Functional testing an expanding field in glaucoma

November 1, 2005

Newport Beach, CA—White-on-white perimetry remains the gold standard for functional testing in glaucoma management, but that methodology has been improved by some recent software developments, and other new functional tests can be useful supplements for patient evaluation, said Donald L. Budenz, MD, MPH.

Newport Beach, CA-White-on-white perimetry remains the gold standard for functional testing in glaucoma management, but that methodology has been improved by some recent software developments, and other new functional tests can be useful supplements for patient evaluation, said Donald L. Budenz, MD, MPH.

For standard automated perimetry (SAP) with the Humphrey Field Analyzer II (Carl Zeiss Meditec), two useful new software programs have become available over the past several years-the Swedish Interactive Threshold Algorithm (SITA) and the Glaucoma Progression Analysis (GPA).

Since SITA greatly reduces the duration of the visual field test, it lessens patient fatigue and thereby makes the test results more reliable. Whereas the examination time for a full threshold test with conventional SAP can range between 12 and 15 minutes, SITA takes only 5 or 6 minutes.

"There is no compromise in the information obtained with SITA. This first change to the algorithm for computerized perimetry since the early 1980s really represents a major improvement in terms of efficiency of visual field testing. Yet, relatively few eye-care providers have adopted this software into their practices," Dr. Budenz said.

The GPA software for the Humphrey Field Analyzer II is designed to identify clinically significant progression automatically and to differentiate it from random variability. It is based on criteria used to diagnose visual field progression in the Early Manifest Glaucoma Trial (EMGT). In that study, progression was defined as statistically significant deterioration of three or more test points at the same location on three consecutive field tests.

"This 'rule of threes' was shown very useful for establishing progression in the EMGT, although its sensitivity and specificity remain to be tested in an independent data set. Still, by placing some statistical parameters on the visual field change, the GPA provides us with a tool that can add certainty to our subjective judgments about whether glaucoma is worsening by visual field criteria," Dr. Budenz said.

Two newer techniques

Frequency-doubling technology (FDT) and short-wavelength automated perimetry (SWAP) are two newer diagnostic techniques in functional assessment. Those tests were designed to target specific populations of ganglion cells so as to avoid redundancy in the visual system and thereby detect early abnormalities due to glaucoma sooner than SAP.

However, both have yet to be proven in rigorous trials to diagnose glaucoma or its progression earlier than standard methods.

Introduced commercially in 1997, FDT has been most widely performed to date using the original portable instruments for the purpose of community glaucoma screenings.

"The test is completed in about 90 seconds and there is evidence to show it is much better at predicting glaucoma than IOP measurement," Dr. Budenz explained.

Recently, a second-generation FDT perimeter, the Humphrey Matrix, became available. Due to its size, that technology is office-based, but it features new test patterns with more, smaller-sized targets that make it better able to detect early glaucomatous visual field loss and progression. New test patterns include 10-2, 24-2, and 30-2 threshold tests; the latter test has 69 stimuli.

SWAP was also first introduced a number of years ago, and it has been demonstrated as able to detect glaucoma sooner than SAP. Available evidence also suggests it may have a benefit for earlier detection of progression.

However, SWAP testing in its original format required 20 minutes per eye, and so it failed to gain widespread acceptance because it was so time-consuming. That limitation will be overcome soon when SITA-SWAP becomes available. With testing time reduced to 6 minutes or less using the new algorithm, SWAP may see growing popularity.

Dr. Budenz said that while FDT and SWAP are very promising modalities, he believes their best role currently is as adjuncts to standard assessment based on evaluation of the optic nerve and SAP.