Congenital Abnormalities and Metabolic Diseases Affecting the Conjunctiva and Cornea
Nadia K. Waheed
Nathalie Azar
CONGENITAL ABNORMALITIES
Clinical Aspects
Congenital anomalies of the cornea are the result of abnormal corneal development and are evident as alterations in the morphology of the cornea at birth. This is in contrast to metabolic diseases of the cornea and to corneal dystrophies, which occur in previously normal tissue and appear clinically only some time after birth.
Absence of the Cornea and Agenesis of the Anterior Segment
Agenesis of the cornea is unknown as an isolated abnormality. In such cases, there is usually variable absence of other ocular structures derived from the surface ectoderm (1). The result is a scleral shell lined with choroid, retinal pigment epithelium, and retina, but lacking cornea, anterior chamber, iris, ciliary body, and lens. The affected eye is usually small. Ultrasonography may be used to distinguish this condition from true cryptophthalmos. Embryologically, this abnormality occurs when the optic vesicle forms and invaginates to form the optic cup, but the anterior segment fails to differentiate. This abnormality is a form of microphthalmos because it occurs after the formation of the optic vesicle and because the affected eye is usually small.
Cryptophthalmos
True cryptophthalmos (ablepharon) happens when the lids fail to form (1, 2, 3). The exposed cornea undergoes metaplasia to skin and so appears to be absent. Because the skin covering the eye is essentially metaplastic corneal tissue, the brows and lashes are absent, which allows for easy differentiation of cryptophthalmos from pseudocryptophthalmos (total ankyloblepharon), in which brows and lashes are present. In true cryptophthalmos, the lacrimal gland and puncta are likely to be missing as well, and the anterior segment of the globe is usually disorganized (4). Affected patients have a layer of skin extending from the forehead to the malar region. An incomplete form in which only the nasal aspect of the lid fold is involved is recognized. Still another presentation is the abortive form, in which the upper eyelid is replaced by a fold of skin that is adherent to the upper third of the cornea; the lower eyelid is normal (5). Even in the complete form, the underlying eye moves and may even show some reaction to bright light in the form of contractions of the periocular skin. However, attempts to treat it are futile, because any incision into the overlying skin will result in entry into the malformed eye. The only advantage of a cutaneous incision may be some slight cosmetic benefit. Embryologically, cryptophthalmos results from a failure of formation of the eyelid folds.
Cryptophthalmos is rare, with only about 50 cases reported. It is usually transmitted as an autosomal-recessive trait and may be unilateral or bilateral. When it is unilateral, the other eye may have symblepharon or coloboma of the eyelid. Males and females are affected equally.
When cryptophthalmos occurs in association with other systemic abnormalities, as it often does, the condition is described as cryptophthalmos syndrome. Associated systemic abnormalities include, most commonly, syndactyly, genitourinary anomalies, and craniofacial anomalies. Less commonly, spina bifida, deformed ears or teeth, cleft palate or lip, laryngeal or anal atresia, ventral hernias, cardiac anomalies, displacement of the nipples or umbilicus, basal encephaloceles, and mental retardation may also occur (2,5, 6, 7). Renal agenesis has been documented in siblings of patients with this syndrome.
Abnormalities of Size
Megalocornea
Megalocornea is characterized by a primarily enlarged diameter of the cornea (more than 13 mm in horizontal diameter) in the absence of previous or concurrent elevated intraocular pressure (1, 2, 3). The corneal enlargement may occur as an isolated anomaly (simple megalocornea) or in association with enlargement of the ciliary ring and lens (anterior megalophthalmos) (9). Simple megalocornea is usually a nonprogressive, usually symmetric, inherited condition. Megalocornea is usually X-linked recessive, with 90% of cases in males, although all forms of inheritance have been reported, including occasionally dominant, less often recessive and germ-line mosaicism also reported (10).
