Visual Acuity A Key to Understanding Visual Function

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Visual Acuity A Key to Understanding Visual
Function

• Cheryl J. Reed, O.D.

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• 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

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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.

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Anatomy of Eye

• Cornea
• Aqueous
• Crystalline lens
• Vitreous
• Retina
• Macula
• Fovea
• Optic Nerve
• Brain

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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

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To get good visual acuity contd

• Good visual acuity is necessary for
• Reading small print
• Recognizing
• people at distance
• Reading signs at distance

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To get good visual acuity contd.
Can You See This?
Can You See This?
Can You See This?

• Factors which affect acuity
• Environmental factors
• lighting, optotype, crowding, position of chart
• Patient factors
• Fatigue, nervousness, eye movements, fixation,
  motivation

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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)

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• 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)

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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)

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• 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

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• 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

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• 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

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• 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

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Eye Disease
Visual Behavior
Rehabilitation Strategy

• 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

Age Related Macular Degeneration (Wet Form)
Random head movement to the right
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

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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
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Visual Behavior
Rehabilitation Strategy
Eye Disease
Views with chin down and eyes in upward gaze

• Use reading stand to position page
• Desktop CCTV often helpful
• Bifocal not helpful

Congenital Nystagmus

• 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

Views with head to left and eyes in right gaze
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Eye Disease
Visual Behavior
Rehabilitation Strategy

• Encourage patient to shift gaze and turn head to
  right
• Consider referral for application of base right
  prism to glasses

Right Homonymous Hemianopsia
Skips letters on right side of chart
Under Corrected Myopia or Nearsightedness
Patient pushes glasses close to face to read chart

• Refer for refraction

Patient squints when trying to read eye chart
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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
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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
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Visual Behavior
Rehabilitation Strategy
Eye Disease

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

Congenital Toxoplasmosis
Patient uses stable eccentric gaze