The clarity of vision (acuity) is dependent on the refraction of parallel light rays and the sharp focus of this light on the retina. Normally the ciliary muscles of the lens can remain relaxed and distant objects can be seen clearly, albeit small depending on the distance. This normal vision is known as emmetropia. To see near objects clearly the muscles have to contract and alter the lens shape and therefore the refractive index of the lens. The parallel rays of light can then be focused clearly on the retina. This is known as accommodation. Some of the more common vision problems in both children and adults is due to a refractive error. This means that the light is not focused on the retina with the sharpness that it should be. The image either falls short of the retina or beyond it and the vision appears blurred. A person may, however, see with some degree of clarity if an object is very near or very far away.
What is myopia?
Myopia or myopic vision means that a person can clearly see objects that are near. It is also commonly known as nearsightedness or short sight. Myopia is one of the more common refractive errors that causes vision problems. Distant objects appear blurred and the eye is not able to accommodate in any way to correct it. Therefore corrective lenses, whether spectacles or contact lens, or procedures like LASIK is needed. Myopia is a not serious disorder of the eye and will not lead to blindness in the vast majority of people despite that fact that it is often progressive over years. However, people with severe myopia do have an increased risk of other eye disorders like glaucoma and macular degeneration but overall this risk is small.
How does myopia occur?
The lens of the eye is biconvex when the ciliary muscle is relaxed. This curvature is gradually reduced as the ciliary muscles contract and pull on the lens. In this manner light can be sharply focused on the retina. The cornea also plays an important role in refraction of light, but unlike the lens, its refractive index cannot be changed instantaneously. In myopia the eyeball is too long or the cornea is too curved. Despite the best efforts of the ciliary muscles to alter the lens shape and even flatten it, the sharpest image falls short of the retina. There is no other mechanism in the eye that can alter this but as the object moves closer to the eye, there is a distance at which the image can be sharply focused on the retina. This distance is known as the “far point”. As the object moves closer, accommodation will be able to maintain the clarity. Therefore the “far point” is the defining limit for clear vision.
Causes of Myopic Vision
The exact cause in every case of myopia is unclear. Two important factors though are a family history of short sightedness or visual stress. Other risk factors which may also play some role in the development or worsening of myopia are conditions like diabetes mellitus and being a premature baby. The visual disturbances in diabetes mellitus, however, may not be permanent.
Myopia affects about one-third of the adult population. In most instances it starts in childhood, around school-going age, or sometimes later in the teen years. The condition progressively worsens as the eyeball is still developing and by early adulthood this progression halts. In most cases, the eyesight does not get any worse throughout adulthood although prolonged visual stress and disorders like diabetes mellitus can exacerbate it. In fact it can even cause myopia in an adult with previously normal vision.
It has long been known that there is an increased risk of developing myopia if at least one parent is myopic. This risk increases substantially if both parents are short sighted which is not uncommon in this day and age. While the discovery of certain genes like RASGRF1 confirms a genetic link for myopia, it does not explain every case of familial short sightedness. This may be due to as yet unidentified genes or a combination of genetic and environmental factors.
The mechanism of myopia associated with visual stress is better understood. It is more likely to arise in a person with familial tendencies for myopia. Visual stress in this context refers to prolonged viewing of objects at a close distance as is the case with long hours of reading, computer use or watching TV and playing video games. These situations can also cause a temporary myopia known as pseudomyopia which is associated with acute eye strain. Normal vision may return after rest. However, if these activities are repeated on an ongoing basis, the visual disturbance may become permanent.
Signs and Symptoms
The main feature of myopia is the inability to see distant objects clearly. While adults are more likely to accurately report blurred vision, children generally are not able to understand the visual disturbances. Other findings related o myopia may include squinting to see objects at a distance and headaches. The latter is associated with eye strain. Sometimes there may be a burning sensation or discomfort of the eye after prolonged straining although there is usually not any pain.
Myopia in Adults
The presentation is the same both in adults and children. However, adults need to take note of situations where the myopia may worsen and be associated with eyestrain, particularly with computer use and excessive reading. Children are less likely to report this temporary myopia. Although myopia starts in childhood and generally stabilizes in early adulthood, adults who are noting a progressive worsening of the vision should consider visual stress as being a precipitating factor. Conditions such as diabetes mellitus and cataracts also need to be investigated as the deterioration in vision may be related to other underlying disorders.
Myopia in Children
Children may believe that short sightedness is normal and therefore not report it. Instead changes in personality and daily activities may be an indication of myopia. Difficulty in reading the board, avoiding activities that may require far viewing and positioning the head very near to a book or page when reading or writing are all signs of myopia. Poor performance in school is sometimes the warning signal that a visual disturbance is present.
There is no physiological reason for myopic children to be more introverted, however, this is often reported by concerned parents. Initially the poor eyesight may hamper the child’s play and interaction with others to some extent although this is dependent on the severity of the condition. Later the use of eyeglasses may be the focus for mocking and ridiculing on the playground which may be the reason for the child being withdrawn. Eyeglasses may also be limiting in certain physical activities and it is not uncommon for children to fear breaking or losing the glasses. This can further restrict activity and contribute towards a more introverted personality. However, as the prevalence of myopia has increased over the past three decades, more children are using corrective lenses and there is wider acceptance of eyeglass use.