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Donnell A Knighten, Wadih M Zein, Catherine A Cukras, Brett G Jeffrey; Color vision in Retinitis Pigmentosa: A Quantitative Analysis of Farnsworth Dichotomous Panel D15 Test. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1389.
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Color vision may be altered in the early phases of retinitis pigmentosa (RP). The present study evaluated a quantitative method for assessing error scores from the Farnsworth Dichotomous Panel Test (D-15) in RP patients.
Seventy D-15 exams were randomly selected from 56 patient charts (male=36, female=20) and retrospectively analyzed. Scores from the right eyes were calculated from the moment of inertia of color difference vectors. Derived parameters were 1) Confusion index, a measure of color confusion relative to a perfect arrangement of caps; 2) The angle of color confusion and 3) Selectivity index, a measure of error polarity or lack of randomness in cap arrangement. Data for the normal range of each parameter were obtained from Vingrys. Patients with no crossing errors (n=14) were excluded from calculations as were subjects considered to be congenital dichromats (n= 2). The derived parameters were correlated with visual acuity.
The confusion index was correlated with visual acuity (p<0.001). Forty percent of RP patient with 20/20 acuity or better had a confusion index elevated above the normal range; Overall 91% of patients with 20/25 or worse had an elevated confusion index. Angle of confusion results indicate that subjects with better acuity (20/20 - 20/100) show loss of color vision primarily along the tritan axis. At lower acuity (20/125 - 20/500), the angle of confusion is less consistent but in many subjects aligned closer with the protan axis. The selectivity index was negatively correlated with acuity (p<0.013). For subjects with good acuity, crossing errors conform tightly to the tritan axis but as acuity worsens crossing errors become increasingly anarchic.
As acuity worsens the severity of dyschromatopsia rapidly increases. In RP color vision worsens from minor tritan crossings to anarchic patterns. In many patients, presumable tritan errors can be evaluated within anarchic D-15 scores. This supports that short-wave cone function may be lost early relative to acuity and widespread cone dysfunction occurs progressively. Assessment of color vision loss with the D-15 can be useful in early detection of RP.  Vingrys, AJ et al. IOVS 1988 29:50-63.
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