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Casting some new light on visual field test

02/07/2019
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Visual field tests have changed significantly throughout my career. Despite the progress, in many ways the current technology lags behind modern research.

In the mid 1960s Friedmann developed a tangent perimeter, which I used as a student. The early version was a manual device; complex to use but popular with optometrists.

Automated perimetry originated in the 1980s, with the Humphrey Company commercialising some of the very first units. These devices were large bowls calibrated for light output, and automation took care of the test staircase. In those days, perimetry required some 8 to12 minutes for testing a normal eye, and 15 to 20 minutes when testing an abnormal eye. My experience is that patients find the test both lengthy and onerous.

Over the years, bowl size has shrunk and test time shortened with the SITA suite of Bayesian thresholding: SITA standard, SITA fast (30% faster) and now SITA faster (50% faster). Although these developments are speeding up testing to about 4 minutes per eye, the cost of these devices remains high and they require space in clinics, as well as trained operators.

What is most surprising is that the current generation of automated perimeters do not include enhancements learned over the past 30 years. In fact, the grids and test approaches (spot size) are the same as those of the past. In this aspect nothing has changed, and we are still using the dated approaches of Goldmann.

"What is most surprising is that the current generation of automated perimeters do not include enhancements learned over the past 30 years"

There is now a large body of evidence that shows that the standard (24-2 and 30-2) grids under-sample the macula, the region of highest density of retinal ganglion cells, so in many cases clinicians need to put patients through a second test, the 10-2. What a pain for both the patient and clinician.

Carl Zeis Meditec recently addressed macula under-sampling by developing the 24-2C grid. This new grid has an extra 10 points within the central 10 degrees for a total of 22 macula test points. However, this compares poorly to the 68 points of a 10-2 grid over the same area. Given that the 24-2C locations have been optimised for glaucoma, will they cope with the more common causes of central vision loss such as AMD or macula oedema?

I believe, as many others also do, that this past knowledge can seed better perimeters, as is the case of our tangent device Melbourne Rapid Field (MRF).

As clinicians, both Dr George Kong (who is my business partner, a glaucoma surgeon and IT expert) and I appreciate the expense that perimetric bowls place on a practice. We wanted to create a compact device that would fit into modern rooms yet be portable for vision testing; anywhere, anytime, by anyone.

Designs for Vision
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Our approach surveyed modern technology to find the ideal stimulus generator. This yielded goggles, smart-phones and tablets. We chose a tablet because they are portable, compact, ubiquitous and most folks know how to use them. The only other device to share these characteristics is a smart-phone, but its size limits its use for perimetry.

We wanted a test for all applications but realising that vision loss varies with disease, we optimised designs to include three forms of acuity that identify refractive/optical changes, retinal or brain losses and four grid patterns specific to: glaucoma, maculopathy, diabetes and neural diseases.

The novel implementations in these designs are: a spot size that increases with eccentricity to reduce threshold variability; an expanding test grid to define edges of vision loss and a Bayes threshold predictor with neighbourhood logic to return fast, reliable thresholds (2-3 minutes).

We have also designed the test for your high-risk patients to use at home as part of a self-monitoring program. This includes a voice-over set of instructions that guides patients through the test. Their results are transmitted by broadband to secure servers (HIPAA compliant) where the data is stored and analysed. If change is confirmed, the clinician is notified. This allows intervention tailored to each patient’s disease with ongoing charges being less than a glass of beer for each visual field test.

In contrast to present commercial devices, the MRF is based on advances made over the past 30 years of vision and perimetry testing. I believe it heralds a new era of comprehensive vision assessment and patient monitoring, using cheap and portable tablet technology. I hope that our work can make a significant contribution to eyecare professions.

Name: Algis J Vingrys
Qualifications: BSc(Optom), PhD
Workplace: The University of Melbourne
Position: Professor, Department of Optometry & Vision Sciences; Founding Director GLANCE Optical Pty Ltd.
Location: Melbourne
Years in the profession: 35

 

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