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Introduction
Myopia that generates a high risk of ocular morbidity is a condition characterized by excessive elongation of the eyeball (axial length (AL) >26 mm), associated with pathological changes in the fundus of the eye, giving rise to refractive errors with a spherical equivalent (SE) of -6.00 D or more1-3, so it is also known as high myopia. As posterior staphylomas have been described in eyes without high myopia, an international panel of researchers recently proposed a classification system in which pathological myopia was defined as eyes with chorioretinal atrophy equal to or more severe than diffuse atrophy, i.e., without considering the magnitude of the refractive error or the increase of AL4. Other common names are myopia magna or degenerative, progressive, or malignant myopia.
Due to degenerative changes, this type of myopia causes an irreversible decrease in best-corrected visual acuity5, which is why it causes low vision and blindness, especially in Eastern Asia4, although the evidence indicates that also in Europe (7%) and in other Asian populations (12-27%)6. It is considered as the third most frequent cause of blindness or low vision, after glaucoma and cataracts7,8.
Therefore, the objective of this review was to describe the characteristic anatomical alterations of eyes with high myopia and to identify the prevalence of this condition in different populations.
Anatomical alterations in pathological myopia
When there is progression of myopia, there is an axial elongation of the eyeball, which exerts a biomechanical stretch in the posterior pole, which leads to pathological changes in the posterior pole and in the peripheral retina, such as myopic crescent and tiger-striped appearance, posterior staphyloma, lacquer cracks, choroidal neovascularization (CNV), retinoschisis, epiretinal membrane, myopic maculopathy, dome-shaped macula and peripheral retinal lesions9.
Myopic crescent and tiger-striped appearance
The scleral expansion that results from axial lengthening leads to a depigmentation around the optic disk called myopic crescent, and the most common presentation is in the temporal quadrant. These structural changes lead to retinal pigment epithelium atrophy, which causes a striated appearance of the fundus10.
Posterior staphyloma
Posterior staphyloma is a protrusion of a delimited area of the posterior segment of the eye11. It has been identified that there are differences in the prevalence of macular anatomical alterations in high myopia with posterior staphyloma (53.65%) or without it (22.41%), and that the most frequent findings in eyes with staphyloma are foveoschisis, vascular traction and epiretinal membrane12. These changes predict alterations that threaten vision considerably. It has been identified that the prevalence of posterior staphyloma is 10%, a figure that increases with age13,14.
There is an objective classification of posterior staphyloma based on 3D magnetic resonance, with 5 types according to the appearance of the edge and the location of the staphyloma, including whether it is wide (I), narrow (II), peripapillary (III), nasal (IV), inferior (V) or others. The most common type of posterior staphyloma is type I (74%). However, the described findings do not occur in all eyes with higher AL11.
Lacquer cracks
The mechanical stretching of the choroid produces ruptures in Bruch's membrane in the macular area, whose appearance is yellow and linear, reticular or stellate. Its presentation is more frequent in the temporal quadrant (44%) and occur generally in adults with an AL >29 mm15.
Choroidal neovascularization (CNV)
Pathological myopia is one of the main causes of CNV under 50 years of age (62%)16. It consists of a flat, gray-colored subretinal membrane17, associated with abnormal growth and invasion of choroidal vessels through Bruch's membrane18. It has a high impact on visual acuity and approximately 5% to 11% of subjects with degenerative myopia develop CNV19. Although CNV can occur in any degree of myopia and even without the presence of the typical fundus changes, irregular atrophy and lacquer cracks may be predisposing factors for CNV development20,21.
Retinoschisis
Myopic macular retinoschisis is described as a traction maculopathy, which appears more commonly in the external plexiform layer and, less frequently, with detachment of the internal limiting membrane22, which can lead to vitreous detachment, macular holes and foveal detachment. This complication can occur in 9% of highly myopic eyes with posterior staphyloma10,21.
Epiretinal membrane
It is composed of three layers, of which the innermost is composed of vitreous, the intermediate by fibroblast-like cells, and the outermost corresponds to the internal limiting membrane of the retina. A prevalence of 11.2% was found in eyes with high myopia associated with posterior staphyloma12.
Myopic maculopathy
The presence of maculopathy may involve irreversible changes in visual acuity. The signs begin with anatomical changes in the choroid and retina, followed by a compromise of the macular area9. Although the incidence is of 0.05%, the rate of progression in patients with maculopathy can rise to 35%23.
There is a current classification proposed in the META-PM study, with 5 categories according to the presence of pathological changes in the posterior pole. The classification includes: no myopic retinal lesions (Category 0), tessellated fundus only which indicates a mosaic color pattern in the fundus (Category 1), diffuse chorioretinal atrophy with a yellowish appearance generally surrounding the optic disc (Category 2), patchy chorioretinal atrophy with a defined grayish-white lesion in the macular area or around the optic disc (Category 3), and macular atrophy (Category 4)24.