Some patients with megalocornea are myopic because of increased corneal curvature, although the curvature may also be normal. With-the-rule astigmatism is often present when the corneal curvature is increased. Megalocornea can be differentiated from congenital glaucoma by the normal intraocular pressure, the clarity of the cornea, and the normal optic nerve in simple megalocornea. Moreover, megalocornea demonstrates normal endothelial cell population densities on specular microscopy, whereas in congenital glaucoma, these are diminished, ostensibly because of corneal distention (11). Studies have also suggested the use of A-scan ultrasonography to distinguish features of megalocornea that are not present in glaucoma, including increased anterior chamber depth, posterior lens and iris positioning, and short vitreous length (12). Some people believe megalocornea represents congenital glaucoma that has been arrested, but a histopathologic report on an eye with megalocornea did not show the angle abnormalities classically seen in congenital glaucoma. However, both megalocornea and congenital glaucoma have been reported in the same families and even in the same person (13,14).
Anterior megalophthalmos is associated with enlargement of the lens-iris diaphragm and ciliary body in addition to the cornea. This condition may be associated with a large myopic astigmatic error. The iris may demonstrate iris transillumination defects. The condition is usually harmless except for three complications that may appear later in life: ectopia lentis due to the abnormal architecture; glaucoma secondary to lens subluxation; and cataract, which is usually posterior subcapsular but may be nuclear or peripheral. Other associated abnormalities include Marfan’s syndrome, Apert’s syndrome, and mucolipidosis type II (1, 2, 3,14,15).
Megalocornea is probably the result of a failure of the anterior tips of the optic cup to grow sufficiently close to one another, the remaining space being taken up by the cornea. Other possible explanations are that it represents an exaggeration of the normal tendency for the cornea to be large, relative to the rest of the eye, from embryonic life to the age of 7 years; an atavistic regression to the tendency for nonhuman mammals to have larger corneas relative to their globes; or spontaneously arrested congenital glaucoma.
TABLE 39-1. ABNORMALITIES ASSOCIATED WITH MEGALOCORNEA | ||||||||||||||||||||||||||||||||||||||
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Table 39-1 lists other abnormalities associated with megalocornea.
Microcornea
A microcornea is one that has an adult horizontal diameter of less than 11 mm (1,3). Note that the cornea usually reaches its adult size around 2 years of age. Microcornea can occur as an isolated anomaly, or the whole anterior segment may be small, in which case the term anterior microphthalmos applies. Nanophthalmos indicates an eye that is small but otherwise normal, and microphthalmos refers to a small eye that is also malformed in other ways.
Patients with microcornea are likely to be hyperopic because their corneas are relatively flat, but any kind of refractive error is possible owing to variations in length of the globe. Open-angle glaucoma develops later in life in 20% of patients, and some predisposition to narrow-angle glaucoma also exists because of the shallow anterior chamber with crowding of the anterior chamber structures seen in anterior microphthalmos. Congenital glaucoma coexists occasionally. An eye with microcornea (or microphthalmos) is sometimes misinterpreted as being normal in comparison to its fellow (actually normal) eye, which is thought erroneously to have corneal enlargement from congenital glaucoma. Certain somatic abnormalities have been described in conjunction with microcornea and anterior microphthalmos, including dwarfism and Ehlers-Danlos syndrome. Table 39-2 lists other problems that are sometimes associated with microcornea (1,16,17).
Microcornea is thought to be caused by an overgrowth of the anterior tips of the optic cup, leaving less than normal
space for the cornea. It may be transmitted as a dominant or recessive trait, the former being more common.
space for the cornea. It may be transmitted as a dominant or recessive trait, the former being more common.
TABLE 39-2. ABNORMALITIES ASSOCIATED WITH MICROCORNEA | ||||||||||||||||||||||||||||||||||||||||||||||||
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Abnormalities of Shape
Horizontally Oval Cornea
The cornea is normally horizontally oval when viewed from in front, with the horizontal diameter approximately 1 mm larger than the vertical diameter (although it is round when seen from the back). The oval appearance is caused by greater scleral encroachment above and below than in the horizontal meridian. An exaggeration of the normal oval shape usually indicates the presence of some degree of sclerocornea (1,3).
A horizontally oval, bifid cornea attributed to maternal ingestion of large amounts of vitamin A throughout pregnancy has been reported (18). The condition was unilateral and manifested a cornea and iris having roughly the shape of an hourglass lying on its side. Reduplicated but clear crystalline lenses were also present.
Vertically Oval Cornea
A vertically oval cornea sometimes occurs in association with iris coloboma, Turner’s syndrome (ovarian dysgenesis, XO karyotype), or intrauterine keratitis (usually from congenital syphilis) (1,13,16,19,20). It is interesting that the luetic interstitial keratitis can appear before or after the observation of the abnormal shape of the cornea.
