Myopia: causes, risk factors
Causes and factors associated with myopia
The causes and factors associated with the development of myopia have been the subject of many studies for several decades. The'Increase in the frequency of myopia In the population of industrialized countries, particularly In Asia Where we talk about a real ' Outbreaks of myopia", encourages to find the causes and mechanisms involved in theexcessive elongation Of the eye, which is the main anatomical factor responsible for myopia.
Before reviewing the main factors, it should be recalled that at the present time the mechanisms involved in the onset and progression of myopia remain for many mysterious. Parcel information has been acquired over time, but it is difficult to establish cause-and-effect linkages, sorting between risk factors and causal mechanisms. This explains that there is still a need to devise effective strategies that would prevent or slow myopia. The Genetic studies Have revealed more than twenty gene loci that are able to promote the occurrence of myopia (some on the Chromosome 15): But the genetic traits identified Certainly interact with the environment To modulate the expression of the myopia phenotype.
The study of the causes and risk factors of myopia is punctuated by twists and turns; Some theories feed on the results of experimental studies, and suggest in return the realization of new experiments... whose conclusions sometimes go against the initial postulate. There are still many mysteries to be pierced to understand the origin of myopia. At the end of the years 90 and the beginning of the years 2000, factors Lifestyle related As the Time spent outside in childhood were put forward as Protective elements against myopia. Thus, as some previous studies have already suggested, the time spent practicing outdoor activities in childhood and adolescence would have a protective effect on the incidence of myopia error (Guggenheim JA, Northstone K, McMahon G, Ness AR, Deere K, Mattocks C, St Pourçain B, Williams C. Time outdoors and physical activity as predictors of incident myopia in childhood: A prospective cohort study.) Invest invest Vis Sci. 2012 apr 6.
A study published in May 2015 confirms that The European continent is also reached by this "epidemic of myopia", and that the Level of education is positively correlated with myopia (Williams KM, Bertelsen G, Cumberland P, et al. Increasing prevalence of myopia in Europe and the Impact of Education. Ophthalmology. 2015 May 13).
NB: Listen to the programme of France Culture «public Science», of 22 May 2015, dedicated to the "epidemic of myopia" that this European multicentric study highlighted old continent.
See the documentary "Tomorrow, All myopic", Arte, Réal: F. Papillon & C. Killian, scientific adviser of Ganeshan
Listen to the show "From cause to effect, environmental magazine", France Culture, January 14, 2018
Listen to the show " Great good you do«, France Inter, 15 January 2018, devoted to myopia
Myopia and Visual deprivation
Experimental studies conducted in animals revealed an important principle: the induction of a Reduced vision (occlusion) or Fuzzy (Interference with improper corrective glass) in the animal results in a Eye elongation ; We're talking about nearsightedness of sensory deprivation. This phenomenon of myopia of deprivation concerns both chickens, guinea pigs, dogs or great apes (Irving et al. Inducing myopia, hyperopia, and astigmatism in chicks. Optom Vis Sci 1991; 68: 364-368 / Sivak et al. Bilateral experimental myopia in chicks. Optom live Sci 1989; 66: 854-858. 87 / HD & McFadden SA Howlett. Form-deprivation myopia in the guinea pig (Cavia porcellus). Vision Res 2006; 46: 267-283 / DL, Greene PR & Scholz RT Guyton. Dark-rearing interference with emmetropization in the rhesus monkey. Invest invest live Sci 1989; 30: 761-764).
In addition, if the 'sensory deprivation' is interrupted early enough, so a move towards the progressive correction of the refractive defect induced can be observed; for example, the eyeball growth slows)Troilo and Wallman. The regulation of growth and refractive eye state: year experimental study of emmetropization. Vision Research, 1991; 33:1237 - 1250).
It is suspected that the induction of a Extended retinal Blur Could regulate the development of myopia in animals (Wildsoet CF. Active emmetropization - evidence for its existence and ramiﬁcations for clinical practice. Ophthalmic Opt 1997 Dec; 17: 279-290). The fact that this phenomenon is widespread within a large number of animal hopes suggests that it is involved in human myopia. These data demonstrate in all case that there is one (or more) "vision-dependent" mechanism involved in the development of refractive error such as myopia. A "Error Signal" retinal Would be at the origin of a physico-chemical mechanism causing elongation of the eyeball.
