Microcornea

Microcornea refers to a clear cornea of normal thickness with a diameter of less than 10 mm (or <9 mm in a newborn). The cause is unknown and may be related to fetal arrest of corneal growth in the fifth month of gestation. Alternatively, it may be related to overgrowth of the anterior tips of the optic cup, which leaves less space for the cornea to develop. Microcornea is inherited as an autosomal dominant (most commonly) or recessive trait and has been associated with mutations in paired box gene 6 (PAX6). There is no sex predilection. Because the cornea is relatively flat in microcornea, these eyes are usually hyperopic, and there is an increased incidence of angle-closure glaucoma. Open-angle glaucoma develops later in life in 20% of patients in whom angle-closure glaucoma does not occur. Important ocular anomalies associated with microcornea include persistent fetal vasculature (PFV), congenital cataracts, anterior segment dysgenesis, and optic nerve hypoplasia. Significant systemic associations include myotonic dystrophy, fetal alcohol syndrome, achondroplasia, and Ehlers-Danlos syndrome.

Table 5-1 Developmental Anomalies of the Anterior Segment

When microcornea occurs as an isolated finding, the visual prognosis is excellent if spectacles are used to treat the hyperopia resulting from the flat cornea. Concurrent ocular pathologic conditions such as refractive amblyopia, cataract, PFV, and glaucoma may require medical or surgical treatment.

Wang P, Sun W, Li S, Xiao X, Guo X, Zhan Q. PAX6 mutations identified in 4 of 35 families with microcornea. Invest Ophthalmol Vis Sci. 2012;53(10):6338–6342.

Megalocornea

Megalocornea is a bilateral, nonprogressive enlargement of the cornea. It may be inherited as an X-linked recessive trait and is associated with mutations in the chordin-like 1 gene (CHRDL1) (see Table 5-1). Rare cases of autosomal recessive and autosomal dominant inheritance have been reported. The cornea is histologically normal but measures 13.0–16.5 mm in diameter (Fig 5-1). Males are affected more often than females, but in heterozygous females, corneal diameter may be slightly increased.

The etiology may be related to failure of the optic cup to grow and of its anterior tips to close, leaving a larger space for the cornea to fill. Alternatively, megalocornea may represent arrested buphthalmos and exaggerated growth of the cornea in relation to the rest of the eye. An abnormality in collagen production is suggested by the association of megalocornea with systemic disorders of collagen synthesis (eg, Marfan syndrome).

Associated ocular anomalies may include iris translucency (diaphany), miosis, goniodysgenesis, cataract, ectopia lentis, arcus juvenilis, and glaucoma (but not congenital glaucoma). Nonocular and systemic associations may include craniosynostosis, frontal bossing, hypertelorism, facial anomalies, facial hemiatrophy, dwarfism, intellectual disability, hypotonia, Down syndrome, Marfan syndrome, Alport syndrome, osteogenesis imperfecta, mucolipidosis II, or occasionally other genetic syndromes.

The differential diagnosis is primarily congenital glaucoma (discussed later in the chapter), which can be ruled out by intraocular pressure measurement and careful biomicroscopy. The presence of pigmentary dispersion, iris transillumination defects, and/or a Krukenberg spindle and the absence of Haab striae and previous breaks in Descemet membrane can also help distinguish megalocornea from congenital glaucoma. Ultrasonography may be of value in demonstrating the short vitreous length, deep lens and iris position, and normal axial length that distinguish megalocornea from buphthalmos caused by congenital glaucoma. Myopia and with-the-rule astigmatism, which are common in megalocornea, are managed as in patients without this anomaly.

Lens instability and subluxation, iridodonesis, and poor zonular integrity are also associated with megalocornea and present additional risks and challenges in the management of cataracts, particularly in terms of the selection and placement of an intraocular lens (IOL) after cataract surgery. If the lens is of normal size, an IOL can be inserted in the capsular bag. A capsular tension ring may be placed to reduce stress on the zonular fibers (Video 5-1). Standard-sized posterior chamber lenses are typically too short to be fixated in the ciliary sulcus, and anterior chamber lenses are similarly problematic in the enlarged anterior chamber. If zonular support is insufficient or capsular volume too great to allow stable IOL centration, an iris clip or iris-sutured lens may be the best option.

VIDEO 5-1 Phacoemulsification with intraocular lens implantation for megalocornea.

Courtesy of Robert W. Weisenthal, MD.

Access all Section 8 videos at www.aao.org/bcscvideo_section08.

Smith JEH, Traboulsi EI. Malformations of the anterior segment of the eye. In: Traboulsi EI, ed. Genetic Diseases of the Eye. 2nd ed. Cary, NC: Oxford University Press; 2011:92–93.

Webb TR, Matarin M, Gardner JC, et al. X-linked megalocornea caused by mutations in CHRDL1 identifies an essential role for ventroptin in anterior segment development. Am J Hum Genet. 2012;90(2):247–259.

Welder J, Oetting TA. Megalocornea. EyeRounds.org. September 17, 2010. Available at www.EyeRounds.org/cases/121-megalocornea.htm. Accessed February 3, 2017.

Cornea plana

Cornea plana, literally “flat cornea,” is a rare condition in which the radius of curvature is less than 43 D and keratometry readings of 30–35 D are common (see Table 5-1). Additional hallmarks of cornea plana include high hyperopia (usually >10 D) and peripheral corneal haze or arcus. Corneal curvature that is the same as the curvature of the adjacent sclera is pathognomonic of this condition. Sclerocornea also features flat corneas, but it is distinguished by the loss of corneal transparency (see Fig 5-6).

Both autosomal recessive and dominant forms of cornea plana have been associated with mutations of the KERA gene (12q22), which codes for keratan sulfate proteoglycans (keratocan, lumican, and mimecan). These proteins are thought to play an important role in the regular spacing of corneal collagen fibrils. Investigators have speculated that mutations in the KERA gene cause an alteration of the tertiary structure of the keratan sulfate proteoglycans that leads to the cornea plana phenotype.

Cornea plana is often seen in association with sclerocornea (discussed later in the chapter) or microcornea. Other associated ocular or systemic abnormalities may include central corneal clouding, cataracts, anterior and posterior colobomas, and Ehlers-Danlos syndrome. Because of the morphologically shallow anterior chamber, angle-closure glaucoma occurs; open-angle glaucoma may develop because of angle abnormalities. Most isolated cases of cornea plana appear in patients of Finnish ancestry.

Treatment of cornea plana consists of correction of refractive errors and control of glaucoma, either medically or surgically. Loss of central corneal clarity may require deep anterior lamellar keratoplasty (DALK) or penetrating keratoplasty (PK), which has the added risk of endothelial rejection.

Khan AO. Cornea plana. In: Traboulsi EI, ed. Genetic Diseases of the Eye. 2nd ed. Cary, NC: Oxford University Press; 2011:85–91.

Lehmann OJ, El-Ashry MF, Ebenezer ND, et al. A novel keratocan mutation causing autosomal recessive cornea plana. Invest Ophthalmol Vis Sci. 2001;42(13):3118–3122.

Tahvanainen E, Villanueva AS, Forsius H, Salo P, de la Chapelle A. Dominantly and recessively inherited cornea plana congenita map to the same small region of chromosome 12. Genome Res. 1996;6(4):249–254.

Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.