Nearsightedness, medically known as myopia, is the most common cause of poor eyesight, especially in younger adults. It refers to being able to see well at close range, but not at a distance. It is the opposite of farsightedness (hyperopia). Focusing ability can start to decline in childhood and continue to slide through the teenage years, usually stabilizing in the early 20s. After that, most people find they don't need to change their glasses or contact lens prescriptions very often, if at all.
Myopia affects about 30% of the population of Canada. We know that some ethnic groups are more prone than others. People of Chinese, Japanese, and Southeast Asian descent are especially likely to be myopic.
Myopia is caused by an eye whose shape doesn't permit it to focus on distant objects. The cornea is a special layer on the surface of the eye that does most of the work of bending light as it enters the eye. Light must be bent properly so that it creates a sharp image. When parallel light rays pass through the cornea, they should be bent just enough to meet and focus on the retina (the light-sensitive membrane that lines the back of the eye). The lens is responsible for fine adjustments of focus. In myopia, either the cornea is too curved or the eye is too long front to back, so the light rays meet and cross in front of the retina. By the time the image reaches the retina, it's blurred.
Nearsightedness is at least partly genetic. It isn't a disease. Just as some people's noses are longer than others, so are some people's eyes. Some research suggests that people who do a lot of work requiring that they focus closely may be more likely to develop nearsightedness over time. Children who are born prematurely often develop eye conditions that affect the shape of the eye and may be more likely to develop nearsightedness.
People usually first notice myopia in their teenage years, when they may experience headaches and it becomes apparent that other people can see distant objects better than they do. This may come as a shock to a young person who has forgotten that they used to see more clearly as a child. Such a teenager may be amazed at the clarity of the world after they get their first lens prescription - often they were unaware that the human eye is capable of functioning that well. Other clues that a person may be developing nearsightedness include the need to squint to see clearly, frequent blinking and eye rubbing. A person who is nearsighted may also hold books close to read.
Myopia almost never worsens rapidly. Some diseases, like diabetes, can make nearsightedness develop faster, but underlying diseases more usually cause trouble at the back of the eye. This makes it hard for an adult to notice if their prescription needs changing. The only way to be sure is to get regular eye tests.
Myopia can lead to poor performance at school. If a student's grades start dropping off around age 10 or 12, there's a chance eyesight is involved, although certainly plenty of other factors can cause 12-year-olds to have problems at school.
Among adults, severe myopia can cause disability, primarily by preventing driving. This can usually be corrected.