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Developmental Corneal Anomalies of Size and Shape
Key Concepts
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Developmental abnormalities of the cornea can occur as isolated defects or in association with other ocular or systemic anomalies.
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Megalocornea must be distinguished from buphthalmos due to congenital glaucoma, in which other features such as elevated intraocular pressure, Haab striae, optic disk changes, and enlargement of the entire globe are also present.
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Microcornea is diagnosed when the horizontal corneal diameter is less than or equal to 10 mm in an otherwise normal-sized globe as compared with anterior microphthalmos (the entire anterior segment is small), microphthalmos (the entire eye is small and disorganized), and nanophthalmos (the entire eye is small but otherwise normal).
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Sclerocornea is characterized by a flat cornea with progressive scleralization peripherally, then centrally; it often requires corneal transplantation.
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Patients with keratoglobus have a diffusely thinned cornea and are at risk for perforation with mild trauma.
This chapter is devoted to specific diseases appearing predominantly in early infancy or childhood as the result of disruption in the normal development of the cornea and its associated structures. These developmental abnormalities may arise because of one or a combination of various genetic, infectious, inflammatory, toxic, metabolic, traumatic, or mechanical processes and may occur at any time during tissue induction, differentiation, and maturation. Most of the etiologic factors are believed to exert their influence either during the period of organogenesis (between the fourth and sixth gestational weeks) or during the period of anterior segment differentiation (between the sixth and sixteenth gestational weeks). Developmental insults with an earlier onset result in more severe and extensive injury than those that take place at a later intrauterine date. Therefore not only the nature but also the timing of these insults contributes to determining the degree of damage and may aid the clinician in deducing the chronology of embryologic events. The exact time points of developmental insult as well as the pathophysiologic mechanisms and specific causative elements involved are often speculative and unknown.
Developmental disorders may be manifest at birth and refer to those conditions occurring secondary to some alteration or alterations of normal growth and differentiation ( Box 55.1 ). The descriptive term congenital refers to any condition that is evident at the time of birth and carries no implications as to etiologic process, mechanism of injury, or hereditary status. Therefore developmental abnormalities of the cornea should be considered as a separate and distinct subcategory of congenital disorders, since not all congenital corneal anomalies result from errors in development.
BOX 55.1
Developmental Corneal Anomalies of Size and Shape
Absence of the Cornea
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True absence of the cornea
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True cryptophthalmos (ablepharon)
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Pseudocryptophthalmos (total ankyloblepharon)
Anomalies of Corneal Size
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Megalocornea
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Microcornea
Anomalies of Corneal Shape
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Oval cornea
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Horizontal
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Vertical
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Astigmatism
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Sclerocornea (cornea plana)
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Posterior keratoconus
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Generalized
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Circumscribed
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Keratoglobus
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Congenital anterior staphyloma and keratectasia
Although rarely encountered in routine ophthalmic practice, developmental corneal abnormalities are important to recognize in the newborn or child for several reasons. Providing the correct diagnosis can inform the clinician concerning the natural history of the condition, indicate the necessary medical or surgical treatment, and determine appropriate scheduling of follow-up care. Prompt examination can also alert the physician to the various ocular and systemic complications that may accompany the disorder and warrant additional investigation by other subspecialists. Finally, accurate identification and analysis of the disease will help parents deal with the realities of the prognosis and guide them in seeking the proper genetic counseling when indicated.
The multitudes of other congenital anomalies causing corneal opacities that are not considered to be truly developmental in origin are addressed in Chapter 18 , and anterior chamber cleavage syndromes are discussed in Chapter 57 . The present chapter focuses on the developmental corneal anomalies of size and shape.
Developmental corneal anomalies of size and shape represent an interesting collection of conditions caused by a defect in development resulting in a departure from normal corneal structure. These alterations in normal development often involve the neighboring structures of the anterior chamber angle, iris, and lens. Some of the ocular and systemic associations with the various corneal anomalies involve the anterior segment and occur because of the close embryologic relationship between these structures and the cornea. However, some of these associations do not have any connection with the developmental process and merely reflect reported cases of various accompanying characteristics.
Absence of the Cornea
Although absence of the cornea does not reflect an actual anomaly of size or shape, it is discussed in this chapter because it represents an extreme deviation of normal corneal structure. True absence of the cornea is a very rare condition that is always accompanied by agenesis of various other anterior segment structures. Manschot described a case of primary congenital aphakia that was associated with a missing cornea. Congenital aphakia can present as an isolated anomaly or in combination with multiple somatic or complex anterior chamber anomalies. True absence of the cornea can never be an isolated finding due to its close relation to the embryologic differentiation of other anterior segment structures. The absence of the cornea, iris, lens, and other anterior structures lies on a spectrum of agenesis, with anophthalmos (absence of the entire eye) representing the most extreme example.