After the age of 6 years, the corneal curvature changes more (Zadnik K, Manny RE, Yu JA, et al. Ocular component data in schoolchildren as a function of age and gender. Optom Vis Sci. 2003;80:226–236) , and so the onset of myopia evolving into adulthood is due to elongation of the eyeball.
Things are not so clear, because if we generalise this reasoning, the blur induced by a strong hyperopia (inverse defect from that of myopia, the eye is too short) should lead to the gradual correction of it. This was not observed in the animal, or the blur associated with high Hypermétropique defocusing does not appear to trigger excessive eye elongation; So there are other factors that are able to modulate this control loop.
The optical correction of nearsighted children should not be "corrected" (child remaining slightly nearsighted), or sur-corrigée (child becoming slightly farsighted with its correction in glasses or lenses), in order to allow the retinal image to be optimal. The concept of deprivation myopia caused some theories to explain the causes of the development of nearsightedness, whose theory related to a lack of accommodation.
Myopia and accommodation
THEaccommodation is the ability of the eye to perform the focus to see net of close targets (e.g. reading book). Accommodation has been incriminated as possible causal or triggering factor of myopia. At the origin of this suspicion was the fact that myopia is often found in individuals doing work closely in a prolonged manner, and that a certain degree of "accommodative laziness" ("accommodative lag" in English) had been put into Evident in myopic (Gwazda et al. Myopic children show insufficient accommodative response to blur. Invest invest Vis Sci, 1993,34: 690-694).
A lack of accommodation that could induce "retinal blur", the current myopia could be the fruit of a mechanism close to that involved in the myopia of "deprivation". Accommodation was also suspected of increasing the intraocular pressure, and induce a gradual distension of sclera, both of which are synergistic to increase the axial length of the eye and thus promote myopia. However, no study has shown that improving the quality of accommodation (e.g. progressive glasses in children) does not slowing down myopia. It is possible that short-sighted accommodate less because accustomed to perceiving the world in a fuzzy way, they benefit from neuro-cortical strategies to better interpret the retinal images and thus tolerate a slight blur close. If there is an association between myopia and accommodation, the existence of a causal link has not been demonstrated.
Myopia and outdoor activities
From 2008, an Australian study showed that children who practice littleOutdoor Outdoors Activities Are on average more myopic (or less farsighted) than those who practice outdoor activity on a regular and prolonged basis. Children who in addition to a low outdoor activity regularly practice activities that seek close vision (reading, video games on portable consoles, etc.) had Three times more likely to be myopic That those who practice a lot of outdoor activities and little reading activity (Rose et al. Outdoor activity reduces the prevalence of myopia in children. Ophthalmology 2008 115: 1279-1285).
These data have been backed up by the results of a study led to Singapore on teenagers, who were all less myopic (or more farsighted) that they were practicing outdoor activities)Dirani et al. Outdoor activity in andmyopia Singapore teenage children. BR J Ophthalmol2009; 93: 997-1000).
It seems to be the Time spent outside Which is a protective element against myopia, rather than the practice of sport in itself (no influence of indoor sport on the prevalence of myopia). This was corroborated more recently by the study by Guggenheim et al. (Guggenheim JA, Northstone K, McMahon G, Ness AR, Deere K, Mattocks C, St Pourçain B, Williams C. Time outdoors and physical activity as predictors of incident myopia in childhood: A prospective cohort study. Invest invest Vis Sci. 2012 apr 6. [Epub ahead of print])
In addition, the influence of a reduction of the lighting on the progression of myopia has recently been identified in an animal model)Cohen et al. Dependency between light intensity and refractive light-dark cycles under development. Exp Eye Res 2011; 92: 40-46).
The biological mechanism at the origin of this protective effect is not clearly identified; It could involve a reduction of the retinal blur (Pupillary constriction linked to the high brightness and improvement of the depth of field). The secretion of Dopamine At the retinal level is stimulated by light, and dopamine has an inhibitory effect on ocular growth (ashby RS & Schaeffel F. The effect of bright light on lens compensation in chicks. Invest invest Vis Sci 2010; 51: 5247-5253).
