The cornea is the clear structure at the front of the eye. When
it is functioning normally it is not visible, just as a clean window
is not visible as you look through it. The cornea and the lens focus light
rays on the retina. The cornea has a rich supply of sensory nerves
from the fifth cranial nerve (V1), and injuries or dysfunction of
the cornea may therefore be quite painful. Corneal abnormalities
in children are rare, but important due to their potentially severe
effects on vision.
The cornea is approximately 0.5 mm thick. It has 5 layers (Figure 28–1). The anterior layer is the corneal epithelium, a
cellular layer that can regenerate rapidly (which is why corneal
abrasions usually heal quickly). Beneath the epithelium is the thin,
acellular Bowman’s membrane. The bulk of the cornea is
composed of the stroma, which is formed by a regular pattern of
collagen fibers that allow light rays to transmit clearly. Descemet’s
membrane lies posterior to the stroma. It is another very thin acellular
layer. The corneal endothelium lines the back of the cornea. The endothelium
functions as a pump to keep excess fluid from accumulating in the
stroma. It is critical in maintaining corneal clarity.
Normal cornea. The layers are (1) epithelium, (2) Bowman’s
membrane, (3) stroma, (4) Descemet’s membrane, and (5)
endothelium. (Photograph contributed by Morton Smith, MD.)
The normal corneal diameter is approximately 10 mm in newborns
and increases to 12 mm in adults. The normal cornea has no blood
vessels, which is another reason the cornea is clear and light rays
can be focused through it. Because there are no blood vessels in
the cornea, nutrition must be obtained via the tears anteriorly
and the aqueous humor posteriorly.
The cornea begins to form embryologically during the fourth week
of gestation when the lens vesicle separates from the surface ectoderm.
The ectoderm forms 2 layers. The innermost layer secretes collagen
that forms the corneal stroma. Mesenchymal cells derived from the
neural crest then migrate across the posterior surface of the cornea
to form the endothelium.
The cornea must be clear and regular to focus light properly,
and therefore most corneal diseases affect vision due to opacification.
The rich supply of nerves to the corneal epithelium may cause significant
discomfort in patients with corneal diseases.
Corneal diseases result from a variety of causes, including infection,
trauma, metabolic diseases, developmental anomalies, inherited dystrophies,
and as secondary effects of other ocular problems, such as glaucoma
or tear film insufficiency.
Many corneal abnormalities present with visible clouding of the
cornea. The opacities may be localized or diffuse. If the corneal
epithelium is involved, ocular discomfort will usually be present.
In children, this typically is manifest by light ...