Myopia is a general clinical term for what we commonly refer to as “near-sightedness”, where the eye is able to focus on objects at a short distance but has trouble focusing on objects that are far away, making them look blurry. It is the opposite of hyperopia (far-sightedness). Myopia has been scientifically correlated with intelligence and level of study, although there is no evidence to support a cause-effect relationship. In its different forms, near-sightedness may appear at any point in a person’s life, from birth to late adulthood.
Research has shown that myopia is a leading cause of visual impairment worldwide, although it affects different populations very differently. Worldwide estimates of myopia are between 800 million to 2.3 billion people. On average, the frequency of near-sightedness is highest in Asian countries (70-90% of the population), followed by Europe and North America (30-40%) and then Africa (10-20%).
Myopia has been the subject of many research studies which attempt to understand its associated causes and risk factors, which to this day remain unclear and indeed, many of these studies have conflicting results.
In this article, we discuss the anatomy of myopia vision disorder, what causes myopia and how myopia is treated.
Anatomy of myopia: the optics of vision
When we open our eyes, light reflected off the objects in our field of vision enters our eye, passing through the cornea and into the pupil. The cornea is a transparent dome which covers the pupil and iris and then merges with the sclera (the white of the eye). After entering the eye through the pupil, light must travel through a fluid called aqueous humor before reaching the lens. The lens of the eye functions like a somewhat flexible version of a lens you might find on a magnifying glass, microscope, eye glasses, etc. Ciliary muscles control the curvature of the lens in order to focus on objects at varying distances. In normal vision, the light (image) is focused by the lens onto the retina, a small area of light-sensitive tissue near the back of the eyeball which sends chemical and electrical signals to the brain via the optic nerve, allowing us to “see” everything in our field of vision.
In myopia, however, images of objects at far distances are not focused onto the retina by the lens, but slightly in front of it, creating a blurry image. There are several anatomical anomalies that can cause this to happen. For example, the curvature of the lens and/or cornea may create optics that are too powerful; that is, they cause the image to come into focus before it should. An elongated eyeball can also cause the retina to be farther away from the lens so that the image focuses in front of it rather than on it.
Causes of myopia
A variety of causes and risk factors have been studied. A “primary cause” of near-sightedness has yet to be identified, as there has been significant variation in the roles identified by several genetic and environmental factors in different studies.
Most researchers agree that there is a small but significant genetic (i.e. hereditary) factor that causes or at least creates a predisposition for myopia. This conclusion is drawn from studies showing that, under similar environmental circumstances, identical twins and other close relatives showed similar rates of myopia. The genetic component, however, is believed to be outweighed by external factors because of the dramatic increase in myopia cases today compared to several decades ago (except in congenital myopia, in which infants are born near-sighted and this condition persists throughout their infancy; in this case, genetic factors may play a more important role, along with conditions in the womb during fetal development). The latest genetic research into myopia has identified at least 18 different genetic loci on 15 different chromosomes that are correlated with myopia, suggesting that there is no one single gene that is responsible for myopia. It seems more likely that a variety of genetic mutations working in concert are responsible for the genetic component of near-sightedness.
Regarding environmental factors, one popular hypothesis is that “near work”, i.e. sustained activity focusing on objects at a close distance, is a significant risk factor for myopia. This hypothesis seems to be supported by statistics indicating much higher incidence in students than in the general population – a relationship that appears to be proportional to the level of education. For example, while the frequency of myopia in the general population in China is approximately 31%, the frequency in Chinese high school students is around 70% and as high as 80% in college students. This hypothesis could also explain the recent dramatic increase in myopia in industrialized countries; the amount of time spent on near work activities, such as working on the computer and using mobile devices, has increased for the general population in these societies in recent decades.
However, a second hypothesis exists which is supported by the same data. Some researchers postulate that lack of exposure to sunlight and/or dynamic visual stimuli contribute to the anatomical changes that lead to myopia. In general, students (particularly high-achieving students) spend more time indoors studying and tend to get less exposure to sunlight than their less studious peers. In industrialized countries, a larger portion of the general population works inside away from sunlight and dynamic visual stimuli compared to several decades ago. Studies have shown a lower incidence of myopia among people who regularly participate in outdoor activities and sports, and outdoor play has been associated with a lower risk of near-sightedness in children. A Brazilian study reported that approximately 13.3% of students in the city of Natal were near-sighted, compared to 6.4% in the general population and 2.7% in some indigenous communities.
Prevention and treatment of myopia
Because the exact causes of myopia aren’t fully understood, there is no official recommendation for its prevention. However, one could reasonably expect to reduce their risk of becoming near-sighted, at least moderately, by regularly engaging in outdoor physical activity and seeing natural environments in full daylight. It might also help to take frequent breaks when performing near work, like reading, studying, sewing, etc.
The standard treatment for myopia is corrective lenses, i.e. glasses or contacts. Lenses to correct near-sightedness are concave in order to reduce the optic power of the lens so that far-away images can be focused on the retina instead of in front of it. Both glasses and contact lenses are appropriate for people with near-sightedness between -0.50 and -6.00 diopters; however, for people with high myopia (-6.00 or more), the strong eye glasses prescription necessary to correct their vision can cause chromatic aberrations, causing them to see color fringing that doesn’t exist when looking at something that isn’t perfectly centered on the lens. This phenomenon does not occur with contact lenses because the lens is always centered on the person’s gaze. Using corrective lenses has not been shown to have a significant effect on the progression of myopia; in other words, wearing glasses or contact lenses doesn’t slow down myopia, but it doesn’t make it get worse any faster, either. This is true whether lenses are worn all the time, part-time or almost never.
Laser vision correction surgeries are also an option to treat myopia. Different techniques are available, each with their own risks and benefits.
In children younger than 18, certain topical medications have been shown to slow the progression of myopia, including pirenzepine, cyclopentolate and atropine. Side effects reported in these studies included sensitivity to light and blurred near vision.
Finally, some people report improvement in near-sightedness using alternative techniques like eye exercises and biofeedback techniques. However, scientific evidence supporting the effectiveness of these techniques is limited at best.