Finally, in addition to the light intensity gap between the outside and the interior, the difference in spectral composition between natural and artificial light could play a role in the genesis of myopia (reduction in Blue Light which is the most abundant radiation in the outdoor environment - see below). These data suggest a causal relationship between the occurrence or worsening of myopia and the less time spent in outdoor activities for children.
A study conducted in October 2012 reported a very low rate of myopia in a population of schoolchildren in the province of Vintiane in Laos (Southeast Asia): The prevalence of refractive error (hyperopia, myopia, astigmatism) was 3.6% for a cohort of 2842 children (6 to 11 years) examined: The overall prevalence of myopia was only 0.8%. The children were of different ethnicities; Of these, the Yao (South China Population group) accounted for a risk factor of 11.3 for myopia, emphasizing the likely influence of Genetic factors. Nevertheless, the authors of this study stress that its results are consistent with those of previous studies, because the Lao children studied did not seek much their vision closely, compared to those of other countries with the infrastructure More developed schools (little or no school books, blackboard teaching) (Casson et al. Exceptionally low prevalence of refractive error and visual impairment in schoolchildren from Lao people democratic republic.) Ophthalmology, 2012:119:2021 - 2027).
Finally, a meta-analysis (a global analysis carried out from the collection of about twenty studies related to the subject) was also published in October 2012: It confirms the protective effect on the onset and progression of the myopia of time spent in Outside, suggesting that to curb myopia, it might be appropriate to have children practice outdoor activities (e.g. one hour per day of outdoor activity as an outdoor sport). However, the authors of the study remain cautious about the possibility of misinterpretation of their causal result: it might be possible for the association between time spent outside and less myopia to be linked to the fact that the Myopic children prefer other types of recreation and are less inclined to practice some outdoor activities (not to mention the fact that urban populations, where the prevalence of myopia is stronger, have fewer outside spaces). However, this meta-analysis reinforces the hypothesis of a Beneficial effect of regular outdoor activities To reduce the onset of myopia (Sherwin and al. The association between time spent outdoors and myopia in children and adolescents. Ophthalmology, 2012; 119:2141 - 2151). These activities however do not seem to reduce the speed of evolution of the short-sightedness once it installed)Wu PC et al. Outdoor activity during class recess reduces myopia onset and progression in school children. Ophthalmology. 2013; 120 (5): 1080-1085).
A randomized prospective study was conducted in China in children of 6 years; less impact of myopia was observed after 3 years, for the Group of children for whom a daily outdoor activity of 40 minutes had been established. (He Mr. Effect of Time Spent Outdoors at School on the Development of Myopia Among Children in China: A Randomized Clinical Trial.) JAMA. 2015,15; 314 (11): 1142-8.)
More recently, the value of time spent on outdoor activities among children aged 3 to 9 years was confirmed by another study carried out in Europe (Shah RL, Huang Y, Guggenheim JA, Williams C. Time Outdoors at Specific Ages During Early Childhood and the Risk of Incident myopia. Invest Ophthalmol Vis Sci. 2017,1; 58 (2): 1158-1166).
Myopia and activities close (reading)
An Australian study showed that the Work closely, quantified by time spent and reading distance, had an indisputable influence on the existence of myopia in children aged 12 years. Children who Were reading more than 30 minutes a day Continuously with more risk of being myopic than those who read less than 30 minutes a day. Those reading less than 30 cm had a two and a half times greater probability of being myopic than those who read at a greater distance, and tended to exhibit a higher myopia (IP et al. Role of near work in myopia: ﬁndings in a sample of Australian school children. Invest invest live Sci 2008; 49: 2903-2910). Another study showed that children who read more than two pounds per week on average were more myopia (nearsightedness greater than 3 diopters) than those who read less)Saw et al. Component dependent risk factors for ocular parameters in Singapore Chinese children. Ophthalmology 2002; 109:2065 - 2071). The axial length averaged 0.17 mm larger in children who read more than two pounds per week.
