Myopia: frequently asked questions
Myopia is the most common refractive defect. Pages collected here are the questions generally asked to characterize and quantify the importance of short-sightedness, its mechanisms, and means of prevention.
(1) myopia: which definition?
Myopia is defined as a refractive error related to the fact that parallel light rays (from a point light source located at infinity) are focused in front of the retina. However, this concise definition has several extensions: anatomical, functional, optical,... Indeed, the finding of rays which converge too quickly and cross in front of the retina can be the result of various causes, but he suggests implicitly that a length of the eye (axial length) too important as the most frequent cause of myopia. Should however not forget that for a given axial length, the cornea and the lens must have an optical power able to focus the incident rays in the plan of the retina. In excess of vergence case (excess of refraction of the cornea and the crystalline lens), myopia may appear while the axial length is not increased. There is no 'one' standard axial length or normal, even only a single ocular vergence. That is why it is not possible to quantify the myopia as excessive axial length with respect to the standard, because even if the average axial length in the non-sighted population is close to 23.5 mm, there is a relatively large spread of values. A study on the measurement of the dimensions of normal eyes (in this definition, we accept the presence of myopia < 5 d), axial length was 23.67 mm (0.9 mm, extreme deviation: 21.68-27.36). (Oliveira et al. Axial length and optic disc size in normal eyes. BR J invest 2007; 91 (1): 37-39)
What to remember is that the eye is not nearsighted, is emitted by a distant object light rays are focused at the right distance, that is to say in terms of the photoreceptors of the retina (see the pages dedicated to a more basic description of the vision to the)optics of the eye). After the birth, the eye grows and this extension should be "adjusted" so that it coincides with the distance where the light rays are focused. If the elongation of the eye is excessive, a myopia will appear. The onset of myopia in a child or adolescent is the result of excessive growth of the length of the eye: it is not excessive towards a 'standard', but with respect to the focus distance (distance from home) of the crystalline + horny couple (see page more specialized to the)study of the optical power of the eye paraxial)
So the myopic eye is an eye which the axial length is excessive in relation to the conjugate of the couple 'horny + lens' optical power: insist on the fact that this definition implicitly assumes that the observed light source is located at the infinite, that there be no cristallinienne accommodation, and that the incident light rays are parallel. Indeed, if the source is close, the incident light rays are no longer considered as parallel, but diverge when they meet the eye. This divergence that rays converge further, and that for a certain divergence - IE not infinite for a certain distance, refracted rays can be focused in the retinal plan to form a sharp image the eye did not have need to accommodate.
The axial length of the eye is the distance between the top of the cornea and the layer of retinal photoreceptors, and even if it varies between individuals, there is overall a good correlation between myopia and axial length. More the eye is "long", more likely that the eye is nearsighted is increasing: myopia associated with the elongation of the eye is known as axile myopia: it is the most frequent of the myopia. However, if we measure a look, and that found a length slightly above average (ex; 24.2 mm), could not be certain the presence of myopia. Indeed, if the cornea was less convergent than average ('flatter' cornea), compensation would be possible and the light from distant sources could be focused in the retinal plan.
Conversely, a look he's short-sighted, and for observing and more a small corneal curvature (reduction of the) corneal power called 'power k') is an eye which the axial length is certainly significantly higher than average: despite a lesser convergence of the incident rays (low power k), these are still focused in front of the retina () plansee simulations in ray tracing of a myopic eye)
In 1866, the German scholar Helmholtz stated a corollary to the definition of short-sightedness: for a myopic eye, the plan which must be a point light source to be seen net (without accommodation) is located at a distance finished, and not infinite. The distance from this plane of sharpness allows to quantify the importance of myopia: more this plan is close to the eye, and most nearsightedness is strong. We can then quantify the myopia with the distance of the plane where the objects are seen net by the myopic eye.
This distance (d), where an object can be seen by the nearsighted eye, is specific to the degree of myopia of the concerned eye: its inverse (1/d = D, with d expressed in meters) corresponds to the nshadow of diopters needed to correct nearsightedness. By convention, we express this figure with a negative sign. For example, a myopic eye see blur in the distance, but when the object gets closer, it ends up being net at a distance of 2 meters: nearsightedness is equal to 1/2 = 0.5 diopters. If the distance to which the object becomes net is 50 cm, myopia is 1 / 0.5 = 4 diopters. A myopic 10 diopters can see net... If you place the object of interest to... 10 cm! This figure (diopters) is that the optician is used to make corrective lenses (concave); by convention, we use a negative sign (ex: prescription-3D).
For example, the severity of myopia can be estimated by the power of the corrector glass : myopia of - 10 d is more severe than a myopia of - 6 d.
This simple quantification of myopia to escape values of optical power of the cornea, the length of the eye or its excess... Glass corrective action is to pretend that the light rays from infinity (far) were in fact issued since the distance where the vision is clear. This distance is known as the 'punctum point. " The inverse of this distance in meter is equal to the degree of myopia expressed in diopters.