Abnormalities of Curvature
Cornea Plana
Cornea plana (flat cornea) is seldom an isolated entity. It is more often seen in association with microcornea or sclerocornea (1,4,21). The sclerocornea is likely to be more prominent above and below, so that the cornea appears to be horizontally oval as well. The limbus in cornea plana is usually indistinct, whereas it is typically well defined in simple microcornea.
Cornea plana often produces hyperopia, but the refractive error is unpredictable because the length of the globe varies. The cornea itself must have a radius of curvature of less than 43 diopters if it is to be designated as a cornea plana, but measurements of 30 to 35 D are more common. A keratometry reading of as low as 23 D has been reported (4). A corneal curvature that is the same as that of the sclera is almost pathognomonic for cornea plana. The cornea is even flatter than the sclera in some cases. The anterior chamber is shallow, and angle-closure glaucoma is not uncommon. The incidence of open-angle glaucoma is also increased. Other possible abnormalities are listed in Table 39-3 (1,3,4,21).
Cornea plana is thought to be the result of a developmental arrest in the fourth month of fetal life, when the corneal curvature normally increases relative to that of the sclera. The heredity may be dominant or recessive. The recessive form is more severe and can be complicated by the presence of central corneal opacities. Cornea plana is especially likely to occur in patients of Finnish extraction (4).
TABLE 39-3. ABNORMALITIES ASSOCIATED WITH CORNEA PLANA | ||||||||||||||||||||||||||||||||||||||||||||||
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Anterior Keratoconus and Keratoglobus
Anterior keratoconus usually develops during the first two decades of life and is only rarely evident at birth. Keratoglobus (globular cornea) is not infrequently congenital, but it, too, can appear after birth. Both of these corneal ectasias are usually classified as corneal dystrophies and so are not discussed here, although some of their features are summarized in Table 39-4.
Generalized Posterior Keratoconus
In this condition, the entire posterior surface of the cornea has an increased curvature, that is, it has a shorter radius of curvature and so is more strongly curved, whereas the contour of the anterior surface remains normal (1,22, 23, 24). The differential features of anterior keratoconus, keratoglobus, generalized posterior keratoconus, and circumscribed posterior keratoconus (discussed later) are given in Table 39-4 (1,22, 23, 24).
Generalized posterior keratoconus is the least common of the four disorders. It probably represents a developmental arrest, as the posterior surface of the cornea is normally more curved during fetal life (24). Generalized posterior keratoconus is usually unilateral. All examples have been in women, but there is no evidence of hereditary transmission. Central corneal thinning is present, but the condition is nonprogressive, and the vision is normal unless there is associated clouding of the cornea, which seldom occurs.
Keratectasia
Keratectasia is characterized by the presence of a bulging, opaque cornea that protrudes through the palpebral aperture (1,3,25). Most cases are unilateral and are probably the result of intrauterine keratitis; corneal perforation in utero causes the cornea to undergo metaplasia to tissue resembling skin (dermoid transformation). The metaplasia involves only the cornea and does not extend over the entire eye to the area of the lids, as occurs in cryptophthalmos.
TABLE 39-4. COMPARATIVE FEATURES OF ANTERIOR KERATOCONUS, POSTERIOR KERATOCONUS, AND KERATOGLOBUS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Some examples of keratectasia may be caused by a failure of mesenchyme to migrate into the developing cornea, resulting in subsequent corneal thinning, bulging, and metaplasia, with or without preceding perforation.
Congenital Anterior Staphyloma
Abnormalities of Structure
Anterior Segment Dysgenesis
Anterior segment dysgenesis (ASD) was formerly called mesodermal dysgenesis (anterior chamber cleavage syndrome), but evidence indicates that the affected embryonic tissues probably originate from the neuroectoderm of the neural crest rather than from the mesoderm (26,27). These problems may be thought of as a spectrum in which any of several abnormalities may exist alone or in various combinations (1,14,22,25,28, 29, 30). Some of the more frequently
occurring combinations are given eponymic designations such as Rieger’s anomaly, Peters’ anomaly, and others.
occurring combinations are given eponymic designations such as Rieger’s anomaly, Peters’ anomaly, and others.