It is necessary to keep from inferring from this work that reading promotes the onset of myopia; These studies only show that there is more chance of finding myopic children among readers who are more avid, and/or practicing less outdoor activity. Moreover, a longitudinal study conducted for 5 years on a sample of 1318 children aged 6 to 14 showed that the number of weekly hours of reading or computer screen work was not different before the occurrence of myopia, Between children who became myopic and those who were not. This study tends to show that there is no link between the causal work closely with development of myopia)Jones-Jordan et al. Visual activity before and after the onset of juvenile myopia. Invest invest live Sci, 2011; 52: 1841-1850).
Yet, various mechanisms to explain how close extended vision could induce myopia have been proposed, as the occurrence of an increase in intraocular pressure (action of the ciliary muscle, which contracts and increases intra injection pressure), or the action of the extra eye muscles on the sclerale wall, etc.
The duration of studies would alone be a risk factor for myopia, more than the coginitives performance itself, according to a study published in 2016 (Mirshahi A, Ponto KA, Ubert-Reh D, Rahm B et al. Myopia and Cognitive Performance: Results From the Gutenberg Health Study. Invest Ophthalmol Vis Sci. 2016,1; 57 (13): 5230-5236).
The explosion of mobile devices contributes to the time spent reading or viewing content "in near vision". Between 2009 and 2013, the number of smartphones used by the Korean population increased from 500,000 to 33 million (Korea Communication Commission. Statistics on wired and wireless communication service subscribers, 2013). There may be differences between the type of electronic terminal used and the myopic risk (more pronounced when caseusing a smartphone than a tablet (Liu S, Ye S, Xi W, Zhang X. Electronic devices and myopic refraction among children aged 6-14 years in urban areas of Tianjin, China.Ophthalmic Physiol Opt. 2019 Jul;39(4):282-293).
Myopia and blue and violet light
Incriminating (without formal evidence) as a risk factor for certain dégénratives ocular pathologies such as AMD (age-related macular dégénrescence), Blue Light, which includes radiation from the most energetic visible light spectrum (the shortest wavelengths are perceived as violets – 360 to 400 nanometers) could be a factor whose chronic deficit is a risk factor for the onset and The evolution of myopia (Torii, H. et al. Violet Light Exposure Can Be a Preventive Strategy against myopia Progression. Ebiomedicine, 2017:15, 210 – 219) . Thus, myopia has evolved less rapidly in a group of myopic children equipped with transparent corrective contact lenses for Violet Light (some lenses, such as Aqua/Alcon Dailies are transparent to violet, while others, such as Acuvue True eye/Johnson and Johnson, filter about 50%). These results are strengthened by those of a study carried out in strong myopic adults operated by the installation of two types of Phake implant (ARTISAN ® vs ARTIFLEX ®). Patients receiving the ARTIFLEX implant benefited from lesser elongation of the eyeball over time: However, the transmission of this implant for Violet light is greater than that of the ARTISAN (Torii H, Ohnuma K, Kurihara T, Tsubota K, Negishi K. Iolet Light Transmission is related to myopia Progression in Adult High myopia. Sci Rep. 2017 6; 7 (1): 14523). These results corroborate observations that point towards an association between the risk of myopia and the time spent outdoors: in fact, short blue (and violet) radiations are not present in confined environments, but abound in Outside. They strongly question the interest of blue filters on eyeglass glasses and the lenses prescribed for the correction of nearsightedness in eyeglasses in children and young adults (but could logically be case Preferentially indicated for the correction of the hyperopia, the defect opposite that of myopia, and where it is hoped an elongation of the eyeball.