Thus,. the blur of the myopic eye respect to distant objects. On the other hand, a myopic eye is able to see clearly at a distance "finished.", and this without any accommodation: his punctum point (the point where the vision is clear without effort) is located at a distance "finished" (below 4 or 5 meters in practice).
Since a non-myopic eye is naturally set to net "in the distance", at infinity (in practice beyond 5 metres), this eye accommodates to see net: bomb the lens to increase the optical power of the eye and focus the light rays emitted by the close target that is seen on the retina. We can consider that a not myopic eye which accommodates becomes 'temporarily' myopic, time to see net close! So, if it "froze" a not myopic eye in a permanent accommodation, he would see blur by far (see the) blurred vision of the myopic). This is what is happening in photography when the update is made on a subject near: more distant elements appear blurry, as in a myopic vision. So, not to overestimate the myopia, to perform a measurement of refraction with a technique that does not stimulate the accommodation (so-called "interference" techniques). The instillation of eye drops "cycloplegic" puts the lens at rest, and prevents him to accommodate; that is why it instills this type of eye drops during the balance of the refraction in children (which the eye accommodates easily and many), and young adults during the assessment of prior consultation refractive surgery.
A confusion between the diopters and the tenths and often made; Why not use a number of 'tenths' lost visual acuity to quantify myopia? Visual acuity by far No corrected is certainly reduced in the short-sighted, but the number of tenths 'lost' depends on other factors such as the diameter of the pupille at time of review (himself subject to various influences such as the brightness of the examination room, the stress of the patient, etc.), the ability of the subject to "guess" the presented letters. Even blurred, we can distinguish the contours of a N and z more easily than those of a E and f. It is indeed difficult to establish a perfect correlation between diopters of myopia and tenths Visual acuity: However, we can establish only a myopia of-1.25 D enough to penalize the Visual acuity of... eight tenths. Thus, a myopic of-1.25 D (low myopia) sees only 2 tenths (2/10) about.
In conclusion, it seems that use the power of glass myopia correction in diopters as 'standard - meter' or a least bad method, although two eyes each myopic 3 dioptres may have features (like power k or axial length) significantly different. Moreover, the estimation of the degree of myopia can be rendered more difficult in case ofastigmatism pronounced.
Knowing that the degree of myopia is estimated by the power of the corrector glass to wear, from what correction is nearsighted? Beyond-0.50 D for most of the specialists; in deca, even if the vision can be improved by a slight correction (-0.25D), it is so weak that it is not required in a way permanent and hinders that little daytime Visual function.
The difference in minimum correction at which the human eye is subjectively sensitive (blur printing accented) is-0.25 about D.
(2) what are the economic consequences of myopia?
Myopia is a public health problem, whose cost is far from negligible to society. The percentage of short-sighted people in the world's population is growing steadily: the prevalence of myopia increases. The percentage of myopia in adolescents is between 25 to 50% in Western countries, and between 60% and... 100% on the Asian continent. At this rate of progress, we can predict that nearly a billion earthlings will be short-sighted in 2020.
In Asia, the rate of nearsighted subjects in the general population is higher than that present in the population of the country Western (or populated by a majority of subjects of Western origin). Singapore, estimated that about 11 percent of ethnic Chinese children from 6 months to 6 years are short-sighted. This proportion believes strongly in adolescence... Since three out of four adolescents are short-sighted in Singapore! In the urban areas of the South Korea, 96.5% adults 19 years of age are nearsighted! The prevalence of myopia has strongly believed in this country over the past decades)Kim and al. Prevalence and Risk Factors for Refractive Errors: Korean National Health and Nutrition Examination Survey 2008-2011. PLoS One. 2013; 8 (11): e80361). This growth is attributed to a radical change in the mode of education in Korean children, who are generally subject to a teaching and education very framed school.
Most nearsightedness is pronounced in adulthood, more she began early in life in general.
Environmental and genetic factors contribute to nearsightedness: for example, the Chinese are among Asians to be more exposed to short-sightedness, and Asians are on average more nearsighted than Westerners. The prevalence of myopia in adults South Koreans aged 20 to 39 years (75.1% is more than 3 out of 4 adults!) is superior to that of the white and black populations American (50%) (Vitale and al. Prevalence of refractive error in the United States, 1999-2004. Arch invest, 2008; 126: 1111-1119).
In Australia, a study estimated that 15% of adults were myopic)Attebo K, Ivers RQ & Mitchell P. Refractive errors in year older population: the Blue Mountains Eye Study. Ophthalmology 1999; 106: 1066-1072), while in Japan, 41.8% of adults have myopia greater than-0.50 D according to a recent)Sawada A, Tomidokoro Saudi M, Iwase A & Yamamoto T. Refractive errors in year elderly Japanese population: the Tajimi study. Ophthalmology 2008; 115: 363-370.e3).
