Frequency of colour vision defects among Zulus in Natal.
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A genealogical link was established six generations back between a family living in England and Australia, and one of the families reported originally by Sorsby et al (1949) as suffering from autosomal dominant inflammatory macular dystrophy (fundus dystrophy). The onset--in the fifth decade of life--and the progress of the condition, which usually ends in blindness, has been observed in a number of patients and the prodromal development of a colour vision deficiency in some of them confirmed. This defect is fundamentally different from the X-linked colour vision defects and merits further investigation.
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Normal colour vision is trichromatic and is mediated by the blue, green and red visual pigments present in the corresponding blue, green, and red cone cells of the retina. The red and green pigment genes have evolved from an ancestral pigment gene and reside in a head-to-tail tandem array on the long arm of the X chromosome. This arrangement and a high degree of homology predispose to illegitimate recombination between the red and green pigment genes explaining the various forms and the high frequency of red-green colour vision defects.
The Farnsworth-Munsell 100-hue test was employed to determine whether there was any defect of colour vision in 29 confirmed male heroin addicts who had been successfully detoxified. Forty age-matched males served as controls. A typical normal error score on the FM test is about 40 and an error score of over 100 indicates poor colour discrimination; 86.2% of the eyes of the control group had an error score below 100 while only 17.2% of the eyes of heroin addicts had an error score below 100. The colour confusion among the heroin addicts was in the blue-purple (475-495 mu) range. These results indicate that colour vision defects are more common in heroin addicts.
Panel tests of color vision (eg FM100-Hue test) lack a common quantitative method for the scoring of cap arrangements. We describe a scoring method applicable to all panel tests that makes use of a novel technique to analyze test cap data, namely the calculation of a moment of inertia from the Color Difference Vectors (CDVs) of any arrangement pattern. Using the Farnsworth D-15 panel, as an example, we specify how to determine CDVs and demonstrate the benefits of calculating a moment of inertia for the analysis of these vectors. Moment of inertia analysis yields three factors which quantify cap arrangements: the first is the confusion angle which identifies the type of color defect; the second is the Confusion index (C-index) which quantifies the degree of color loss relative to a perfect arrangement of caps; and the third is the Selectivity index (S-index) which quantifies the amount of polarity or lack of randomness in a cap arrangement. A retrospective study on the result of 53 normal and 66 congenitally color defective observers is reported and provides normative data. We show that the technique differentiates between different types of color defect and provides useful clinical information regarding a loss of color vision. Likewise, a similar observation is made on a smaller sample of FM100-Hue results. A BASIC computer program is provided for anyone wishing to use the technique.
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