Myopia and computer
Many studies point The growing time spent on screen, especially by children and teenagers: the time spent viewing a computer screen or smartphone (time spent on the Internet) for younger generations is now higher than that spent in front of television. Consulting a computer or tablet screen will require close vision, and should logically be associated with an increased prevalence of myopia. A study was conducted between February 2010 and March 2011 in Taiwan by measuring the refraction of military conscripts aged between 18 and 24 years. A biometrics (measuring the length of the eye) was also carried out. For the diagnosis of myopia, the refraction had to be less than 0.5 diopters (D). A total of 2316 participants were registered. The prevalence of myopia was 86.1% with an average refractive error of-3.66 D, and an average axial length of 25.40 mm. Frequent consultation of the computer has been found associated with a higher axial length: this correlation is quite eloquent considering that myopia is primarily related to an excess of growth and axile length of the eyeball. Other risk factors associated with myopia were: parental myopia, higher levels of study, more time spent reading, shorter reading distance, less time spent in outdoor activities, (Lee and al. What factors are associated with myopia in young adults? A survey study in Taiwan Military Conscripts. Invest invest Vis Sci. 2013 Feb 5; 54 (2): 1026-33)
Myopia and month of birth
Some studies have shown a link between month of birth and risk of developing myopia. Indeed, the prevalence of myopia is higher in subjects born in spring or summer than in those born in autumn. The mechanics behind this variation in vision are not well understood, and may be related to variations in day length during the development of vision in infants (see:Lee DC et al. An epidemiological study of the risk factors associated with myopia in young adult men in Korea.Sci Rep. 2018 Jan 11;8(1):511). This data suggests that the development of myopia may be influenced by the conditions surrounding the first few weeks of life (Matsuda K, Park K. Recent trend of increasing myopia can be traced to infancy. Med Hypotheses. 2019 Jul;128:78-82).
Myopia and level of education
Many studies have shown a strong Positive correlation between level of education, graduate studies and myopia (Katz et al. Prevalence and risk factors for refractive errors in year adult inner city population. Invest invest live Sci 1997; 38: 334-340 / Wang et al. Refractive status in the Beaver Dam Eye Study. Invest invest live Sci 1994; 35: 4344-4347 / Tay et al. Myopia and educational attainment in 421,116 young Singaporean males. Ann Acad Med Singapore 1992; 21: 785-791).
It was even reported a positive correlation between the total duration of the studies and length of the eye at a Chinese origin Singaporeans: the increase in the length of the eye was 0.60 mm for each decade of study)Wong et al. Variations in ocular biometry in year adult Chinese population in Singapore: the Tanjong pay Survey. Invest invest live Sci 2001; 42: 73-80).
A Relationship between myopia and education was also found in European myopic (Williams KM1, Bertelsen G2, Cumberland P3, et al. Increasing Prevalence of Myopia in Europe and the Impact of Education. Ophthalmology. 2015 May 13)
The North American continent is no exception to this association between myopia and level of education (Nickels S et al Myopia is associated with education: Results from NHANES 1999-2008. PLoS One. 2019 Jan 29;14(1)). The absence of correlation between study level and corneal curvature suggests that the mechanisms involved in this association influence the ocular elongation or optical power of theCrystalline lens.
The superior study level and the duration of the studies are certainly closer to the risk represented by reading factor. However, there are also the hypothesis of common genes for intelligence and myopia.
Myopia and Anthropometry
In children and adolescents, a Stature (size) More important was also found to be a risk factor associated with myopia. The correlation between size and myopia was absent in adults (Rim TH et al. Body Stature as an Age - Dependent Risk Factor for Myopia in a South Korean Population.) Semin invest. 2016 apr 8:0.). This study found a correlation between level of education and myopia among adults (South Korea). The increased prevalence of myopia is found in patients with Marfan syndrome, which also exhibit a higher average size (TR1, Zetterstrom Konradsen C.) Descriptive study of ocular characteristics in Marfan syndrome. ACTA invest. 2013; 91 (8): 751-5). Despite a sometimes significant elongation of the eyeball, the degree of high myopia is not significantly higher due to a lower average keratometry (corneal power reduction partially offset the increase in the ocular axial length).
Nevertheless, it appears that the prevalence of high myopia is higher in taller, thinner patients (low BMI: body-mass index) (Lee DC et al. An epidemiological study of the risk factors associated with myopia in young adult men in Korea.Sci Rep. 2018 Jan 11;8(1):511). On the other hand, obesity would be more correlated with the risk of hyperopia; the presence of excess orbital fat preventing the elongation of the eyeball has been cited to explain this feature.
