What is myopia?
Otherwise known as nearsightedness, myopia occurs when the eye grows too long from front to back. Instead of focusing images on the retina—the light-sensitive tissue in the back of the eye—the lens of the eye focuses the image in front of the retina. People with myopia have good near vision but poor distance vision.
People with myopia can typically see well enough to read a book or computer screen but struggle to see objects farther away. Sometimes people with undiagnosed myopia have headaches and eyestrain from struggling to clearly see things in the distance.
Myopia also can be the result of a cornea – the eye’s outermost layer – that is too curved for the length of the eyeball or a lens that is too thick.
In a normal eye, the light focuses on the retina. With myopia, the eye is too long and focuses light in front of the retina.
Why does the eyeball grow too long?
Scientists are unsure 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. Alone, each gene has a small influence on myopia risk; however, the researchers found that individuals carrying greater numbers of the myopia-prone versions of the genes have a up to tenfold increased risk of myopia.
Although genetics plays a role in myopia, the recent dramatic increase in the prevalence of myopia documented by several studies in the U.S. and other countries points to environmental causes such as lack of time spent outdoors and greater amount of time spent doing near-work, such as reading, writing, and working on a computer.
In 1999, NEI-funded researchers initiated the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE),2 a long-term study following the eye development of more than 1,200 children ages 6 to 14, the age range during which myopia typically develops. CLEERE researchers found that children who spent more time outdoors had a smaller chance of becoming nearsighted.3 The researchers also showed that time spent outside is independent from time spent reading, providing evidence against the assumption that less time outside means more time doing near work.
Researchers are unsure why time outdoors helps prevent the onset of myopia. Some suggest natural sunlight may provide important cues for eye development. Other researchers suggest that normal eye development may require sufficient time looking at distant objects. Curiously, once myopia has begun to develop, time outdoors does not appear to slow its progression, the researchers found.
What is high myopia?
Conventionally, an eye is considered to have high myopia if it requires -6.0 diopters or more of lens correction. Diopters indicate lens strength. High myopia increases the risk of retinal detachment. The retina is the tissue in the back part of the eye that signals the brain in response to light. When it detaches, it pulls away from the underlying tissue called the choroid. Blood from the choroid supplies the retina with oxygen and nutrients.
High myopia can also increase the risk of cataract and glaucoma. Cataract is the clouding of eye’s lens. Glaucoma is a group of diseases that damage the optic nerve, which carries signals from the retina to the brain. Each of these conditions can cause vision loss.
Pathological myopia can cause damage to the retina, choroid, vitreous, and sclera. Image National Eye Institute.
What is pathological myopia?
A condition called pathological myopia (also called degenerative or malignant myopia) sometimes occurs in eyes with high myopia when the excessive elongation of the eye causes changes in the retina, choroid, vitreous, sclera, and/or the optic nerve (see image). The vitreous is the gel-like substance that fills the center of the eye. The sclera is the outer white part of the eye.
Symptoms of pathological myopia typically first appear in childhood and usually worsen during adolescence and adulthood. Treatment cannot slow or stop elongation of the eye; however, complications such as retinal detachment, macular edema (build-up of fluid in the central part of the retina), choroidal neovascularization (abnormal blood vessel growth), and glaucoma usually can be treated.
How common is myopia?
About 42 percent of Americans ages 12-54 are nearsighted, up from 25 percent in 1971.4 A recent review5 reports that myopia prevalence varies by ethnicity. East Asians show the highest prevalence, reaching 69 percent at 15 years of age. Blacks in Africa had the lowest prevalence at 5.5 percent at 15 years of age. Children from urban environments are more than twice as likely to be myopic as those from rural environments.
How is myopia diagnosed?
An eye care professional can diagnose myopia during a comprehensive eye exam, which includes testing vision and examining the eye in detail. When possible, a comprehensive eye exam should include the use of dilating eyedrops to open the pupils wide for close examination of the optic nerve and retina.
How is myopia corrected?
