Definition of Myopia
Myopia, commonly known as being nearsighted (American English) and shortsighted (British English), is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. This causes the image that one sees when looking at a distant object to be out of focus, but in focus when looking at a close object.
Eye care professionals most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, though there are cases of associated side effects. The corrective lenses have a negative optical power (i.e. have a net concave effect) which compensates for the excessive positive diopters of the myopic eye.
Causes of Myopia
Axial myopia is attributed to an increase in the eye’s axial length.
Refractive myopia is attributed to the condition of the refractive elements of the eye. Borish further subclassified refractive myopia:
- Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea. In those with Cohen syndrome, myopia appears to result from high corneal and lenticular power.
- Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.
Elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens as a result of sorbitol (sugar alcohol) accumulating in the lens. This edema often causes temporary myopia (nearsightedness).
Signs and Symptoms of Myopia
Myopia presents with blurry distance vision, but generally gives good near vision. In high myopia, even near vision is affected as objects must be extremely close to the eyes to see clearly, and patients cannot read without their glasses prescribed for distance. On fundoscopic examination of the eye, The optic nerve appears to be tilted and an area of white sclera could be seen on next to the disc with a line of hyperpigmentation separiting this area from normal retina. The macula will have some retinal pigmentary changes and sometimes will have subretinal heamorrages. The retina in myopic patients is thin and thorough evaluation of the periphery might show retinal holes and lattice degenration. In addition, myopic patients might develop choroidal neovascularization in the macula.
Risk Factors for Myopia
The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors.
The prevalence of myopia has been reported as high as 70–90% in some Asian countries, 30–40% in Europe and the United States, and 10–20% in Africa. Myopia is less common in African people and associated diaspora. In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.
A number of studies have shown the incidence of myopia increases with level of education, and many studies have shown a correlation between myopia and a higher intelligence quotient (IQ).
A 2008 literature review reported studies in several nations have found a relationship between myopia and higher IQ and between myopia and school achievement. A common explanation for myopia is near-work. Regarding the relationship to IQ, several explanations have been proposed. One is that the myopic child is better adapted at reading, and reads and studies more, which increases intelligence. The reverse explanation is that the intelligent and studious child reads more, which causes myopia. Still another explanation is that pleiotropic gene(s) affect the size of both brain and eyes simultaneously. According to the two most recent studies, higher IQ may be associated with myopia in schoolchildren, independent of books read per week.
Other personal characteristics, such as value systems, school achievements, time spent in reading for pleasure, language abilities and time spent in sport activities correlated to the occurrence of myopia in studies.
Heredity appears as an important factor associated with juvenile myopia, with smaller contributions from more near work, higher school achievement and less time in sports activity.
Diagnosis of Myopia
A diagnosis of myopia is typically confirmed during an eye examination performed by a specialized doctor who is an expert in refractive conditions of the eye, the optometrist, or by an ophthalmologist or orthoptist. Frequently an autorefractor or retinoscope is used to give an initial objective assessment of the refractive status of each eye, then a phoropter is used to subjectively refine the patient’s eyeglass prescription.
Prevention from Myopia
The National Institutes of Health says there is no known way of preventing myopia, and the use of glasses or contact lenses does not affect its progression. There is no universally accepted method of preventing myopia; proposed procedures have not been studied for effectiveness.
Commonly attempted preventive methods include wearing reading glasses, eye drops and participating in more outdoor activities as described below. Some clinicians and researchers recommend plus power (convex) lenses in the form of reading glasses when engaged in close work or reading instead of using single focal concave lens glasses commonly prescribed. The reasoning behind a convex lens’s possible effectiveness in preventing myopia is simple to understand: Convex lenses’ refractive property of converging light are used in reading glasses to help reduce the accommodation needed when reading and doing close work. Although accommodation is irrelevant in Medina’s quantitative model of myopia, it reaches the same conclusion. The model teaches a very simple method to prevent myopia.
For people with presbyopia, whose eyes’ lenses can not accommodate enough for very near focus, reading glasses help converge the light before it enters the eye to complement the refractive power of the eye lens, so near objects focus clearly on the retina. By reducing the focusing effort needed (accommodation), reading glasses or convex lenses essentially relax the focusing ciliary muscles and may consequently reduce chances of developing myopia. Inexpensive nonprescription reading glasses are commonly sold in drug stores and dollar stores. Alternatively, reading glasses fitted by optometrists have a wider range of styles and lens choices.
A Malaysian study reported in New Scientist suggested undercorrection of myopia caused more rapid progression of myopia. However, the reliability of these data has been called into question. Many myopia treatment studies suffer from any of a number of design drawbacks: small numbers, lack of adequate control group, failure to mask examiners from knowledge of treatments used, etc.
