Other Perimetric Tests
Other psychophysical tests of visual function include frequency-doubling technology (FDT), short-wavelength automated perimetry (SWAP), and flicker-defined form (FDF) perimetry (Fig 15-6). These tests aim to target subpopulations of retinal ganglion cells (RGCs) by evaluating specific aspects of visual function (eg, motion perception, contrast sensitivity, color vision) and thereby reduce the ability of the visual system to use other pathways to compensate. It has been hypothesized that in its early stages, glaucoma may damage predominantly magnocellular RGCs projecting to the magnocellular layers of the lateral geniculate nucleus (ie, the magnocellular [M] pathway), although whether such preferential loss indeed occurs is still unclear.
Figure 6-15 Findings from a right eye with glaucoma. A, Inferior retinal nerve fiber layer (RNFL) thinning is visible on optical coherence tomography (OCT). B, Standard automated perimetry (SAP) shows a small corresponding superonasal visual field defect that is more pronounced on frequency-doubling technology perimetry (C) and Heidelberg Edge Perimetry (D).
(Courtesy of Felipe A. Medeiros, MD, PhD.)
FDT perimetry, which tests the patient’s ability to detect a flickering stimulus at a high temporal frequency, was designed to selectively evaluate the M pathway, but evidence shows that the response to motion, including the flicker of FDT, may be generated by many ganglion cell types and is cortically mediated. Nevertheless, FDT has shown promise for glaucoma detection, and longitudinal studies have shown that abnormalities on FDT may precede detectable SAP changes in many patients. The FDT Matrix (Carl Zeiss Meditec) is a commercially available perimeter that features FDT stimuli. It uses sinusoidal grating targets smaller than the original FDT perimeter to enable 24-2 and 30-2 patterns like those used in SAP. Although the first-generation FDT perimeter was able to evaluate visual field sensitivity in only 17 or 19 locations, the FDT Matrix is capable of testing 54 (24-2 pattern) or 69 (30-2 pattern) locations. This technology potentially improves the ability of monitoring progression over time; however, it is not widely used and is limited by the absence of analytic tools for detecting progression.
SWAP is designed to stimulate the small bistratified ganglion cells, which project their axons to the koniocellular layers of the lateral geniculate nucleus. It uses a blue-violet stimulus (440-nm wavelength) against a bright yellow background. SWAP generally does not have better diagnostic ability than SAP and is now performed in clinical practice only in rare cases.
The FDF stimulus has been proposed as an alternative method for detecting glaucomatous vision loss. FDF perimetry is believed to stimulate the M pathway, and there is emerging evidence that it may be useful for early glaucoma diagnosis.
Although psychophysical tests such as FDT, SWAP, and FDF perimetry attempt to minimize potential input from other pathways, it is unlikely any stimulus can be 100% specific for a single visual pathway or a single subset of RGCs. Furthermore, it is unlikely a single ganglion cell type is always affected first in glaucoma. Perimetric tests are also subjective examinations, and therefore responses may vary on repeat testing, or during the same test, reducing the ability to confidently detect genuine early abnormalities.
Other tests that measure the integrity of the visual field include contrast sensitivity perimetry, flicker sensitivity, microperimetry, visual evoked potential, and multifocal electroretinography. These tests are not commonly employed in the evaluation of patients with glaucoma. Several of these tests are discussed in greater detail in BCSC Section 12, Retina and Vitreous, and in Section 5, Neuro-Ophthalmology.
Meira-Freitas D, Tatham AJ, Lisboa R, et al. Predicting progression of glaucoma from rates of frequency doubling technology perimetry change. Ophthalmology. 2014;121(2):498–507.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.