In trying to understand this subject, it is helpful to review some of the embryology of the anterior segment of the eye (3,22,31). After separation of the lens vesicle, the surface ectoderm forms a layer that becomes corneal epithelium. Three waves of tissue then invade the primary mesenchyme that lies behind the surface ectoderm: the first wave gives rise to corneal endothelium, the second forms corneal stroma, and the third becomes the iris stroma. These waves of tissue (secondary mesenchyme) were once thought to be mesodermal, thus giving rise to the concept of mesodermal dysgenesis, but are now widely held to be of neural crest origin and the term most commonly used now is that of anterior segment dysgenesis.
During early development, there is no anterior chamber, the entire area being filled with primary or secondary mesenchyme. This gradually recedes, and its remnant in the form of the pupillary membrane begins to undergo atrophy at about the seventh month. The angle recess does not become fully opened until sometime during the first year after birth.
Three hypotheses have been proposed to explain the disappearance of mesenchyme and the consequent formation of the anterior chamber (25,26,28,32,33). The first idea was that the mesenchyme disappears by means of atrophy and absorption. The next explanation was that it is pulled apart passively as a result of different growth rates of the anterior tissues; there is no evidence to support this idea and so the term anterior segment cleavage syndrome is rarely used. The latest, and most plausible, explanation for the abnormalities that occur in conjunction with the development of the anterior chamber is that they represent abnormal migration, proliferation, or final differentiation of secondary mesenchymal cells that originate from the neural crest (26). This concept accounts also for the fact that associated abnormalities of the head and face are often present.
The various ASDs are now classified as follows: (a) abnormalities of neural crest cell migration (congenital glaucoma, posterior embryotoxon, Axenfeld’s anomaly and syndrome, Rieger’s anomaly and syndrome, Peters’ anomaly, and sclerocornea); (b) abnormalities of neural crest cell proliferation (essential iris atrophy, Chandler’s syndrome, and Cogan-Reese iris nevus syndrome); and (c) abnormalities of neural crest cell final differentiation (congenital hereditary endothelial dystrophy, posterior polymorphous corneal dystrophy, congenital cornea guttata, and Fuchs’ corneal dystrophy). Other abnormalities such as prominent iris processes, dysgenesis of the iris, circumscribed posterior keratoconus (and, perhaps, generalized posterior keratoconus), goniodysgenesis with glaucoma, and iridogoniodysgenesis with cataract are probably also abnormalities of neural crest cell migration or differentiation.
Posterior Embryotoxon
Posterior embryotoxon is an exaggeration of the normal Schwalbe’s ring. This structure is a collagenous band that encircles the periphery of the cornea on its posterior surface (34). The collagen fibers of Schwalbe’s ring course circumferentially (parallel to the limbus), whereas the fibers elsewhere in the cornea run radially. Schwalbe’s ring is bounded anteriorly by the termination of Descemet’s membrane and posteriorly by the trabecular meshwork. Gonioscopically, it is seen just above the meshwork and is then referred to as Schwalbe’s line. It may be flat and indistinct, or elevated and ridgelike.
In most persons, Schwalbe’s ring is not visible biomicroscopically because it lies behind the opaque portion of the limbus; if it is sufficiently prominent and anteriorly displaced as to be visible, it is called a posterior embryotoxon and is present in 15% to 30% of normal eyes. It appears clinically as an arcuate or scalloped translucent membrane on the posterior surface of the cornea just inside the limbus. It is usually seen in the horizontal meridian, nasally and temporally, but may encircle the entire cornea.
Posterior embryotoxon is inherited as a dominant trait. The eye is usually otherwise normal unless Axenfeld’s anomaly or syndrome (discussed below) is present. A prominent Schwalbe’s line may be associated with other disorders, including primary congenital glaucoma, Alagille’s syndrome (arteriohepatic dysplasia), megalocornea, aniridia, corectopia, and Noonan’s syndrome.
Even a Schwalbe’s ring that is not anteriorly displaced may be visible without gonioscopy if there is a sectoral deficiency of the normal extension of sclera into the superficial tissues of the limbus. This extremely rare anomaly is called the partial limbal coloboma of Ascher and exposes Schwalbe’s ring and the meshwork to direct view (1).
