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.
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.
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 sensitivity, excess tearing, and squinting of the eyes in bright light.
Depending on the etiology, corneal opacification may be accompanied by other signs or symptoms. Traumatic or infectious lesions, for instance, will usually have associated ocular inflammatory changes, such as conjunctival or eyelid erythema and swelling. Patients with metabolic diseases usually have systemic manifestations of the underlying disorder. The specific findings for different diseases are described in the following sections.
The normal corneal diameter at birth is approximately 10 mm. Microcornea is present if a newborn’s corneal diameter is less than 9 mm (Figure 28–2), and megalocornea if the diameter is greater than 12 mm (Figure 28–3). These disorders can be familial, and may be associated with other ocular abnormalities. If isolated, ocular function may be otherwise normal.
The diagnosis of most infantile corneal lesions is based on the morphology of the opacity, as well as the patient’s history and associated systemic abnormalities. Infantile corneal opacities may cause profound amblyopia, similar to that which occurs in patients with infantile cataracts. Prompt diagnosis and treatment is needed to maximize the patients’ visual potential.
Sclerocornea presents at birth with extension of the white sclera onto the corneal surface (Figure 28–4). The scleralization does not always cover the entire cornea, and clear areas may be present centrally. Most patients have other ocular abnormalities. Corneal transplantation may be indicated if the opacity involves the central cornea, but the visual prognosis is guarded.
Peter’s anomaly results from disruption of the central corneal endothelium. The appearance is the opposite of sclerocornea, in that the peripheral cornea is usually clear in Peter’s anomaly, and there is a central circular area of corneal clouding. Strands of iris tissue often are attached at the border of the opacity (Figure 28–5). Patients with Peter’s anomaly may also have cataracts (Figure 28–6). In severe cases, the lens may be adherent to the corneal defect (Figure 28–7). Glaucoma occurs in approximately 50% of patients with Peter’s anomaly.
The visual prognosis for Peter’s anomaly depends on the size and density of the opacity, and whether cataracts or glaucoma is present. The vision may be good if the opacity is small. Larger lesions, particularly those involving the lens, have poor vision potential. Unilateral Peter’s anomaly is usually an isolated defect. Bilateral lesions are more likely to be associated with systemic abnormalities.
Anterior segment dysgenesis includes a spectrum of abnormalities of the cornea and iris. The mildest form is posterior embryotoxon, in which Schwalbe’s line (the anterior edge of the trabecular meshwork) is anteriorly displaced. Schwalbe’s line cannot usually be seen, but it is visible in patients with posterior embryotoxon as a thin white strip at the peripheral cornea (Figure 28–8A and B). Posterior embryotoxon itself does not cause visual problems. It is a common feature of arteriohepatic dysplasia (Alagille syndrome).
Axenfeld-Rieger syndrome is a more severe form of anterior segment dysgenesis, in which posterior embryotoxon is accompanied by strands of iris to the anteriorly displaced Schwalbe’s line, and variable degrees of iris hypoplasia (Figure 28–9). Glaucoma develops in approximately 50% of patients.
Different genetic causes of Axenfeld-Rieger syndrome have been identified, including mutations of the PITX2 and FOXC1 genes. It is usually transmitted as an autosomal dominant disease. Systemic manifestations may include dental, umbilical, and cardiac abnormalities.
Corneal dermoids are choristomas that present as elevated nodules at the junction of the cornea and sclera inferotemporally (Figure 28–10). They are usually not large enough to directly block vision, but they may cause amblyopia secondary to induced astigmatism. They may be isolated or occur in association with Goldenhar syndrome. In addition to dermoids, other ocular abnormalities in Goldenhar syndrome include upper eyelid colobomas and Duane retraction syndrome. Surgical excision of corneal dermoids may be performed if the lesions cause decreased vision or irritation, or to improve appearance (Figure 28–11). Even with successful removal, patients usually have persistent astigmatism and require continued treatment for amblyopia.
Corneal forceps injuries occur due to compression of the eye by forceps during difficult deliveries. Affected newborns usually have associated bruising and swelling of the eyelid and periocular tissue (Figure 28–12A). The corneal opacities in forceps injuries result from tears in Descemet’s membrane and damage to the corneal endothelium. The normal pump mechanism of the endothelium is disrupted and fluid accumulates in the corneal stroma, producing clouding. The corneal clouding is usually diffuse in the immediate postpartum period, and hemorrhages may be visible (Figure 28–12B).
FIGURE 28–12
Corneal forceps injury. (A) Patients usually present with associated bruising and swelling of the eyelids due to the forceps. (B) Initial appearance of cornea in the newborn period. Note clouding of the cornea and linear corneal hemorrhages (following the path of forceps compression). (C) Late appearance. Linear Haab striae (due to linear tears in Descment’s membrane).
As the cornea heals and the endothelial pump regains its function, Haab striae become visible. These are parallel lines at the edges of the breaks in Descemet’s membrane (Figure 28–12C). In forceps injuries they are oblique, following the same angle as the forceps. Haab striae are also common in infantile glaucoma, but in glaucoma they are curvilinear (Figure 28–13, see discussion below). The corneal damage in forceps injuries often causes marked astigmatism, and patients are at high risk for amblyopia. Careful ophthalmic monitoring with patching and glasses increases the chances for a good outcome.
Corneal opacities may result from intrauterine or postpartum infections. A leading cause of congenital corneal infections is ophthalmia neonatorum, in which organisms are usually acquired during passage through the birth canal. Most forms of ophthalmia neonatorum present primarily as conjunctivitis, without direct corneal involvement. Neisseria gonorrhoea, however, may directly infect the cornea, leading to potentially severe consequences such as ocular perforation. Affected infants are also at risk for systemic infection, including sepsis and arthritis. Prompt systemic antibiotic treatment is indicated.
Neonatal herpes simplex virus (HSV) may affect the cornea, as well as the conjunctiva, retina, and lens. Corneal involvement may include the corneal epithelium or stroma. Infants with congenital HSV have a high risk of disseminated disease and systemic complications. Intravenous treatment with antiviral medication, such as acyclovir, is required.
These are rare disorders that present with cloudy corneas in infancy. Patients with congenital hereditary endothelial dystrophy (CHED) have diffusely thick and opacified corneas. The appearance may be similar to that of infantile glaucoma (Figure 28–14). Congenital hereditary stromal dystrophy (CHSD) presents with clouding of the stroma. This is distinguished from CHED by a normal corneal thickness and normal corneal epithelium. CHED occurs in autosomal recessive and autosomal dominant forms. CHSD is autsomal dominant.
