Colour vision deficiency and inherited retinal disease: SAQ

This two-part stem sets the two ends of the spectrum against each other, the benign and the common colour vision deficiency of the schoolboy and the severe and the treatable congenital retinal dystrophy of the infant, and the task is to outline the assessment and the management of the colour defect, and then to discuss the diagnostic pathway and the treatment of the congenital retinal dystrophy, contrasting the two with precision boards reward. ^[6][1]

Question 1 (10 marks)

Outline the assessment and the management of the colour vision deficiency in this six-year-old boy. ^[6]

A full-mark answer covers the interpretation of the Ishihara test, the distinction of the congenital from the acquired defect, the inheritance and the counselling, and the management. ^[6]

The Ishihara test and its interpretation (3 marks). The boy reads four of the fourteen screening plates, which is below the threshold of twelve or more that defines the normal, and the pattern of the misreading indicates the red-green defect, with the deuteranomaly as the commonest form. The Ishihara is the excellent screen for the red-green defect, and it is the poor test for the blue-yellow tritan defect, which the Hardy-Rand-Rittler plates and the Farnsworth tests detect better and the anomaloscope measures as the gold standard. ^[6]

The distinction of the congenital from the acquired (2 marks). The normal acuity, the full fields, the healthy fundus and the stable course confirm the congenital defect, while the acquired defect would be the new and the progressive change pointing to the optic neuropathy, the macular disease or the drug toxicity, with the tritan blue-yellow pattern as the common acquired form. The absence of the progression and the healthy fundus settle the question in favour of the congenital form. ^[6]

The inheritance (2 marks). The red-green colour deficiency is the X-linked recessive trait of the OPN1LW and the OPN1MW opsin genes on the X chromosome, and it affects roughly one in twelve males and well under one percent of the females. The mother is the carrier and the son is the affected male, and the family history of the colour-blind maternal relatives supports the inheritance. ^[6][8]

The management (3 marks). The colour vision deficiency has no treatment and it needs none, because it is stable and it carries no threat to the sight. The management is the honest explanation, the reassurance that the acuity and the field are normal, and the vocational advice that guides the boy away from the careers that demand the perfect colour discrimination, such as the electrical work, the aviation and the railway signalling. The tinted lenses may aid the discrimination but they do not correct the defect. ^[6]