- The most common Visual Acuity Chart that is Snellen chart is designed to perform mostly at 6 meters or 3 meters.
- Sometimes patient’s Visual Acuity less than 6/60 that is maximum size.
- In such cases, to identify patient’s visual acuity we can follow 3 techniques:
- Checking Finger Count Ability.
- Bringing the chart Closer to the patient
- Taking the patient closer to the Chart.
Now, if a chart tis designed for 6-meter distance but Visual Acuity is checked from 2 meters then how to identify the Equivalent or Actual Visual Acuity?
- The formula we can follow to identify equivalent VA acuity is:
For 6 meters chart: Equivalent VA = (Denominator X 6) / Reading Distance in meter
For 20 feet chart: Equivalent VA = (Denominator X 20) / Reading Distance in feet
Here,
- Denominator = Denominator of Patients current Visual Acuity.
- 6 = Identifiable Chart Distance (6-meter Chart).
- Reading Distance = Distance from where patient read.
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Example-1: A 30-year-old patient visits your clinic for an eye examination. During visual acuity testing ,the patient was not able to read 6/60 size letter, so you shift the chart at 2 meters and patient was able to read 6/12 size. What will be the actual visual acuity of the patient at 6 meter?
Here;
- Patient’s VA Denominator = 12
- Identifiable Chart Distance = 6 meters
- Reading Distance = 2 meters
So, the Equivalent VA will be:
- Equivalent VA = (Denominator X 6) / Reading Distance
- Equivalent VA = 12 x 6 / 2
- Equivalent VA = 72/ 2
- Equivalent VA = 36
So, Patients Actual or Equivalent VA is 6/36.
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Example-2: A patient was not able to read any letter in the 20 feet Snellen chart, so you shift the chart at 12 feet and patient was able to read 20/200 size. What will be the actual visual acuity of the patient at 20 feet?
Here;
- Patient’s VA Denominator = 200
- Identifiable Chart Distance = 20 feet
- Reading Distance = 12 feet
So, the Equivalent VA will be:
- Equivalent VA = (Denominator X 20) / Reading Distance
- Equivalent VA = 200 x 20 / 12
- Equivalent VA = 4000/ 12
- Equivalent VA = 333.33
So, Patients Actual or Equivalent VA is 20/333.33 size.
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