Pirenzepine eyedrops had a limited effect on retarding myopic progression in a recent, placebo-controlled, double-blind, prospective-controlled study.
Researchers at the University of Cambridge have found that a lack of outdoor play could be linked to myopia.
Treatment of Myopia
Eyeglasses, contact lenses, and refractive surgery are the primary options to treat the visual symptoms of those with myopia. Lens implants are now available offering an alternative to glasses or contact lenses for myopics for whom laser surgery is not an option. Orthokeratology is the practice of using special rigid contact lenses to flatten the cornea to reduce myopia. Occasionally, pinhole glasses are used by patients with low-level myopia. These work by reducing the blur circle formed on the retina, but their adverse effects on peripheral vision, contrast and brightness make them unsuitable in most situations.
Glasses may have the potential to make the eyes worse, as they increase the accommodation needed by the eyes to focus. Evidence of this can be seen when people with higher prescriptions have a harder time with activities like reading because their eyes grow tired faster. Stronger prescriptions require a higher accommodation by the eyes to focus through them, which can, over time, worsen eyesight, requiring yet another prescription, in a continuous but quickening cycle. Contact lenses of equivalent prescription may not result in the same effect as eyeglasses, as they are closer to the eyes and may require less accommodation.
Glasses work by using optical lenses bringing the image a viewer closer so that it can be focused by their myopic eyes. Large amounts of near work while wearing glasses can be very detrimental to the eyes and can be a cause of worsening nearsightedness. However, the eyestrain caused by not wearing glasses when they are needed can also be a risk factor. The best way to avoid needing new lenses is by reducing the amount of near work, which forces the eyes into a continuous near-focusing position that eventually causes or increases myopia, by taking frequent breaks from near work, and by only wearing glasses when they are needed. Reading glasses can also be worn during near work to decrease the strain on the eye, especially when already wearing corrective lenses, as they work in the opposite fashion to normal lenses. Using this practice may have the potential to prevent nearsightedness or slow its progression.
For people with a high degree of myopia, very strong eyeglass prescriptions are needed to correct the focus error. However, strong eyeglass prescriptions have a negative side effect in that off-axis viewing of objects away from the center of the lens results in prismatic movement and separation of colors, known as chromatic aberration. This prismatic distortion is visible to the wearer as color fringes around strongly contrasting colors. The fringes move around as the wearer’s gaze through the lenses changes, and the prismatic shifting reverses on either side, above, and below the exact center of the lenses. Color fringing can make accurate drawing and painting difficult for users of strong eyeglass prescriptions.
Strongly nearsighted wearers of contact lenses do not experience chromatic aberration because the lens moves with the cornea and always stays centered in the middle of the wearer’s gaze.
Various methods have been employed in an attempt to decrease the progression of myopia. Dr Chua Weihan and his team at National Eye Centre Singapore have conducted large scale studies on the effect of atropine of varying strength in stabilizing, and in some case, reducing myopia. The use of reading glasses when doing close work may provide success by reducing or eliminating the need to accommodate. Altering the use of eyeglasses between full-time, part-time, and not at all does not appear to alter myopia progression. The American Optometric Association’s Clinical Practice Guidelines for Myopia refers to numerous studies which indicated the effectiveness of bifocal lenses and recommends it as the method for “Myopia Control”. In some studies, bifocal and progressive lenses have not shown significant differences in altering the progression of myopia. More recently, robust studies on children have shown orthokeratology and centre distance bifocal contact lenses may arrest myopic development.
Scleral reinforcement surgery is aimed to cover the thinning posterior pole with a supportive material to withstand intraocular pressure and prevent further progression of the posterior staphyloma. The strain is reduced, although damage from the pathological process cannot be reversed. By stopping the progression of the disease, vision may be maintained or improved.
A number of alternative therapies exist including eye exercises and relaxation techniques, such as the Bates method. However, the efficacy of these practices is disputed by scientists and eye care practitioners. A 2005 review of scientific papers on the subject concluded that there was “no clear scientific evidence” that eye exercises were effective in treating myopia.
In the 1980s and 1990s, biofeedback created a flurry of interest as a possible treatment for myopia. A 1997 review of this biofeedback research concluded “controlled studies to validate such methods … have been rare and contradictory.” One study found that myopes could improve their visual acuity with biofeedback training, but that this improvement was “instrument-specific” and did not generalize to other measures or situations. In another study, an “improvement” in visual acuity was found, but the authors concluded this could be a result of subjects learning the task. Finally, in an evaluation of a training system designed to improve acuity, “no significant difference was found between the control and experimental subjects”.