Myopia, also known as nearsightedness, is a common type of refractive error where close objects appear clearly, but distant objects appear blurry.
What is myopia?
When you look at an object, light rays reflect off that object and pass through the cornea and the lens of the eye, which bend (or refract) the light and focus it on the retina at the back of the eye. If you have perfect vision, the rays focus directly on the surface of the retina. But in a myopic eye, the eyeball is usually too long from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. This makes distant objects blurry.
Myopia can also be the result of a cornea that is too curved for the length of the eyeball or a lens that is too thick. For some people, their myopia may be caused by a combination of problems in the cornea, lens, and length of the eyeball.
What are the symptoms of myopia?
If you have myopia, you have trouble seeing things far away, but you can see nearby things clearly. This is why myopia is commonly called nearsightedness.
If you can see well enough to read what’s on your laptop or in a book, but you struggle to see what’s on the television or a movie screen, you may be nearsighted.
Sometimes people with undiagnosed myopia will have headaches and eyestrain from struggling to clearly see things in the distance.
Why does the eyeball grow too long?
What causes the eyeball to grow too long isn’t completely known, but researchers are exploring a number of factors. For many people, myopia appears to be an inherited condition – in other words, if you have a parent with myopia you are at higher risk for developing it. Researchers are also looking at the effects of sex, age, ethnicity, and environmental exposures – such as sunlight and the amount of time spent doing close-up work – on the development of myopia. More recently, scientists have been considering the influence of circadian rhythms (sometimes referred to as our biological or body clock), which regulate systems in the body according to the daily cycles of light and dark, as a factor in the development of myopia.
How common is myopia?
Based on a study published in 2009, experts at NIH estimate that at least 41 percent of Americans are nearsighted. According to the same study, the number of Americans with myopia has increased significantly from the 1970s to the early 2000s. The prevalence of myopia has also been increasing in many other countries around the world. It is particularly prominent among school-aged children living in urban areas in some Asian countries.
In the past, people thought children might become myopic from spending too much time reading and writing, which require close-up vision, or from reading in poorly lit rooms. Recent studies, however, suggest that the increase of myopia in children could be related to a decrease in the amount of time they spend outdoors.
How is myopia diagnosed?
An eye care professional can diagnose myopia during an eye exam, which usually begins with a visual acuity test. This test uses a standardized chart or card with rows of letters that decrease in size from top to bottom. Covering one eye, you will be asked to read out loud the smallest line of letters that you can see. When done, you will test the other eye. If the vision test shows that you are nearsighted, your doctor will use a retinoscope to shine light into your eyes and observe the reflection off the retina to determine the amount of refractive error you have.
What kinds of treatments are available for myopia?
The most common way to treat myopia is to prescribe eyeglasses or contact lenses. Refractive surgery, once the eyes have stopped growing, has become another option for many people.
To find out the amount of myopia you have, an eye care professional uses a device called a phoropter to place a series of lenses in front of your eyes until you are seeing clearly. The combination of the results from both eyes is written as a prescription that will correct your vision to make it as normal as possible.
Eyeglasses use curved lenses to refocus light rays onto the retina, instead of in front of it.
Contact lenses correct vision in the same way as eyeglasses, except they rest directly on the eye.
Refractive surgery changes the shape of the cornea to correct myopia. There are different types of refractive surgery, but the most common are LASIK and PRK.
LASIK removes tissue from the inner layers of the cornea. To do this, a 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 of outer tissue 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 removes a thin layer of tissue from the surface of the cornea to change its shape. This allows light to focus more accurately on the retina. Like LASIK surgery, with PRK there is a limit to how much tissue can safely be removed and the amount of nearsightedness that can be corrected.
Many people will experience dry eye symptoms after refractive surgery and a small number may develop chronic dry eye syndrome. Some people may also develop vision symptoms such as double vision/ghosting, starbursts, glare, and halos, especially at night. Ask your eye doctor to discuss with you the risks and benefits of LASIK or PRK surgery before you undergo either procedure.
Phakic intraocular lenses (IOLs) are a new 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.
What is high myopia?
High myopia is a severe form of myopia in which the eyeball continues to grow and becomes very long from front to back. It can increase the risk for retinal detachment, early development of cataracts and glaucoma.
What is degenerative myopia?
Degenerative myopia (also called pathological or malignant myopia) is a rare and mostly inherited type of myopia that begins in early childhood.
In degenerative myopia, the eyeball elongates rapidly and causes severe myopia, usually by the teenage or early adult years. Degenerative myopia may also progress far into the adult years. People with the condition have a significantly increased risk of retinal detachment and other degenerative changes in the back of the eye, including choroid neovascularization (abnormal blood vessel growth), and glaucoma.
What research is being done?
From 1996-2013 the COMET (Correction of Myopia Evaluation Trial) made many contributions to our understanding of why myopia develops and how it can be treated. One of the most recent COMET reports, published in 2014, looked at seasonal differences in how quickly myopia progresses in young children.
The Pediatric Eye Disease (PED) Consortium is a recently funded project that brings together all existing population-based studies of eye diseases among preschool children around the world. It creates a repository of population-based survey data on vision health among more than 17,000 children between the ages of six and 72 months. PED Consortium researchers are using the data to look for demographic, behavioral, and clinical factors associated with the development of moderate to severe refractive errors.
A number of NEI-funded studies are investigating how natural light exposure and daily circadian rhythm patterns influence the development of the eyeball. These studies are especially important since abnormal growth in the length of the eyeball is what causes myopia. Early studies with animal models have shown that environments that exclude natural daylight, or are solely lit by artificial light, or that switch day for night, influence how long the eyeball grows. Better understanding of the relationship between circadian rhythms and light exposure could provide new ways to decrease the risk for developing myopia.
NEI-supported researchers are also exploring the genetic causes of degenerative myopia. One study is working with members of a large, extended family, many of whom have degenerative myopia. The researchers are using genetic linkage analysis to identify possible causative genes, and then exploring what these genes do in eye tissue. Understanding the underlying mechanisms of degenerative myopia will help scientists work toward better treatments.
LASIK has become a popular surgical method to correct myopia in the United States, but not much is known about the long term outcomes of LASIK surgery. In October of 2009, the U.S. Food and Drug Administration (FDA), the National Eye Institute (NEI), and the Department of Defense (DOD) launched a collaborative study to look at how people fared after LASIK surgery. These studies – Patient Related Outcomes with LASIK (PROWL I and II) laid the groundwork for a computer based questionnaire used to assess how many, of a select group of people who all received the same standardized LASIK treatment, experienced significant problems such as dry eye or vision irregularities (double vision, haloing, starbursts, or glare) over a period of 3 months following the procedure. The data gathered from this survey will give us a much better idea of the long term risks and benefits of LASIK.
Courtesy: National Eye Institute, National Institutes of Health (NEI/NIH).