A Clinical Study of Colour Vision Status of Female Relatives of Indian Males with Defective Colour Vision NC01-NC04
Dr. Tejaswini Prashant Khandgave,
A8, 301, Mangal Bhairav, Nanded City, Sinhgad Road, Pune-411041, Maharashtra, India.
Introduction: Around 15% of females are carriers of congenital colour blindness. Variable colour vision status exists in these heterozygous carrier females due to various factors such as retinal mosaicism and the likelihood of the presence of an additional cone with different photosensitivity. These heterozygous carriers are considered to be tetrachromats.
Aim: To determine the colour vision status of female relatives of Indian colour blind males.
Materials and Methods: In this cross-sectional observational study, screening of males was done by Ishihara plates, and those with Colour Vision Defects (CVD) underwent a digital Farnsworth Munsell hue test (FM 100 test). First degree female relatives and a control group of normal females underwent the same tests. Statistical analysis was done using one-way ANOVA test, Tukey’s test and unpaired t-test.
Results: Thirty one colour blind males accounted for 3.04% of the 1017 males screened. All participating female relatives, i.e., 31 obligatory carriers (23 mothers and 8 daughters) and 12 sisters, had normal test results on Ishihara plates as well as FM 100 test, barring one mother. FM error score above 70 is considered to be abnormal. The FM error scores in the obligatory carrier females ranged from 39 to 62, while that in sisters' group ranged from 36-67. This error score was lower in control group with range from 14 to 49. Although, all female relatives made errors within the normal range, they had statistically significant higher FM error scores than the controls.
Conclusion: Obligate carriers have normal colour vision on Ishihara plates. Higher error scores, albeit within normal limits, were seen on FM 100 test in obligate carriers than controls. Poor performances of genetic tetrachromats on the FM 100 test can be attributed to the fact that these tests are designed specifically for trichroamatic normal individuals, and thus, may not be sensitive enough to detect the superior colour discrimination abilities of tetrachromats. Detection of tetrachromacy will require customisation of colour vision tests as well as their availability to clinicians.