CORNEA
Cross-section of a corn
A plug of dead skin cells extends through the
epidermis into the dermis, which has a nerve
supply. Pressure on the plug can cause pain.
cornea
The
transparent,
thin-walled
surface
that forms the front of the eyeball. The
cornea has two main functions. It helps
to focus light rays on to the
retina
at the
back of the eye, and protects the front
of the eye from debris and injury.
The cornea is joined at its circumfer-
ence to the
sclera
(white of the eye);
the black
pupil
and the coloured
iris
are
visible beneath it. There are five layers.
The outermost layer (the epithelium)
protects the eye and absorbs oxygen
and nutrients from tears. The central
layer (the stroma) is by far the thickest
and gives the cornea its form. The inner
layer (the endothelium) expels excess
fluid from the cornea, thus keeping the
tissues transparent.
For the cornea to stay healthy, it must
be kept moist and clean. It is kept moist
by a film of
tears
,
w hich are produced
by the
lacrimal gland
and by the mucus-
and fluid-secreting cells in the eyelids
and
conjunctiva
.
Further protection is
provided by the eyelids, w hich blink or
close to keep out debris. In addition,
the cornea is very sensitive, and imme-
diately registers the presence of any
injury or foreign body.
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
of
a
corneal
abrasion
includes covering the eye with a patch
and
analgesics
to relieve pain. If the cil-
iary muscles go into spasm, eyedrops
containing cycloplegic drugs may be
used to paralyse the
ciliary muscle.
Antibiotic
eyedrops are usually given to
prevent bacterial infection (w hich can
lead to a
corneal ulcer
) .
corneal graft
The surgical transplantation 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 gener-
ally high, because the cornea has no
blood vessels; this reduces access for
white blood cells, w hich could other-
wise cause
rejection
of the donor tissue.
corneal transplant
See
corneal graft.
corneal ulcer
A break, erosion, or open sore in the
cornea
.
It usually affects the outer layer
of the cornea, but in some cases may
penetrate down to the middle layer.
Corneal ulcers are commonly caused
by a
corneal abrasion
.
They may also be
due to chemical damage, or infection
with
bacteria
,
fungi
,
or
viruses
(partic-
ularly
herpesvirus
) .
Eye conditions such
as
keratoconjunctivitis sicca
and
eyelid
deformities such as
entropion
or
ectropi-
on
increase the risk of an ulcer.
Ulcers are revealed by introducing
fluorescein
dye into the eye. Infections
and predisposing eye conditions are
DISORDERSOFTHE CORNEA
Various injuries or conditions can affect
the sensitive cornea.
Injury
A
corneal abrasion
(scratch) can become
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 such as an acid or alkali.
In
actinickeratopathy,
the outer layer
of the cornea is damaged by ultraviolet
light. In exposure keratopathy, damage
occurs as a result of reduced protection
from the tear film and blink reflex.
Infection
The cornea can be infected by viruses,
bacteria, or fungi. Some infections can
cause ulceration, the
herpes simplex
virus being especially dangerous.
Inflammation
True inflammation of the cornea (called
keratitis
) is uncommon because the
cornea contains no blood vessels.
Congenital defects
Rare congenital defects include
microcornea (a cornea that is smaller
than normal) or megalocornea
(one that is bigger than normal) and
buphthalmos,
or “ox-eye’’, in which
the entire eyeball is distended as a
result of
glaucoma
(raised fluid
pressure in the eyeball).
Degeneration
Degenerative conditions of the cornea
include calcium deposition, thinning,
and spontaneous ulceration. Such
conditions occur mainly in elderly
people, and are more common in
previously damaged eyes.
Other disorders
Other disorders include:
keratomalacia,
which may result from vitamin A
deficiency;
keratoconjunctivitis sicca
(dry eye);corneal dystrophies, such
as
keratoconus,
in which the cornea
becomes thinner and cone-shaped;
and oedema, in which fluid builds up
inside the cornea and impairs vision.
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