Myopia is most often due to an eye too long. The focus of the image of a distant object is thus formed in front of the retina and the distance vision is blurred. On the other hand, the near vision is clear. It allows reading without glasses. The stronger it is, the closer the text should be. For a normal eye the punctum remotum (which is the farthest point of the eye that can be seen clearly) is at infinity. For a short-sighted eye it is closer to less than 5 meters. This net reading distance is inversely proportional to myopia. For example for -3 diopters it is 0.33 m, for -5 diopter the punctum remotum is 0.20 m.
Myopia usually occurs in childhood around 6 to 8 years of age and progresses to the age of 20-25 years and then stabilizes. However, it can sometimes appear earlier (neo-natal or congenital strong myopia) or on the contrary later, after 20 years or stabilize earlier. The earlier it begins, the more it will be evolutionary and important. Two forms of strong myopia exist: one that evolves progressively until 30 years, the other continuously up to 60 years. The myopia usually encountered is not a disease. Only the strong myopia has a true ‘myopia disease’ which is accompanied by a gradual growth of the size of the eye throughout life and major alterations of the visual retina.
Myopia does not preserve presbyopia. The nearsightedness of presbyopia sees blurry when he wears to read his glasses from afar. On the other hand, when he removes them, he can read at a different distance according to the importance of myopia which suggests that he becomes presbyopic after others or, falsely, that myopia improves with age.
The word myopia comes from the contraction of two Greek words muein (se clos) and ôps (oeil). In the adult the disturbance in far vision is eloquent. The short-sighted can improve his vision from afar by narrowing his eyes. In children the diagnosis may be more difficult, as most children do not complain about this defect. It is therefore important to carry out an ophthalmological examination on a regular basis. Myopia can be detected by teachers who notice a difficulty of vision from a distance on the board. The fact of getting closer to his notebooks or the television can also make it evoke. It can also occur later in a student who has had good vision.
It can be expressed either by encrypting the necessary correction in diopter (number preceded by a minus sign for myopia) or by quantifying the visual acuity in tenths.
– The diopter represents the power of a corrective glass to be placed in front of the eye so that it can see the best possible. It corresponds to the figures recorded on the prescription of the glasses. For example, a short-sighted I-diopter can see distinctly without glasses objects located within a distance of one meter, but what is located further is blurry. He needs a glass of -1 diopter to see net.
– The tenths represent the size of the letters read off. They measure visual acuity. The smaller the letters read, the larger the tenths.
There is an approximate relationship between the diopters that measure the importance of myopia and the tenths that measure vision without correction:
• For 0 diopter of myopia, visual acuity is 10/10 tenths
• For – 0.75 diopters, visual acuity is 5 to 7/10 tenths
• For -1.50 diopter, visual acuity is 2 to 4/10 tenths
• For – 2.50 diopters, visual acuity is 1/10 tenths
• For -300 diopters, visual acuity is less than 1/10 tenths
• For – 6.00 diopters, visual acuity is less than 1/20
– Myopia is low between -1 and -3 dioptres (about a third of myopes), average between -3 and -6 dioptres (about two-thirds of myopes), strong beyond (5% of myopes), very rare Above -20 diopters.
– As can be seen, the vision is no longer quantifiable in tenths for an average myopia greater than -3 dioptres.
– Moreover, all tenths do not have the same importance: there is much more difference of vision between 1/10 and 2/10 than between 7/10 and 10/10.
– It is possible theoretically to have a visual acuity greater than 10/10 tenths, that is to say 12/10 tenths, 15/10 tenths, even 20/10 tenths.
– Even well corrected a myopic very rarely reached 10/10 tenths (relative amblyopia).
– Anisometropia is defined by a difference of more than one diopter between the two eyes. A myopic anisometropia of more than 5 diopters results in an amblyopia in the child, ie a decrease in visual acuity by default of development of visual cortical cells. This amblyopia is considered irreversible when it has not been corrected in childhood (usually before 5 years). The prescription of corrective lenses thus remains ineffective and the visual acuity limited despite all means of correction.
Causes of myopia
• Genetic factors.
Myopia is partly of genetic origin and often appears in families where there are myopes with sometimes a generation leap. A child has more risk if one of the parents is short-sighted (about one in three) and even more if both parents are short-sighted (one in two), half of the short-sighted have their sibling brother or sister. Studies carried out on series of monozygotic twins (which share the same genetic heritage) have also established the genetic influence. It is less important in dizygotic twins who share the same environment. The mode of genetic transmission is still poorly known but several genes associated with myopia have been discovered. An identified disability in 2010 is crucial for the retinal function and the consolidation of visual memory. This discovery of a genetic origin is particularly interesting because the eye is an organ of choice for gene therapy. Indeed, the fact that it is of low volume and that it is closed allows the treatment to remain located inside in a highly concentrated volume. Another discomfort has been demonstrated in populations of Japanese and Chinese myopes.
