Chapter 28

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.

###### FIGURE 28–1

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.

### Embryology

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 ...

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