Myopia and intelligence
Many studies report the existence ofAssociations between myopia, intelligence, academic success e.g: Williams et al. Comparison of measures of reading and intelligence have risk factors for the development of myopia in a UK cohort of children. BR J invest. 2008; 92 (8): 1117-21). Short-sighted have on average higher scores on intelligence tests and test (cognitive)Cohn and al. Myopia and intelligence: a pleiotropic relationship? UM. Genet, 1988; 80: 53-58).
These results suggest a genetic part in myopia, because intelligence is partly transmitted hereditary way. However, it cannot be concluded that there would be one or more common genes between intelligence and myopia. In most published studies, tests where short-sighted are the most successful are those who rely mainly on tests where reading and language play a prominent role. In fact, studies also show that short-sighted read more and longer than non short-sighted, and the influence of reading in these results is certainly present. A possible but unproven theory suggests that the short-sightedness and intelligence may share a common genetic background. The naturally gifted intellectually people read more, which would explain the link between myopia and extended reading. Reading certainly promotes the intellectual, and if a cause and effect link between myopia and extended reading (an association demonstrated between reading and myopia is without prejudice of a direct causal link), it could also explain these data, without resorting to genetics.
Myopia and sex ratio
Are you there more myopic among boys or girls? Published studies do not determine with certainty, and the results found in an age category are not necessarily valid in a sample of age (different)JM Hirsch. The change in refraction between the ages of 5 and 14, theoretical and practical considerations. Am J Optom Arch Am Acad Optom, 1952, 29:445 - 459). A more recent study found a prevalence of nearsightedness greater among men in the population aged)Tan et al. Prevalence and risk factors for refractive errors and ocular biometry parameters in year elderly Asian population: the Singapore Longitudinal Aging Study (SLAS). Eye, 2011, 25 (10): 1294-301).
Short-sightedness of the parents
An Australian study showed that the Presence of myopia in one of the parents Multiplied by two the risky for a child to be myopic. When both parents are myopic, the risk is multiplied by eight! This risk is also greater if the myopia of one or more parents is pronounced (IP and colll. Ethnic differences in the impact of parental myopia: ﬁndings from a population-based study of 12-year-old Australian children. Invest invest live Sci 2007; 48: 2520-2528.73).
The axial length of the eyes of children is greater when parents are myopic)Zadnik et al. The effect of parental history of myopia we children' Sandy size. JAMA 1994; 271: 1323-1327). If the prevalence of myopia is stronger in myopic children, it is tempting to blame the influence of genetic factors. However, because other factors of risk above mentioned, myopic parents tend to raise their children in an environment may be more "myopiegene": fewer outdoor activities, more intellectual reading activities)Edwards MH. Effect of parental myopia on the development of myopia in Hong Kong Chinese. Ophthalmic B.j. Opt 1998; 18: 477-483). The protective effect of the outdoor activities seems to exercise as much about the children of myopic parents than not myopic parents.
Myopia and peripheral refraction
The fovea is the seat of the fine vision, and occupies about 1 degree in the Center of the visual field. Myopia is defined as an excessive refraction of the light rays emitted by the distant sources observed (the image of these sources is then projected on the fovea). The vision embraces the entire visual field and the sources located outside the central degree are also imaged on the retina. The way in which the light rays emitted by peripheral light sources (ex: 15 ° eccentricity) are focused on the extra fovea retina concerns the study of the Peripheral refraction (as opposed to the central fovea refraction). Experimental studies conducted in animals have shown that the peripheral retina plays an important role in the process of emmétropisation (control of the growth of the eyeball designed to maintain a balance between focal power of the couple Cornea Crystalline lens and distance from the retina) (Smith et al. Effects of foveal ablation we emmetropization and form-deprivation myopia. Invest invest live Sci 2007; 48: 3914-3922).
On average, peripheral refraction is less myopic (more hyperopic) than central refraction. A study conducted in 822 children aged between 5 and 14 years showed that the eyes of myopic children have a higher relative hyperopia in the periphery than the eyes of children Emmétropes and farsighted) (Mutti et al. Peripheral refraction and ocular shape in children. Invest invest live Sci 2000; 41: 1022-1030). These findings may seem surprising, but optically, due to the peripheral retinal curvature, and the impact of peripheral sources from rays, these can then quite be defocused more 'far' behind the retinal plan (peripheral farsightedness). This peripheral defocus is potentially the source of eye elongation, and a development of myopia.