Dome-shaped macula
It is characterized by an anterior protrusion of the macular area. The incidence of this complication is of 8%25. The most relevant pathological causes are related to choroidal or scleral thickening10,26 and serous retinal detachment27, attributed to choroidal vascular changes28.
Peripheral retinal lesions
The peripheral retina may be affected in high myopia, with anatomical changes such as palisade degeneration (lattice) and peripheral tears29 both of which are risk factors for rhegmatogenous retinal detachment, if the vitreous passes through the holes or tears and separates the retina from the retinal pigment epithelium. It is frequent that the degeneration has the appearance of an elongated area of internal retinal thinning, with its axis parallel to the ora serrata. In some cases, crossed white lines are observed, which correspond to vessels covered with glial or hyaline tissue. The prevalence of these lesions may be up to 8% in the general population, and of 15.6% in eyes with high myopia30-32.
Methodology
To identify the observational studies on the prevalence of high myopia, a search was made in Medline (PubMed) using the terms «myopia AND (prevalence OR epidemiology)» supplemented with the references of systematic reviews on the topic6,33,34. The search for information specifically in Spanish or Portuguese was done in LILACS.
Prevalence data were based on studies since 1990, with populations of more than 1,000 subjects. If the studies reported crude and adjusted prevalence rates, the latter were considered with their respective confidence intervals. The global prevalence of high myopia and some figures disaggregated by sex or race are reported.
Prevalence of high myopia
A 2016 meta-analysis that used data published since 1995 estimated that in 2000, there were 163 million people with high myopia (SE <-5.00 D), corresponding to 2.7% of the world population at the time (95% CI: 1.4-6.3). With these values, for the year 2050 a prevalence increase to 938 million people was estimated (9.8%, 95% CI: 5.7-19.4). It is estimated that by 2020, 2030, 2040 and 2050 the prevalence of degenerative myopia will be 5.2, 6.1, 7.7 and 9.8%, respectively, with significant regional differences33.
The difference in the frequency of high myopia between different groups has been evidenced: in Caucasians the figures range from 2.71 to 7.8%, in Asians and Indians, from 1.8 to 21%, and in Africans, from 4.3 to 5.5%35.
The prevalence of high myopia (≤-6.00 D), standardized by age, is relatively low in Europe (2.7%, 95% CI: 2.69-2.73), with higher values in young people between 15 and 19 years old (5.9%, 95% CI: 1.3-10.5), according to a meta-analysis based on 15 studies and grouping a total of 61,946 subjects34. The study published by the UK Biobank Eye and Vision Consortium, which included subjects from different races, found a prevalence of high myopia three times higher in Chinese. The modelling adjusted for gender, age and educational level, among other variables, indicated that people of Chinese origin are between 2.6 and 4.5 times more likely to suffer from high myopia compared with Caucasians36.
In U.S.A. the prevalence of high myopia has been reported 5 times higher in Chinese subjects compared to Hispanics. This is the same tendency observed for myopia globally, although of greater magnitude. As a result, people of Chinese origin are at greater risk of ocular complications that severely affect vision, such as myopic macular degeneration37.
Table 1 shows the prevalence of high myopia reported in different populations. It is not possible to make global comparisons because the studies used different procedures for the measurement of refractive errors, with different age groups and in different decades.
