Definition and classification of myopia.
When it comes to myopia, it seems familiar to everyone. In fact, myopia is indeed a common disease in China, Japan and Southeast Asia. According to rough estimates, the prevalence of myopia among urban junior high school students in Taiwan and Singapore, China has been as high as 80%, and the prevalence of myopia among 8-year-old children in Hong Kong, China is 37.5%, while the prevalence of myopia among children older than 11 years old is 53.1%. The prevalence of myopia among urban students in Guangzhou, China, reached 78.4% at age 15.
In most Western countries, the prevalence of high myopia is usually less than 3%, while the prevalence of high myopia in Asian populations reaches 10%. Globally, the Chinese have the highest prevalence of myopia. Although the results of numerous studies vary. However, it is generally accepted that the prevalence of myopia is higher in Asian populations than in European populations and higher in urban populations than in rural populations.
First of all, what is myopia?
I. Definition
Myopia (myopia) is a condition in which parallel light from a distance enters the eye and is focused on the front of the retina without the use of accommodation.
In this case, as you can imagine, the light from a distance cannot be focused on the retina, that is, it cannot form a clear image of the object. However, when looking at near objects, the scattered light from the near target enters the eye and can be focused on the retina, so the near vision is found to be normal and the near objects are still clear.
II. Classification
There are various ways to classify myopia, but there are four main types.
1.Classification according to the cause of myopia.
According to the classification of myopia etiology, it can be divided into two categories: primary (meaning that myopia is not caused by a known eye disease or systemic disease) and secondary (meaning that myopia is secondary to a known eye disease or systemic disease). Primary myopia can be divided into two categories: pure and pathological.
(1) Simple myopia (simple myopia):
Myopia starts in children and adolescents, and ends when it reaches a certain level. The final myopic refraction is below -6.00D, the corrected visual acuity is normal, and the fundus is generally normal, with at most a narrow arcuate patch and leopard’s eye. The eye axis may be prolonged, but still within the normal range. The cause of the disease is related to both genetic and environmental factors (prolonged near eye use and lack of outdoor activities), and is multi-factorial genetic.
(2) Pathologic myopia.
Pathologic myopia starts in childhood, progresses rapidly, and stabilizes or remains relatively unchanged until adulthood. The final myopic refraction is greater than -6.00 D. The eye axis is significantly prolonged, with posterior electrogloss and obvious fundus degeneration, including annular and large arcuate spots, lacquer cracks, retinal splitting in the macula, macular hemorrhage, Fuchs’ spot and choroidal retinal degeneration, and complications such as retinal detachment, glaucoma and cataract can occur. Visual function is significantly impaired, and corrected visual acuity may be lower than normal. Visual field, light perception and contrast perception are mostly abnormal. The cause of the disease is mainly related to genetic inheritance, which has been found to be autosomal dominant, recessive and sex-linked. Therefore, if we encounter parents with pathological myopia in the clinical setting, we usually recommend that they bring their children in for an examination in order to detect and treat them early.
2. The degree of myopia and the degree of myopia
According to the diopter classification of myopia, myopia is divided into low myopia (-0.25D~-3.00D), moderate myopia (-3.25D~-6.00D) and high myopia (-6.00D or more), and there is also a separate category of -9.00D, called super high myopia.
Generally speaking, myopia terminating at low to moderate is simple myopia, while hyperopia is pathological myopia. High myopia of -6.00D to -9.00D in China may include milder pathological myopia and heavier simple myopia determined by both genetic and environmental factors.
3. Classification according to the change of refractive elements
Refractive elements: Refractive elements of the eye include the length of the eye axis, the curvature of the cornea, the curvature of the lens and the refractive index of each refractive medium. Changes in each of these elements can cause myopia.
(1) Axial myopia is caused by the lengthening of the eye axis and is mainly seen in primary myopia and some secondary myopia.
(2) Curvature myopia refers to the increase in refractive power due to the shortening of the radius of curvature of the cornea or lens. It is mainly seen in corneal diseases (congenital microcornea, conical cornea, etc.) and lens diseases (small spherical lens, conical lens, etc.).
(3) Refractive index myopia (index myopia) refers to myopia caused by an increase in the refractive index of the refractive medium of the eye. The most common type of myopia is myopia caused by an increase in the refractive power of the lens caused by sclerosis of the crystal nucleus in old age and further development of nuclear cataract.
4. Other types of myopia.
Patients in the clinic often ask: Is this true myopia or pseudomyopia in my child? How can this be determined? This true pseudophakia is a classification method, divided into three categories.
(1) Pseudomyopia: refers to the usual performance of myopia, the use of atropine and other ciliary muscle paralyzing agents forced to cancel the regulatory tension after the disappearance of myopia, presented as orthopia or hyperopia have, less common, accounting for about 10% of our youth myopia.
(2) True myopia: refers to the use of ciliary muscle paralytic agents after the refractive error is unchanged, more common, accounting for about 40%.
(3) Semi-true myopia: refers to the use of ciliary muscle paralyzing agent after the myopia degree is lower, but still myopic, more common, accounting for 50%.
These three types of myopia are not so clearly divided from each other, and a myopic patient may experience all three in succession during the course of his myopia. Later, as the disease progresses and the refractive error deepens, the role of regulatory factors becomes smaller and the role of organic factors (mainly the lengthening of the eye axis) becomes larger, and the number of pseudomyopia decreases. If the refractive error is above -3.00D and the disease duration is 3 years or more, basically there is no more pseudomyopia, and all of them are true or semi-true myopia.