ACHROMATOPSIA AND MONOCHROMATOPSIA
Complete lack of colour perception, in the first case due to the lack of cone cells in the retina and, in the second case, due to the presence of only one type of them.
Dyschromatopsia is defined as a deficiency in colour vision. It may be either congenital, as it is the case of colour blindness, or acquired.
It’s a genetic defect classified under dyschromatopsia which causes problems in colour distinction. The disorder is also named “daltonism”, after the mathematician and physicist John Dalton, who suffered such disorder. Colour blindness has highly variable degrees, going from the failure to distinguish any colour (achromatopsia) to a slight trouble in distinguishing some hues in red and green colours, which may prove problematic in several areas of daily life.
It is a hereditary disorder associated to X chromosome, reason why women are less likely to suffer from it. The disorder types may be classified according to the colour involved and the severity degree.
Only one of the three cone cell pigments is in place and vision is reduced to only one colour.
It is a moderately severe malfunction where there is a lack or dysfunction of one of the three basic colour mechanisms or cell cones. There are three types of dichromacy:
Lack of receptors that allow the identification of long wavelengths responsible for red colour. Red resembles dark beige and green looks a lot like red.
It is the most common type of dichromacy and it consists in a lack of the receptors that allow the identification of medium wavelengths responsible for green colour. The effects are similar to those of protanopia, but reds seem brighter.
Lack of the receptors that allow the identification of short wavelengths responsible for blue colour. Blue and green are easily confused; yellows may be affected and not be distinguished or look similar to reds.
People who suffer from anomalous trichromacy do have the three different types of cell cones, but have functional deficiencies. Therefore, colours are easily confused. It is the most common group among colour-blind, and the effects are similar to the ones suffered by dichromatic people, except they are milder. There are three types of anomalous trichromacy corresponding to the types of dichromacy.
Disturbances in the receptors that allow the identification of long wavelengths responsible for red colour. Protanomaly effects are similar to the ones of protanopia, but milder.
It is the most common type of anomalous trichromacy and it consists in disturbances in the receptors that allow the identification of medium wavelengths responsible for green colour. The effects are similar to those of protanomaly, but reds are brighter. Deuteranomaly is also similar to deuteranopia, but milder.
Disturbances in the receptors that allow the identification of short wavelengths responsible for blue colour. Blue and green are easily confused; yellows may also look similar to reds. Tritanomaly is also similar to tritanopia, but milder.
Acquired deficiencies may affect equally men and women and are usually monocular, which means they only affect one eye. In this case, the most common deficiencies are those related to the disturbance in the perception of blue colour and they usually affect elderly people and children.
They may be due to infectious diseases (like syphilis) or to non-infectious diseases, such as cataracts, glaucoma or macular degeneration, as well as to binge drinking, excessive smoking or drug abuse.
DIAGNOSIS AND TREATMENT
These alterations may be detected by means of specific visual tests such as the Ishihara test, consisting in coloured plates where a number must be identified.
Farnsworth test can also be used. Its a test that consist on colored cards of up to 1000 different numbered tones on the front. The patient must arrange them according to the color of tonalities.
There is also the anomaloscope, an apparatus that uses spectral colours that have been obtained by prisms and that decompose white light. It is a very precise device to detect deficits and degrees of alteration of the vision of the colours.
Many attempts have been made to improve the perception of colours, especially in dichromatic patients, but none of them had obtained the same vision that trichromatos enjoy. The first attempts consisted of monocular red filters, but the lack of aesthetics and limited effectiveness led to reduced use. Later, it was the X-Crom ™ contact lenses and the Chromagen ™ contact lenses, but none of them has achieved the stated goal.
But it seems that the computer revolution begins to provide programs of help for the visualization of colours with which perhaps, in the future, a solution to the chromatic anomalies of the vision will be found.