During World Glaucoma Week, it is helpful – and instructive – to listen to discussions within the medical community about issues that are critically important to people with glaucoma.
One such issue relates to a major challenge in the management of glaucoma: how to best determine the severity of the disease, estimate the rate of glaucoma progression, initiate appropriate treatment, and adjust treatment when necessary.
In persons with glaucoma, it is the visual field that is the most important functional measure of the severity of the disease and its progression. Currently, several groups of researchers are debating the reliability and accuracy of the visual field index, a new parameter first proposed in 2008, to detect and measure the progression of the visual field over time.
The most recent study, entitled The Relationship between Visual Field Index and Estimated Number of Retinal Ganglion Cells in Glaucoma, was presented at the American Glaucoma Society 24th Annual Meeting, with initial results published in PLoS ONE, an international, peer-reviewed, open-access online journal, published monthly by the Public Library of Science (PLoS). The PLoS is a non-profit organization of scientists and physicians who are committed to making the world’s scientific and medical literature a freely available public resource.
The authors are Amir H. Marvasti, Andrew J. Tatham, Linda M. Zangwill, Christopher A. Girkin, Jeffrey M. Liebmann, Robert N. Weinreb, and Felipe A. Medeiros, who represent the following institutions: the University of California, San Diego; Boston University School of Medicine; University of Alabama, Birmingham; and New York Eye and Ear Infirmary.
What Is the Visual Field Index?
The visual field index for the calculation of glaucoma progression is a recent concept, first introduced in 2008. It is a standard of measurement that describes the status of the visual field in glaucoma as a percentage of a normal visual field [i.e., without glaucoma], adjusted for age-related changes.
In other words, the visual field index represents the entire glaucomatous visual field as a single percentage of a normal visual field. It is automatically calculated by the current software contained in the Humphrey Field Analyzer, a computer-based visual field analysis tool.
About the Research
From Visual Field Index Overlooks Neural Loss in Early Glaucoma, via Medscape.com:
The visual field index underestimates the amount of neural loss in eyes affected by glaucoma, particularly in early disease, a new study suggests. “In early disease, we should rely less on the visual field index,” said [author] Amir Marvasti. Underlying ganglion cell loss is used to calculate the index, “however, we found that in early disease, the visual field index overestimates the amount of neural tissue left in glaucoma.” In other words, “it underestimates the amount of neural loss,” he explained.
[Editor’s note: Ganglion cells, also called retinal ganglion cells, are neurons, or nervous system cells. They are located near the inner surface of the retina and give rise to optic nerve fibers that transmit information from the retina to several regions in brain.]
Marvasti presented the study findings at the American Glaucoma Society 24th Annual Meeting. First results were published in PLoS One.
“It’s a useful tool, but the [visual field index] is really most useful in the more moderate stages of disease,” said session moderator Louis Cantor, MD. “I think doctors are relying on it, probably more than the evidence supports that we should, especially in early disease. One of the hardest things to do in practice is to segregate our patients and look at them differently.”
“We should consider disease severity when we look at rates of change in visual field index. Estimates of ganglion cells may be a better method of measuring rates of progression in glaucoma,” Marvasti explained.
Glaucoma: the Basics
Glaucoma is a group of eye diseases that can lead to blindness by damaging the optic nerve. The eye continuously produces a fluid, called the aqueous, that must drain from the eye to maintain healthy eye pressure.
In the most common type of glaucoma, Primary Open Angle Glaucoma, the eye’s drainage canals become blocked, and the fluid accumulation causes pressure to build within the eye. This pressure can cause damage to the optic nerve, which transmits information from the eye to the brain.
Glaucoma results in peripheral (or side) vision loss initially, and the effect can be like looking through a tube or into a narrow tunnel. This “tunnel vision” effect makes it difficult to walk without bumping into objects that are off to the side, near the head, or at foot level.
Glaucoma is an especially dangerous eye condition because most people do not experience any symptoms or early warning signs at the onset. Glaucoma can be treated, but it is not curable. The damage to the optic nerve from glaucoma cannot be reversed. However, lowering pressure within the eye can prevent further damage to the optic nerve and further peripheral vision loss.
Starting as early as age 35, an eye pressure check for glaucoma should be an essential part of your annual routine eye examination.
More about the Study from PLoS
From the article abstract:
Purpose: To evaluate the relationship between visual field index and the estimated number of retinal ganglion cells in glaucoma.
Methods: A multicenter study of 1,245 healthy, glaucomatous, and suspected glaucomatous eyes of 1,245 subjects recruited from the Diagnostic Innovations in Glaucoma Study and the African Descent and Glaucoma Evaluation Study. All eyes underwent standard automated perimetry [i.e., visual field testing] and optical coherence tomography [i.e., high-resolution imaging]. Estimates of retinal ganglion cell count and percentage of retinal ganglion cells remaining, compared to age-matched healthy eyes, were calculated from optical coherence tomography.
Results: The relationships between the visual field index and both estimated retinal ganglion cell counts and the percent remaining retinal ganglion cells were nonlinear [i.e., did not match or directly relate]. A unit number of visual field index loss corresponded to substantially greater loss of estimated retinal ganglion cells and estimated percentage of retinal ganglion cells remaining in early compared to late disease.