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New York-Some imaging devices may help predict which patients will develop glaucoma, but "they will not supplant clinical assessment of the optic disc by a trained observer," said Christopher A. Girkin, MD, MSPH, to attendees of the Glaucoma 2006 meeting here.
"A variety of imaging techniques have shown promise in detecting early glaucoma, but that doesn't necessarily mean that they have the ability to detect the development of glaucomatous changes," said Dr. Girkin, associate professor of ophthalmology and director of the glaucoma service, department of ophthalmology at Callahan Eye Foundation Hospital, University of Alabama at Birmingham.
He spoke on the use of imaging devices to predict glaucoma during the educational symposium, sponsored by The New York Eye and Ear Infirmary, the UMDNJ-Center for Continuing Education, The Academy of Healthcare Education Inc., and CME2 , an independent subsidiary of Advanstar Communications Inc., publisher of Ophthalmology Times. Pfizer Inc. provided an unrestricted educational grant for the program.
"But there have been relatively few longitudinal studies looking at how to define the progression of glaucoma and how to predict progressive disease," he said.
Progressive studies are admittedly difficult, Dr. Girkin acknowledged.
"The primary reason is that glaucoma is a chronic disease," he said. "It takes a long time to develop in an individual and a long time to progress in an individual. The optimal change parameters both in function and structure are not known and the endpoints we use to determine progression in glaucoma are less than optimal."
Variability in the standard visual field (the primary endpoint in most clinical trials) limits the ability to detect progression accurately, Dr. Girkin said.
"When we look at a normal area in a visual field, there is a wide variation in threshold values obtained at one point compared with another," he said. "To make matters worse, if we look at a diseased area of a visual field, in or adjacent to an area of damage, this variability expands tremendously."
This in part fuels the impetus to develop methods to image the optic disc to detect glaucoma earlier, to predict those patients whose glaucoma will progress, and to detect progression in those patients, Dr. Girkin said. There are two types of studies that differ significantly, he said.
Prediction versus progression
"Prediction studies use structural parameters obtained at baseline to pick out which patients in a population are at risk to develop progressive glaucomatous injury or to develop glaucoma," Dr. Girkin said. "Progression studies look at changed parameters over time to try to determine what structural characteristics seen in imaging devices indicate true glaucomatous injury."
One landmark paper came from the Ocular Hypertension Treatment Study and was published in 2005, according to Dr. Girkin (Zangwill LM, Weinreb RN, Beiser JA, et al. Baseline topographic optic disc measurements are associated with the development of primary open-angle glaucoma: the Confocal Scanning Laser Ophthalmoscopy Ancillary Study to the Ocular Hypertension Treatment Study. Arch Ophthalmol 2005;123:1188-1197).
A subset of patients underwent confocal scanning laser ophthalmoscopy at baseline. The researchers looked at baseline characteristics and topographic parameters to see if it could be predicted whether patients would go on to develop glaucomatous field defects or change in the optic disc.
"The investigators found that if the patient had an abnormal Moorfields regression analysis, the patient would be more likely to go on to develop glaucoma," Dr. Girkin said. This was one of the first studies to show that an imaging device can be useful clinically to determine which patients will develop glaucoma.