The average cost of myopia was estimated to about 150 US Dollars per year in Singapore)Lim et al. Direct costs of myopia in Singapore. Eye (Lond). 2009; 23 (5): 1086-9). In the U.S., expenditures related to the medical care of nearsightedness (consultations, optical costs, management of complications, refractive surgery, etc.) is between 4 and 7 billion dollars about)Vitale S, Cotch MF, Sperduto R & Ellwein L. Costs of refractive correction of distance vision impairment in the United States, 1999-2002. Ophthalmology 2006; 113: 2163-2170. It is estimated about 140 billion dollars annual cost of care and correction of eye diseases (including optical defects, diseases of the eye, etc.). This figure is of the same order of magnitude as that of the cost of diabetes ($ 250 billion) or heart disease (310 billion).
Many research programs are so devoted to the study of the mechanisms responsible for myopia, in order to find a prevention for stop the epidemic spread of myopia, including in Asia.
(3) what are the causes of myopia?
The causes of myopia are not completely elucidated. Myopia is an attribute often associated with the image of the 'learned' or the intellectual: the astronomer Kepler had he not himself put his myopia on account of time spent studying? (Kepler J (1604) Ad Vitellionem paralipomena, quibus Nicolaus leave optica traditur... Tractatum visionis modo luculentum, & humorum oculi usu, contra ópticos & anatomicos. Frankfurt: Claudius Marnius & heirs of Joannes Aubrius.)
An important mechanism seems to have recently been identified, because he found in various epidemiological studies: the lack of time spent outdoors would be associated with the prevalence of nearsightedness. Excessive time inside, a deficit of outdoor activity seem to promote the onset of myopia. Why this influence of time spent outdoors on the development of myopia? A likely hypothesis is that exposure of the eye to the outside light (of several order of magnitude greater than that of a lit room) promotes the effect of dopamine at the level of neural retinal transmission. Dopamine has an inhibitory effect on the growth of the eye (increase in axial length) in some animal models)Norton et al. Light levels, refractive development, and myopia - a speculative review. Exp Eye Res. 2013; 114:48 - 57).
Similarly, the outside view in the workplace provides a more harmonious retinal simulation in terms of distance and perspective of the objects observed: the distance between the objects of an outdoor stage (horizon, landscape, distant lines) is more homogeneous than in a closed room and closer objects. Reading a book, a screen, suppose it to be seen net, while other objects located nearby in the lateral Visual field can then be defocused (seen blurry by the retina). This peripheral defocus was incriminated as a cause of refractive errors occurred in some studies (experimental)Sankaridurg and al. Practical applications to modify and control the development of ametropia. Eye (Lond). 2013 Dec 6).
Children who spend more time indoors than outdoors would be more likely to develop myopia.
This environmental explanation comes naturally associated genetic factors. 68 genes have been identified as associated with myopia: their individual role is complex, because they are involved in the expression of different biological and metabolic factors. The existence of myopia among parents exposed to an increased risk of myopia in children, and Asian ethnicities are more exposed to short-sightedness. However, the role of genetic factors is more encouraging that truly causal on myopia: the Indians of Singapore for example have a prevalence of myopia estimated 70%, against only 10 percent for Indians living in India!
(4) can prevent or slow down myopia?
The intimate mechanisms leading to the onset of myopia are not completely elucidated. They generally reflected the cumulative factors above influence as time spent indoors vs. outdoors. For about a decade, an information campaign has been implemented in Singapore, to recommend to children and adolescents not to perform Visual tasks close to way too long, and make the 'breaks' in regularly spending time in outdoor environment. These recommendations seem to bear fruit since the progression of myopia seems jammed in this country, while she was growing in the previous decades. However, spending time outdoors does not appear to slow down myopia in children who are already short-sighted.
The detection of nearsightedness is important, because earlier and more precise correction avoids a more rapid progression of myopia. The examination of children by an ophthalmologist or an optometrist (in countries where this profession is permitted) is important, even the campaigns of school screening. It is better to correct myopia in full, than to leave a slight under correction, for example prescribing corrective lenses a little weak, contrary to a trend that was popular some decades ago.
Some studies to assess the interest of the instillation of eyedrops to Atropine 0.01% (muscarinique agent) seem to conclude to a relatively effective (slowing the progression of myopia): but the side effects of this type of eye drops are enough (dilation, photophobia) important. Another molecule to similar effects, better tolerated, the pirenzepine, also seems to be able to slow the progression of myopia, but further studies are needed to confirm these data)Chua et al. Atropine for the treatment childhood myopia. Ophthalmology. 2006; 113:2285 - 91 /Tan et al. One-year multicenter, double-masked, placebo-controlled, parallel safety and efficacy study of 2% ophthalmic gel pirenzepine in children with myopia. Ophthalmology. 2005; 112:84 - 91)
. Progressive eyewear lenses, the port of rigid contact lenses, the day and/or night (Orthokeratology) has been studied, the results are preliminary but encouraging)Gwiazda. Treatment Options for Myopia. Optom Vis Sci. 2009; 86 (6): 624-628)
In the future, strategies combining one or more of these approaches will certainly help to slow or stabilize the extension of myopia in some populations. One page is specifically devoted to the factors to slow down myopia.