Optimizing Contrast Sensitivity Perimetry for Clinical Use

Perimetric testing is used clinically to detect visual field abnormalities and to monitor changes during the course of management. Conventional automated perimetry (CAP), which employs small (0.43 degree) targets (Fig. 1), is hampered by high test-retest variability. Consequently, many tests are required in order to determine whether a patient is stable or progressing. Decreasing test-retest variability would enable clinicians to detect progression with fewer tests.1 Test-retest variability for CAP is inversely related to retinal sensitivity, which varies as a function of distance from fixation and in the presence of disease.2,3 Test-retest variability is also influenced by the use of high stimulus contrast which can saturate ganglion cell responses and by variations in prereceptoral factors such as refractive status, pupil diameter, and density of the crystalline lens.4,5 For CAP, there is an inverse relationship between optical blur and sensitivity. Hence, patients require accurate refractive correction prior to testing.6-11 Pupillary diameter (area) and density of the crystalline lens are factors that contribute to retinal illuminance (the amount of light originating from a stimulus and background that reaches the sensory retina). For subjects with clear ocular media and pupil diameter of 3 mm or greater, adherence to Weber’s law ensures that sensitivity will be relatively unaffected by changes in retinal illuminance in conventional perimetry. FDT perimetry, a form of contrast sensitivity perimetry, uses larger (5-10 degree), 0.25–0.50 cycle/degree targets with rapid (18/25 Hz) temporal counterphase flicker, for which high mean luminances are required to reach the Weber region. Previous studies have demonstrated that, while variability for FDT perimetry does not increase as a function of sensitivity,12 results can be dramatically affected by changes in retinal illuminance (e.g., lenticular density or