Frequency-doubling technology (FDT; Carl Zeiss Meditec) perimetry was developed as a fast, effective way to detect visual field loss. FDT was introduced in the late 1990s, and a second-generation instrument with significant enhancements to its diagnostic capabilities (Humphrey Matrix; Carl Zeiss Meditec) was released in 2005. The consensus, derived from numerous studies of the original FDT instrument, was that it predicted the development of defects that were subsequently detected with standard automated perimetry (SAP).

Studies disagree on whether a threshold test in the Matrix, which uses a 24-2 test pattern, is more sensitive in detecting early disease, said Christopher A. Girkin, MD, MSPH.

Describing the technology, he explained that, theoretically, conventional perimetry is nonselective, obtaining input from all the cell types in the layers of the retina; FDT, by contrast, was designed preferentially to stimulate only a subset of retinal ganglion cells in the magnocellular (M-cell) pathway. FDT capitalizes on the fact that the M-cell pathway is involved in the detection of motion. Patterns of flickering black-and-white bars incite a reaction in these cells and create the perception that twice as many bars are present. It is believed that a group of M-cells, the My cells, are lost in early glaucoma and that, therefore, the frequency doubling illusion could be a useful diagnostic tool. According to Dr Girkin, however, the degree of isolation of this pathway is a matter of debate, and he referred to several studies evaluating the effectiveness of the original instrument.

Where does it fit in practice?

"In a 2004 study,¹ functional abnormalities detected by FDT perimetry were predictive of the future onset and location of SAP visual field loss among glaucoma suspect patients," Dr Girkin noted. "An abnormal FDT examination at baseline predicted development of SAP visual field conversion, with the abnormalities preceding the SAP visual field loss by as much as four years.

"In 2008," he continued, "a study² showed that the 24-2 FDT threshold test performed similarly in tests of visual function to the first-generation FDT N-30 test. The study also demonstrated that the FDT 24-2 was better than SAP at discriminating between healthy and glaucomatous eyes."

By contrast, studies using a newer algorithm (Swedish Interactive Threshold Algorithm [SITA]; Carl Zeiss Meditec) have not demonstrated greater sensitivity of the FDT over SAP. "Interestingly. the tests seem to identify abnormal visual function in a different subset of eyes with early glaucoma," Dr Girkin said.

Emerging evidence also hints that FDT could monitor progression by following damaged regions. Test-retest variability in 24-2 FDT seems lower in damaged areas, possibly because the FDT stimuli sample a larger region.

"In a clinical setting, the FDT 24-2 may complement the use of the newer algorithm with SAP in determining whether eyes with suspicious-appearing optic nerves truly have early glaucoma," Dr Girkin suggested. Corroboration of FDT results with repeat testing and with optic nerve structure is, however, required to rule out false positives.

FDT 24-2 has promising characteristics that may help in the detection of progression, but longitudinal data are needed to define its relative efficacy, Dr Girkin concluded.

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Special Contributor
Christopher A. Girkin, MD, MSPH is director of the glaucoma service and professor of ophthalmology at the University of Alabama at Birmingham. He may be reached by E-mail: cgirkin@uab.edu
Dr Girkin has a financial relationship with Carl Zeiss Meditec.

References

1. F.A. Medeiros, et al. Am. J. Ophthalmol. 2004;137:863–871.

2. L. Racette, et al. Invest. Ophthalmol. Vis. Sci. 2008;49(3):954–960.