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Blue Cone Monochromatism



Blue Cone Monochromatism Characteristics and Features


Because the blue cone detects wavelengths of lower magnitude than the red and green cones, the vision of an incomplete achromat is limited to the scotopic region and the lowest limits of the photopic area. A complete achromat has no working cones at all and can completely see within the scotopic region. Someone with normal color vision will show a maximum sensitivity at 530 nm, while for incomplete achromats, there is no discrimination of wavelength beyond 520 nm. Since the S-cones are not responsible for our perception of brightness, blue cone monochromats are not only deficient in color perception, but they are also hypersensitive to light. People affected by this condition describe being “dazzled” and “blinded” by bright light. In broad daylight, their vision can easily become blurry and then completely washed out. This extreme sensitivity to light is called photophobia, also known as hemeralopia or day blindness. Achromats are known to develop an aversion to bright sunlight. This is because their rods saturate at low light conditions, blinding the viewer. Often, achromats squint and blink frequently as a compensatory behavior so as not to saturate the rods.


Blue Cone Monochromatism Definition


Blue cone monochromatism, also known as S-cone monochromacy, is a recessive disorder that is linked to the X-chromosome. The affected person's green and red cones are missing while the blue cone mechanism is dominant and functioning properly. Cases like these have been called incomplete achromatopsia, wherein color vision is only impaired and not wholly absent.


Blue Cone Monochromatism Prevalence


Most kinds of congenital colorblindness are caused by defects found in the X chromosome. Since females have two X chromosomes and males have an X chromosome and a Y chromosome, there is a dramatic preponderance of colorblindness in males. For females to exhibit colorblindness, they must have defects in both chromosomes. To be a carrier of colorblindness, a female must have the colorblindness defect in one X chromosome. Male children of a female carrier are as likely to be colorblind as male children of a male who is colorblind, while male children of a male with colorblindness and a female carrier are extremely likely to be colorblind. An estimated 5% to 8% of the men and 0.5% of the women of the world are born colorblind. That is one out of twelve men and one out of two hundred women. People who are protans (weak against red) and deutans (weak against green) make up 99% of this group.


Blue Cone Monochromatism Symptoms and Signs


Patients with blue cone monochromatism have profound color impairment, but due to the presence of some functioning cone cells in the retina, they have a small residual amount of color vision and slightly better visual acuity. They are often called yellow-blue disorders, but “blue-green disorder” is more accurate since they affect the ability to distinguish colors in the short- and middle-wave regions of the color spectrum.


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