cheryl j. reed, o.d.. snellen visual acuity a measure of smallest high contrast symbol that patient...

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

• Examples20/20 Test distance = 20 feet

Distance at which letter subtends 5’ arc = 20 feet

10/200 Test distance = 10 feetDistance at which letter subtends5’ arc = 200 feet

- Visual acuity refers to smallest letter size that patient gets half or more correct on line.

•Anatomy of EyeCorneaAqueousCrystalline lensVitreousRetina

MaculaFovea

Optic NerveBrain

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?

Can You See This?

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, eccentric2. Stable, wandering, nystagmus, unsteady3. Head or body movement4. Squinting or shutting one eye5. Use of glasses (peeking over glasses, viewing

through bifocal segment)

Chart Observation

1. Missing or skipping letters2. Confusion of similar letters3. 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 seeb) Eccentric gaze or head movementc) Instruction in eccentric fixation helpfuld) Scotoma to right – reading slow across linee) 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 helpfuld) Lighting more helpful than magnificatione) 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 monocularb) May try to squint dominant eye shut

5. Strabismus

a) Observe eye turnb) May report double vision c) May squint one eye shutd) May use head turn to try to align eyes or

block one eye

6. Homonymous hemianopsia

a) History of stroke, head trauma, or tumorb) Observe head turn in direction of field lossc) May (or may not) miss letters on one side of

chart

Eye Disease Visual 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 Disease Visual Behavior Rehabilitation 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 Behavior Rehabilitation 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 Disease Visual Behavior Rehabilitation 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 Disease Visual 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 Disease Visual Behavior Rehabilitation Strategy

Cone Dystrophy Patient 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 Behavior Rehabilitation Strategy

Congenital Toxoplasmosis

Patient uses stable eccentric gaze

•May help to position CCTV or computer monitor opposite direction of gaze