Keeping Up with Glaucoma

Event and trend analysis are critical components in the management of glaucoma — both support earlier diagnosis and more accurate tracking of progression.

In managing glaucoma, the physician's primary goal is minimizing the risk of visual disability during the lifetime of the patient. According to Robert N. Weinreb, MD, achieving this goal requires a comprehensive strategy that includes staging the disease, estimating life expectancy and evaluating the past rate of disease progression based on visual fields, the retinal nerve fiber layer (RNFL) and changes in the optic disc.

Progression can be evaluated in the clinic by one of two approaches. One approach, known as event-based analysis, involves evaluating change from baseline. The second approach, known as trend-based analysis, involves estimating the rate of change. Change from baseline is typically more sensitive than rate of change; however, rate of change is essential for judging whether a patient is at risk for vision loss during his or her lifetime. Historically, estimating rate of change, i.e., rate of progression, has been more difficult.

Why Determining Rate of Disease Progression is a Challenge

Landmark research efforts, such as the Ocular Hypertension Treatment Study (OHTS) and the Early Manifest Glaucoma Trial (EMGT), have been of some help in this regard. Data analysis from OHTS identified risk factors, such as larger vertical cup/disc ratio, thinner central corneal thickness, older age and higher IOP, that can help in estimating the potential risk of a patient's progression from ocular hypertension to glaucoma. Similarly, EMGT showed that certain risk factors can help in predicting which patients with established glaucoma will move through the disease continuum. "Rate of progression is highly individual among patients, and not all patients progress at vision-threatening rates," says Dr. Weinreb, distinguished professor of Ophthalmology, Morris Gleich MD Chair of Glaucoma, and director of the Hamilton Glaucoma Center at the University of California San Diego.

Furthermore, says Dr. Weinreb, "It has been fairly time-consuming to do a sufficient amount of testing to estimate progression. We have not had the analytic methods necessary to efficiently estimate rate of progression in clinical practice. Physicians have had to rely on evaluation of visual field tests and their clinical examination of the optic disc or RNFL." Also, it is difficult to differentiate true progression from variations in patient physiology and test performance and clinician interpretation.

A Welcome Technological Advance

Ideally, physicians should be able to go beyond estimating glaucoma progression and quantitatively and reproducibly measure it, Dr. Weinreb comments. Today, after years of working with glaucoma experts and other scientists, ophthalmic device manufacturers have been able to deliver meaningful advances in this area. "We now have software available for both structural and functional testing that can provide clinicians with the relevant quantitative and statistical analysis to objectively measure progression," Dr. Weinreb says.

For example, Guided Progression Analysis (GPA) software is available for two Carl Zeiss Meditec instruments: The Humphrey Field Analyzer and the Cirrus HD-OCT. GPA enables physicians using Humphrey visual field testing to specifically assess change from baseline and the rate of change of visual field loss over time. They can see at a glance whether vision has changed for each individual patient and can subsequently identify how rapidly visual field loss is progressing.

Building on the proven performance of Cirrus HD-OCT in RNFL measurement reproducibility (within a standard deviation of 1.2 μm for glaucomatous patients) the addition of GPA alerts physicians to statistically significant changes in RNFL thickness as well as when a rate of RNFL change reaches statistical significance.

The ability to measure rates of disease progression across both structure and function is crucial for optimal management of glaucoma patients, Dr. Weinreb says. "Structural and functional information with current testing techniques are not interdependent. If a change in the visual field is observed, there is not necessarily a change in the optic disc. Structure can predict function in many patients, but that is not always the case. In some patients, function can predict structure. Structural and functional testing are complementary. One should not be used at the exclusion of the other."

Safeguarding Patients' Vision

Dr. Weinreb says the latest glaucoma progression analysis tools help physicians to meet one of the most important challenges in treating patients with glaucoma, which is to detect patients whose disease is worsening rapidly. "It is important to identify them because they can benefit from more frequent examination or more aggressive therapy," he says. "It is equally important to know which patients are progressing slowly because they may not need to be examined as often and might not need as intensive therapy. While these new tools should never replace the clinical judgment of the physician, the improved understanding of the rate of disease progression they offer provides the best chance of minimizing visual disability for our patients." OM