16 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

18 What’s next? Emmetropia (normal vision) Myopia (near-sighted) Hyperopia (far-sighted) –Inability of the lens to accommodate adequately for near vision Presbyopia Astigmatism

19 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

20 Errors of Refraction Figure 49-12; Guyton and Hall Normal vision Far sightedness Near sightedness

21 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?

22 Correction of Myopic Vision Figure 49-13; Guyton and Hall Myopia corrected with concave lens

23 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

24 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

25 Errors of Refraction Figure 49-12; Guyton and Hall Normal vision Far sightedness Near sightedness

26 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

27 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 years of age. Near point of focus recedes beyond 22 cm (9 inches).

29 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

30 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

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

32 Astigmatism Correction Cylindrical optical refractive correction –Glasses –Contact lenses Surgery –LASIK Laser-assisted in situ keratomileusis

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

34 Cataract

35 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

38 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.

40 Eyes & Visual Pathways Ishihara 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

41 Color Vision Colorblind individuals should see the yellow square. Color normal individuals should see the yellow square and a "faint" brown circle.

42 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.

43 Now, Do You See Things More Clearly???