Myopia (word of Greek origin, muôpia) is a disorder of the vision where the person is more vague Items with their removal. In other words, the nearsighted see better almost as far. This can be corrected by glasses or a contact lens. About 26% of the world population is myopic.
Optical myopic eye
The first stage of the vision is to create an image of the retinal optic surrounding objects. This first stage is based on the universal laws of propagation of light. An object will be better defined as the spread of light rays results in an image close to the retina. In emmetropia, ie in the absence of disorder refraction, the image coincides with the retinal.
In myopia, the focus of an object to the "infinite" is in front of the retina. The image that will impress the retina will be spread over more blurred.
So it should be to infinity (5 meters in practice), the punctum remotum (the most distant yet seen net) is in the nose ( "myopic as a mole") and 5 meters, according to the degree of myopia.
The myopia is measured in diopters, which give negative power glasses. Myopia of 2 diopters (too) needs glasses -2 diopters. By misnomer myopia is measured by its correction.
The determination of the distance from punctum remotum also gives the degree of myopia: the degree of myopia is the inverse of the distance from the punctum remotum expressed in meters. For example, if the most distant yet seen net is 10 cm, 0.1 meter therefore, myopia is 10 diopters, it is 2 meters myopia is 0.5 diopters.
Failure to focus the image in myopia results from a mismatch between the length of the eye and its power diopter.
* In most cases the eye is too long (more than 23 mm in length) and the image is formed in front of the retina. (Ie where the retina should be when the size of the eye is normal).
* Sometimes it's too much curvature of the cornea (the limit Keratoconus) or crystalline (myopia transient spasm ciliary or certain forms of scleritis earlier)
* Sometimes is an increase of refractive index lens, as in the late myopia and cataract nuclear debutante. The lens is the only eye (and body) that continues to grow throughout life like the concentric layers of tree trunks that indicate their age. This increase in intra-cristallinien diopters is usually offset by a concomitant reduction of the curvature of the capsule (Creedmoor principal). This increase lens is also a factor in the emergence, almost physiological, and cataract after 75-80 years. The lens is devoid of vessels metabolism will be more difficult by simple diffusion from the aqueous humor. The myopic nature with very big lens, cataract will be home earlier.
Most often, therefore, myopia did not affect the reading. Even at 5 diopters playback can be done to 20 centimeters. Nevertheless, it is better to keep its correction to play the muscles of accommodation. The myopia is often associated with astigmatism and a exophorie.
Contrary to a common idea, myopia does not delay presbyopia. The opposite is also true among non-myopic corrected that develop a ciliary muscle atrophy. What is true is that the myopic presbyopic may withdraw his glasses, happened near correction, unlike the emmetropia presbyopia that must fit his glasses "wolves" for the reading glasses that will make by Moreover myopic!.
There is a rough relationship between diopters and visual acuity in the distance in tenths of myopic correction:
* - 0.75 d: 5 to 7 / 10
* - 1.50 d: 2 to 4 / 10
* - 2.50 d: 1 / 10
* - 3.00 d: <1 / 10
* - 6.00 d: <1 / 20
Correction of myopia
To get a clear image of distant objects should be postponed to be focused on the retina. The correction of myopia therefore aims to change the path of light rays to converge on the retina:
* By concave lenses, so recognizable to board thick
* By contact lenses with a clear visual field more than glasses and a field of light preserved because of the mobility of the lens concurrently with the eye.
* By a myopic which is to change the curvature of the cornea by excimer laser or by corneal rings and high myopia, to develop an implant inside the eye.
o The excimer laser optics corrects the appearance of myopia by performing a "sanding" micrometer of the cornea to make it less convex. The excimer laser can be achieved on the surface of the cornea (PKR, LASEK, epi-LASIK) or more deeply after cutting a strand (lasik).
o intracornéens rings are inserted into the thickness of the cornea.
o The implant can be placed in your room before or after removal or not after the lens.
These operations have indications, indications and against specific risks should be carefully weighed before any decision. Also a possible change myopique post-operative home cristallinienne is not preventable, but remain low.
