Megalocornea

Megalocornea is a rare nonprogressive enlargement of the cornea to 13 mm or greater; in the setting of normal intraocular pressure. The cornea and limbus are enlarged, but the cornea itself is histologically normal and of normal thickness. Megalocornea is usually seen as an isolated finding, known as simple isolated megalocornea. It may be associated with other ocular and systemic findings, including megophthalmos anterior and/ or dysgenesis of the iris, lens, and ciliary body.

Megalocornea is shown in the image below.

Megalocornea is a developmental anomaly of unknown etiology. Postulated mechanisms of development include a defect in formation of the optic cup in which the anterior tips of the cup fail to fuse, allowing more space for the developing cornea. This is considered a primary overgrowth of the cornea, producing a normal endothelial cell density. In congenital glaucoma, the main differential diagnosis, the cell density is low due to distention of the cornea from elevated intraocular pressure.[1] The cornea does not reach its full adult size until age 12 months; thus, a definitive diagnosis of megalocornea cannot be ruled out until after this time.

Frequency

United States

No data are available.

International

No data are available.

Mortality/Morbidity

Morbidity and mortality can be secondary to the many systemic conditions found in association with megalocornea. See Special Concerns.

Ocular morbidity is associated with anterior megalophthalmos. An enlarged ciliary ring causes zonular stretching, leading to phacodonesis, ectopia lentis, iridodonesis, iris stromal hypoplasia and transillumination defects, Krukenberg spindles, and trabecular meshwork pigmentation. Other findings are posterior embryotoxon, Rieger anomaly, goniodysgenesis, and cataracts. Goniodysgenesis and pigment dispersion can contribute to glaucoma.

Sex

Males account for 90% of cases because X-linked recessive inheritance is most common.

Age

Megalocornea is present from birth.

Megalocornea is present from birth.

A family history of megalocornea may be present.

The mother may have slightly enlarged corneas.

Findings of simple megalocornea

See the list below:

• Symmetric

• Dome-shaped cornea

• Normal corneal thickness

• Normal endothelial cell density, usually bilateral

• Usually good visual acuity

• With-the-rule astigmatism common

• Corneal diameter usually 13.0-16.5 mm

• Corneal thickness normal and stroma clear; may have central mosaic dystrophy (see the image below)

  Central mosaic dystrophy sometimes noted in megalocornea.

• Limbus sharply demarcated

• Keratometry usually normal but may be steeper than normal

• Anterior chamber depth increased

• Lens-iris diaphragm positioned posteriorly

• Vitreous length short

• Intraocular pressure normal

Findings of anterior megalophthalmos

See the list below:

• Megalocornea

• Ciliary body band wider than the trabecular meshwork and scleral spur on gonioscopy[2]

• May have any of the following:

  • Iridodonesis

  • Iris stromal hypoplasia

  • Transillumination defects

  • Phacodonesis

  • Ectopia lentis

  • Cataracts

  • Krukenberg spindles

  • Posterior embryotoxon

  • Excessive mesenchymal tissue in the angle

  • Glaucoma (but not congenital glaucoma)

Findings of congenital glaucoma not found in megalocornea

See the list below:

• Elevated intraocular pressure

• Corneal edema and Haab striae

• Increased axial length

• Decreased endothelial cell density

• Flatter cornea

• Decreased lens thickness

• Asymmetry

• Optic disc cupping

Megalocornea is inherited as an X-linked recessive trait (90% of cases).

The gene locus for X-linked megalocornea is in band Xq12-q26, most likely band Xq21-q23.

Autosomal dominant, autosomal recessive, and sporadic inheritance have been reported.

Gonioscopy

Widened ciliary ring indicates anterior megalophthalmos.

Check for excessive mesenchymal tissue in the angle.

A-scan ultrasound biometry

Buphthalmos may indicate congenital glaucoma.[3]

Findings with megalocornea but not congenital glaucoma - Increased anterior chamber depth, posterior lens-iris diaphragm position, and short vitreous length

Specular microscopy

Normal endothelial cell density in megalocornea

Decreased endothelial cell density in congenital glaucoma

The cornea is histologically normal.

Medical care for patients with megalocornea includes correction of refractive error and thorough evaluation for findings of anterior megalophthalmos. Also, routine examination for the development of cataracts and glaucoma is necessary.

Surgical amelioration of glaucoma and cataract is performed when necessary.

Cataract is common with megalocornea, and the cataract surgery is difficult. The zonules supporting the lens capsule surrounding the cataract are often weak. Weakened zonules make it more difficult to support an artificial intraocular lens (IOL). This leads to increased surgical complications, such as vitreous loss and loss of lens material to the back of the eye during cataract surgery.[4, 5, 6]

The large size of the anterior segment makes IOL selection difficult as standard IOLs are too small. A standard lens will often not work. Oetting and Newsom at the University of Iowa reported on the use of a special IOL called the Artisan lens (see the image below) that clips onto the iris.[7]

Because this IOL clips onto the iris rather than fitting into the angle, it allows for a safe and stable placement, even with the large anterior segment. Basti and colleagues reported using suture based iris fixation to allow placement of a standard IOL in these large eyes.[8]

Pediatric developmental specialist consultation is always indicated to evaluate for the many associated systemic conditions. These include albinism, Down syndrome, Marfan syndrome, Alport syndrome, craniosynostosis, and many others.

Patients with megalocornea should have regular examinations to detect glaucoma or cataract. Patients should also receive counseling regarding megalocornea as an X-linked recessive trait.

Complications of megalocornea may include the following:

• Lens subluxation

• Early cataracts

• Glaucoma (not congenital)

The prognosis is excellent in simple megalocornea. Patients with anterior megalophthalmos will always be at risk for glaucoma and cataracts, which can be difficult to treat in these patients. The implementation of CRISPR (clustered regularly interspaced short palindromic repeats) may offer hope in the prevention of this disease.

Advise parents of the potential for ocular complications and associated systemic conditions. Emphasize the importance of a thorough developmental evaluation and regular medical and ophthalmic follow-up care.