visual acuity a key to understanding visual function

1. Visual Acuity �A Key to Understanding Visual Function Cheryl J. Reed, O.D.

2. 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

3. Examples 20/20 Test distance = 20 feet Distance at which letter subtends 5� arc = 20 feet 10/200 Test 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.

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

5. 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

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

7. 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

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

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

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

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

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

13. Strabismus Observe eye turn May report double vision May squint one eye shut May use head turn to try to align eyes or block one eye

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