| Acute congestive glaucoma Source: Daljit, Singh, M.D.
Many of us take an event-based approach to managing our glaucoma patients. We base our decisions regarding the management of individual patients on the occurrence of certain events. An event might be a higher-than-acceptable IOP measurement, a change in visual field status from one test to another, or a change in optic nerve head appearance from one examination to another.
Recognizing these events is an important part of glaucoma management, to be sure. But this event-based approach to the disease provides us with no information about the rate of progression over time. How rapidly is Mrs. Smith’s glaucoma progressing? What is her rate of change? Or, phrased in the language of risk assessment, what is her likelihood of progressing to the point of visual disability in her expected lifetime? Determining the rate of progression should be a goal of managing all of our patients, said Claude F. Burgoyne, M.D., senior scientist and research director, Optic Nerve Head Research Laboratory, Devers Eye Institute, Portland, Ore. “Determination of the rate of progression can help us to identify those patients at the highest risk of vision loss,” he said. “A patient’s current rate of progression may inform us about future progression rates. If we then factor in life expectancy, we can begin to assess the risk of vision loss within a given patient’s lifetime.”
Balwantray Chauhan, Ph.D., professor, department of ophthalmology and visual sciences, Dalhousie University, Halifax, Nova Scotia, agrees. “Identifying fast progressors should be a priority in the management of patients with glaucoma.”
How do we determine the rate of progression of our glaucoma patients? We can use data acquired from visual field tests, from optic nerve imaging devices, or a combination of both.
Rate of structural change
Structural change is worth considering because it is widely held that structural optic nerve changes precede functional visual field loss in the majority of subjects with progressing glaucoma. This dogma was recently confirmed by a study conducted by Robert N. Weinreb, M.D., director, Hamilton Glaucoma Center, University of California, San Diego, and colleagues.
In their longitudinal study involving a cohort of glaucoma suspects followed prospectively for many years, “Changes in the optic nerve were very predictive of a subsequent change in visual function,” Dr. Weinreb said.
Dr. Weinreb is the principal investigator of the Confocal Scanning Laser Ophthalmoscopy Ancillary Study to the Ocular Hypertension Treatment Study (OHTS). This study seeks to evaluate the effectiveness of the Heidelberg Retina Tomograph (HRT, Heidelberg Engineering, Vista, Calif.) in detecting the presence and progression of glaucomatous optic nerve changes and in predicting visual field loss. The HRT evaluates the topographical features of the optic nerve head and provides a number of descriptive parameters, including the area of the neuroretinal rim. A recent analysis of data from this study revealed a striking finding.
“Among the subjects in the OHTS, the rate of change in rim area in subjects who ultimately developed primary open-angle glaucoma was five times faster than in subjects who did not develop glaucoma,” he said.
Rate of functional change
In clinical practice, visual fields are often interpreted by event analysis; they are either stable or worse compared to prior tests. Rate-of-change analysis, also called trend analysis, can be more informative.
“Trend analysis of visual field data can give us information about the rate of vision loss,” said Joseph Caprioli, M.D., professor of ophthalmology, Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles.
Dr. Caprioli described a new feature of the Humphrey visual field software package that will do this automatically. “The Glaucoma Progression Index, or GPI, is a number between 0 and 100 that is generated for each visual field test,” he explained. “It is based on the pattern deviation but weighs each point by its location in the visual field, with peripheral points being weighed less than central points,” he said. Plotting the GPI over time, which the software does automatically, yields a line with the slope representing the rate of change of the visual field.
Unless we formally assess rate of change in our patients, we often don’t have a true sense of how they are doing. Assessing rate of change can produce some clinical surprises. “When we reviewed this software in our Glaucoma Diagnostic Laboratory we reanalyzed a series of seemingly stable patients. It was unsettling to find that some had relatively rapid rates of change and regions of progression not revealed by our previous software tools,” said Robert Fechtner, M.D., director, Glaucoma Division, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark. “The most accessible of the progression tools will be ones such as this that can use existing test data.”
If you don’t have access to the new software, fret not. Dr. Chauhan explained that rate of visual field change between two tests can be straightforwardly calculated by dividing the change in mean deviation by the time between the tests in years. This yields a rate of change parameter in units of decibels per year.
“A fast progressor might be someone who is progressing at a rate greater than one decibel per year,” he said. “Progressing at a rate of two or more decibels per year might be even more worrisome.” Whether the automated or manual methods of determining rate of change are used, the results are equally useful, said Dr. Chauhan. “Once the rate of visual field change has been determined for a given patient, we can then predict the age at which that patient will experience clinically relevant vision loss or blindness, assuming that the rate of progression remains constant over time,” he said.
Implementing rate-of-change analysis in clinical practice
Ideally, clinical evaluation of glaucoma progression would be based on a combination of both structural and functional assessments. This approach has practical limitations given the need for multiple tests to detect progression.
For instance, visual fields have a lot of within-test and between-test variability, so multiple tests are required to reduce the noise. The same is true for optic nerve imaging tests.
“For this reason, visual field trend analysis requires many tests and takes longer to reach a conclusion,” said Dr. Burgoyne.
“The key is to perform frequent examinations—whether visual fields, optic nerve imaging, or both—within the initial follow-up period,” said Dr. Chauhan. He said that six to seven tests within the first two years is probably the minimum number needed to detect rapidly progressing patients.
Performing six visual fields and six imaging tests within two years may not be practical, he acknowledged. “It comes down to which test generally drives your clinical decision-making,” he said. “If you tend to decide on progression based on visual fields, then get six over two years. If you tend to decide based on imaging, then get imaging tests instead.”
In this era of cost-conscious medical practice, even this many fields may be impractical. And our patients won’t be thrilled at the prospect, either. “Our greatest experience and validation of rate of progression is with visual field testing. It is doubtful that my patients will cheerfully accept field testing two or three times yearly. I look forward to the commercial availability of meaningful progression software for structural testing where patient acceptance will likely be much greater,” Dr. Fechtner said.
Editors’ note: The physicians interviewed did not indicate any financial interests related to their comments.