Tritanopia

Tritanopia is a type of dichromacy in which the retina completely lacks functional short-wavelength (S) cones. These cones are normally most sensitive to light at around 420 nanometers, corresponding to the blue-violet portion of the spectrum. Without S-cones, the visual system relies solely on L-cones and M-cones to interpret color, producing a perception in which blues and yellows are the most affected. Tritanopia is sometimes described as "blue-yellow" color blindness, though this name can be misleading since it is specifically blue perception that is lost.

People with tritanopia see the world in shades of red-pink and blue-green (cyan), with the blue and yellow portions of the spectrum most distorted. Blues may appear greenish or teal, while yellows can look pinkish, light gray, or nearly white. The distinction between blue and green becomes particularly difficult, and purple can appear reddish because the blue component is not perceived. Visual acuity and brightness perception are generally normal, as S-cones contribute relatively little to overall luminance sensitivity.

Hereditary tritanopia is caused by mutations in the OPN1SW gene on chromosome 7, which encodes the S-cone opsin protein. Because this gene is on an autosome rather than the X chromosome, the condition is not sex-linked and affects men and women equally. Most familial cases follow an autosomal dominant pattern. Acquired tritanopia is relatively more common than acquired red-green deficiency and can result from age-related yellowing of the crystalline lens, retinal diseases affecting the macula, glaucoma, or exposure to certain chemicals such as organic solvents.

Standard Ishihara color plates are designed to detect red-green deficiency and typically miss tritanopia. Specialized tests are required, including the Farnsworth D-15 panel, the Lanthony desaturated D-15, or the HRR (Hardy-Rand-Rittler) pseudoisochromatic plates which include tritan-specific plates. The Cambridge Colour Test and computerized anomaloscopy can also identify and quantify tritan defects. Because acquired tritan deficiency can be a sign of eye disease, a thorough ophthalmic examination is recommended alongside color vision testing.

Because tritanopia is so rare, awareness among the general public and even among some eye care professionals is limited. People with tritanopia may go many years without a correct diagnosis, especially since the standard screening tests in schools and workplaces focus on red-green deficiency. Once identified, most people with tritanopia find that the practical impact on daily life is manageable. Blue-yellow confusion is less likely to affect traffic safety than red-green confusion, as critical signals like traffic lights and brake lights use red and green rather than blue and yellow.

Anyone who notices difficulty distinguishing blues from greens or yellows from whites should ask their eye care provider for a tritan-specific color vision test. This is especially important if the difficulty develops gradually in adulthood, as acquired tritan deficiency can be an early sign of age-related macular degeneration, diabetic retinopathy, or glaucoma. Regular comprehensive eye exams that include color vision screening are recommended for people over 50 and for those with a family history of eye disease.