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Frequency-doubling technology (FDT) has a role as a diagnostic and monitoring instrument for glaucoma, although it may be best used to complement the use of other testing methods, particularly with the newer version of this technology. Studies suggest that the FDT may detect early glaucoma in a different subgroup of patients than other testing devices, and long-term studies will be needed to confirm its effectiveness in detection of progression.
Atlanta-Frequency-doubling technology ([FDT] Carl Zeiss Meditec) perimetry was developed as a fast, effective method for detection of visual field loss. FDT was introduced in the late 1990s, and a second-generation instrument (Humphrey Matrix, Carl Zeiss Meditec) was released in 2005 with significant enhancements to its diagnostic capabilities. The consensus derived from numerous studies of the original FDT instrument was that it predicted the development of defects subsequently found with standard automated perimetry (SAP).
Describing the technology, he explained that, theoretically, conventional perimetry is nonselective, obtaining input from all the cell types in the layers of the retina, whereas FDT was designed to preferentially 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. The degree of isolation of this pathway is a matter of debate, however, Dr. Girkin said.
A 2008 study by Racette et al. showed that the 24-2 FDT threshold test, introduced with the second-generation FDT instrument, 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.
Additional studies, however, have not clearly demonstrated greater sensitivity for the FDT over SAP using a newer algorithm (Swedish Interactive Threshold Algorithm [SITA], Carl Zeiss Meditec) strategy.
"What is interesting is that these tests appear to be identifying abnormal visual function in a different subset of eyes with early glaucoma," Dr. Girkin said.
Emerging evidence also hints that using FDT could be advantageous in following damaged regions for progression. Two studies have shown that test-retest variability in 24-2 FDT is lower in damaged areas, likely due to a larger region of sampling with the FDT stimuli.
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, he said. Corroboration of FDT results with repeat testing and with optic nerve structure, however, is required to rule out false positives.
FDT 24-2 has promising characteristics that may help in the detection of progression, Dr. Girkin continued, but longitudinal data are needed to define its relative efficacy.