Axenfeld’s Anomaly and Syndrome
Axenfeld’s anomaly is the combination of posterior embryotoxon with prominent iris processes. The iris processes extend across the angle and insert into the prominent Schwalbe’s line. Axenfeld syndrome is the name given to Axenfeld anomaly occurring along with glaucoma (30,38). Both the anomaly and the syndrome are dominantly inherited. Hypertelorism is occasionally present. Systemic abnormalities are rare (10).
Rieger’s Anomaly and Syndrome
Rieger’s anomaly consists of the changes found in Axenfeld anomaly plus hypoplasia of the anterior iris stroma (28,30,33). Peripheral anterior synechiae, corectopia, and pseudopolycoria are often present also, as is glaucoma in 50% to 60% of cases. Reiger’s syndrome is present when the Reiger’s anomaly is accompanied by skeletal abnormalities,
such as maxillary hypoplasia, microdontia, and other limb and spine malformations (35). Some patients are mentally retarded.
such as maxillary hypoplasia, microdontia, and other limb and spine malformations (35). Some patients are mentally retarded.
Rieger’s anomaly and syndrome are usually dominant but are occasionally sporadic. One case showed a presumptive isochromosome of the long arm of chromosome 6 (36), and another had a pericentric inversion of chromosome 6 (37). Various systemic associations have been described, such as Down syndrome, Ehlers-Danlos syndrome, Franceschetti’s syndrome, Noonan’s syndrome, Marfan’s syndrome, oculodentodigital dysplasia, and osteogenesis imperfecta.
Examination of these patients must include gonioscopy and tonometry: this not only helps make the differential diagnosis, it also helps direct treatment (especially if intraocular pressure is elevated). The pneumotonometer or Tonopen is preferable to the Perkins, or Schiötz tonometers because the presence of associated corneal abnormalities or small radius of corneal curvature may give false intraocular pressure readings. Also, a thorough assessment of the optic nerve is critical in determining the overall visual prognosis and deciding on the course of future treatment.
Medical therapy can be useful when intraocular pressure is high and needs to be decreased in an urgent manner. However, this disorder generally has a relatively poor surgical prognosis, both for glaucoma control and for corneal opacities, if present. Deciding on the correct balance between chronic administration of medications and performing surgery is difficult. The advent of effective use of antimetabolites for filtration in children may tip the balance in favor of surgery when the optic nerve is threatened significantly. However, this type of treatment in the maturing eye of a child is, itself, embryonic.
Goniodysgenesis with Glaucoma
Iridogoniodysgenesis with Cataract
Iridogoniodysgenesis with cataract differs from Rieger’s anomaly in that cataract is present, posterior embryotoxon is absent, and the heredity is autosomal recessive (30,38). Iridogoniodysgenesis is not associated with systemic abnormalities, but Conradi’s syndrome (congenital stippled epiphyses) sometimes manifests other forms of ASD in association with cataract.
Peters’ Anomaly
Peters’ anomaly is characterized by a central corneal opacity with corresponding defects in the stroma, Descemet’s, and endothelium. Two variants of Peters’ anomaly have been described: in the so-called mesodermal form (or, probably more properly, the neuroectodermal form) or type I Peter’s anomaly, the central cornea has a congenital leukoma with strands of iris adherent to it (1,28,30,33,40, 41, 42). The adhesions usually, but not invariably, arise from the iris collarette and represent persisting remnants of the pupillary membrane. The lens, which is ectodermal in origin, is clear in the classic and purely mesodermal (neuroectodermal) form of the anomaly. It is most often sporadic but may be transmitted recessively or as an irregularly dominant trait (39). Approximately 80% of cases are bilateral, and about half include glaucoma. Other associated abnormalities such as microcornea and sclerocornea may be present, although they usually are not. This form of Peters’ anomaly is caused by abnormal development of the tissues associated with the central portions of the iris, anterior chamber, and cornea. Descemet’s membrane and endothelium are generally absent at the site of the leukoma, as is true also of the other two forms of the anomaly that are discussed below.
Peters’ anomaly type II, or the surface ectodermal form of the anomaly, is the result of faulty separation of the lens vesicle from surface ectoderm. In addition to the features of the mesodermal (neuroectodermal) type, anterior cataract (polar, subcapsular, or reduplication) is present. This form is usually bilateral and is almost always associated with other more severe manifestations, both ocular and systemic. Fifty percent to 70% of patients have concomitant glaucoma, and other associated abnormalities include microcornea, microphthalmos, cornea plana, sclerocornea, colobomas, aniridia, and dysgenesis of the angle and iris.
