Chicken and Egg

There are many excellent automated fields screeners and analysers available. Very few, beyond perhaps the Oculus Easyfield, lend themselves to be transported from room to room, practice to practice or for domiciliary visits.
Optometrist Graham O'Regan has many years' experience and has already developed interesting ways of adapting everyday equipment to assist in such assessment (optician November 19 2004). To add to this, he has developed over the past few years a computerised fields assessment programme which he hopes will prove useful, not only for domiciliaries, but also for pre-screening areas and also wherever a patient has difficulty accessing a conventional automated screener (perhaps due to disability).
A key theme has been to make the programme stimulating and visually interesting for both patient and practitioner. Hence, he decided to name the programme the 'Chicken and Egg Visual Field Screener' and throughout it maintains this user-friendly style.
It cannot be denied that access to repeatable fields assessment of a degree of reliability is limited, certainly for those unable to attend for a full examination at a practice, so such developments must be deemed useful.

Setting Up
The system is Powerpoint based, but requires a minimum of computer literacy as, once running, the various slides may be toggled through either using the mouse wheel or the up and down keyboard arrows. The programme may be used on a laptop (the better the screen quality, the better the result, but a 15in size is minimum) or a desktop (17in size minimum). The screen size dictates the working distance and 40cm is appropriate for a 15in screen and the patient needs to be corrected for this distance as with any central screening test. The test runs best in mesopic conditions and it is essential that any point glare sources are avoided. Where screen sizes vary, a useful method of establishing best working distance is to get a number of people to view the chicken with one eye (blind spot chart shown in Figure 1) and to move towards the screen until the egg disappears. The average for these distances measured should be used as the working distance for that particular screen. O'Regan has also included a near duochrome target if a check on focusing is required.

Omelette Test
There is then a choice of tests. The so-called 'omelette test' is a sequence of different tests, including ones to orientate the patient to the tests, screening for gross loss of hemianopias (Figure 2), nasal step assessment for glaucoma (Figure 3), a gross peripheral assessment and a version of a maculopathy screening test.

Chicken and Egg Test 32-Point Screening Test
This begins, as with many of the tests, with a title screen on which an animated chicken is fixated by the patient as it is heard to crow in the background. The charts are then displayed on which an egg will appear at some point on the screen and the patient reports having seen it. A control slide (without an egg) helps validate fixation as, when the slide is about to appear (signalled by a bell) the patient's fixation may be closely monitored to see if it moves around the screen looking for the nonexistent egg.
If defects are suspected (eggs missed), then the 100 egg test should be used. A plot of the field loss found this way in a glaucoma patient is shown in Figure 4.

Glaucoma Specific Tests
There is a sequence of tests specific to glaucoma screening. These use blue eggs on a yellow background in an attempt to simulate short wavelength automated perimetric tests found elsewhere. These use targets in the regions vulnerable to early glaucoma field loss (paracentral loss and temporal horizontal midline differences) such as the 'expanding rainbow test' where the patient expands a blue rainbow and notes if any of the dots disappear (Figure 5).

Macular Function
A variety of tests are used to assess macular function. Many of these are variations of an Amsler-style target with grid lines used to detect areas of central loss or distortion. One such test, the 'graded maculopathy test', begins with a central grid which expands outwards as a square (Figure 6) and, by changing the direction of mouse wheel movement, may be brought back to the central grid again. The record of macular defects of a diabetic patient before and after laser photocoagulation are shown in Figure 7. The larger red field indicates the reduced macular sensitivity after treatment.

Trials
Over the coming weeks optician will be testing the system at a variety of locations. It seems that the software is very simple to run, allows repeatable assessment in a variety of environments previously unavailable to fields assessment, and should have sufficient sensitivity to detect significant field loss from a variety of conditions. The results of our trial will be published later in the year.

Further details are available on 01453 832924