Visual Acuity Testing

Visual acuity testing measures the sharpness or clarity of your vision. It's the most fundamental part of any eye examination and is typically done using an eye chart with letters or symbols of decreasing size.

Understanding Visual Acuity Numbers

What 20/20 Means

The number "20/20" means:

• At 20 feet, you can read what a person with normal vision reads at 20 feet
• This is considered "normal" visual acuity

Other Numbers

20/40

• At 20 feet, you can read what a normal person reads at 40 feet
• You need to be closer than normal to see clearly
• This is typically the minimum for driving in most states

20/200

• At 20 feet, you can read what a normal person reads at 200 feet
• This is the definition of "legal blindness" (with best correction)

20/15 or 20/10

• Better than "normal"
• Can see at 20 feet what others need to be at 15 or 10 feet to see

How the Test Is Done

Standard Testing

1. Cover one eye with occluder or palm
2. Read letters on chart from top down
3. Continue until you can't read accurately
4. Repeat with other eye
5. Test with glasses/contacts if you wear them

Types of Eye Charts

Snellen chart

• Most common
• Letters decrease in size
• "E" at top

LogMAR chart (ETDRS)

• More precise
• Same number of letters per line
• Used in research and clinical trials

Picture charts

• For children or non-readers
• Use shapes or pictures

Tumbling E

• For non-readers
• Patient indicates direction E points

Testing Conditions

Distance Vision

• Tested at 20 feet (or 6 meters)
• Or at calibrated distance with mirrors

Near Vision

• Tested at reading distance (14-16 inches)
• Uses near card with different size text
• Important for reading glasses prescription

With and Without Correction

• Tested without glasses first
• Then with current glasses
• Then with refraction (best possible correction)

When Visual Acuity Changes

Sudden Changes

May indicate:

• Optic neuritis
• Retinal problems
• Stroke affecting vision
• Eye emergency

Gradual Changes

May indicate:

• Cataracts
• Macular degeneration
• Refractive error changes
• Diabetic eye disease

Visual Acuity in Neuro-Ophthalmology

Visual acuity is crucial for diagnosing and monitoring:

• Optic neuritis—often reduced acuity
• Ischemic optic neuropathy
• Papilledema—may be preserved until late
• Compressive lesions
• Toxic/nutritional optic neuropathy

Important Considerations

• Acuity may be normal despite significant optic nerve disease
• Color vision and visual field may be affected before acuity
• RAPD can be present with normal acuity
• Always test each eye separately

Limitations of Visual Acuity

Visual acuity only measures:

• Central (foveal) vision
• High contrast recognition
• At specific distances

It does NOT measure:

• Peripheral vision (needs visual field test)
• Contrast sensitivity
• Color vision
• Night vision
• Depth perception

Recording Visual Acuity

Standard notation:

• OD: Right eye (oculus dexter)
• OS: Left eye (oculus sinister)
• OU: Both eyes (oculus uterque)

Example: "VA: OD 20/25, OS 20/20, OU 20/20"

Special Situations

Can't See Chart at All

If unable to read any letters:

• Counting fingers (CF): Can count fingers at specific distance
• Hand motion (HM): Can see hand movement
• Light perception (LP): Can see light
• No light perception (NLP): Cannot see light at all

Pinhole Testing

• Looking through small hole
• Eliminates refractive error effects
• If acuity improves with pinhole, glasses may help
• If doesn't improve, problem is not refractive