The ultimate goal of glaucoma management is the preservation of the patient’s visual function and quality of life. Visual function is a very complex concept that can be measured in a variety of ways. For many years, the standard measurement has been clinical perimetry, which measures differential light sensitivity, or the ability of the subject to distinguish a stimulus light from background illumination. As usually performed in glaucoma examinations, the test uses white light and measures what is conventionally referred to as the visual field. The classic description of the visual field given by Harry Moss Traquair (1875–1954) is “an island hill of vision in a sea of darkness.” The island of vision is usually described as a 3-dimensional graphic representation of differential light sensitivity at different positions in space (Fig 3-18).
Perimetry refers to the clinical assessment of the visual field. Perimetry has traditionally served 2 major purposes in the management of glaucoma:
identification of abnormal fields
quantitative assessment of normal or abnormal fields to guide follow-up care
Quantification of visual field sensitivity enables detection of initial loss by comparison with normative data. Regular visual field testing in known cases of disease provides valuable information for helping to differentiate between stability and progressive loss. It is likely that in individual patients, different tests will show abnormalities at different times. Some methods may be better for identification than for following the progression of defects, and vice versa.
Over the last 2 decades, automated static perimetry has become the standard for assessing visual function in glaucoma. With this procedure, threshold sensitivity measurements are usually performed at a number of test locations using white stimuli on a white background; this is known as standard automated perimetry (SAP), or achromatic automated perimetry. Assessment of threshold sensitivity by SAP traditionally uses simple staircase algorithms that employ a bracketing approach to estimate threshold. Recently, other technologies have become available that may be useful in evaluating the visual field. Evidence from detailed investigations using these newer perimetric tests—such as short-wavelength automated perimetry, high-pass resolution perimetry, and frequency-doubling technology perimetry—strongly suggests that they may provide beneficial clinical information.
A description of these newer perimetric tests follows:
Short-wavelength automated perimetry (SWAP): This is also known as blue-yellow perimetry. Standard perimeters are available that can project a blue stimulus onto a yellow background. This method is sensitive in the early identification of glaucomatous damage. Several studies suggest that the rate of development of perimetric defects in early glaucoma may be higher with blue-on-yellow (short-wavelength) testing than with conventional (achromatic) white-on-white visual fields.
Frequency-doubling technology (FDT) perimetry: This visual field testing paradigm uses a low spatial frequency sinusoidal grating undergoing rapid phase-reversal flicker. Commercially available instruments employ a 0.25 cycle per degree grating, phase-reversed at a rapid 25 Hz. When a low spatial frequency grating is presented in this manner, it appears to have twice as many alternating light and dark bars than are actually present—hence the term frequency doubling. It is believed that the stimuli employed in this test preferentially activate the M cells and may be more sensitive in the detection of early glaucomatous loss.
Visually evoked cortical potentials and electroretinography: Cortical (VECP; also VEP or VER) or retinal (ERG) electrical responses to a stimulus, such as a reversing pattern of light and dark squares or a flickering light, are recorded. The multifocal ERG and multifocal VECP may be a useful objective test for assessing RGC function. Although these tests require visual attention, they do not require a subjective response.
Other measures of visual field include contrast sensitivity, flicker sensitivity, and high-pass resolution perimetry. Several of these tests are discussed in greater detail in BCSC Section 12, Retina and Vitreous.