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Visual function-specific tests are useful in clinical practice because they can identify early vision loss and provide more detailed information about the impact of glaucoma on visual function than tests such as standard automated perimetry (SAP). Function-specific perimetry, combined with SAP or with another type of function-specific test, also can verify that loss of vision identified in an initial test is not due to artifact or variability.
Las Vegas-Clinicians may be better able to detect glaucoma if they have access to more than one type of test, because it is unlikely that one functional test assesses the whole dynamic range, said Pamela A. Sample, PhD, who spoke at a Glaucoma Subspecialty Day pre sentation at the American Academy of Ophthalmology annual meeting.
"We find that there is no one particular test that always identifies glaucoma first," said Dr. Sample, professor of ophthalmology, Shiley Eye Center, and director, visual function laboratory, University of California, San Diego, School of Medicine. "If you use two or more tests, you pick up more of the individuals who have evidence of glaucomatous optic neuropathy."
Using more than one test also is critical for verification of initial results. Researchers have found that when visual field loss is pres ent on more than one type of perimetry, it is in the same quadrant in 95% of eyes, Dr. Sample said.
Visual function-specific perimetry is a type of functional testing that identifies early vision loss and gives more information about glaucoma's impact on visual function in an individual, she added. Combining a function-specific test and another type of test, such as standard automated perimetry (SAP), or two function-specific tests, improves early detection and verifies the area of the retina affected by glaucoma, confirming that the loss is not due to artifact or variability.
Visual function-specific tests target specific ganglion cell types, whereas SAP is not selective for a particular ganglion cell type. Targeting specific ganglion cell types reduces redundancy in the visual system and provides more information about lost functions, Dr. Sample said.
Investigators have learned that all of the ganglion cell types involved in primary visual pro ces ses are affected by glaucoma, she added. "We thought originally that there might be one type that was more susceptible than others, and that's turned out not to be the case. It's possible in any given individual for any of the cell types to be affected first."
The stimuli for short-wavelength automated perimetry (SWAP) are most easily detected by the short-wavelength cones, and the stimulus information is then processed through the blue-yellow ganglion cells, also known as the small bistratified ganglion cells. Frequency doubling technology (FDT), another commercially available function-specific test, most likely targets the magnocellular ganglion cells and their pathways, where as high-pass resolution perimetry (HPRP) stimuli apparently are most easily processed by the parvocellular ganglion cells.
SAP has been used for years to detect glaucoma. Function-specific perimetry is newer and less familiar, and many ophthalmologists' offices do not have screening tools such as FDT and HPRP. Most have a specific system for glaucoma detection and management (Humphrey Visual Field Analyzer, Carl Zeiss Meditec), which can be used for both SAP and SWAP.
SWAP has several advantages over SAP, such as its ability to detect visual field loss early in the course of disease and to detect progression, Dr. Sample said. SWAP also is more sensitive than SAP in distinguishing visual field loss due to causes such as migraine, neuro-ophthalmic diseases, macular edema, and prediabetic retinopathy from vision loss caused by glaucomatous optic nerve damage.
SWAP has drawbacks as well, Dr. Sample said. It is strongly influenced by the presence of cataract, which reduces the dynamic range available for testing in more advanced cases of glaucoma. Also, there is slightly higher inter and intraindividual variability. The introduction of proprietary technology to streamline the visual field testing process (Swedish interactive threshold algorithm, Carl Zeiss Meditec), however, has enhanced SWAP by reducing test times from 20 minutes to 4 or 5, increasing the dynamic range, and slightly improving variability.
"It's much easier now to do two tests on an individual in one visit without the person getting overly tired," Dr. Sample said.