Eye Optics and Refractive Errors By John J

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

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

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

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

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

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

Abbreviations • C/o – complaining of or complains of 7

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

Snellen Charts http: //store 6. yimg. com/I/sightmart-eye-care-products_1753_2381891 accessed 9/5/03 9

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 10

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.

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

Image Reception • Optics/Refraction – Air anterior Cornea • 2/3 of the refractive power of the eye – Posterior Cornea – Iris / pupil aqueous humor • Variable aperture – Aqueous humor – Posterior lens anterior lens vitreous humor 13

Image Reception • Convex refraction – Refractive index – Convergence – Image reversal • Perception • Blind spot 14

Refractive Principles of a Lens • Convex lens focuses light rays Figure 49 -2; Guyton and Hall

The Refractive Principles of a Lens Figure 49 -8; Guyton and Hall 16

Refractive Principles of a Lens • Concave lens diverges light rays. Figure 49 -3; Guyton and Hall

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

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 19

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

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? 21

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

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 23

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 24

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

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 26

Correction of Hyperopic Vision Hyperopia corrected with convex lens Figure 49 -13; Guyton and Hall 27

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).

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 29

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 30

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

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

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

Cataract 34

Cataract Correction • Surgical – The lens is replaced – Induces presbyopia – Frequently dramatically improves far vision 35

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

Each Cone is Receptive to a Particular Wavelength of Light Figure 50 -7; Guyton & Hall 38

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.

Eyes & Visual Pathways Ishihara Test for Color Blindness http: //www. toledo-bend. com/colorblind/Ishihara. html, 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. 40

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

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. 42

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