MYOPIA

What is myopia?

Myopia, known more commonly as “nearsightedness”, is a disorder in which the refractive power of the eye’s optical media (i.e., the cornea and lens) is too strong relative to the length of the eyeball. Because of this, the image is focused in front of the retina rather than on it, resulting in a blurred image. There are two types of myopia:

Refractive myopia, which results when the optical media refract light too powerfully; and

Axial myopia wherein the eyeball is too long for eye’s high refractive power.

Myopia affects a large percentage of the North American population (approximately 25%) and appears to be affected by both genetic and environmental variables. It runs in families; children with myopic parents tend to have elongated eyeballs, and identical twins are more similar than are fraternal twins. But it also occurs more frequently in groups of people that are educated (i.e., people who do a lot of reading) or do other types of near work. For example, the prevalence of myopia is as high as 70% among Asian children who experience heavy schoolwork demands.

Animals deprived of form perception also tend to develop overly long eyeballs. Together, the research findings suggest that there is a genetically programmed feedback mechanism that is affected by environmental stimuli. They also highlight the importance of early visual experience in the normal development of the eyeball. Individuals with severe myopia are also at higher risk for retinal detachment relative to the general population.

Below is a simulated view of how myopes see the world. Compare the view seen with the normal, emmetropic eye with the view seen with the myopic eye. Notice the sharp focus of the bushes in the foreground in contrast to the blurry barn in the background.

How does myopia affect vision?

In contrast to the normally emmetropic eye, the myope's eye brings light into focus in front of the retina (pictured right). Because it is optically powerful, the myope’s eye can bring nearby objects to focus; more distant ones, however, are blurred. By bringing more distant objects closer, the focal plane is moved back toward the retina, image blur is reduced. The distance at which stimuli can be brought to focus on the retina is termed the far point (i.e., the farthest distance at which clear vision can be obtained).

Treatment

Myopia can be treated with corrective lenses, and more recently, by refractive surgery. To correct myopia optically, the eye care professional (optometrist or ophthalmologist) determines the strength of diverging (convex) lenses needed to move the focal plane back to the retina. Clinically, this type of lens is termed a negative sphere lens. The strength of the lenses (measured in diopters) depends on each eye’s the far point; an eye with a far point at 10 cm would need a –10.0 diopter negative sphere (1/0.10 m) and an eye with a far point at 100 cm (= 1m) would need a negative sphere corrective lens of only a –1.0 D (1.0/1.0 m).
Myopia can also be treated surgically by flattening the cornea to reduce its optical power. Several different procedures have been developed in recent years. These include:

Radial Keratotomy (RK) in which four to eight cuts are made in the cornea to reduce its refractive power. Caution has to be used with this procedure, as many people will experience over- or under-correction of vision, and for some the myopia may return.

Photorefractive Keratotomy (PRK) wherein the epithelium (outer layer) of the cornea is removed and an excimer laser is used to reduce the curvature of the cornea.

Laser Assisted in Situ Keratomileusis (LASIK) in which a hinged “flap” is lifted back from the outermost layer of the cornea and an excimer laser is used to reshape the underlying cornea. The flap is then put back in place. By lifting and returning this outer corneal flap, visual symptoms are minimized and recovery is more rapid.

EXPLORE MYOPIA: PAUL CÉZANNE

EXPLORE MYOPIA: AUGUSTE RENOIR