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New technology is enabling clinicians to perform quantitative as well as qualitative assessments of glaucoma patients. As this technology becomes more widely available, it will be important to match the output from the new imaging devices to clinical needs as well as to deal with practical issues such as the cost of buying and updating new devices.
Examination of the optic disc and the nerve fiber layer are critical and important elements in the assessment of the glaucoma patient, he said. "But I think it's important to recognize that this qualitative assessment is probably suboptimal in the accurate follow-up of glaucoma patients. We really need to do something more quantitative," he added.
Quantitative assessment probably is best performed if the images being recorded can be archived. This is typically done with conventional fundus photographs, either analog photographs that have been converted and digitized or with digital acquisition and, more commonly, from some of the modern imaging techniques, Dr. Chauhan said.
Two types of analysis strategies exist: cross-sectional analysis and longitudinal analysis. A cross-sectional analysis, such as the Moorfields Regression Analysis (MRA), exploits a population characteristic of normal subjects. When an examination shows that this characteristic has moved from within normal limits to outside of them over time in the same patient, it is classified as change.
Although this approach seems intuitive, it has limitations. Dr. Chauhan cited an example of an MRA performed with a specific confocal laser scanning system (Heidelberg Retina Tomograph, Heidelberg Engineering) with the change analysis based on the relationship between disc area and rim area.
"We can see that when a patient's optic disc goes from within normal limits to outside normal limits, the disease has legitimately changed. However, we can have another situation where the optic disc undergoes the same degree of change and still remains within normal limits," he said. "And conversely, a patient whose disc is outside of normal limits to start with, progresses, and still remains outside of normal limits."
A longitudinal analysis is a more logical approach and can be either trend- or event-based, Dr. Chauhan said. In an event-based analysis, baseline data are obtained, and the limits of "no change" can be derived from either the baseline measurements or population parameters. As the patient is tested subsequently, it will be apparent if the measurements slip outside of the variability limits for measurements obtained within that particular disc.
A trend-based analysis uses the same type of data but relies on regression analysis to derive the rate of change. A criterion such as a given percentage of rim area lost per year or decibels of visual field per year could be used.
At the moment, lacking a precise definition of glaucoma, it is easy to identify those on the side of the spectrum who have unquestionable moderate or severe disease and in whom demonstration of change is not necessary to establish a diagnosis of glaucoma, Dr. Chauhan said.
"What we really do need to work on is the side of the spectrum where there is an equivocal optic disc and a normal, conventional visual field. Because there is no reference or validated quantitative standard for the definition of a glaucomatous disc, this disc can be classified as normal, questionable, or glaucomatous depending on what clinical or study criterion is employed for the definition. However, if we can demonstrate change in any disc, then the diagnosis of glaucoma becomes more convincing," he said.