Achromatopsia – the hereditary disease caused by mutations of various genes coding proteins of photoreceptors of a retina and which is characterized full (a bacillary monokhromaziya) or partial lack of perception of flowers. At the same time black-and-white sight is kept though there are also other violations of operation of the visual device – a day blindness (gemeralopiya), , decrease in visual acuity, sometimes squint. For diagnostics use as techniques of modern ophthalmology (Ishikhar's tests, Rabkina, an elektroretinografiya), and geneticists – studying of the hereditary anamnesis, the sekvenirovany sequences of the genes associated with pathology. Today specific treatment does not exist, there are developments of gene therapy of some forms of achromatopsia.
Achromatopsia (bacillary monochromatism, color blindness) is hereditary pathology at which work of flasks – the photoreceptors of a retina responsible for perception of flowers is broken. Full achromatopsia is characterized by loss of receptor function by all three types of flasks – "blue", "red" and "green". The sticks providing black-and-white sight remain the only photoreceptors of a retina at the same time. Traditionally allocate partial achromatopsia at which to light only the certain type of flasks – "green" or "red" is unreceptive. Strictly speaking, partial achromatopsia is monochromatism of "blue" flasks, at the same time some colors and shades drop out of perception, but in general color sight remains.
On average, occurrence of achromatopsia makes about 1:300000, at the same time more than 70% are the share of a full form of a disease. Full bacillary monochromatism is inherited on autosomno-recessive type therefore equally strikes both men, and women. Partial achromatopsia is the hereditary pathology linked to the H-chromosome, because of it the vast majority of patients make men. It is known that the famous scientist D. Dalton therefore in his honor such violation is often called daltonism suffered from monochromatism of "blue" flasks. Now check of color sight is an obligatory component of any preventive medical examination in ophthalmology, and the persons having problems with perception of flowers are limited in obtaining the driving license, work on some productions.
Full achromatopsia, or bacillary monochromatism, represents group of hereditary pathologies, each of which is called by a mutation of a certain gene, all of them are inherited on autosomno-recessive type. These genes code proteins which cause intracellular information transfer from a rodopsin to a cage. Change of a komformation of a rodopsin under the influence of cages through a chain of proteins (G-squirrels – - a fosfodiesteraza) leads to hyperpolarization of a membrane of a photoreceptor, it slows down allocation from it of a glutamate, causing excitement of bipolar cages. The last finally give action potential to fibers of an optic nerve. Violation of structure of proteins transmitters in flasks because of a mutation leads to the fact that in response to influence of light and change of a komformation of a protein photopigment there is no reaction of a cage and, as a result, a nervous impulse.
In this situation sticks which photopigment has no sensitivity to a certain color therefore waves of light of various length perceive these receptors as various shades of gray color remain the only operating photoreceptors of a retina. Besides black-and-white sight, lack of flasks at full achromatopsia leads to the central scotoma – in the field of a yellow spot there are practically no sticks. It, in turn, leads to visual acuity violation as it is provided with focusing of light on the central part of a yellow spot. As sticks in general are more sensitive than flasks (normal they provide night sight), at full achromatopsia photosensitivity sharply increases.
Partial achromatopsia, or daltonism, is caused by a mutation of the genes localized on the H-chromosome. These genes code red (OPN1LW gene) or green (OPN1MW gene) photopigments of flasks. As a result of violation of structure the pigment is not able to perceive any more light because of what there is a violation of perception of certain flowers and shades. As at this disease only about a third of flasks drops out of work, color sight remains, though is a little changed. Also much more weakly visual acuity, not suffers as at full achromatopsia, the photophobia is expressed.
Classification of achromatopsia
Full achromatopsia can be caused by mutations of several genes, on this basis and make classification of various forms of a disease. Taking into account that clinically these forms have no distinctions, for the practicing doctor geneticist and the more so, the ophthalmologist, such division, at first sight, not essentially. However it is important for the researchers running for search of methods of treatment of achromatopsia and also in genetic diagnosis of a disease.
The ACHM2 type – is caused by a mutation of the gene of CNGA3 located in the 2nd chromosome and coding alpha 3 - a G-squirrel – a secondary messenger of flasks.