The factor responsible for structural changes in the strong myopic eye has yet to be identified in order to design an effective preventive treatment against the aggravation of myopia and its blinding complications.
• Environmental factors.
The increase in the prevalence of myopia in recent decades seems to be mostly associated with environmental factors: reduced time spent on outdoor activities and increased activities in close-up vision to artificial light. The genetic factors involved in myopia are not likely to cause such a rapid change in prevalence. The gradual increase in instruction and activities in near vision require accommodation on the one hand and on the other hand increase exposure to artificial light. There seems to be a feedback loop ‘visual information-growth of the eye’ that would cause the eye to modulate its growth according to the quality of the image received. Thus, in the case of a preponderant near vision, the axial length of the eye increases. In animals during growth, experimental myopia can be created by depriving an eye of an image by occlusion. This phenomenon may be partially reversible depending on the duration of the experiment. The modification of the size of the eye is accompanied by a modification of the structure of the sclera.
– A first hypothesis is that the probability of becoming short-sighted is proportional to the time spent at work or to focus on close objects (reading, computer, television, video games). Myopia increases in industrialized countries, with the level the economic level, socio-educational, the number of years of studies and in the work in near vision. It is proportional to the average daily duration of this close-up vision, which is much more currently solicited by reading, precision work or computer screens. The myopes are generally more educated, more intellectual, more intelligent according to some studies of the intelligence quotient in the short-sighted. Is this a consequence of short-sightedness that encourages reading or close-up work rather than outdoor sports activities or a factor favoring increased solicitation of accommodation during reading? Both hypotheses have been advanced. In the second hypothesis, the antidote would be to look away, outdoors.
The role of defocusing has been advanced: to compensate for the focus of vision on the central retina in myopia, there is a defocusing in the peripheral retina which is a factor of scleral growth of the eyeball.
An under-correction of myopia can cause an elongation of the eyeball.
• A recent article (Lancet, 2012) highlights another risk factor: lack of natural light. A neurotransmitter produced in the retina under the effect of light, dopamine, plays a role would avoid excessive growth of the eye during childhood. If spending hours reading, playing or working on a computer or smartphone promotes short-sightedness, it would be because you spend much less time outdoors in natural light. Prevention involves asking children to spend more time outdoors in natural light. 2 to 3 hours a day are sufficient. See: http://www.ophtalmologie.fr/le-figaro-4mai2012.html
• Another recent epidemiological study of 300,000 people shows that babies born in June and July are 24% more likely to develop myopia than those born in December and January. A child born in winter would be less likely to become short-sighted than a child born in summer. This would be due to exposure from the first days of life to exposure to increased natural luminosity. The same finding was made in children sleeping with a night light before the age of two who are more likely than those who sleep in the dark.
• Myopia may be related to the spatial restriction associated with an enclosed environment, occurring in a prison setting or in submarine crews.
• A recent Australian study of thousands of children between the ages of six and twelve that outdoor sports were accompanied by a reduction in myopia
• A recent study showed increased prevalence of myopia in elderly mothers, intrauterine growth retardation and smoking during pregnancy.
• Other causes
• Myopia of curvature may be due to distortion of the lens or cornea (keratoconus).
• Myopia of index is late onset due to the increase in the refractive index of the lens and then reflects the existence of a cataract (nuclear cataract).
• Nocturnal myopia is due to an excess of accommodation in reduced luminosity. An accommodation spasm can also cause false myopia. In these cases, the excessive convergence of the lens creates myopia.
• Myopia is sometimes due to medication or diabetes. In these cases it is reversible.
Taking into account these different factors, treatment trials have been proposed to try to slow down the evolution: atropine instillation, total optical correction or with positive lenses to inhibit the growth of the peripheral sclera, wearing hard lenses, Outdoor activities, high luminosity. No results have been achieved, apart from atropine which would be the best inhibitor of the growth of the globe, but it causes unacceptable side effects and its efficacy disappears after three years.
