Use of prosthetic cornea avoids problems of tissue
rejection
Two completely blind men can now see after being fitted with prosthetic
corneas
by Emory corneal transplant surgeon David Palay.
Both men had corneal blindness and between them had undergone nearly a
dozen
corneal transplantations with donor corneas. All prior surgeries failed
because
their bodies rejected the tissue transplant of the cornea, the clear
outermost
covering of the front of the eye. Although it is transparent and not very
visible, the cornea serves two main functions -- it both protects the eye
and
helps focus light rays onto the retina in the back of the eye. Corneal
grafts
are considered when most of the patient's eye is healthy and the patient
has
good vision potential, but with substantially impaired vision because of
a
corneal disorder.
Palay decided to apply the technique he learned from prosthetic cornea
pioneer
Claes H. Dohlman of Harvard to treat three difficult cases. The procedure
worked "miraculously" in two of the three cases.
The procedure is called keratoprosthesis. It is more of an implantation
than
transplantation, since human tissue is not involved. The collar
button-shaped
prosthesis used during the procedure is made of a plastic called
polymethylmethacrylate (PMMA).
Implications of the procedure
Keratoprosthesis is important because of implications that reach beyond
the
United States. Corneal blindness is fairly rare in the United States and
in
other industrialized countries, but it is one of the most common causes
of loss
of sight in developing parts of the world because of parasitic diseases.
Vitamin A deficiency, common in severely undernourished children, can
cause a
condition called keratomalacia, in which the cornea becomes soft and
often
perforates. Keratomalacia is a major cause of blindess in some tropical
countries. Corneal blindness in the United States, however, is often due
to
trauma and inherited corneal diseases.
Made up of five layers, the cornea has inner cells that pump excess
water out
of the cornea to keep it clear and transparent. If those cells, which in
adults
cannot reproduce themselves, are severely damaged, there may be permanent
clouding of the cornea.
Corneal transplantation
Palay also is one of a few eye surgeons in the state performing corneal
transplantation on infants and children. He uses a "cookie cutter" type
of tool
called a trephine to remove a circle of corneal tissue. Donor tissue cut
into a
circle is sutured into the opening. Palay performs this on infants and
children
as well as adults. He is beginning to use the immune suppressant
cyclosporine
topically in infants and children as they have a high risk of organ
rejection
-- and he is getting good results.
--Lorri Preston