Visual Acuity & Refractive Errors.ppt
- 1. 1 Eye Optics and Refractive Errors By: John J. Beneck MSPA, PA-C
- 2. 2 Case 1 • 14 year old boy comes to primary care office c/o inability to see the blackboard in school
- 3. 3 Case 2 • 51 year old man presents c/o difficulty reading the news paper: “My arms are too short!”
- 4. 4 Case 3 • 6 year old girl presents with mom who states she squints when looking at anything more than 2 feet away.
- 5. 5 Objectives • Understand the optics of the eye • Understand visual acuity assessment • Understand common refractive errors • Understand color perception assessment
- 6. 6 Objectives (Cont.) • Understand common refractive errors in terms of: – Etiology/pathology – Clinical presentation – Course and prognosis (when appropriate) – Diagnosis – Interventions/treatments
- 7. 7 Abbreviations • C/o – complaining of or complains of
- 8. 8 Visual Acuity • Use Snellen chart – Positioned 20 feet away • Each Eye Alone, Then Together • With Corrective Lenses (If indicated)
- 10. 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
- 11. 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. 12 Further Acuity Assessment/Diagnosis • Optometric examination – Cornea – Anterior chamber – Posterior chamber • Retinal examination and imaging
- 13. 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. 14 Image Reception • Convex refraction – Refractive index – Convergence – Image reversal • Perception • Blind spot
- 15. Refractive Principles of a Lens • Convex lens focuses light rays Figure 49-2; Guyton and Hall
- 16. 16 The Refractive Principles of a Lens Figure 49-8; Guyton and Hall
- 17. Refractive Principles of a Lens • Concave lens diverges light rays. Figure 49-3; Guyton and Hall
- 18. 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. 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. 20 Errors of Refraction Figure 49-12; Guyton and Hall Normal vision Far sightedness Near sightedness
- 21. 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. 22 Correction of Myopic Vision Figure 49-13; Guyton and Hall Myopia corrected with concave lens
- 23. 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. 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. 25 Errors of Refraction Figure 49-12; Guyton and Hall Normal vision Far sightedness Near sightedness
- 26. 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. 27 Correction of Hyperopic Vision Figure 49-13; Guyton and Hall Hyperopia corrected with convex lens
- 28. 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).
- 29. 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. 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. 31 Exaggerated Astigmatic Corneal Shape Notice the difference in the degree of curve of the cornea in 2 planes Cornea: face-on
- 32. 32 Astigmatism Correction • Cylindrical optical refractive correction – Glasses – Contact lenses • Surgery – LASIK • Laser-assisted in situ keratomileusis
- 33. Cataracts • Cataracts – cloudy or opaque area of the lens – caused by coagulation of lens proteins • More to come
- 34. 34 Cataract
- 35. 35 Cataract Correction • Surgical – The lens is replaced – Induces presbyopia – Frequently dramatically improves far vision
- 36. 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
- 37. 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. 38 Each Cone is Receptive to a Particular Wavelength of Light Figure 50-7; Guyton & Hall
- 39. 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. 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. http://www.toledo-bend.com/colorblind/Ishihara.html, 2001
- 41. 41 Color Vision Colorblind individuals should see the yellow square. Color normal individuals should see the yellow square and a "faint" brown circle.
- 42. 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. 43 Now, Do You See Things More Clearly???