CORNEA, DISORDERS OF
CORONARY ARTERY
the
retina
at the back of the eye and to
protect the front of the eye. It is kept
moist by tears produced by the
lacrimal
gland
and the mucus- and fluid-secret-
ing cells in the eyelids and
conjunctiva
.
cornea, disorders of
Injuries or dis-
eases affecting the cornea, the outer
shell of the eyeball. Injuries include
corneal abrasions,
which sometimes be-
come infected and progress to a
corneal
ulcer
. Penetrating corneal injuries can
cause scarring, which may lead to im-
pairment of vision. Chemical injuries can
result from contact with a corrosive
substance and require immediate flush-
ing of the eye with water.
In actinic
keratopathy
, the outer layer
of the cornea is damaged by ultraviolet
light. In exposure keratopathy, damage
is due to reduced protection by the tear
film and blink reflex. The cornea can also
be infected by viruses, bacteria, and fungi,
the
herpes simplex
virus being especially
dangerous. True inflammation of the cor-
nea (called
keratitis
) is uncommon as
the cornea contains no blood vessels.
Other disorders include:
keratomalacia
as a result of vitamin A deficiency;
ker-
atoconjunctivitis sicca
(dry eye); corneal
dystrophies such as
keratoconus
; and
oedema, in which fluid builds up in the
cornea and impairs vision. Rare con-
genital
defects
include
microcornea
(smaller cornea than normal) or mega-
locornea (bigger than
normal) and
buphthalmos
,
or “ox-eye'', in which the
entire eyeball is distended as a result of
glaucoma
. Degenerative conditions of the
cornea such as calcium deposition, thin-
ning, and spontaneous ulceration occur
mainly in the elderly, and are more
common in previously damaged eyes.
corneal abrasion
A scratch or defect
in the
epithelium
(outer layer) of the
cornea
caused by a small, sharp particle
in the eye (see
eye, foreign body in
) or
by an injury. Corneal abrasions usually
heal quickly but may cause severe pain
and
photophobia
. Treatment includes
covering the eye with a patch,
analgesic
drugs
to relieve pain, and, if the eye
muscles go into spasm, eyedrops con-
taining cycloplegic drugs (which paralyse
the ciliary muscle, preventing
accommo-
dation
).
Antibiotic
eyedrops are usually
given to prevent bacterial infection,
which can lead to a
corneal ulcer
.
corneal graft
The surgical transplanta-
tion of donor corneal tissue to replace a
damaged
cornea
. In most grafts, tissue is
taken from a human
donor
after death.
The success rate of corneal grafts is
generally high, because the cornea has
no blood vessels; this reduces access
for white blood cells, which can cause
rejection
of the donor tissue.
corneal transplant
See
corneal graft.
corneal ulcer
A break, erosion, or open
sore in the
cornea
commonly caused by
a
corneal abrasion
. It may also be due to
chemical damage, or infection with
bac-
teria, fungi,
or
viruses
(particularly with
the
herpes viruses
). Eye conditions such
as
keratoconjunctivitis sicca
and eyelid
deformities such as
entropion
or
ectro-
pion
increase the risk of an ulcer.
Corneal ulcers are revealed by intro-
ducing
fluorescein
dye into the eye.
Infections and predisposing eye condi-
tions are treated according to their
cause. A superficial, noninfectious ulcer
usually heals quickly; if it fails to do so,
it may be treated with a “bandage'' con-
tact lens or
tarsorrhaphy
.
coronary
Any structure that encircles
like a crown. The term usually refers to
the
coronary arteries.
It is also some-
times used as a nonmedical term for a
heart attack (see
myocardial infarction
).
coronary artery
Either of the 2 main
arteries
that supply the tissues of the
heart with oxygen-rich blood. These are
CORONARY ARTERY
Superior
Aorta
vena cava
Pulmonary
veins
Left main
coronary
artery
Left
circumflex
artery
Right
main
coronary
artery
Inferior
vena cava
Left
anterior
descending
artery
Coronary
vein
Descending
aorta
146
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