The Cornea

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Last modified 08/03/2015

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6 The Cornea

THE NORMAL CORNEA

The cornea has three primary functions. These are:

To achieve these functions the cornea has evolved as an avascular structure with metabolic requirements supplied by diffusion. The main supply of oxygen to the epithelium and stroma is provided by atmospheric oxygen dissolved in the tear film with a small contribution from the limbal vessels peripherally; the endothelium obtains oxygen primarily from the aqueous. Glucose is similarly supplied from the tear film and aqueous. Aerobic metabolism produces carbon dioxide, which either diffuses away through cell membranes or is converted to bicarbonate and pumped into the aqueous by a carbonic anhydrase-dependent pump at the endothelial cell surface. Lactic acid, produced by anaerobic metabolism, cannot easily diffuse through the epithelial cell barrier and most diffuses posteriorly into the aqueous; corneal hypoxia leads to an accumulation of lactic acid and metabolic acidosis which contributes to corneal oedema.

The corneal epithelium is derived from surface ectoderm but the other corneal structures are formed from neural crest. After invagination of the lens vesicle, a layer of loose collagen fibrils between the ectoderm and the lens represents the corneal stroma. Mesenchymal cells from the perilimbic cell mass begin to form endothelium, and the stroma is invaded by perilimbic fibroblasts (future keratocytes) at about 6 weeks of gestation. At birth the cornea is relatively large compared to the rest of the globe and adult size is attained by about 2 years of age.

The cornea has a horizontal diameter of 11–12 mm that is reduced to 9–11 mm vertically by encroachment of the limbus. The central corneal thickness is 0.52 mm (normal range 0.49–0.56 mm) increasing to 0.7 mm peripherally. The central cornea has an anterior radius of curvature of 7.8 mm (43.5d) (normal range 7.0–8.5 mm, 39.5–48d) and a posterior radius of curvature of 6.8 mm (49.5d). The posterior surface faces the aqueous, which has a lower refractive index (1.336) so that the refractive power of this surface is about –6D. The average refractive index of the cornea is 1.376 and the axial refractive power is approximately 43D, which is about 74 per cent of the total dioptric power of the human eye.

NORMAL CORNEAL TOPOGRAPHY

The central 4 mm of the corneal surface is spherical but progressively flattens towards the periphery (prolate curvature). Keratometry measures the average curvature of the central 3 mm along its two principal meridians. Topography, which can be performed by reflection, projection or interference based systems, measures the shape (curvature, power or elevation) of the whole cornea.

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Fig. 6.9 With topography (see Ch. 1) a series of concentric rings is projected on to the surface of the corneal tear film. A difference in the relative distances between these rings compared to a calibrated spherical surface allows the corneal curvature to be measured from the visual axis to the periphery; this is then converted to dioptric power and displayed as a colour-coded topographical map in which colours towards the red end of the spectrum represent increasingly steep dioptric powers. A small range of dioptric powers can be seen on this normal cornea with flattening toward the periphery.

AGE-RELATED DEGENERATION

Involutional changes as a result of ageing must be distinguished from pathological changes.

CONGENITAL CORNEAL ANOMALIES

Congenital abnormalities of corneal diameter that are not associated with abnormality of thickness are rare and usually inherited. A small-diameter cornea (microcornea) may be associated with a small anterior segment of a small eye (nanophthalmos). A large-diameter cornea (megalocornea) is normally X-linked and must be distinguished from buphthalmos. Associated ocular and systemic abnormalities are common.

Anterior segment dysgenesis produces a spectrum of anomalies. The current clinical classification does not reflect the underlying genetic defect. Both the Axenfeld–Rieger and Peter’s anomalies can be caused by abnormalities of at least four different genes. Posterior embryotoxon, the mildest expression, represents a centrally displaced Schwalbe ring. It is commonly a normal variant and not associated with glaucoma. The Axenfeld–Rieger syndrome (see Ch. 8) consists of posterior embryotoxon with anterior iris adhesions, corectopia and iris hypoplasia, probably due to arrest of neural crest development. Peter’s anomaly does not usually have posterior embryotoxon or peripheral anterior iris adhesions but there is a central iris to cornea adhesion with a defect of the endothelium and posterior stroma. The significance of the Axenfeld–Rieger syndrome and Peter’s anomaly lies in their association with childhood glaucoma, buphthalmos, corneal oedema and blindness. They may also be associated with systemic defects such as dental and cranial anomalies and malformations of the upper limbs and spine. Posterior keratoconus causes thinning of the posterior stroma with overlying haze and may be congenital; it has therefore been classified as a dysgenesis although many cases are thought to result from trauma. Congenital absence of the limbus is often associated with flattening of the cornea, as is seen in sclerocornea and cornea plana.

CORNEAL DYSTROPHIES

The corneal dystrophies are inherited corneal diseases that exhibit a remarkable degree of phenotypic and genetic heterogeneity (Table 6.1). Molecular biology has given new insights into the pathogenesis of these diseases. While at present they are classified clinically or histopathologically, their future classification is likely to be genotypic. It is evident, however, that the distinction between some dystrophies is not as clear as had been thought as apparently different clinical appearances can be caused by mutations in the same gene. Conversely, clinically similar dystrophies can result from different genes on separate chromosomes determining different protein products.

Table 6.1 Inheritance of corneal dystrophies

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Many dystrophies are extremely rare and only the more common types are illustrated here. Dystrophies that involve the epithelium and anterior stroma may present with painful recurrent epithelial erosions and reduced vision from scarring; those involving the deeper cornea present only with loss of acuity. Endothelial dystrophies may progress to cause corneal oedema. Early or subclinical cases are sometimes found on routine examination or by examining family members.

EPITHELIAL DYSTROPHIES

STROMAL DYSTROPHIES

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