The ACHM3 type – a mutation of a gene of CNGB located in the 8th chromosome it codes beta 3 a G-squirrel.
The ACHM4 type – a mutation of the gene of GNAT2 located in the 1st chromosome and coding alpha 3 - a squirrel of a transdutsin.
The ACHM5 type – a mutation of a gene of PDE6C located on the 10th chromosome it codes alpha fosfodiesteraza.
The ACHM6 type – a mutation of the gene of PDE6H located on the 12th chromosome and coding gamma fosfodiesteraza.
Thus, full achromatopsia is caused by violation of information transfer from a rodopsin to other structures of a photoreceptor because of violation of structure of intermediary proteins. As these proteins are identical in all flasks, all three types of these cages suffer. Unlike full achromatopsia, at daltonism the mutation arises in a gene actually of photopigments which type divides flasks into three groups. Such types of partial achromatopsia are most often observed:
- Protanopiya – the immunity of red color caused by a mutation of a gene of OPN1LW located on the 28th locus of the H-chromosome.
- Deyteranopiya – immunity of green color, a mutation arises in a gene of OPN1MW located in the same 28th locus of the H-chromosome.
The majority of symptoms of achromatopsia subjective though there is also a number of objective indicators of a disease. At full achromatopsia besides black-and-white sight also far-sightedness often is found in patients . Because of tolerance to light of the photoreceptors of a yellow spot (presented generally by flasks) visual acuity falls, is frequent to level 0,1 below. Achromatopsia results also in hypersensibility of eyes to light and the photophobia developing on this background and a day blindness (gemeralopiya). Quite often above-stated symptoms are complicated by squint. An important role at full achromatopsia is played also by psychological factors – often lowered look down (because of hypersensibility to light), need of wearing dark glasses can depress the patient.
At incomplete achromatopsia all above-stated symptoms are observed, but in most cases they are expressed much more weakly. It occurs because only a part of photoreceptors (about a third of flasks) therefore visual acuity seldom decreases lower than 0,3 suffers, the photophobia and a gemeralopiya are less intensive, far-sightedness quite seldom develops. Violation of color perception – for example will be a subjective symptom, sick incomplete achromatopsia in case of a protanopiya does not distinguish green and red colors.
Diagnosis of achromatopsia
Diagnosis of achromatopsia is made on the basis of inspection at the ophthalmologist, and also by studying of the hereditary anamnesis and genetic researches. Violation of color perception which is found by means of Rabkin's tables will be the main ophthalmologic display of a disease. In the presence of achromatopsia patients become incapable to distinguish figures and other images on the pictures made of circles of a certain color. Using different types and colors of tables, the ophthalmologist can define an achromatopsia form – full or incomplete (a protanopiya or a deyteranopiya). There will be Besides, changes at an elektroretinografiya – reduction of amplitude of kolbochkovy peaks up to them before total disappearance at full achromatopsia.
Genetic methods of diagnostics come down to a sekvenirovaniye of the sequences of the genes associated with achromatopsia for the purpose of identification of mutations. The research of the family anamnesis for clarification of nature of inheritance of pathology can help with detection of a disease. Differential diagnostics should be carried out with some forms of a pigmentary abiotrophy of a retina – initial displays of this disease can have symptoms, similar to achromatopsia.
Treatment of achromatopsia
Specific treatment of any form of achromatopsia does not exist today. The conducted researches in the field of gene therapy allowed to return partially perception of flowers at laboratory animals. For reduction of other symptoms of achromatopsia (far-sightedness, a gemeralopiya) use correcting and sunglasses. Helps to improve perception of flowers at incomplete achromatopsia glasses wearing with neodymium glasses a little – in some countries sick daltonism allowed even driving of motor transport on condition of use of such points.
In the presence of full achromatopsia the forecast more often adverse as violations of sight at such disease tend to progressing. However the same points and other actions for improvement of quality of life of the patient can slow down these processes. Also the supporting therapy on the basis of vitamins A and E, vasodilating means can be appointed. In case of incomplete achromatopsia violation of perception of flowers and insignificant deterioration in sight do not result in serious condition of the patient.