SenSory SyStemS 281or a natural cornea from a donor (a cadaver) is stitched in
place. Within a year, the patient is fitted with eyeglasses or
contact lenses. Similarly, cataracts often can be surgically
corrected by removing the lens and replacing it with an
artificial one.
Severely nearsighted people may opt for pro cedures that
eliminate the need for corrective lenses. So-called Lasik (for
laser-assisted in situ keratomileusis) and lasek (for laser-
assisted subepithelial keratectomy) use a laser to reshape
the cornea. All or part of the surface of the cornea is peeled
back and then replaced into position after the defect being
treated is corrected. Conductive keratoplasty (CK) uses radio
waves to reshape the cornea and bring near vision back
into focus.
Retinal detachment is the eye injury we read about
most often. It may follow a blow to the head or an illness
that tears the retina. As the jellylike vitreous body oozes
through the torn region, the retina lifts away from the
underlying choroid. In time it may leave its blood supply
behind. Early symptoms include blurred vision, flashes of
light that occur in the absence of outside stimulation, and
loss of peripheral vision. Without medical help, the person
may become totally blind in the damaged eye.
A detached retina may be treatable with laser coagula-
tion, a painless technique in which a laser beam seals off
leaky blood vessels and “spot welds” the retina to the
underlying choroid.it can spread along the optic nerve to the brain, the affected
eye often is removed surgically. If both eyes are involved,
radiation therapy may be used to try to save one of them.
aging increases the risk of some types
of eye disorders
Clouding of the eye’s lens, or cataracts, is associated with
aging, although an injury or diabetes can also cause them
to develop. The under lying change may be an alteration
in the structure of transparent proteins that make up the
lens. This change in turn may scatter incoming light rays
(Figure 14.23A). If the lens becomes totally opaque, no light
can enter the eye.
Even a normal lens loses some of its natural flexibility as
we grow older. This normal stiffening is why people over
40 years old often must start wearing eyeglasses.
In macular degeneration part of the retina breaks
down and is replaced by scar tissue that results in a “blind
spot” (Figure 14.23B). Most cases of macular degeneration
are related to advancing age. Treatment is difficult unless
the problem is detected early. Glaucoma results when too
much aqueous humor builds up in the eyeball. Blood ves-
sels that service the retina collapse under the increased
pressure, and vision deteriorates as blood-starved neurons
of the retina and optic nerve die. Although chronic glau-
coma often is associated with advanced age, the problem
really starts in a person’s middle years. If detected early, the
fluid pressure can be relieved by drugs or surgery before
the damage becomes severe.
medical technologies can remedy some
vision problems and treat eye injuries
Today many different procedures are used to correct eye
disorders. In corneal transplant surgery, a defective cornea
is removed; then an artificial cornea (made of clear plastic)
Figure 14.23 Cataracts and macular degeneration
obscure vision. A Cataracts produce overall fuzzy
vision. b Macular degeneration causes a blind spot
in the center of the visual field.A
© National Eye Institute, U.S. National Institute of Healthb
© National Eye Institute, U.S. National Institute of HealthFigure 14.22 Conjunctivitis may be due to a bacterial
infection or an allergy.
Dr. P. Marazzi/Science SourceCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).