Table 1 Prevalence of high myopia in different regions and ethnic groups
| Authors | Year | Population | n | Age | High myopia definition (SE) | Refraction method used | Cycloplegia | Prevalence of high myopia (% and 95% CI) |
|---|---|---|---|---|---|---|---|---|
| Asia | ||||||||
| Wong, et al.44 | 2000 | Singapore (Chinese residents) | 1.232 | 49-79 | <−5.00 D | Autorefraction | No | 9.1% (7.2-11.2) Men 7.5% (5.1-10.4), women 10.5% (7.6-13.3) Age-adjusted |
| Lin, et al.52 | 2004 | Taiwan | 10.889 | 18 | <−6.00 | Autorefraction | Yes | 21% |
| Raju, et al.47 | 2004 | India | 2,508 | >39 | <−5.00 D | Subjective | No | 4.32% Age and gender-adjusted |
| Bar Dayan, et al.45 | 2005 | Israel | 919,929 | 16-22 | <−6.00 | Autorefraction | No | Men 2.05%, women 2.4% |
| Krishnaiah, et al.53 | 2009 | India (Andhra Pradesh Eye Disease Study) | 3,642 | ≥40 | <−5.00 D | Subjective | No | 4.5% (3.8-5.2) Age, sex and area-adjusted. Men 5.2% (4.1-6.1), women 4.5% (3.5-5.4) |
| Wang, et al.41 | 2009 | Taiwan | 3,709 | First-year university students | <−6.00 D | Autorefraction | No | 36.9% Men 36.6%, women 37.2% |
| Liu, et al.54 | 2010 | China (Beijing Eye Study 2001) | 4,319 | >40 | ≤−6.00 D | Subjective | No | 2.5% of the eyes |
| Gao, et al.55 | 2011 | China (The Handan Eye Study) | 6,603 | ≥30 | <−5.00 D | Subjective | No | 2.1% > 40 years: 1.8% (1.5%-2.2%) |
| Tan, et al.56 | 2011 | Singapore | 1,835 | 55-89 | <−6.00 D | Autorefraction | No | 3.1% |
| Asakuma, et al.57 | 2012 | Japan | 1,892 | ≥40 | <−5.00 D | Autorefraction | No | 5.7% |
| Pan, et al.58 | 2012 | Singapore | 3,400 | >40 | <−5.00 D | Autorefraction | No | 4.8% (4.0-5.7) Age and gender-adjusted |
| Kim, et al.59 | 2013 | Korea | 21,356 | >20 | <−6.00 D | Autorefraction | No | 4.0% (3.7-4.3) |
| Lee, et al.60 | 2013 | Korea | 2,805 | 19 | <−6.00 D | Autorefraction | Yes* | Men 6.8% (5.8-7.7) |
| Lan, et al.40 | 2013 | China | 2,478 | 3-6 | ≤−6.00 D | Autorefraction | Yes* | 0.08% |
| You, et al.46 | 2014 | Great Beijing | 15,066 | 7-18 | ≤−6.00 D | Autorefraction | Yes* (1082 children) | 4.3% (4.0-4.7) |
| Gao, et al.61 | 2015 | China | 1,565 | 6-21 | ≤−6.00 D | Autorefraction | Yes* | 2.9% (2.1-3.7) |
| Gupta, et al.62 | 2015 | Singapore | 28,908 men | 17-29 | ≤−6.00 D | Autorefraction | No | 8.9% |
| Joseph, et al.63 | 2018 | India | 4,351 | ≥40 | ≤−6.00 D | Subjective | No | 2.0% |
| Australia | ||||||||
| Attebo, et al.64 | 1999 | Sidney | 3,654 | 49-97 | <−5.00 D | Autorefraction | No | 1.8% |
| McCarty, et al.65 | 1997 | Melbourne | 3,262 | ≥40 | <−5.00 D | Autorefraction | No | 2.2% |
| Vongphanit, et al.66 | 2002 | Sidney (Blue Mountains Eye Study) | 3,654 | ≥49 | ≤−5.00 D | Lensometry Subjective | No | 2.2% |
| North America | ||||||||
| Vitale, et al.67 | 2009 | USA (National Health and Nutrition Examination Survey -NHANES) | 1971-1972: 4,436 1999-2004: 8,339 | 12-54 | ≤−7.9 D | Lensometry, retinoscopy or autorefraction | No | 1971-1972: 0.2% 1999-2004: 1.6% (1.3-2.0) |
| Villarreal, et al.68 | 2003 | Mexico | 1,035 | 12-13 | ≤−5.00 D | Autorefraction | Yes** | 1.4% Boys 1.5%, girls 1.4% |
| Pan, et al.37 | 2013 | USA | 4,430 | 45-84 | ≤−5.00 D | Autorefraction | No | 4.6% Chinese 11.8%, white 5.4%, black 3.1%, hispanic 1.8% |
| Willis, et al.69 | 2016 | USA (several races) | 8,865 | >18 | ≤−6.00 D | Autorefraction | No | 3.91% (3.44-4.45) Men 2.99% (2.47-3.63), women 4.8% (3.98-5.77) |
| Varma, et al.42 | 2017 | USA (Chinese) | 4,144 | ≥50 | <−5.00 D | Autorefraction | No | 7.4% (6.6-8.3) Men 7.2%, women 7.5% |
| Africa | ||||||||
| Ezelum, et al.48 | 2011 | Nigeria | 10,687 | ≥40 | <−5.00 | Autorefraction | No | 2.1% (1.8-2.4) Excluding significant opacities from the lens: 0.7% (0.5-0.9) |
| Europe | ||||||||
| Matamoros, et al.35 | 2015 | France | 100,429 (Caucasians) | 0 to > 80 | <−6.00 D | Autorefraction | Not in adults, yes* in children | 3.9% (3.8-4.0) |
| Cumberland, et al.36 | 2015 | UK | 107,452 | 40-69 | ≤−6.00 D | Autorefraction | No | 4% (3.9-4.1) |
| South America | ||||||||
| Cortinez, et al.70 | 2008 | Argentina | 1,518 | 25-65 | ≤−5.00 D ≤−6.00 D | Subjective | No | 2.30% 1.60% |
| Galvis, et al.71 | 2017 | Colombia | 1,228 | 8-17 | ≤−6.00 D | Retinoscopy/Subjective | No | 0.2% |
*1% cyclopentolate cycloplegia
**0.5% tropicamide cycloplegia
Discussion
In high myopia there may be progressive anatomical changes that affect the layers of the eyeball. These morphological changes include posterior staphyloma, lacquer cracks, CNV, myopic maculopathy and retinoschisis, among others21. Posterior staphyloma is a frequent pathological finding with a great clinical relevance, since its appearance can predict the occurrence of maculopathies, which would irreversibly affect central vision38.
