myopia is defined as poor vision in the eyes, although it cannot be defined as a visual disease that has caused great harm in common society.
If you are nearsighted, you will usually have difficulty reading road signs and seeing distant objects clearly, but you will be able to see well for tasks up close, such as reading and computer use.
Other signs and symptoms of nearsightedness include squinting, eye fatigue, and headaches. Feeling fatigued while driving or playing sports can also be a symptom of uncorrected nearsightedness.
If you experience these signs or symptoms while wearing glasses or contact lenses, schedule a complete eye exam with your optometrist or ophthalmologist to see if you need a stronger prescription.
What Causes Myopia?
Nearsightedness occurs when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus on a point in front of the retina, rather than directly on its surface.
Nearsightedness can also be caused by the cornea and/or lens being too curved for the length of the eyeball. In some cases, nearsightedness is due to a combination of these factors.
Nearsightedness typically begins in childhood. You may be at higher risk if your parents are nearsighted. In most cases, nearsightedness stabilizes in early adulthood, but sometimes continues to progress with age.
Why does the eyeball grow too long?
Scientists are not sure why the eyeball sometimes grows too long. In 2013, the Consortium for Refractive Error and Myopia (CREAM), an international team of vision scientists, discovered 24 new genetic risk factors for myopia.1 Some of these genes are involved in nerve cell function, metabolism, and eye development. Each gene alone has a small influence on the risk of nearsightedness; however, the researchers found that individuals who carry a greater number of versions of the myopia-prone genes have up to a tenfold increased risk of nearsightedness.
Although genetics plays a role in nearsightedness, the recent dramatic increase in the prevalence of nearsightedness documented by several studies in the U.S. and other countries points to environmental causes such as lack of outdoor time and increased time spent near work, such as reading, writing and working with a computer.
In 1999, NEI-funded researchers initiated the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE)2, a long-term study that followed the eye development of more than 1,200 children ages 6 to 14, the age range during which myopia typically develops. The CLEERE researchers found that children who spent more time outdoors were less likely to become nearsighted.3 The researchers also showed that the time they spent outdoors was independent of the time they spent reading, which provided evidence against the assumption that less time outdoors meant more time working close to work.
Researchers aren't sure why outdoor weather helps prevent the onset of nearsightedness. Some suggest that natural sunlight may provide important signals for the development of the eyes. Other researchers suggest that normal eye development may require enough time to look at distant objects. Interestingly, once nearsightedness has begun to develop, outdoor weather doesn't seem to slow its progression, the researchers found.
What is pathological myopia?
A condition called pathologic myopia (also called degenerative or malignant myopia) sometimes occurs in eyes with high myopia when excessive eye lengthening causes changes in the retina, choroid, vitreous, sclera, and/or optic nerve (see picture). The vitreous is the gelatinous substance that fills the center of the eye. The sclera is the outer white part of the eye.
How common is nearsightedness?
About 42 percent of Americans aged 12 to 54 are nearsighted, compared to 25 percent in 1971.4 A recent review5 reports that the prevalence of nearsightedness varies by ethnicity. East Asians show the highest prevalence, reaching 69% by age 15. Blacks in Africa had the lowest prevalence, at 5.5 percent at age 15. Children in urban settings are more than twice as likely to be nearsighted as those in rural settings.
Generally, two types of myopia are distinguished: simple myopia, which is usually less than 6 diopters, and severe, high or pathological myopia, when the patient's prescription exceeds 6 diopters and is due to excessive lengthening of the eyeball. In some cases, magnetic myopia, considered a visual pathology, can cause degenerative changes associated with excessive lengthening of the eyes, especially at the retina level, seriously compromising the vision of the person suffering from it.
Simple nearsightedness is the most common type and cannot be prevented, although it must be detected early through a comprehensive eye exam and corrected with glasses, contact lenses, or different types of surgical techniques.