Cheryl J. Reed, O.D.

Snellen Visual Acuity A measure of smallest high contrast symbol that patient can see and recognize Test Distance / Distance at which letter subtends 5 minutes of arc or detail subtends 1 minute of arc

Examples 20/20 Test distance = 20 feet Distance at which letter subtends 5’ arc = 20 feet 10/200Test distance = 10 feet Distance at which letter subtends 5’ arc = 200 feet - Visual acuity refers to smallest letter size that patient gets half or more correct on line.

Anatomy of Eye Cornea Aqueous Crystalline lens Vitreous Retina Macula Fovea Optic Nerve Brain

To get good visual acuity:  Light must pass through cornea, aqueous, lens, and vitreous  Light must focus on retina  Image must “land” on macula  Retina must respond to visual stimuli by generating photochemical reaction  Electrical stimuli must be transmitted from retina to brain  To get good binocular acuity, two eyes must accurately point to image and the two images fuse into a single image  Higher processing areas must interpret image

To get good visual acuity cont’d:  Good visual acuity is necessary for :  Reading small print  Recognizing people at distance  Reading signs at distance

To get good visual acuity cont’d.:  Factors which affect acuity:  Environmental factors –  lighting, optotype, crowding, position of chart  Patient factors –  Fatigue, nervousness, eye movements, fixation, motivation Can You See This?

To learn about acuity and functional vision, must observe patient and observe chart. Test R.E, L.E., O.U. Visual behaviors 1. Central, eccentric 2. Stable, wandering, nystagmus, unsteady 3. Head or body movement 4. Squinting or shutting one eye 5. Use of glasses (peeking over glasses, viewing through bifocal segment)

Chart Observation 1. Missing or skipping letters 2. Confusion of similar letters 3. Reading speed (especially note if large, supra- threshold letters are read with same difficulty as threshold letters 4. Note any observation made by patient (i.e. distortion, hallucination, blurred areas)

Expected visual behaviors during acuity testing with: 1. Large central or paracentral scotoma a) Suprathreshold letters easier to see b) Eccentric gaze or head movement c) Instruction in eccentric fixation helpful d) Scotoma to right – reading slow across line e) Scotoma to left – line returns difficult, may miss first letter(s)

2. Large scotoma with small area of sparing a) Patient complaints worse than acuity would suggest b) Small letters may be easier to see than larger letters c) Getting close not necessarily helpful d) Lighting more helpful than magnification e) Reading slow and loss of place common

3. Multiple small scotomata around fixation a) depends on size, location, and density of scotoma b) combination of above 4. Dominant eye is poorer eye a) Binocular acuity may be poorer than monocular b) May try to squint dominant eye shut

5. Strabismus a) Observe eye turn b) May report double vision c) May squint one eye shut d) May use head turn to try to align eyes or block one eye

6. Homonymous hemianopsia a) History of stroke, head trauma, or tumor b) Observe head turn in direction of field loss c) May (or may not) miss letters on one side of chart

Eye DiseaseVisual Behavior Rehabilitation Strategy Age Related Macular Degeneration (Wet Form) Random head movement to the right Teach eccentric fixation to right or upper right Instruct patient to point past word when reading Use CCTV which scrolls word to left as patient reads Random head movement to left Teach eccentric fixation to left or upper left Keep left thumb at beginning of line and use as line return guide

Eye DiseaseVisual BehaviorRehabilitation Strategy Age Related Macular Degeneration (atrophic) Fixation is central or slightly paracentral Use minimal magnification Use bright illumination to maximize contrast Typoscope helpful to keep place More visual complaints than would be predicted by relatively good visual acuity

Eye Disease Visual BehaviorRehabilitation Strategy Congenital Nystagmus Views with chin down and eyes in upward gaze Use reading stand to position page Desktop CCTV often helpful Bifocal not helpful Views with head to left and eyes in right gaze Position CCTV or computer monitor to right Position student in classroom in front and left of center Consider referral for prism glasses or extra ocular muscle surgery

Eye DiseaseVisual BehaviorRehabilitation Strategy Right Homonymous Hemianopsia Skips letters on right side of chart Encourage patient to shift gaze and turn head to right Consider referral for application of base right prism to glasses Under Corrected Myopia or Nearsightedness Patient pushes glasses close to face to read chart Patient squints when trying to read eye chart Refer for refraction

Eye DiseaseVisual Behavior Rehabilitation Strategy Under corrected Hyperopia or Farsightedness Patient lets glasses slip down nose or pulls farther from face Refer for refraction Patient views though bifocal, trifocal or lower part of progressive addition lens

Eye DiseaseVisual BehaviorRehabilitation Strategy Cone DystrophyPatient squints, drops head or shades eyes with hand Use dark amber or red/orange sunglasses Use ball cap or visor Patient squints May turn head (see congenital nystagmus) Use dark amber or grey sunglasses Use ball cap or visor Albinism

Eye Disease Visual BehaviorRehabilitation Strategy Congenital Toxoplasmosis Patient uses stable eccentric gaze May help to position CCTV or computer monitor opposite direction of gaze