The inflammatory form follows intrauterine inflammation and so is nonhereditary. The inflammation can interfere with surface ectodermal or neuroectodermal development or both. There is no definitive way to make the diagnosis, although signs of inflammation may still be present after birth, and the iris adhesions are extensive and do not arise only from the vicinity of the collarette. Cases of inflammatory Peters’ anomaly nearly always fulfill the criteria for use of the term von Hippel’s posterior corneal ulcer, namely inflammatory signs in association with congenital defects of Descemet’s membrane and endothelium. We have already seen that some examples of circumscribed posterior keratoconus have these same features and so may also be referred to as cases of von Hippel’s ulcer; in fact, there seems to be little, if any, difference between the inflammatory form of Peters’ anomaly and the inflammatory form of circumscribed posterior keratoconus.
The management of patients with Peters’ anomaly is complex, and the outcome of a keratoplasty depends on the ability to control the associated glaucoma.
Prominent Iris Processes
Although prominent iris processes are not corneal anomalies, they should be mentioned because they are part of
the spectrum of ASD and because it is necessary to determine their relationship to the peripheral cornea in order to evaluate their pathologic importance (14).
the spectrum of ASD and because it is necessary to determine their relationship to the peripheral cornea in order to evaluate their pathologic importance (14).
It is normal for some slender processes (usually fewer than 100) to extend from the peripheral iris to the scleral roll (also known as the scleral spur) at the posterior edge of the trabecular meshwork or, occasionally, even to the central portion of the meshwork itself. Extensions to or beyond Schwalbe’s ring are abnormal and are referred to as prominent iris processes. Abnormal processes are often more numerous, in addition to being more prominent and anteriorly displaced, than are normal processes.
Prominent iris processes can occur with any of the other manifestations of ASD. They are also seen in many cases of primary congenital glaucoma and in several systemic disorders that are associated with congenital glaucoma: phakomatosis; homocystinuria; rubella; and Marfan’s, Lowe’s, Pierre Robin, Hallermann-Streiff, Rubenstein-Taybi, and Turner’s syndromes (14).
Anterior Segment Dysgenesis of the Iris
In addition to prominent iris processes, ASD may be associated with congenital peripheral anterior synechiae and a variety of abnormalities of the iris itself, including atrophy of the iris stroma, corectopia, pseudopolycoria, and congenital ectropion uveae.
Posterior Polymorphous Dystrophy
Congenital Cornea Guttata
Cornea guttata can occur, rarely, as a congenital anomaly. It is sometimes familial. A dominant pedigree with associated anterior polar cataract has been described, which suggests an abnormality in the secondary mesenchyme that helps to separate the lens from the surface ectoderm at about the sixth to eighth week of embryonic life (45). Congenital cornea guttata is also described elsewhere in this book.
Congenital Hereditary Endothelial Dystrophy
Congenital hereditary endothelial dystrophy (CHED) is now classified as an ASD. It is characterized by the presence, at birth or soon thereafter, of bilateral corneal edema that is often slightly worse centrally and that is not associated with vascularization or inflammation (44,46, 47, 48, 49). Epithelial edema is not prominent, but the stroma may be swollen to two or three times its normal thickness. Descemet’s membrane, when visible, is seen to be thick and opaque, but guttate changes are not present. Intraocular pressures are normal. The corneas are not enlarged.
Two types of CHED with different modes of transmission are recognized (46,49,50). The recessive form is more common, and is usually more severe, than the dominant form. In the recessive disease, the corneas are cloudy at birth, and nystagmus is common. The condition is essentially nonprogressive and is asymptomatic except for severely decreased vision. Deafness is sometimes present; otherwise, there are no related systemic abnormalities (51).
Corneal edema in dominant CHED may not become apparent until sometime during the first or second year after birth (44,46,50). Nystagmus is absent. The edema is likely to progress slowly, and some patients develop pain, photophobia, and tearing.