The prevalence of myopia
It has continued to increase in recent decades, especially among children and adolescents. This phenomenon is observed in certain regions (South-East Asia, especially Japan in South Korea and especially in China), and is found in the Americas, Australia and even in Europe. The prevalence of sharp myopia is twice as high among populations from the Far East than among Westerners. Myopia reaches record highs in Asia: nearly 80% of the population in some regions in 2012. The phenomenon becomes crucial in this continent, where it is spoken of in terms of an epidemic. The number of Asian children afflicted increases, myopia starts younger and its power is higher. To date, the pathogenesis is not identified. Genetic and environmental factors are called into question. Myopia is also higher in urban areas, according to the results of a study carried out in Taiwan in 1983 and 2000: the prevalence at age 12 has increased in the past 20 years from 37% to 61%; It was found that the number of children affected increased, that myopia began earlier and that it was stronger; Moreover, it occurs 3 times more in highly urbanized areas than in rural areas.
Recent statistics show that in the United States the myopia rate, which was 25% in 1970, rose to 42% in the early 2000s. In France, myopia is estimated at 39% of the population. Ophthalmologists of France), with a female predominance, figures observed in other countries of Europe and in the United States.
Complications of myopia
The sharp myopia makes the eye more fragile than a normal eye. This is the fourth cause of low vision in Europe. It represents 5% of myopes in the United States and Western Europe, 10% in some Asian countries. High myopia is a degenerative disease, a progressive myopia-disease defined by figures greater than -8 diopters and an axial length (corneal-retina distance) greater than 26 mm (23 mm for a normal eye), up to 35 mm for An extremely strong myopia. The elongation of the globe takes place at the expense of the posterior part of the eye.
A severe nearsightedness is a severe condition because it translates a much larger eye and consequently, because of the lengthening of the eyeball, the retina is stretched, distended, thinned, susceptible to tear. It is badly irrigated, the nourishing layer, the choriocapillary, bringing it less blood flow.
• Impact of the macula
The attack of the macula, the central zone of the retina, is particularly to be feared because it jeopardizes the central visual retinal function, that is to say the possibilities of reading. In addition to the decrease in visual acuity, the sign of alarm which can evoke the attack of the macula is the existence of metamorphopsies (deformations of the lines). OCT (Optical Coherent Tomography) is a fundamental examination that makes it possible to highlight and monitor the state of the retina in case of strong myopia.
High myopia can lead to macular pallor, myopic staphyloma (backward distortion of the globe), ruptures of the visual retinal layers (rupture of Bruch’s membrane), deep chorio-retinal atrophy, choroidal neovascularization.
Foveoschisis is common. It is due to the existence of intraretinian cysts. It may be accompanied by an epiretinal membrane stretching the retina forward as the staphyloma stretches it backwards, a macular detachment, a partial lamellar or complete macular hole and lead to a detachment of retina.
Chorioretinal atrophy is a frequent complication leading to a progressive decrease in visual acuity.
The macular hole may also lead to decreased vision on reading with deformities (metamorphopsies). Surgery is possible.
Choroidal neovascularization refers to the passage of choroide vessels through the retina resulting in edema and haemorrhage of the macula. They can be treated with focused argon laser, dynamic phototherapy (PDT) or intra-vitreous anti-VGEF injections.
Much progress has been made recently in the diagnosis and treatment of these retinal complications involving dynamic phototherapy, anti-angiogenic factors and endocular surgery.
• Impairment of the peripheral retina
The periphery of the myopia retina should be monitored regularly by a 3-mirror glass examination or a panoramic lens. There may exist at the periphery of the retina palisades, holes and tears to be treated preventively by the argon laser to avoid retinal detachment more frequent in the myopic.
It is important to note that it is not uncommon to discover these dangerous retinal lesions, predisposing to detachment of the retina, in a nearsighted person, not being alarmed because not complaining of any visual disturbance, no Pain, consultant for trivial reasons like the prescription of a pair of glasses or contact lenses, and which thus requires an argon laser treatment in order to prevent a detachment of the retina. Sometimes there are phosphenes, lightning, floating bodies, a visual veil. In case of proven retinal detachment, the treatment can only be surgical.
• Floating bodies
The myopic often complains of ‘flies’ or floating bodies, small particles that move slowly with the position of the gaze. This phenomenon is due to the liquefaction and the change of structure of the vitreous with the age but often occurring earlier in case of myopia. There is no need to worry about the perception of floating bodies, flying flies, filaments … but their appearance requires a thorough examination of the retina especially if these floating bodies multiply or if flashes appear , Fixed in a well-defined place and even persist with closed eyes. If there are lesions of the peripheral retina that may be complicated by retinal detachment, they should be treated preventively by argon laser.
