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Problems in Color-Vision Screening of Diabetic Patients

Recently, our laboratory has been involved in a research program designed to assess the effect of variations in viewing conditions on performance, with several versions of the pseudoisochromatic (PIC) tests of color vision. This work could have strong implications for diabetologists, because several recent articles have addressed the utility of color-vision testing in the management of diabetic patients." PIC tests are generally assumed to be equivalent and hence interchangeable measures of color vision. It has been determined, however, that the performance of color-normal observers on several popular PIC tests is differentially affected by variations in exposure duration, viewing distance, and stimulus clarity. Long et al.' examined the effects of variations in viewing distance and duration on performance with three PIC tests used to detect congenital defects: the American Optical Corporation Pseudo-Isochromatic Plates for Testing Color Perception (AOC), the Dvorine Pseudo-Isochromatic Plates, and the Ishihara Tests for Colour-Blindness. They found that increasing viewing distance and decreasing viewing duration significantly increased the number of errors made by color-normal observers. In addition, the three tests were found to differ markedly between themselves, such that the AOC and Dvorine tests produced significantly more misdiagnoses than the Ishihara test. In a second experiment, Long et al. found that decreasing stimulus clarity (by imposing target blur) generally produced more errors. However, the effects of both experimental manipulations were not equivalent across the three tests, with the AOC and Dvorine adversely affected and the Ishihara relatively insensitive to the manipulations. Recently, we extended this line of research to two tests developed to screen for acquired deficiencies in addition to congenital defects: The City University Colour Vision Test (CUCVT) and the Standard Pseudoisochromatic Plates (Part 2, for Acquired Color Vision Defects) (SPP-A). The findings from these studies are particularly interesting to diabetologists because at least two recent articles suggested that a large percentage of patients with diabetes may suffer from acquired color defects. Results from our work with colornormal subjects indicate that the CUCVT is adversely affected by variations in viewing distance >46 cm. The SPPA appears to be especially sensitive to the manipulations of both viewing distance and exposure duration (unpublished observations). Under a 76-cm viewing condition, 100% of the color-normal subjects produced at least one error on the SPP-A and would be misdiagnosed as color defective. Even under the conditions specified exactly by the test publishers, 90% of the subjects produced some errors on the SPP-A. The combined results of the studies described above strongly indicate that these PIC tests are not equivalent measures of color vision under a range of conditions likely to be encountered in applied settings. The findings clearly demonstrate the need for careful selection of the tests that assess color vision of diabetic patients. We recommend that the Ishihara test be used to screen for congenital defects, because this test is relatively impervious to variations in viewing conditions. Bresnick et al. have noted that, given the incidence of congenital defects in the general population (8 10% of men), it seems likely that some diabetic patients will exhibit this form of color-vision deficiency. Thus, screening patients for congenital defects seems warranted. The selection of a reliable PIC measure to assess acquired deficiencies may be more difficult. In addition to our findings, other investigators have reported that several of the PIC tests available for screening tritan defects often yield inconsistent data and demonstrate poor diagnostic sensitivity for acquired defects when used in evaluating color-defective individuals. Therefore, we recommend that diabetologists employ a small battery of tests. The value of a diverse test battery is that testing with a variety of color-vision measures allows examiners to determine more accurately the color-discriminative ability of the patients under evaluation, while effectively avoiding any weaknesses idiosyncratic to a single test. Such a battery could include multiple PIC measures of both congenital and acquired defects. Furthermore, perhaps a very different type of test should also be included in this battery. The Farnsworth D-15 test is a color-arrangement test (rather than a PIC measure) and may not be susceptible to the same kind of variations in viewing conditions. However, additional research is necessary to identify the most effective components of this battery.