Advances in perimetry help speed up testing time

Frequency-doubling technology (FDT) Matrix (Carl Zeiss Meditec) and SITA-SWAP (short-wavelength automated perimetry incorporating the Swedish interactive threshold algorithm, Carl Zeiss Meditec) are two of the newest advances in glaucoma testing. In comparisons of these devices with earlier versions, the newer instruments have performed favorably, and it has been shown that FDT Matrix permits improved characterization of the pattern of visual field defects, said Dr. Crowston, professor and director of glaucoma research, University of Melbourne, Center for Eye Research Australia, Victoria, Australia.

SWAP and FDT are function-specific tests that attempt to isolate subpopulations of retinal ganglion cells in an attempt to identify early visual field defects. SWAP is specific for the small bistratified retinal ganglion cells of the koniocellular pathways. The bright yellow background of the instrument desensitizes green and red cones, and the high luminance also desensitizes rods, leaving only the blue cones to respond. Longitudinal studies have shown that SWAP deficits appear on average 3 to 5 years before standard automated perimetry (SAP), Dr. Crowston said.

"This also increases the dynamic range of the instrument, perhaps allowing us to use this more in advanced cases of glaucoma," Dr. Crowston said.

FDT involves presentation of phase-reversing gratings to assess visual field loss; contrast sensitivity is tested using spatial frequency-doubled stimuli. FDT isolates the magnocellular M-cells that encode low-contrast, high-temporal frequency stimuli (3% to 9%). Cross-sectional and longitudinal studies have shown that FDT deficits appear earlier than SAP.

The FDT Matrix is the second generation of the FDT thresholding program and uses similar test patterns to the Humphrey Field Analyzer II (Carl Zeiss Meditec), such as 30-2, 24-2, 10-2, and macula threshold. The FDT Matrix uses a smaller test target-5° compared with the 10° of the original FDT. This permits better characterization of the pattern of field loss compared with the earlier version without affecting inter-test variability, Dr. Crowston said. In addition, vertical and horizontal midlines are bracketed to increase detection of typical neurological deficits.

The FDT Matrix uses Zippy estimates of sequential testing, an algorithm based on Bayesian logic, which reduces the test time to about 4 minutes. The normative database includes 270 individuals aged 18 to 85 years.

The instrument has a larger display than the previous model that includes video eye monitoring, and the upgraded device offers improved data handling and storage.

Studies have begun to appear comparing the new and earlier models of these instruments. In 2006, Medeiros et al. reported results of a prospective observational case control series based on the DIGS data set comparing FDT 24-2 versus SITA-SAP in 174 eyes of 110 subjects with primary open-angle glaucoma and 196 eyes of 101 control subjects. They incorporated covariates (age and IOP) in ROC regression models for the two instruments.

"They showed that the FDT 24-2 was superior for detecting glaucomatous field loss, especially for early disease," Dr. Crowston said. "They also showed that age and disease severity had a significant impact on the diagnostic performance of the instrument."

In another recent study, Bengtsson and Heijl compared SITA-SWAP with full-threshold SWAP or SAP-SITA-Fast in diagnosing early glaucoma. The study included 101 subjects with mild lens opacity <2 LOCS. SAP was used for diagnostic categorization of subjects as ocular hypertensive, glaucoma suspects, or early manifest glaucoma. Endpoints were the number of significantly reduced test locations or clusters of reduced test locations.

The number of abnormal visual fields was equivalent for all three testing programs.

"They concluded that SITA-SWAP was at least as sensitive as full-threshold SWAP, and interestingly the SAP-SITA-Fast had similar sensitivity to both the SWAP programs," Dr. Crowston said.