Spatial frequency doubling excellent tool to predict progression in glaucoma

Canberra, Australia-Spatial frequency doubling (FD) stimuli have an excellent ability to predict progression of visual field loss and optic disc abnormalities for about 3 years in patients with glaucoma, but the predictive power of one assessment on its own drops off over time, said Ted Maddess, PhD, a senior fellow (associate professor) in the visual sciences group, Research School of Biological Sciences, Australian National University, Canberra, Australia.

Dr. Maddess and his colleague Ivan Goldberg, MBBS, FRACO, reported 12-year follow-up data on 218 subjects from a group of 330 first assessed in 1991-1992. Dr. Goldberg is a clinical associate professor, Department of Ophthalmology, University of Sydney, and director of Eye Associates, Glaucoma Services, Sydney Eye Hospital, Sydney, Australia.

Based on the results of the initial tests, patients were diagnosed as having glaucoma or suspected glaucoma; the suspected glaucoma was classified as "weak," "moderate," or "strong." To study the predictive as well as the diagnostic capability of FD, subjects also were identified as having progression or nonprogression at the end of the study.

Over the years, patients were followed for changes in visual field and vertical cup-to-disc ratios as measured by the Humphrey Field Analyzer (HFA, Carl Zeiss Meditec) and optic disc appearance, respectively. The HFA measures were mean defect (MD) and pattern standard deviation (PSD), which were derived from 52 thresholds per eye.

Overall, FD thresholds were just as good as or better than HFA at determining whether the patient had glaucoma or progression, Dr. Maddess said. "This is a bit surprising given that the mean defect and pattern standard deviation of the HFA were based on 104 thresholds instead of the FD's two."

Rate of success

Both methods had a high rate of success for the first 2 or 3 years in predicting progression in any hemifield. "With either one of these methods, you can predict with about 80% accuracy or better who is going to have progression, which may be important in the clinic to determine whom to treat more aggressively," Dr. Maddess said.

Although both FD and HFA predicted cup-to-disc ratio progression at about 80% accuracy in moderate suspected glaucoma, the FD measures outperformed HFA for patients with glaucoma, he added.

Visual field progression accuracies of 90% to 100% were achieved for the first 3 years but fell to 70% or better for FD stimuli after 12 years in the moderate and strong suspected groups. MD and PSD performed relatively poorly. The long-term data also showed that both FD and HFA discriminated HFA visual field progression from non-progression at 60% to 80% even after 12 years, with the rate depending on the patient group.

FD, however, was poor at predicting progression in the hemifield where the threshold was measured. The data suggest that it is possible to predict progression but not to predict accurately where it will occur, Dr. Maddess said.

FD and measures of progression appear to be less correlated in the inferior hemifield than in the superior hemifield, a finding also noted in another study. "Maybe there is something going on in the superior field that is a bit different from the inferior field," Dr. Maddess said.

Although neither FD nor HFA was very good at predicting progression after the first few years, findings could have important implications on the nature of the disease.

"Maybe glaucoma is not strictly a progressive disease but is really more episodic or stochastic such that progression in a particular retinal area is triggered by other influences-hence it is not predictable from baseline conditions," Dr. Maddess suggested.

Spatial FD as a means of glaucoma diagnosis was first performed in Dr. Maddess' lab, he said. He holds two patents for FD stimuli, which are under license to the manufacturer of the FDT and Matrix perimeters and marketed by Carl Zeiss Meditec.