Histopathologically, the anterior (“banded”) portion of Descemet’s membrane is normal, but the posterior nonbanded layer (which is formed later during development) is abnormal and consists of a variably thickened, or occasionally thinned, layer of aberrant collagen (47,52,53). Guttate excrescences do not form. Endothelial cells are absent or atrophic. The primary abnormality is presumed to be with the endothelial cells and must manifest itself during or after the fifth month of gestation, at which time the endothelial cells begin to form the posterior nonbanded portion of Descemet’s membrane.
Asymptomatic relatives of patients with CHED may show corneal changes resembling posterior polymorphous dystrophy (54). An attempt should be made to identify such persons because their children seem to run a greater risk of having CHED.
Circumscribed Posterior Keratoconus
Circumscribed posterior keratoconus may be a localized form of Peter’s anomaly. It is characterized by the presence of a localized, crater-like defect (convex toward the stroma) in the posterior surface of the cornea (22,30,40, 41, 42,55). Contrary to former belief, Descemet’s membrane and endothelium are usually present in the area of the defect, although the collagen of Descemet’s membrane may be thinned and abnormal in structure and configuration (22,40,42,55). More than one pit may be present. The overlying stroma often has nonspecific opacities. Most cases are in females, unilateral and sporadic, although familial examples have occurred. It is probably the result of abnormal migration or terminal induction of cells of neural crest origin in the area of involvement, perhaps secondary to some problem with separation of the lens vesicle. Some cases show evidence of being related to intrauterine inflammation: corneal infiltrates and vascularization, keratic precipitates, anterior synechiae, and uveitis; these cases often have defects in Descemet’s membrane and endothelium and are
sometimes referred to as von Hippel’s posterior (or internal) corneal ulcer.
sometimes referred to as von Hippel’s posterior (or internal) corneal ulcer.
TABLE 39-5. ABNORMALITIES ASSOCIATED WITH CIRCUMSCRIBED POSTERIOR KERATOCONUS | ||||||||||||||||||||||||
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The characteristics of circumscribed posterior keratoconus, as compared with generalized posterior keratoconus, anterior keratoconus, and keratoglobus, are summarized in Table 39-4. Associated ocular and systemic abnormalities are listed in Table 39-5 (22).
The anterior surface of the cornea is usually normal in these individuals, unless there is enough thinning to cause ectasia. Thus, although the posterior corneal surface may degrade vision to some extent, this is usually not enough to warrant a surgical procedure.
Sclerocornea
Sclerocornea is an abnormality in which the margins of the cornea are not well defined because scleral tissue with conjunctival vessels extends to the margins (56, 57, 58). The scleralization may be only peripheral or virtually complete. Even when it is complete, the central cornea is apt to be slightly less opaque than the periphery. Affected areas have fine, superficial vessels that are direct extensions of normal scleral, episcleral, and conjunctival vessels. Sclerocornea is usually bilateral (57).
Histopathologic studies reveal elastic fibers and collagen fibers of increased and variable diameter in the anterior corneal stroma. The deeper collagen fibers have smaller diameters than do the more anterior ones, as is typical of sclera; the reverse is true of normal cornea (58).
About 50% of cases of sclerocornea are sporadic, and the remainder can be dominant or recessive (57). The dominant forms are less severe than the recessive ones (58). Sclerocornea is occasionally caused by chromosomal aberrations. There is no sex predilection. The most common associated finding is cornea plana. Other ocular and systemic associations are given in Table 39-6 (7,57,59,60,61,62). In brief, sclerocornea is associated with cornea plana in about 80% of patients. Other associated ocular abnormalities include microphthalmos, iridocorneal synechiae, persistent pupillary membrane, dysgenesis of angle and iris, congenital glaucoma, coloboma, and posterior embryotoxon of the fellow eye. Somatic abnormalities sometimes occur along with associated chromosomal abnormalities; they include mental retardation, deafness, and craniofacial, digital, and skin abnormalities.
TABLE 39-6. ABNORMALITIES ASSOCIATED WITH SCLEROCORNEA | ||||||||||||||||||||||||||||||||||||||||||||||
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Other Combined Forms of Anterior Segment Dysgenesis
Although the foregoing disorders are the most frequently encountered combined forms of ASD, it is worth reemphasizing that any combination of features is possible. This is illustrated well by the family reported by Grayson (43), in which some members had all of the following findings: cornea guttata, posterior polymorphous dystrophy, posterior embryotoxon, circumscribed posterior keratoconus, iris atrophy, peripheral anterior synechiae, prominent iris processes, and glaucoma. These patients also developed corneal edema and fibrocalcific band keratopathy.