Forms of myopia
One can distinguish many forms of myopia, depending on the mechanism optical Thursday and has already been mentioned, the age of onset, pathogenic conditions (trauma, scleritis, cataracts). However, the most significant in practice is the degree of myopia since it is usually an indication of greater or lesser size of the eye. Thus, a myopia of less than 3 diopters is a priori benign, and more than 6 diopters a priori severe as it can bring a great eye and therefore tissue distended and likely to tear (like a balloon that the it inflates more it is inflated, over its wall is fine; high myopic often have "beautiful" eyes). There is a correlation between the size of the eye and the degree of myopia.
In most cases, myopia is between 8 and 12 years, progressing slowly to 20-25 years. But it can also begin at all ages, and even after 70 years (cataract novice). Over the top is early myopia will be more severe.
* Genetics - The size of the eye is clearly genetically determined, most researchers think that myopia is hereditary sometimes skipping a generation or even a mutation.Le gene coding for the size of the eye would be located on chromosome 5. The gene coding for the size and / or curvature of the cornea are several different combinations are possible where variability optical final result.
Some studies suggest that myopia could be inherited up to 89%.
* The environment - It is true that too much ciliary muscle tone can lead to myopia by ciliary spasm, more or less sustainable. Similarly the index of refraction of the lens can be changed by exposure to high heat (cristallinienne opacity of glass blowers). Thus, a second theory attributes myopia to environmental factors. It contends that work closely supported reinforces the ciliary muscles that provide accommodation and reduce the curvature cristallinienne (which increases its refractive power) in a sustainable manner. This theory is consistent with cases of myopia or corneal not axil and could be confirmed by new means of specific measures the curvature cristallinienne (UBM ultrasound and OCT).
It is not totally exclude a significant period of work on screen is myopia provider of late, although for the moment, studies show that the eye can result often mentioned as visual disturbance already and not acquired diseases.
* A combination of genetic and environmental factors
Monozygotic twins who have a priori the same genetic heritage are almost never the same degree of myopia (recording artist) like the same psychological. The genes do not explain everything. The nature seems to leave a certain degree of freedom in random factor and "play dice". For example, the genetic factor could be potentiated by an environmental factor in the development of myopia. It was suggested that causes food, especially the rapid consumption of sugars, were responsible for the sharp increase in myopia in recent decades. The anthropological, ethnological, historical and medical support this hypothesis are quite strong.
Risks and complications
Cataracts are among the earliest very myopic.
Glaucoma is somewhat more common among the myopic than in the rest of the population.
The myopic often complains of 'Fly' or floating bodies, which run during eye movements. These myodesopsies are linked to the vitreous liquefaction, earlier in the myopic, and are not serious when they do not present a retinal tear (a review of the retinal periphery is necessary). There is no need to worry about the perception of body floating, flying flies, filaments ..., but their emergence requires a thorough examination of the retina especially if these bodies floating multiply or lightning occur, fixed in a well-established and persist even with closed eyes.
A high myopia can be complicated because it reflects a great eye (axial length of the eye greater than 25 mm) and therefore a distended retina, thinned, poorly irrigated and can tear and cause a detached retina. the prevention of retinal detachment based on retinal photocoagulation lesions peripherques Argon laser (do not confuse this with the Excimer Laser). Achieving the macula is rare but serious because visual acuity may collapse. It is a distortion of objects and especially their edges. The OCT shows that the retina is an in-depth review that monitors the status of the retina in case of high myopia.
The high myopia is the 4th cause of sighted people in Europe.
Treatment of myopia
Wearing glasses (or contact lenses) does not cure myopia, ie the need for glasses. Glasses can not change the actual size of the eye or prevent it from growing. The correction of the degree of curvature of the cornea through micro-incisions made using an excimer laser is an alternative technique, called LASIK (Laser in-Situ Keratomileusis).
The treatment of complications of myopia is mainly the prevention of detached retina and is based on the argon laser (not to be confused with the excimer laser) which saw here the first application of medical lasers. It is not uncommon to discover dangerous retinal injuries, predisposing detachment of the retina, in a myopic subject, not alarmed because not complaining of any visual, no pain, consultant for a reason banal like a pair of prescription glasses or contact lenses, which necessitates a preventive laser treatment for a detached retina. In the case of detached retina found, treatment can only be surgical.
William Bates, New York ophthalmologist from the beginning of the century, dedicated his career developing the method Bates based on eye exercises. This method remains controversial. It would have allowed some people to heal their myopia, and restore sight among other Aldous Huxley wrote a book on his experience.
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