The routine test that most glaucoma patients dislike the most (and find the most difficult) is perimetry or visual field testing. It can be difficult because of the concentration needed to respond every time a light is seen, no matter how dim or bright and, especially after a long day at the hospital sitting in waiting rooms and walking corridors the results can be very variable, which is bad news for those who have to undergo the test and bad news for their doctors because the results can be unreliable or contradictory. Nevertheless, perimetry is the only test we have that can measure a glaucoma patient's functional ability to see and as such it is a vital element in the battery of tests used to monitor glaucoma and to decide upon any necessary changes that may be required in an individual's management plan in order to protect their vision.
How much better would it be if the same functional information could be obtained without the patient having to do anything – nothing to concentrate on, no buttons to click, no decisions to make? An instrument that can measure the visually evoked potential (or the visually evoked response) automatically already exists, in fact various instruments have been under development for several years. They work by measuring the tiny changes in the electrical activity of the brain when the eye 'sees' something, in this case a light from a perimeter type setup.
Researchers at the New York Eye and Ear Infirmary have just published a paper in the Journal of Glaucoma called 'Short Duration Transient Visual Evoked Potentials in Glaucomatous Eyes' that shows that a visual evoked potential instrument can measure a patient's ability to see accurately and objectively and not only that, but that the measurements correlate well with the structural changes in the retina and the optic nerve head that can now be measured using optical coherence technology (OCT). The test itself takes between four and six minutes per eye.
Dr Robert Ritch, MD, one of the study authors is quoted as saying, 'The SD-tVEP results correlated significantly with the severity of visual field damage, but the VEP results were obtained objectively, which helps give eye care specialists more confidence in the findings.'
It remains to be seen when and if this technology will become commonplace in the glaucoma management regimes used in different countries around the world, but this would seem to have the potential to make life a little easier for glaucoma patients and to provide their doctors with more robust information about precisely how well they can see regardless of an individual's level of fatigue or ability to concentrate.
David J Wright FIAM
International Glaucoma Association