Myopia

Forms of myopia

This anomaly is measured in Dioptrie S, which gives the power of the necessary corrective lenses to correct the defect and to make so that the focusing of a distant object is done on the retina.

A myopia of less than 3 dioptres is a priori benign, and of more than 6 dioptres a priori severe because it can translate a large eye and consequently fabrics slack and likely to tear. (image of a balloon which one inflates, more it is inflated more its wall is fine, the strong short-sighted ones often have " beaux" eyes)

One can schematically divide myopias into three groups:

small myopias: up to -4 dioptres
average myopias: of - 4,00 with - 8,00 dioptres
strong myopias: higher than - 8,00 dioptres.

But the fundamental point is always the size of the eye.

There exists an approximate relation between dioptres and the vision with far from the myope in tenth without correction:

For a myopia of - 0.75 dioptres, acuity is from 5 to 7/10
For a myopia of - 1.50 dioptres, acuity is from 2 to 4/10
For a myopia of - 2.50 dioptres, acuity is of 1/10
For a myopia of - 3.00 dioptres, acuity is lower than 1/10
For a myopia of - 6.00 dioptres, acuity is lower than 1/20

Causes

Theories:

the genetics - the size of the eye being obviously genetically determined, the majority of the researchers think that myopia is Héréditaire with sometimes jump of a generation even a change. Certain studies suggest that it could be hereditary up to 89%. Moreover, new studies among twins monozygotes associate myopia with a defective version of gene PAX6 located on chromosome 11.
the environment - It is true that an excess of tonicity of the muscle ciliaire can involve a myopia by spasm ciliaire, more or less durable. In the same way the index of refraction of the crystalline lens can be modified by a strong exposure to heat (opacity cristallinienne of the glass-blowers). Thus a second theory allots myopia to environmental factors. It supports that a work of close constant reinforces the muscles ciliaires which ensure accommodation and would decrease thus the curve cristallinienne (what increases its refringent capacity) in a durable way. This theory would be compatible with the cases of myopia not axiles or corneal and could be confirmed by the new means of precise measurements of the curve cristallinienne (echography UBM and OCT.).

It is not excluded that an important work period on screen is not provider of late myopia.

Combination of genetic factors and environmental - a genetic factor could be potentiated by an environmental factor in the development of myopia.

the prevalence of myopia did not cease increasing during the last decades, in particular at the children and the teenagers. This phenomenon noted in certain areas (Southeast Asia, in particular in Japan and especially in China), would be noted on the American continent, in Australia and even in Europe.

To Taiwan the prevalence at the 12 years age spent these 20 last years from 37% to 61%. It was noted there that the number of children reached of myopia increased, that myopia started earlier and that it was stronger. Moreover, it meets 3 times more in the zones strongly urbanized that in the rural areas. In France, myopia relates to 15 to 20% of the young people of less than 20 years, with a female prevalence, figures observed in other countries of Europe.

evolutionary Approche - It was proposed that food causes, particularly the fast sugar consumption, were responsible for the strong increase for short-sighted during the last decades. The data anthropological, ethnological, historical and medical in favor of this assumption are rather strong.

Risks and complications

The strong short-sighted ones have a risk more high of tear or detachment of retina as well as cataract and of glaucome.

The Cataracte is earlier in the event of strong myopia.

The Glaucome is a little more frequent in the myope than in the remainder of the population.

The myope often complains about floating “flies” or bodies, which move at the time of the ocular movements. These myodésopsies is related to the liquefaction of the vitreous body, earlier in the myope, and is without gravity when they do not translate a retinal tear (an examination of the bottom of eye is necessary).

Myopia makes the eye more fragile than a normal eye. It can exist with the periphery of the retina of the holes and the tears to be treated by argon laser to avoid a detachment of retina.

Treatment of myopia

There is no curative treatment of myopia in the direction where the length of the eye can hardly be shortened and a fortiori the thickened ocular wall is reinforced.

It as should be specified as the port of the glasses does not cure a need for glasses.

The treatment of the complications of myopia is especially the prevention of the detachment of retina and rests on the argon laser (not to confuse with the Laser excimer which treats only the optical aspect of the myopia) which saw here the first medical application of the lasers.

In the event of separation of retina proven, the treatment can be only surgical.

Correction of myopia

To obtain a clear vision, the image of the remote objects must be focused on the retina. The purpose of the correction of myopia is thus to modify the way of the luminous rays so that they converge towards the retina:

by glasses whose concave glass pushes back the image on the retina,

by contact lenses offering a field of vision wider than the glasses,

by an operation of myopia which consists in modifying the curve of the cornea to the Laser excimer or, for strong myopia to set up an implant inside the eye.

Myopia in its optical aspect can be decreased or removed by an surgical operation with the laser. It carries out true a " ponçage" micrometric of the cornea in order to make it less convex. This operation presents a risk (like any operation) not to neglect. In addition a possible post-operative myopic evolution of origin cristallinienne is not avoidable, but will remain weak.

Random links: