Megalocornea


Megalocornea

Description, Causes and Risk Factors:

Congenital anomaly consisting of an enlarged anterior segment of the eye.

The pathogenesis of simple megalocornea and megalophthalmos anterior is unknown. There may be a large cornea (keratodysgenesis), iris and angle abnormalities (iridogoniodysgenesis), or a combination of these.

The differentiation between simple megalocornea, megalophthalmos anterior and primary infantile glaucoma is often difficult, but very important, as in the latter case surgical treatment prevents blindness.

X-linked megalocornea (MGC1) is an ocular anterior segment disorder characterized by an increased cornea diameter and deep anterior chamber evident at birth and later onset of mosaic corneal degeneration (shagreen), arcus juvenilis, and presenile cataracts. We identified copy-number variation, frameshift, missense, splice-site and nonsense mutations in the chordin-like 1 gene (CHRDL1) on Xq23 as the cause of the condition in seven MGC1 families. CHRDL1 encodes ventroptin, a bone morphogenic protein antagonist with a proposed role in specification of topographic retinotectal projections. Electrophysiological evaluation revealed mild generalized cone system dysfunction and, in one patient, an interhemispheric asymmetry in visual evoked potentials. We show that CHRDL1 is expressed in the developing human cornea and anterior segment in addition to the retina. We explored the impact of loss of ventroptin function on brain function and morphology in vivo. CHRDL1 is differentially expressed in the human fetal brain, and there is high expression in cerebellum and neocortex. We show that MGC1 patients have a superior cognitive ability despite a striking focal loss of myelination of white matter. Our findings reveal an unexpected requirement for ventroptin during anterior segment development and the consequences of a lack of function in the retina and brain.

Simple megalocornea can be differentiated from congenital glaucoma by the clarity of the cornea and by a normal intraocular pressure and optic nerve in the former. Simple megalocornea must also be differentiated from keratoglobus. Keratoglobus is a rare bilateral condition resembling megalocornea, with the exception that the cornea is uniformly thinned, particularly peripherally. A familial association between keratoconus and keratoglobus has been made. Rupture of Descemet's membrane may occur, as in keratoconus, but this is not usually the case. Especially in cases associated with Ehlers-Danlos syndrome, type VI, patients must be cautious to avoid even monitor ocular trauma because rupture of the globe can occur easily and repair is difficult.

Symptoms:

Signs:

    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).

Diagnosis:

Diagnosis is based on a thorough examination of the child's eye, often with general anesthesia. Different clinical aspects can be observed: simple megalocornea without ocular or systemic anomalies, megalocornea with ocular and/or systemic anomalies as well as multiple malformation syndromes, dermatological diseases, skeletal diseases, and genetic or chromosomic diseases. A systematic general pediatric examination is therefore necessary. Congenital megalocornea must be considered, first of all, as a differential diagnosis of primary congenital glaucoma and all of its etiologies.

Gonioscopy:

  • Widened ciliary ring indicates anterior megalophthalmos.

  • Check for excessive mesenchymal tissue in the angle.

A-scan ultrasound biometry:

  • Buphthalmos may indicate congenital glaucoma.

  • 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.

Treatment:

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.

Treatment options for managing patients with megalocornea are limited and present a challenge due to a large thinned cornea. Possible treatment modalities include spectacle correction (in patients with clear corneas), hydrogel lenses (although contact lenses are not commonly used due to abnormal corneal topography and increased susceptibility to corneal rupture following minimal trauma), large-diameter or eccentric PK, large-diameter inlay lamellar keratoplasty (LK), tectonic LK followed by a PK, central LK with peripheral intralamellar tuck, deep anterior LK with big bubble technique, limbus-to-limbus epikeratoplasty (EK), limbus-to-limbus EK followed by a PK (in patients with a central corneal scar), and peripheral suturing of a corneoscleral ring graft. The overall visual outcomes in megalocornea patients are not optimal.

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.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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