Assessing visual function by routine eye tests

Overview

Recall questions on visual acuity measured with Snellen and logMAR charts, 6/6 and 6/18 results, pinhole testing, the fovea, the Ishihara test and inherited red-green colour deficiency, and optical coherence tomography in macular degeneration and glaucoma.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define visual acuity.

   Model answer

   Visual acuity is the ability of the eye to distinguish fine detail, that is to see two points close together as separate.

2. 2

   Describe how visual acuity is measured using a Snellen chart.

   Model answer

   The patient stands 6 m from a chart of letters that get smaller line by line and reads down it with one eye covered, and the smallest line read correctly is recorded for each eye separately.

3. 3

   What does a visual acuity recorded as 6/6 mean?

   Model answer

   It means the patient can read at 6 m the line that a person with normal vision can read at 6 m, so their acuity is normal.

4. 4

   A patient is recorded as 6/18 in the left eye. Explain what this tells you.

   Model answer

   The left eye can only read at 6 m what a normal eye can read at 18 m, so its acuity is well below normal.

5. 5

   Give two advantages of a logMAR chart over a traditional Snellen chart.

   Model answer

   A logMAR chart has the same number of letters on every line and equal steps in difficulty between lines, so scores are more reliable and small changes over time can be detected.

6. 6

   Why is each eye tested separately in an acuity test?

   Model answer

   A good eye would compensate for a poor one, so a loss of vision in one eye would be missed if both were tested together.

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