Optician India

Welcome to question of the day #118

I have a computer controlled visual field analyser in my practice and for most patients who require visual field analysis it works well. But some people can’t position their head and/or chin properly and I get erroneous results (false positives-the results suggest a visual field defect when there isn’t one). Other patients don’t understand my instructions and I get false positives. Is there an alternative to computer controlled visual field analysis.

I like technology but it can be expensive to purchase, maintain and replace. It can be diffilcut to use and sometimes overcomplicates what needs to be done. This can be true of computer controlled visual field analysers for some patients and under some circumstances. This is one of the reasons I developed the Aston Perimetry Tool (APT). See figure. It can be obtained from https://store.aston.ac.uk/product-catalogue/life-health-sciences/optometry/aston-perimetry-tool

The APT can be used to make a speedy and assessment of a patient’s visual field. It consists of a white target 5 mm in diameter (0.87° angular subtense) and a red spherical tar get 15 mm in diameter (2.60°). The white and red stimuli are attached to the ends of a thin (2 mm) rod and separated by 330 mm.

Here is a step-by-step procedure for kinetic perimetry with central field analysis using the APT. See figure.

1. The patient should be instructed to cover over their left eye using their left hand. The patient should use a palm cupped over the eye and the clinician should ensure that the patient is not peeking or not pressing on the cornea. The latter would cause residual blurring of vision and make subsequent testing of this eye difficult.

2. The clinician should move her/his head so it is directly in front of and at the same height as the patient’s head.

3. The patient should be instructed to look at the bridge of the clinician’s nose.

4. The clinician should position the white or red target out of the direct line of sight of the patient in the horizontal meridian (i.e. in line with the patient’s eyes) so when they are looking at the bridge of the nose they cannot see the target.

5. The clinician should use the APT to gauge how far away from the patient’s head to position the white or red target. The APT is 33 cm long and this is how far the white or red target should be held from the patients head.

6. The clinician should move the white or red target slowly in an arc centred on the patient’s head and at all times keep the target 33 cm from the patient’s head and remind the patient to keep looking at the bridge of your nose.

7. The clinician should ask the patient to let him/her know when they first see the target (they don’t have to name the colour just to be aware of the stimulus and this should be made clear at the start of the procedure).

8. The clinician should make a mental note of when the patient first detects the target and then compare this to the result that would be expected for a person with a normal visual field.

9. The clinician should continue to move the target in an arc towards the centre of their line of sight and ask the patient to indicate if the target disappears. If the target does disappear the clinician should make a mental note of where it disappears and then to continue moving the target and for the patient to indicate when the target reappears.

10. The clinician should stop moving the target when it reaches the patient’s line of sight for the right eye.

11. Then repeat steps 4 to 10 at 45°, 90°, 135°, 18 0°, 225°, 270° and 360°. 1

2. Then repeat steps 1 to 11 for the left eye with the patient’s right eye covered.