The assessment of functional visual loss is a critical part of the diagnosis and management of glaucoma. Perimetry is the primary means of providing this information, particularly for the midperiphery of the visual field, where glaucomatous damage is most frequently evident. The past few years have brought significant advances in the capabilities of this technology, but many of perimetry's facets still require improvement and refinement. This article presents an overview of the recent progress, describes some of the important questions and intriguing problems that remain, and briefly addresses future directions for the functional assessment of glaucoma.
RECENT ACCOMPLISHMENTS
Despite the introduction of many testing procedures, few are useful clinical diagnostic tools. Short Wavelength Automated Perimetry (SWAP; Carl Zeiss Meditec, Inc., Dublin, CA) has many advantages over standard automated perimetry, although the duration of the former is quite long.1 Several recent studies have successfully applied the Swedish Interactive Threshold Algorithm (SITA; Carl Zeiss Meditec, Inc.) testing strategy (described in more detail later) to SWAP to create SITA SWAP (Carl Zeiss Meditec, Inc.), which has reduced testing time to between 3 and 4 minutes per eye2,3 (Figure 1).
The clinical use of frequency doubling technology perimetry has increased, and the technology is now available in a second-generation device known as the Humphrey Matrix (Carl Zeiss Meditec, Inc.).4,5 This testing strategy has a number of advantages compared with the original frequency doubling technology device, and it is now used by many practitioners worldwide. Rarebit perimetry is a new clinical testing procedure that is also becoming popular. It provides finely detailed mapping of the visual field by presenting one or two small (pixel-sized) targets in critical areas,6 and multifocal visual evoked potentials (mfVEPs) monitor electrophysiological cortical signals to track glaucomatous visual field loss7-9 (Figure 2). More detailed discussions of the aforementioned new techniques may be found in the literature.1-9
Recently, Heidelberg Engineering GmbH (Heidelberg, Germany) introduced the Heidelberg Edge perimeter, which uses flicker-defined form. Small, random dots are presented on a display screen and flicker rapidly. A subset of flickering dots are in counterphase to the other dots (ie, they are light when the others are dark and vice versa), which forms a target of visible dots. Because this test is new, there is limited information on its clinical utility.
Researchers have also developed novel testing strategies for estimating detection thresholds. SITA is a forecasting procedure that is based on Bayesian statistics. It uses prior information from normal participants and patients with glaucomatous visual field loss to initiate and direct the procedure, and the technology employs information from the participant's responses to modify the estimate. In this manner, it is possible to obtain slightly more accurate information in significantly less time compared with older testing methods in a manner that is clinically useful.10,11 The Zippy Estimation of Sequential Thresholds procedure is similar to SITA, but the former can provide more consistent testing times from one person to another and elicit reasonably uniform variability characteristics across the entire operating range of the instrument.12 Tendency oriented perimetry is a spatial averaging procedure that also reduces testing time compared with older methods while maintaining accurate results.13 Each of these procedures has its own unique set of advantages and disadvantages, but they all represent enhancements over previous threshold estimation methods.
CURRENT ENIGMAS IN FUNCTIONAL ASSESSMENTS OF GLAUCOMA
Despite the availability of several new testing procedures for the early detection of glaucomatous functional loss, many other procedures that have been developed do not serve a distinctly useful purpose in the eye clinic. A practical diagnostic testing procedure must be sensitive, specific, reproducible, easy to use, and robust to other (nonpathologic) influences, as noted by Harper and Reeves.14 It has become very difficult to develop new strategies to accomplish these goals, in spite of progress in this area.
After visual field testing is complete, the analysis and interpretation of its results are of high clinical importance. A number of tools such as STATPAK (Carl Zeiss Meditec, Inc.) are available to determine the status of a single visual field,15 and a new method of evaluating the progression of visual field loss in glaucoma recently became available.15 The Glaucoma Progression Analysis (GPA) software (Carl Zeiss Meditec, Inc.) uses information derived from stable glaucoma patients tested four times over a 1-month period in conjunction with results from the Early Manifest Glaucoma Trial.15,16
The GPA software requires two baseline visual fields (full threshold or SITA-Standard) that can then be compared to subsequent follow-up visual fields. The program flags locations on follow-up tests that are worse than the lower 95% confidence limits of variability with a solid triangle. If three or more of these locations are worse than the lower 95% confidence limits on two successive follow-up visual fields, the GPA software designates the result as possible progression. If the same three or more locations are worse than the lower 95% confidence limits on three successive follow-up visual fields, the results are labeled likely progression.
The GPA software thus has a criterion of worsening based on the variable characteristics for each location, a comparison to baseline results, a confirmation of deficits, and a critical number of locations that must be affected. These characteristics provide a technique for determining progression that has high specificity. It is important to note, however, that the designations do not indicate definite or glaucomatous progression.
FUTURE DIRECTIONS
A long-standing dilemma has been the relationship between structural and functional glaucomatous damage. Although technology has dramatically altered the assessment of both clinical structure and function, the general findings have not changed during the past 50 years or more.17 Structure and function correlate in many patients, but they are not as strongly related in others, at least according to existing diagnostic tools.18,19
It would be helpful to devise a means of obtaining information about the structural and functional integrity of the visual pathways together, instead of regarding them as separate entities. A combined structural/functional measure would be of great clinical benefit for managing glaucoma, and new developments that involve the measurement of metabolic activity, the consumption of oxygen, blood flow, and mitochondrial activity appear promising.20,21 Within the next 5 years, there will likely be many approaches that simultaneously measure the structure and function of the visual pathways.
Specific testing and data-analysis methods that elicit more detailed information about glaucomatous pathologic changes are also desirable. Moreover, diagnostic testing procedures that can shed light on how glaucoma damages underlying visual mechanisms will help physicians better understand and manage the disease.
Currently, there is no consensus on a quantitative method to evaluate glaucomatous progression for either structure or function.
CONCLUSION
Clinicians' ability to perform functional assessments of glaucoma has become significantly better during the past 30 years. They can now obtain a greater amount of information that is more accurate and reproducible in a much shorter amount of time. Many challenges remain, however, for improving the functional evaluation of glaucoma. The next 5 years may resolve some of them.
Chris A. Johnson, PhD, is Levitt Visiting Professor with the Department of Ophthalmology at the University of Iowa in Iowa City. He is a consultant to and receives research support from Welch Allyn Medical Products. Dr. Johnson may be reached at (319) 356-0384; chris-a-johnson@uiowa.edu.
