Corneal transplantation surgery takes place
with a local anesthetic to numb the eye and an
intravenous sedative medication for relaxation
and comfort. The operation takes 45 to 60 min-
utes. From the donor cornea, the ophthalmologist
uses a trephine, a device that cleanly punches out
a buttonlike segment of the cornea’s center. Using
a surgical microscope, the ophthalmologist then
removes a similarly shaped segment from the dis-
eased cornea and places the donor button in its
place. Very fine suture, sometimes thinner than a
human hair, secures the donor corneal button in
position and remains in the eye for three months
to one year. The ophthalmologist often removes
the sutures a few at a time as HEALINGprogresses,
using an ophthalmic anesthetic to numb the
affected eye, depending on the rate of vision
improvement. Some sutures may remain in place
indefinitely.
Full recovery typically takes about a year. Some
people will have ASTIGMATISMand require CORREC-
TIVE LENSES following corneal transplantation,
resulting from irregularities in the shape of the
cornea that develop during healing. Corneal trans-
plantation may correct another VISION IMPAIRMENT
such as HYPEROPIA(farsightedness) or MYOPIA(near-
sightedness) because the OPERATIONchanges the
shape of the cornea.
The most common complication of corneal
transplantation is rejection of the transplanted
cornea, which occurs overall in about 15 percent
of corneal transplantations. Rejection is most
likely to take place in the first two years after the
operation. Early detection and prompt treatment
with ophthalmic CORTICOSTEROID MEDICATIONS can
reverse the rejection process. Signs of rejection
include
- diminished VISUAL ACUITY
- PAIN
- redness of the eye
- PHOTOPHOBIA(sensitivity to light)
Other complications that can occur include
INFECTIONand bleeding within the eye.
See also ORGAN TRANSPLANTATION; PHOTOTHERAPEU-
TIC KERATECTOMY(PTK).
corrective lenses Eyeglasses or contact lenses
that alter the focal point of the lightwaves enter-
ing the EYEto correct REFRACTIVE ERRORSof vision,
including HYPEROPIA(farsightedness), MYOPIA(near-
sightedness), ASTIGMATISM (blurred or distorted
vision), and PRESBYOPIA (age-related hyperopia).
The eye’s natural focusing structures, the CORNEA
and the LENS, gather lightwaves and refract (bend)
them toward their centers. The cornea refracts the
lightwaves first. The lens, which can thicken or
flatten to refine its focal efforts, refracts the some-
what focused lightwaves that come to it from the
cornea. In normal vision, this sequence results in
the focal point of the lightwaves striking the
RETINA.
When refractive errors exist the focal point falls
in front of or behind the retina, resulting in
blurred images. Corrective lenses add a third level
of refraction to compensate for the error, bending
the lightwaves before they enter the cornea to
realign their focal point. The direction of refrac-
tion depends on the refractive error:- In myopia, the focal point falls short of the
retina. A lens that corrects for myopia bends
the lightwaves inward, narrowing the span of
light as it enters the cornea to lengthen the
focal point. Such a lens is thicker at the edges
than in the middle (concave); it is a minus
spherical correction. - In hyperopia, the focal point extends beyond
the retina. A lens that corrects for hyperopia
bends the lightwaves outward, broadening the
span of light as it enters the cornea to shorten
the focal point. Such a lens is thicker in the
center than at the edges (convex); it is a plus
spherical correction. - In astigmatism, irregularities in the surface of
the lens cause a second focal point. A lens that
corrects for astigmatism refracts along a specific
axis, realigning the lightwaves. This is a cylin-
der correction.
Corrective lenses can, and often do, combine
spherical and cylindrical corrections. A multifocal
lens further incorporates a correction for presby-
opia in the form of a bifocal, trifocal, or progres-
sive lens. The bottom of the lens is a plus section,82 The Eyes