True cryptophthalmos (OMIM:123570), otherwise known as complete cryptophthalmos or ablepharon, is a rare congenital abnormality in which the eyelids are replaced with skin extending from the forehead to cover the orbit ( Fig. 55.1 ). The cornea and conjunctiva are present in this condition but undergo metaplastic change (termed dermoid transformation) to form skin. Cryptophthalmos is very rare, with an estimated prevalence of 0.20 per 100,000 births from one study in a European population. This condition is usually bilateral, although it can be asymmetric and has been reported to be an autosomal recessive trait and autosomal dominant in isolated cryptophthalmos. A novel homozygous mutation p.Arg 2167Trp in FREM2 has recently been reported in association with an isolated form of cryptophthalmos, which is also accompanied by the absence of lashes or brows, with the lacrimal glands and canaliculi frequently absent as well. , The eye itself usually contains a small or absent anterior chamber; the iris, lens, trabecular meshwork, and Schlemm canal are frequently nonexistent or otherwise replaced by connective tissue. Of the reported associated systemic abnormalities, craniofacial anomalies are the most common; others include syndactyly, spina bifida, deformed ears and teeth, cleft lip or palate, laryngeal or anal atresia, ventral hernias, displacement of the nipple or umbilicus, basal encephalocele, genitourinary anomalies, cardiac anomalies, and mental retardation. Cryptophthalmos can present as isolated eyelid anomalies or as part of Manitoba oculotrichoanal syndrome or Fraser syndrome. Specific criteria, as outlined by Thomas et al., are listed in Table 55.1 . ,
TABLE 55.1
Diagnostic Criteria for Cryptophthalmos Syndrome
| Major Criteria | Minor Criteria |
|---|---|
|
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For diagnosis of cryptophthalmos syndrome, patients must have at least two major criteria and one minor criterion or they may have one major criterion and four minor criteria.
Pseudocryptophthalmos (total ankyloblepharon) is a related condition in which the eyelids form but fail to separate, leaving a normal cornea and conjunctiva totally covered by skin (OMIM 611038). Unlike its true counterpart, in this condition both lashes and brows are present with an otherwise normal eye, and vision is restored by surgically creating a palpebral fissure.
With the exception of pseudocryptophthalmos, the two former conditions are associated with a very dismal visual prognosis and no indicated treatment. Besides proper education and counseling, the only warranted intervention may be for cosmetic purposes. Although pseudocryptophthalmos is associated with an excellent visual potential after surgery, the newly opened eyelids pose a continued challenge in terms of preventing lid reclosure and maintaining a functionally normal eyelid. Although management of cryptophthalmos, isolated or in combination with multiple anterior segment anomalies, is difficult, early surgical reconstruction of eyelid and fornix by amniotic and scleral graft may be an effective strategy to prevent exposure keratopathy in abortive cryptophthalmos. According to report, eyelid reconstruction using the preexisting scleral remnant in incomplete cases of cryptophthalmos has been successfully achieved.
Anomalies of Size
The size of a normal newborn cornea measures approximately 10 mm in horizontal diameter, whereas the size of a normal adult cornea measures approximately 12 mm in horizontal diameter. The most accepted average for measured diameter of the cornea has been reported to be 11.7 by 10.6 mm in horizontal diameter. The measured horizontal diameter of the normal cornea virtually always exceeds the vertical diameter by approximately 1 mm because of the prominent scleral encroachment present in the superior and inferior limbus. The cornea usually reaches adult size by 2 years of age.
Megalocornea
Megalocornea (MGC1; OMIM: 309300), as the name implies, refers to an enlarged cornea defined by a horizontal diameter greater than or equal to 13 mm. It is a nonprogressive and rare condition that is usually bilateral and symmetric. Because of its predominant transmission as an X-linked recessive trait, 90% of cases are found in the male population. Megalocornea is caused by mutations in CHRDL1 on Xq23. , Autosomal dominant, autosomal recessive, and sporadic cases have also been reported.
The clinical characteristics that have been described for megalocornea include an enlarged but clear cornea of slightly below normal or normal thickness ( Fig. 55.2 ). The cornea usually possesses a greater than normal steepness but can be normal by keratometry. It has been shown to be histologically normal in all respects, with a normal endothelial cell density. Although megalocornea is most frequently seen as an isolated condition, many ocular and systemic associations have been documented ( Box 55.2 ). The steeper cornea usually results in with-the-rule astigmatism and myopia. Many of the ocular associated findings involving the lens, iris, and iridocorneal angle are thought to exist secondary to the coexisting anatomic changes of an enlarged anterior segment and ciliary ring, sometimes referred to as a separate condition known as anterior megalophthalmos. The widened ciliary ring presumably causes zonular stretching, resulting in phacodonesis, iridodonesis, and ectopia lentis. The iris also undergoes anatomic stress, which manifests in stromal hypoplasia and transillumination defects along with subsequently increased trabecular meshwork pigmentation and formation of the Krukenberg spindle. These changes, coupled with an abnormal angle containing excess mesenchymal tissue and prominent iris processes, may predispose the eye to glaucoma.
BOX 55.2
Ocular and Systemic Associations of Megalocornea
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