Achromatopsia, also called achromacy, is a rare pathology of the eye which prevents from seeing colors. It has a genetic origin in most cases, but can also result from a brain lesion. The patients concerned see the world in black and white.
What is achromatopsia?
Achromatopsia is the inability of a person to see colors. There are several forms:
Complete congenital achromatopsia
Because of genetic mutations on one or more genes involved in color vision, the retina that lines the back of the eye is devoid of photoreceptor cells called “cones” that allow to distinguish colors and details in light. As a result, the patients concerned see in shades of gray.
In addition, they have a high sensitivity to light and a very low visual acuity. All this, from birth. This color blindness being linked to recessive genes, it only concerns people whose two parents are carriers of the same mutation and have transmitted it. This is why its prevalence is low, in the order of 1 case for 30,000 to 50,000 births in the world;
Partial or incomplete congenital achromatopsia
This form is also genetic, but unlike the first one, it is linked to a mutation on the X chromosome and does not affect all the cones of the retina. It leaves those sensitive to blue. This is why the condition is also called “blue-cone monochromatism”.
Patients with this condition do not see red and green, but do see blue. Due to the lack of sufficient cones, their visual acuity is nevertheless very poor and they are unable to tolerate light. Incomplete achromatopsia is linked to a recessive mutation on the X chromosome and affects mainly boys, who have only one copy of this chromosome (their 23rd pair of chromosomes is composed of an X and a Y). Since girls have two X chromosomes, they must have inherited the mutation from both parents to be achromats, which is much rarer;
It is not of genetic origin. It appears following a cerebral lesion or a cerebral vascular accident (CVA) altering the visual cortex. Patients, whose retina is normal and who used to see colors very well, lose this ability totally or partially. Their brain is no longer able to process this type of information.
What causes achromatopsia?
Congenital achromatopsia is a genetic disease. Several genes may be involved:
- GNAT2 (chromosome 1);
- CNGA3 (chromosome 2);
- CNGB3 (chromosome 8);
- PDE6C (chromosome 10);
- PDE6H (chromosome 12);
- and, in the case of blue-cone monochromatism, OPN1LW and OPN1MW (chromosome X).
While congenital achromatopsia manifests itself from birth, there are a few cases of acquired achromatopsia. They are caused by brain lesions: a trauma or a stroke having affected the visual cortex.
In patients with mutations in one or more of these genes, the retina lacks the cones that allow them to see colors and details. They have only the “rod” cells, which are responsible for night vision, in shades of gray.
What are the symptoms of achromatopsia?
Congenital achromatopsia is characterized by:
- color blindness : this is total in complete achromats, who see the world in black and white, or almost total in monochromats, who still manage to distinguish blue;
- significant photophobia , that is, hypersensitivity to light;
- reduced visual acuity, between 1/10 and 2/10;
- a nystagmus , that is to say an unintended oscillation and jerky eyeball, especially in the presence of a bright light. This symptom may fade with age;
- a small central scotoma , which is one or more small spots in the center of the visual field.
This deficiency is present from an early age, but it is the aversion to light and the abnormal movement of their baby’s eyes that first alerts parents, especially if there are other cases in the family. When the child is old enough to express himself, a color test can be done but it is not enough, since some monochromats manage to adapt, to the point of being able to name certain colors. Only an electroretinogram (ERG), measuring the electrical activity of the photoreceptors of the retina, can confirm the diagnosis. A genetic analysis can then determine the mutation involved.
Congenital achromatopsia is a stable condition. However, progressive degeneration of the macula, the area in the center of the retina, may occur in some patients, especially as they age.
Cerebral achromatopsia is only a sudden loss of color vision after a head injury or stroke.
How to treat achromatopsia?
There is currently no treatment for this pathology, only solutions to relieve the symptoms. The aversion to light and the sensitivity of contrasts in particular can be improved by wearing glasses or contact lenses tinted in red or brown, called “chromogenic”. Magnifying glasses with high magnification can facilitate reading. There are also other aids to help patients become more independent: training in locomotion, adapting the time to take university exams, etc.
Is prenatal diagnosis possible?
Yes, as achromatopsia is a disabling genetic disease, a prenatal diagnosis can be proposed to couples at risk, i.e. couples in which both partners are carriers of mutations linked to this pathology. If they have the same mutation, they have a 25% chance of giving birth to an achromatous child.
In the case of monochromatism, only the mother can transmit the deficient gene to her offspring. If it is a boy, he will be achromatous. If it is a girl, she will be a carrier of the mutation, like her mother.