There are discrepancies in the classification of the stages of a posterior staphyloma. The Ohno-Matsui study proposes an objective classification based on 3D magnetic resonance that includes five types of findings according to the appearance of the edge and the location of the posterior staphyloma, simplifying the subjective classification proposed by Curtin in 197739. This reclassification poses new challenges in research and simplifies its clinical presentation, making monitoring and identification of anatomical changes in the posterior pole more practical.
Regarding the prevalence of high myopia, it varies according to the region and the ethnic group. In this review, prevalence was found from 0.08% in Chinese children40 to 36.9% in university students in Taiwan41. Other reviews have reported that the prevalence in young adults is higher in Asian populations (6.7-21.6%) compared to non-Asian (2.0-2.3%)9. Even a US-based study that only included Chinese individuals reported a prevalence of 7.4%42, much higher than that of other studies conducted in the same country.
After the Asian countries, the highest prevalence has been found in European countries6. The few studies conducted in Australia and Latin America report even lower prevalence rates.
The definition of high myopia varies between studies, with values of SE lower than -5.00 or -6.00 D, so the results of research on prevalence may vary. This has been studied in low myopia, in which changes of -0.25 D in its definition make the estimates vary in the analysis of risk factors43. However, the impact of the reclassification has not been evaluated for high myopia, although it can be assumed that such a decision would also facilitate the performance of future meta-analysis33.
Similarly, since there is no single definition of pathological myopia, some studies may have included subjects based only on the dioptric value, without considering the pathologic fundus changes6. This is another possible bias if the prevalence figures were to be grouped.
When assessing the prevalence of high myopia in different populations of the world, factors such as race, sex, occupation, income, educational level44,45, urban or rural housing46,47, age or the presence of nuclear sclerosis47 must be considered, so that the rates are properly adjusted. For example, even after controlling for educational level, living in an urban environment continues to be associated with a higher prevalence of myopia of any degree, suggesting that there are other contributing factors42. Another difference observed among the populations of the studies was the inclusion of some adult patients with lens opacities, a factor that increases prevalence rates48.
The rapid increase in the prevalence of high myopia appears to be based on the epidemic of school myopia, and studies suggest that it should be differentiated from high myopia of genetic origin. These two types of high myopia, genetic or acquired, differ in the age of onset and in their relationship with environmental risk factors. High myopia of genetic origin begins at 5 years of age, whereas acquired high myopia is identified at 11 or 12 years of age49. Studies suggest that children of 6 or 7 years with school myopia will reach the threshold of high myopia in 5 to 6 years, associated with the increase in educational demands that augment the reading and writing time in indoor spaces and reduce the hours that children are outdoors50.
Based on animal studies, it has been proposed that the protective effect of the outdoors could be due to the high intensity of natural light (which triggers the release of dopamine, an inhibitor of ocular growth), to the spectral composition of daylight (the exposure to blue light would be protective against the development of myopia) or to increased levels of vitamin D in blood as a result of exposure to UVB radiation51.
Myopia should be considered as a public health problem, due to its association with different eye diseases that can lead to blindness2. The prevalence increase estimated in 2050 has important implications for planning health services in terms of preventing complications and visual loss, since this would affect nearly 1,000 million people worldwide33.
Several studies have been carried out on the prevalence of high myopia in Asian countries and, according to a meta-analysis34, there are at least 15 studies in European countries. However, information on populations in the Americas is limited, and they indicate very low figures of high myopia compared to other continents. Therefore, there is a need to conduct population-based studies in Latin America, in different age groups and races, to obtain information for planning preventive public health actions.
Conclusion
The prevalence figures for high myopia vary between geographic regions. The high values identified in Asia pose a greater risk of eye morbidity from retinal disorders that could lead to low vision or blindness. The lack of epidemiological studies on high myopia in Latin American populations is clear.