Congenital Mass Lesions of the Cornea
Metaplasias
A metaplasia is a transformation of tissue from one type to another, usually in response to exposure, inflammation, or trauma (1,63). As mentioned earlier, the cornea can undergo metaplasia to skin (dermoid transformation) in the conditions of keratectasia, corneal staphyloma, and true
cryptophthalmos. The metaplasia is at times of such proportions as to produce an apparent tumor of the cornea.
cryptophthalmos. The metaplasia is at times of such proportions as to produce an apparent tumor of the cornea.
If the metaplasia is strictly ectodermal, the corneal epithelium is replaced by tissue resembling epidermis, sometimes with related derivatives such as hairs and glandular structures being present. Mesodermal metaplasia brings about the appearance in subepithelial stroma of the fibrofatty elements of dermis, and sometimes cartilage or bone. When the metaplasia consists primarily of a hypertrophic overgrowth of fibrous tissue, the term corneal keloid may be used (64).
Choristomas
A choristoma is a mass of tissue that has been displaced during prenatal development from its normal position to a location where it would not normally be found (63).
Corneal, Limbal, and Epibulbar Dermoids
Corneal, limbal and epibulbar dermoids consist of masses of tissue that were destined to become skin but were displaced to the surface of the eye (1,63). They can also occur in the orbit.
Choristomatous dermoids contain ectodermal elements (keratinizing epithelium, hair, sebaceous and sudoriferous glands, nerve, smooth muscle, and, rarely, teeth) and mesodermal derivatives (fibrous tissue, fat, blood vessels, and cartilage) in various combinations. They are called lipodermoids if fatty tissue predominates.
Clinically, an epibulbar dermoid is usually a round or ovoid, yellowish white, dome-like mass. Occasionally, a dermoid may be rather diffuse or may encircle the limbus or peripheral cornea. Hairs often protrude from the lesion. The surface may be pearly or clear and glistening, depending on the presence or absence of epithelial keratinization. The most common location is at the inferotemporal limbus, but dermoids can occur anywhere on the surface of the eye. The central cornea can be affected, although some of these lesions are probably the result of dermoid transformation (metaplastic dermoids) rather than being choristomatous malformations.
The second most common site for a dermoid is the superotemporal orbit. Dermoids usually exhibit little or no growth but do enlarge occasionally, especially at the time of puberty (65). They do not become malignant.
A limbal or corneal dermoid can cause decreased visual acuity by covering the visual axis or by causing astigmatism. An arcuate deposition of lipid often develops along the central (corneal) edge of a limbal dermoid and is another possible source of decreased vision. The lipid sometimes increases as long as the dermoid is present (65).
Limbal dermoids always involve some of the corneal stroma and can even extend into the anterior chamber. Any attempt at excision must be limited merely to shaving away the elevated portion of the lesion, unless one is willing to undertake a corneoscleral graft, which would seldom be indicated. It is important to explain to the parents, or to the patient who is old enough to understand, that the shaving procedure can only eliminate the elevation and that the corneal opacity will remain. The surgery is not without some difficulty and risk. The cornea may be thin in the area of the dermoid, and perforation is possible. I have observed that if the dermoid covers the anterior portion of the sclera, the underlying extraocular muscles and their insertions may be anomalous and may inadvertently be transected if the surgeon is not careful.
Perhaps 30% of patients who have epibulbar dermoids have other abnormalities (65). These most often consist of some or all of the features of Goldenhar’s syndrome (Goldenhar-Gorlin syndrome, facioauriculovertebral sequence, oculoauriculovertebral dysplasia) (2,16,65,66). Goldenhar’s syndrome is the result of maldevelopment of the first and second branchial arches, which give rise to the maxilla, mandible, malar bone, auricle, and structures of the upper neck. The syndrome comprises a triad of findings: epibulbar (usually limbal) dermoid(s), abnormalities of the ear such as auricular appendages or pretragal fistulas, and anomalies of the vertebral column. Other abnormalities that sometimes occur are listed in Table 39-7 (2,16,65,66,68).
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