Dyschromatopsia

Dyschromatopsia is a deficiency in the perception of colours. When an individual has a complete inability to recognise the chromatic scale, this is called achromatopsia.

There are two types of dyschromatopsia depending on the origin: congenital or hereditary and acquired. The term Daltonism refers to congenital dyschromatopsia.

The cells responsible for seeing colours (chromatic vision) are the retinal photoreceptors called cones. They are mainly located in the central portion named the macula. There are three types of cone depending on the colours they are able to perceive: red (with a wavelength of 549 to 570nm); green (522 to 539nm) and blue (114 to 424nm). 

Congenital dyschromatopsias are classified depending on the colours they are capable of perceiving:

• Monochromatic: Only one of the macula pigments exists and it is restriced to one colour dimenstion. 
• Dichromatic: It is due to a change in one of the three essential colour mechanisms. There are three types:

– Protanopia: The cones that perceive the colour red are missing. 

– Tritanopia: The phoreceptors that code the colour blue are missing. It's the most uncommon. 

– Deuteranopia: The absence of the ability of perceive the spectrum of the colour green.

• Trichromacy: The photoreceptors of the three types of colours are present but have an altered function. This is the most common type and may present with severe changes to the perception of colours. Depending on the changes perceived, they can be classified into:

– Protanomalies: Loss of red–green vision due to poor functioning of the cones sensitive to red. 

– Deuteranomalies: Loss of green vision due to a shift in the functioning of the cones sensitive to green. 

– Tritanomalies: Loss of the blue–green spectrum due to a shift in the cones that identify the colour blue. 

The origin of dyschromatopsia may lie in a genetic or acquired disorder. Hereditary dyschromatopsia may be due to a change in the X chromosone. This leads to the disease being transmitted by the woman even though its the man who is suffering from the disorder. 

The acquired causes  for a certain degree of dyschromatopsia are multiple and varied. Within one same pathology there may be many types of shifts in the perception of colour without there being any shift specific to a determined entity. The most common are macula diseases (where there is a very high number of cells responsible for the perspetion of colour, the cones). 

Within these, we must highlight, in particular, age-related macular degeneration, a pathology that changes the anatomy of the macula and, as a consequence, the cells located therein. Other pathologies may include ones of an infectious or toxic origin. Different anomalies like the formation of cataracts or changes to the optic nerve may give rise to a range of types of dyschromatopsias.

It is important to recognise the differences in symptomology between the congenital and acquired dyschromatopsias when it comes to performing a differential diagnosis. The acquired are usually asymmetrical, they are associated with functional shifts other than anomalous perception of colours and they usually affect the red-green and blue-yellow axes. The congenital, on the other hand, usually affect both eyes and are symmetrical, the rest of the visual functions remain intact, the shift usually becomes stable over time and most frequent error is red-green perception. 

There is a range of specific chromatic vision tests. 

We have highlighted the two most commonly used: 

• Ishihara test. These are plates called pseudochromatic plates because they seem to be made of one single colour. They are coloured dots grouped together with two pigments of variable intensity that demarcate numbers that healthy individuals are able to distinguish, but a patient with the disease sees them all in the same colour and cannot differentiate them. The test is performed at a distance of 75cm. The full test has 21 plates although there are some with fewer, which are mainly for children. It is a very useful test for early diagnoss of dyschromatopsia although it is not useful in determining the severity thereof. 
• Farnsworth test. It is has disk shapes that have to be put in order of colour and tonality. It is the most sensitive test for classifying and measuring the degree of severity of the dyschromatopsia.