7.1.5 Assesses patients with impaired visual function and understands the use of specialist charts for distance and near vision and the effects of lighting, contrast and glare

LogMAR/Snellen (Meters)

What is a snellen chart?

Each letter on the chart is designed to subtend 5 minutes of arc at the test distance, meaning the overall height of the letter fits within that visual angle.

Snellen chart

Advantages vs Disadvantages

Advantages

• Readily available

• Sensitive to refractive error

• Can be used to predict and verify magnification for LVA’s

Disadvantages

• Inability to control crowding

• Variable letter legibility

• Non-uniform letter size progression

• Harder to score letter-by-letter acuity

What is a Bailey-Lovie chart?

• Letter sizes decrease in equal logarithmic steps (logMAR scale), allowing for consistent visual demand between lines.

• Letters double in size every 3 lines (0.3 logMAR). Halving distance of chart requires VA addition of 0.3

Bailey-Lovie chart

Advantages vs Disadvantages

Advantages

• More letters presented to a low vision patient

• Equal letters and spacing

• Measurable letter-by-letter acuity

• Standardised letter size progression

Disadvantages

• Large sized chart

• No round letters for x-cyl targets

• Less available / high cost

• Takes longer to assess VA

What is the Pelli-Robson chart?

• Letter sizes decrease in equal logarithmic steps (logMAR scale), allowing for consistent visual demand between lines

• Threshold is the triplet where 2/3 letters are correct or each letter is scored and recorded as 0.05 logMAR.

Pelli-Robson

Advantages vs Disadvantages

Advantages

• Good repeatability and reliability

• Better predictor than VA for performance in everyday tasks

Disadvantages

• Large chart

• Difficult to illuminate evenly to 85cmd-2

• Cardboard chart has limited life span

How can the refraction routine be adapted for patients with impaired visual function?

• Use large aperture for trial lenses

• Use big bracketing steps

  • ±1.00 / 2.00D or even bigger.

  • Smaller changes won’t be noticeable

• Adjust final prescription for test chart distance

  • If the chart is at 4m instead of 6m, 0.25D must be subtracted from the final prescription to account for the extra amount of accommodation that the patient is using

How can the refraction routine be adapted for patients with impaired visual function?

Near visual acuity

Acuity reserve is 2:1 - if a patients best VA is N6, they will comfortably read N12 for a long duration of time.

Bailey-Lovie Keeler VA (set to 25cm working distance)

• MNRead

• Maclure