The ethnic differences between Asian and African children of origin have been found in a U.S. study: the peripheral refraction of eyes of Asian children was generally more hypermetropique than even myopic African children. (Mutti et al. Refractive error, axial length, relative and peripheral refractive error before and after the onset of myopia. Invest invest Vis Sci 2007; 48: 2510-2519).
A more hypermetropique deviation of the peripheral refraction has been also found at children and nearsighted adults of Chinese origin. On the other hand, the farsighted eyes of Chinese adults and children tended to have a more myopic peripheral refraction)Chen et al. Characteristics of peripheral refractive errors of myopic and non-myopic eyes Chinese. Vision Res 2010; 50: 31-35). These data were confirmed by a study recently conducted in children of Chinese origin in Singapore)SNG et al. Peripheral refraction and refractive error in singapore chinese children. Invest invest live Sci, 2011; 52: 1181-1190).
Peripheral refraction is influenced by various factors, including the corneal Asphericity (variation of the edge curvature of the cornea). Only longitudinal studies could accredit the responsibility of asphericity of the cornea in the genesis of myopia. A longitudinal study recently conducted in 187 children ages averaged 7.2 years showed that during their follow-up, that the initial peripheral refraction had no influence on the risk of developing myopia. On the other hand, become myopic eyes have developed a more hypermetropique peripheral refraction at 30 ° in nasal and temporal thus, the initial state of the peripheral refraction does not appear to affect the risk to become myopic, but on the other hand, the appearance of myopia seems to alter peripheral refraction.
Myopia and cataract
Myopia seems to favour the occurrence of certain types of cataracts such as nuclear cataract, and the posterior capsular cataract (see page on cataract). This association has been underlined by two studies (recent)Pan et al. Myopia and Age-related Cataract: A Systematic Review and Meta-analysis. Am J invest. Aug 9, 2013 and Pan and al. Myopia, axial length, and age-related cataract: the singapore malay eye study. Invest invest Vis Sci. 2013 Jul 2; 54 (7): 4498-502). Cataract may in turn increase initially axile myopia (related to the excessive length of the eye). Myopia increase mechanism is related to the increase in the index of refraction of the cristallinien kernel: we talk about "index myopia" (see the page dedicated to the) various types of myopia). Thus, myopia argues with the occurrence of early cataract and faster evolution. Cataract surgery can correct or reduce myopia, by the use of a suitable power implant.
Contact lenses and myopia
The "brake" effect on the myopia of a correction in Contact lenses Rather than in glasses is often debated. Some studies have not been able to show any statistically significant difference (DG, Soni PS, TO Salmon and al Horner. (1999) progression Myopia in adolescent wearers of soft contact lenses and shows. Optom Vis Sci 1999; 76: 474-9 - Katz J, Schein OD, Levy B et al. A randomized trial of rigid gas permeable contact lenses to reduce progression of children's myopia. Am J invest, 2003; 136, 82-90). Some studies even suggest an increase in the speed of development of myopia in the adolescent from glasses to a correction in lens correction)Fulk GW, Cyert the Parker of et al. The effect of changing from glasses to contact lenses soft we myopia progression in adolescents. Ophthalmic B.j. Opt, 2003; 23, 71-7 - Marsh-Tootle WL, Dong LM, Hyman L, et al. Myopia progression in children wearing shows vs. switching to contact lenses. Optom live Sci, 2009; 86, 741-7). Yet the equipment in conventional contact lenses could alter peripheral refraction, inducing a peripheral myopisation rather than a peripheral hypermetropisation, and thus have a beneficial effect on the development of myopia. This theory is at the origin of the development of contact lenses, to the gradient of curvature amended to accentuate this effect of peripheral myopisation multifocal.
