Megalocornea is a rare congenital condition defined by an abnormally large cornea. This enlargement affects both eyes and is present from birth, though it does not typically worsen over time. The condition is often benign, meaning it usually does not interfere with vision or eye health. However, it must be correctly identified and monitored to distinguish it from more serious disorders that also cause corneal enlargement.
Defining Megalocornea and Its Characteristics
Megalocornea is characterized by an enlarged corneal diameter exceeding the normal range of 11 to 12 millimeters (mm). For diagnosis, the horizontal diameter is measured at greater than 12.5 mm in infants or at least 13 mm in adults. Despite this increased size, the cornea remains clear and its structural layers are healthy.
A common feature is a noticeably deeper anterior chamber, the space between the cornea and the iris. Due to the altered anatomy, the iris and the lens are positioned further back than usual. This deep anterior chamber sometimes leads to a slight quivering of the iris, known as iris tremulousness. Although the cornea is enlarged, the overall length of the eyeball, the axial length, usually remains normal.
Genetic Basis and Inheritance Patterns
Megalocornea is a developmental anomaly arising from inherited factors. The most common form of isolated megalocornea follows an X-linked recessive inheritance pattern. This means the responsible gene is located on the X chromosome, and the condition predominantly affects males.
The primary gene implicated in X-linked megalocornea is \(CHRDL1\). This gene produces ventroptin, a protein that regulates eye development. A mutation in \(CHRDL1\) can lead to the overgrowth of the cornea. While X-linked recessive inheritance is most frequent, cases can also appear without a known family history or follow other patterns, such as autosomal recessive.
Distinguishing Megalocornea from Similar Eye Conditions
Differentiating megalocornea from other conditions that cause corneal enlargement is essential, particularly congenital glaucoma, also known as buphthalmos. Both conditions result in an enlarged cornea in infants, but they have vastly different underlying causes and prognoses.
A defining feature of megalocornea is that the intraocular pressure (IOP) within the eye is normal. Congenital glaucoma, in contrast, is characterized by persistently high IOP, which causes the infant cornea to stretch and enlarge. High pressure in glaucoma also leads to symptoms not seen in simple megalocornea, such as corneal clouding, excessive tearing, and sensitivity to light (photophobia).
The corneal structure provides another clear difference. Glaucoma-related enlargement can cause horizontal breaks in the inner corneal layer, known as Haab’s striae, which are absent in megalocornea. Megalocornea is typically a symmetric condition affecting both eyes equally, while congenital glaucoma can present with noticeable asymmetry. Other rare conditions, like keratoglobus, which involves significant thinning and bulging of the cornea, must also be considered.
Clinical Management and Visual Outcomes
Since isolated megalocornea is a non-progressive condition, management focuses on long-term monitoring and vision correction. Patients often experience refractive errors, such as nearsightedness (myopia) or astigmatism, which are managed with prescription glasses or contact lenses. Regular comprehensive eye examinations are necessary to monitor for potential secondary complications that may arise over time.
These long-term complications can include the premature development of cataracts, lens dislocation (ectopia lentis), or the development of glaucoma later in life. Due to the altered anatomy, including the deep anterior chamber and weakened lens support structures, surgical intervention, such as cataract surgery, can be more complex. For simple megalocornea not associated with other syndromes, the overall visual prognosis is favorable with consistent follow-up care.