**1. Introduction**

Myopia is the most common eye disease and is one of the leading causes of vision impairment worldwide [124]. Prevalence of myopia is significantly different among racial groups, although its worldwide prevalence is approximately 30% (3–84%) [41]. The highest prevalence is found in East Asia, such as in mainland China [41]. The prevalence of myopia in the US population was estimated in the early 1970s to be 25% in persons aged 12–54 years [111]. A meta-analysis of population-based studies found a prevalence of 25% in persons over age 40 [61]. The World Health Organization has grouped myopia and uncorrected refractive error among the leading causes of blindness and vision impairment in the world [45].

Myopia (nearsightedness) is a refractive error, in which the eye possesses too much optical power (too much plus powers) for its axial length ; as a consequence, images of distant objects focus in front of the retina, when accommodation relaxed (www.checdocs.org).

Myopia has been recognized as a distinct visual disability for millennia and has been known for more than 2000 years, first described by the ancient Greeks [54]. It was probably the ancient

Greeks who coined the term, using the roots *myein* (to close) and *ops* (eye) to characterize those individuals who narrow their eyelids to improve distance visual acuity, the pinhole effect. The focus of distant parallel rays of light falls anterior to the retinal plane and produces a blurred image in myopia. This situation can arise because either the primary refractive components are too powerful or the globe is too long. Thus, myopia can be due to increased corneal or lenticular curvature, or an increase in the lens index of refraction, as occurs with the develop‐ ment of nuclear sclerosis. More commonly, myopia is the result of axial elongation of the posterior segment of the eye.

Myopia is categorized into two groups : (1) low-to-moderate myopia (≤ 6.0 D myoptic spherical equivalent (SE) with or without astigmatism) and (2) high myopia (≥ 6.0 D of myopic SE with or without astigmatism (www.medpagetoday.com).

Low-to-moderate myopia, known as physiologic myopia, is generally defined as that state in which the eye is rendered myopic by a combination of its components of refraction. In this situation, the refractive power of the eye (corneal power plus lens power slightly modified by anterior chamber depth (ACD)) and the axial length are such that its posterior focal plane lies anterior to the retina. Each component has a value within its normal curve of distribution. These eyes demonstrate normal anatomy and physiology. Whether the absence of correlation among the elements of refraction occurs by chance or is a heritable trait is unknown at present. Low-to-moderate myopia is also considered as low-to-moderate refractive errors defined as myopia less than 6.0 diopters (D).

High myopia, also known pathologic, degenerative or malignant myopia, is related to an eye with an axial length exceeding 25.5 or 26 mm, a refractive error of at least -5.0 D and charac‐ teristic degenerative changes (eachers%20stangov.uk).

Posterior pole abnormalities typical of high myopia include tessellated fundus, lacquer cracks, diffuse atrophy, patchy atrophy, choroidal neovascularization (CNV), macular atrophy, posterior staphyloma but also straightened and stretched vessels, temporal peripapillary atrophic crescent, hemorrhages, and tilting of the optic disk [48, 52]. Recently, high myopia has been defined as a SE refractive error of at least −6 D associated with characteristic degen‐ erative changes which are more seen in eyes with myopic SE exceeding 8 D.

As elongation of the globe is a key feature of pathological myopia, an axial length of ≥26.5 mm has been adopted as a biometric definition in clinical trials [122], with recent studies reporting a mean of 29 mm (range 26.8–31.5 mm) [129, 130]. Limit of 25.5 mm of myopic eye has been arbitrary fixed as 25, 25.5, or 26.5 mm [74] with an inferior limit of −6 to −10 D of refractive error, providing a cornea of +43.5 D as average refractive power, in the absence of spherophakia or nuclear cataract (eachers%20stangov.uk).

Of greater interest is the determination of the best cutoff for high myopia. Criteria for high myopia that have been used in previous studies include −5.0, −6.0, −10.0, and −12.0 D and there is no universal definition for high myopia [85, 132]. It is thought that at this level, the risks of secondary complications, such as retinal detachment and glaucoma may increase [103, 117, 132]. There may also be further deteriorations in visual field, central visual acuity, increased risks of irregular astigmatism, keratoconus, and peripheral visual field defects.

Most cases of myopia are in children of school age and young adults. The etiology of myopia is not clear, but there is evidence that genetic and environmental factors play a role. The chief complaint is difficult reading at a distance. The diagnosis is made by measurement of refractive errors by refraction [106].

The various modes of treatment of myopia(commonly used methods for correcting myopia) include medical therapy options and surgical therapy options (known as refractive surgery).