The most common way to treat myopia is with corrective eyeglasses or contact lenses, which refocus light onto the retina. Contact lenses can cause complications (e.g., dry eye, corneal distortion, immunologic reaction, infection), but may be advantageous for activities where glasses are not practical (e.g., certain sports). An eye care professional can quickly identify lenses that best correct a patient’s vision using a device called a phoropter. In younger children, a technique called retinoscopy helps the eye doctor determine the correction required. The results are written as a prescription.
Refractive surgery is an option once the optic error of the eye has stabilized, usually by the early 20s. The most common types of refractive surgery are laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). Both change the shape of the cornea to better focus light on the retina.
LASIK removes tissue from the inner layer of the cornea. To do this, a thin section of the outer corneal surface is cut and folded back to expose the inner cornea. A laser removes a precise amount of tissue to reshape the cornea and then the flap is placed back in position to heal. The correction possible with LASIK is limited by the amount of corneal tissue that can be safely removed.
PRK also removes a layer of corneal tissue, but does so without creating a surface flap. Instead, the corneal surface cells are removed prior to the laser procedure. For this reason, PRK requires a longer healing time, as the surface cells have to grow back to cover the corneal surface. As with LASIK surgery, PRK is limited to how much tissue safely can be removed.
In the NEI-funded Patient Reported Outcomes with LASIK (PROWL) study, up to 28 percent of people experienced dry eye symptoms after LASIK.6 In the same study, up to 40 percent of patients undergoing LASIK experienced side effects such as ghosting of images, starbursts, glare, and halos, especially at night. Nevertheless, less than 1 percent of patients experienced difficulty performing their usual activities following LASIK surgery due to any one symptom and 95 percent of participants said they were satisfied with their vision.7
Potential side effects of refractive surgery. Image National Eye Institute.
An important consideration for people considering refractive surgery is that a nearsighted person who can comfortably read without glasses will likely require reading glasses if good distance vision is achieved through refractive surgery, so an individual who gets full distance correction with LASIK or PRK might be trading distance glasses for reading glasses.
Phakic intraocular lenses (IOLs) are an option for people who are very nearsighted or whose corneas are too thin to allow the use of laser procedures such as LASIK and PRK. Phakic lenses are surgically placed inside the eye to help focus light onto the retina.
1Verhoeven VJ, et al., 2013. Genome-wide meta-analyses of multi-ancestry cohorts identify multiple new susceptibility loci for refractive error and myopia. Nature Genetics 45:314-318. doi:10.1038/ng.2554.
2CLEERE study: https://clinicaltrials.gov/ct2/show/NCT00000169.
3Jones-Jordan, LA, et al. 2012. Time outdoors, visual activity, and myopia progression in juvenile-onset myopes. Clinical and Epidemiologic Research 53: 7169-7175.
4Vitale, S et al. 2009. Increased prevalence of myopia in the United States between 1971-1972 and 1999-2004. Arch Ophthalmol 127(12): 1632-1639.
5Rudnicka AJ, et al. 2016. Global variations and time trends in the prevalence of childhood myopia, a systematic review and quantitative meta-analysis: implications for aetiology and early prevention. British Journal of Ophthalmology 100(7):10.1136/bjophthalmol-2015-307724.
6Food and Drug Administration [Internet]. Silver Spring (MD); LASIK Quality of Life Collaboration Project; reviewed 2017 September; cited 2017 September 29. Available from: https://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/SurgeryandLifeSupport/LASIK/ucm190291.htm
7Food and Drug Administration [Internet]. Silver Spring (MD); LASIK Quality of Life Collaboration Project; reviewed 2017 September; cited 2017 September 29. Available from: https://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/SurgeryandLifeSupport/LASIK/ucm190291.htm
The National Eye Institute (NEI) is part of the National Institutes of Health (NIH) and is the Federal government’s lead agency for vision research that leads to sight-saving treatments and plays a key role in reducing visual impairment and blindness.