known more commonly as nearsightedness, is a disorder
in which the refractive power of the eyes 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:
myopia, which results when the optical media refract
light too powerfully; and
myopia wherein the eyeball is too long for eyes
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.
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.
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.
does myopia affect vision?
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 myopes 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).
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 eyes 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:
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.
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.
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.
MYOPIA: PAUL CÉZANNE
MYOPIA: AUGUSTE RENOIR
| Cézanne | Degas
| El Greco | Monet
| Renoir | Van