A study of homozygous twins suggested that bifocal lenses may have a beneficial effect by slowing down the development of myopia)Hasebe S, Nakatsuka C, Hamasaki I et al. Downward deviation of progressive addition lenses in a myopia control trial. Ophthalmic B.j. Opt, 2005; 25, 310-14). A laboratory (CooperVision) has developed "double focus" lenses ' MiSight), by the juxtaposition of areas of varying powers, which create a peripheral retinal myopisation, in order to slow the growth of the eye. A study showed that this equipment could slow the growth of the eye ((0.11 ± 0.08mm in avec la lentille à double focus versus 0.22 ± 0.09mm avec les lentilles conventionnelles) ())Anstice NS, Phillips JR (2011) Effect of dual-focus soft contact lens wear on axial myopia progression in children. Ophthalmology, 2011; 118, 1152-61).
Myopia and Orthokeratology
The Orthokeratology is to Wear rigid lenses at night, so as to induce corneal deformation (central flattening) and correct myopia, because the effect of the nocturnal port tends to persist during the day, after removing the lenses in the morning. It is difficult to validate the hypothesis of a braking effect of orthokeratology on myopia, due to methodological problems (absence of control group, transient modification of ocular geometry, etc.). In addition, Orthokeratology exposes microbial complications, particularly in children (Watt K, Swarbrick HA. Microbial keratitis in overnight orthokeratology: review of the first 50 casees. Eye Contact Lens, 2005; 31, 201-8). The assumed braking effect would result from the change in asphéricité of the cornea (less prolate/More oblate geometry), allowing for the inducing of peripheral retinal myopia. Recent studies have certified a beneficial effect of orthokératomogie on the ratentissement of myopia (Leo SW. Current approaches to myopia control. Curr Opin Ophthalmol. 2017; 28 (3): 267-275).
Myopia and keratoconus
The keratoconus represents a particular situation where myopia is acquired later in life (late adolescence, young adulthood) and progresses rapidly. Keratoconus can occur in initially myopic, emmetropic or hyperopic eyes. Myopia induced (or accentuated) by the keratoconus is not related to an elongation of the eye but to the increase in corneal curvature which partly defines the clinical picture of this condition. The increase in the optical power of the central region of the cornea leads to the appearance of myopia (excess corneal vergence with respect to the axial length, whereas the eye concerned had been able to grow without excess and reach emetropia, or to be affected by stabilized myopia).
Recent and sometimes contradictory studies show that the dimensions of the eyeballs are not very different overall from those of healthy subjects, particularly with regard to the posterior segment of the eye (Jian W et al Ocular dimensions of the Chinese adolescents with keratoconus.BMC Ophthalmol. 2018 Feb 13;18(1):43) (Lanier JD et al. Axial length in keratoconus.Cornea. 1992 May;11(3):250-4.)
Patients with advanced keratoconus all have strong myopia of corneal origin: an increase in corneal curvature of 10 Diopters (e. g. keratometry from 43D to 53D) is responsible for a comparable increase in the spherical myopic equivalent. According to the author of this site, keratoconus is caused by eye rubbingexcessive and prolonged. The impact of these frictions on the cornea is responsible for progressive thinning and deformation; their effect on the posterior segment is poorly documented. The classical theory, which states that keratoconus results from a deformation linked to a primitive biomolecular alteration of corneal tissue, is invalidated by the observation of ocular manifestations of Marfan's disease. During this condition, characterized by protein damage responsible for a progressive loss of the elasticity properties of the supporting and ligamentary tissues, the eye becomes distended overall. In addition to axial myopia (which is important for the clinical diagnosis of the condition), there is thinning and flattening of the cornea, not an increase in its arch... as expected due to progressive distention of the corneal stroma.
The precise mechanisms responsible for the excessive elongation of the eyeball, which is responsible for most myopias, remain largely unknown. Studies suggest that epidemiologically, children of Asian origin are more frequently affected by myopia, and that there is a positive (not proven causal) link between prolonged reading and myopia, as well as a negative link between outdoor activities and myopia. The population of the European continent is proportionally less affected by myopia than that of the Asian continent, but the prevalence of myopia is increasing. Outdoor activities could have a protective effect, and it seems reasonable not to deprive our children of their myopia.