• Cataract is more prevalent in cases of high myopia, on average ten years earlier than in the non-myopic subject. It is therefore necessary to operate it sooner, which is the occasion to correct at the same time myopia.
• Glaucoma is significantly more common in strong myope (10%) than in the rest of the population (2%). It must be detected early because it can lead to a deterioration without alarming symptoms.
Google Traduction pour les entreprises :Google Kit du traducteurGadget TraductionOutil d’aide à l’export
Correction of myopia
To obtain clear vision, the image of distant objects must be focused on the retina. The aim of correction of myopia is therefore to modify the path of the light rays so that they converge towards the retina.
• The correction can be made by glasses whose concave glass diverges the image on the retina. The stronger the correction, the smaller the myopic perceives the smaller the reality. The stronger the correction, the thicker the glass, the thicker the edge and the finer the center. This unsightly aspect is reduced by the use of glasses with increased refractive index enabling their thinning. The subject has the choice between lighter, thinner but heavier mineral lenses or lighter organic lenses. The choice of a small frame makes it possible to limit the thickness to the edge of the glasses. The higher the index, the more luminous reflections are formed on the surface. Hence the advantage of anti-reflective treatment for thinned glasses.
• Correction can be done by disposable (daily or monthly) or traditional soft contact lenses or by rigid lenses. The lenses make it possible to retrieve a normal image size because bringing the correction closer to the cornea enlarges the retinal image. The image being larger than with a spectacle lens, visual acuity is often better in cases of high myopia, which can reach 2/10 for myopia of -8 diopters. The visual field is more important, that is to say that the peripheral vision is wider, especially for a strong myopia. With the lenses there is better night vision by increasing the retinal illumination compared to the glasses (25% for a myopia of -10 D). Note that from a certain degree of myopia (-4 diopter) the power of the lens is lower than the power of the spectacle lens (for example -7 glasses diopter corresponds to -6.50 lens diopter ).
The wearing of rigid oxygen-permeable lenses in children promotes visual maturation, improves the quality of life and may slow the development according to some studies. They can be worn at the age of 6-7 years if the child is cooperating. Sometimes earlier in case of severe myopia that may lead to amblyopia or strabismus. It is necessary to choose a material with high transmissibility to oxygen whether with a rigid lens or a soft lens. The braking effect of rigid lenses on evolution has been evoked, albeit unproven. It would be due to the correction of optical aberrations stimulating retinal visual cells and inhibiting the growth of the eyeball. In the future, optical corrections based on the theory of defocus and para-sympathicolytic treatments may become useful.
It is common to observe an intolerance to contact lenses worn for many years, requiring intermittent wearing or abandonment.
• The surgical correction of myopia can be performed by a myopia operation that consists of modifying the curvature of the cornea by excimer laser or, for the strong myopia, to place an implant inside the eye.
The various current operations can be carried out by subtraction or addition.
The subtractive way is to ‘erase’ the laser excimer the surface of the cornea to make it less convergent. This ablation of the corneal tissue can be done superficially (PKR, lasek, epi-lasik) or deep after raising a corneal flap (classic lasik or lasik femtosecond).
The quality of vision of the opera is an important concept, different from visual acuity. Visual acuity can be 10/10 and poor quality vision. Quality of vision means better to see at night, in times of fog, rain, strong light. Improved perception of distances, relief, velocities, absence of side effects: nocturnal halos, split vision, blurry, fluctuating, veil, dazzling.
Myopia surgery went from standard surgery where each patient was treated identically according to his or her number of myopia to customized surgery, optimized, depending on each case.
It is possible to adapt the profile of each treatment to improve the quality of vision and avoid side effects.
One of the means used is the treatment of optical aberrations due to the cornea or the lens that vary from one individual to another. The quantification of these aberrations is done by examining the wavefront which makes it possible to study the irregular behavior of each light ray penetrating the eye and the way it arrives on the retina.
The additive way, for its part, consists in introducing an implant
– in the anterior chamber of the eye in front of the iris (phake implant), or with angular support (abandoned) due to corneal complications or fixed to the iris (Artisan or Artiflex).
– in the posterior chamber behind the iris:
– implant in place of the crystalline lens (extraction of the clear crystalline lens to reserve for the myopias in subjects of more than 50 years because it suppresses the accommodation),
– implant in front of the lens (ICL)
– implant in the cornea (inlay).
Let us recall for the record two operations aimed at flattening the cornea to make it less convergent and no longer practiced: radial keratotomy was the first operation of myopia to be practiced routinely in the 80s, the setting of rings Intra-corneal was also practiced.
Article from the site http://www.ophtalmologie.fr