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	'''Megalocornea''' is a rare condition in which the cornea is larger than usual. In primary congenital megalocornea, the enlarged corneas are present from birth and the intraocular pressure is not raised.

'''Megalocornea''' is a rare eye condition characterised by an abnormally large cornea. The cornea is the transparent, dome-shaped front surface of the eye that helps to focus incoming light. In individuals with megalocornea, the cornea is larger than the average size, which can lead to visual abnormalities and potential complications.
	The condition is usually bilateral, meaning it affects both eyes. It is important to distinguish megalocornea from congenital glaucoma, because glaucoma involves raised eye pressure and needs different management.

== Presentation and Clinical Features ==
Megalocornea is typically diagnosed when the horizontal diameter of the cornea exceeds 13 millimeters in infancy or 12 millimeters in adulthood. However, the exact measurement criteria may vary among studies. Individuals with megalocornea often have an otherwise normal eye structure, including the lens and retina.
	== Features ==
	Megalocornea is usually defined by an enlarged horizontal corneal diameter, often at least 12.5 to 13 mm. The cornea is enlarged but typically clear.

The most common symptom associated with megalocornea is reduced visual acuity, which can be present from childhood. Some affected individuals may also experience astigmatism, which causes blurred vision due to an irregularly shaped cornea or lens. In some cases, the enlarged cornea can lead to increased intraocular pressure and glaucoma, although this is less common.
	Common or recognised features include:

== Genetics and Inheritance ==
Megalocornea can have both non-genetic (non-hereditary) and genetic causes. In the non-genetic form, megalocornea can occur as an isolated eye abnormality without any associated medical conditions. It may also be associated with certain connective tissue disorders or metabolic disorders.
	* enlarged corneal diameter
	* deep anterior chamber
	* mild to moderate short-sightedness in some people
	* light sensitivity
	* iridodonesis, where the iris trembles because support structures are abnormal
	* lens dislocation or lens instability in some cases
	* cataracts developing earlier than usual in some adults
	* risk of secondary glaucoma in some people

In the genetic form of megalocornea, the condition is inherited in an autosomal dominant pattern. This means that an affected individual has a 50% chance of passing the condition on to each of their children. Several genes have been associated with genetic megalocornea, including CHRDL1, KERA, and ADAMTS17. However, the underlying genetic mechanisms and specific gene mutations involved are not yet fully understood.
	Visual outcome varies. Some people have relatively good vision with monitoring and correction, while others develop complications that need specialist eye care.

== Diagnosis and Management ==
Diagnosis of megalocornea is typically made through a comprehensive eye examination, including measurements of corneal diameter and assessment of visual acuity. Additional imaging tests, such as corneal topography or ultrasound, may be used to further evaluate the cornea's size and shape.
	== Cause and Inheritance ==
	Isolated congenital megalocornea is most often linked to pathogenic variants in the [[CHRDL1]] gene. CHRDL1 is located on the X chromosome and is involved in eye development.

Management of megalocornea focuses on addressing visual impairments and preventing complications, such as glaucoma. Corrective lenses, including glasses or contact lenses, may be prescribed to improve visual acuity and correct astigmatism. Regular monitoring of intraocular pressure is important to detect and manage any signs of glaucoma.
	The recognised isolated form is usually inherited in an X-linked recessive pattern. This means males are more commonly affected, while carrier females may have no symptoms or milder findings.

In cases where megalocornea is associated with other systemic conditions or abnormalities, a multidisciplinary approach involving various medical specialists may be required to address the specific needs and potential complications.
	Megalocornea can also appear as part of broader syndromes or other eye-development disorders. When other features are present, clinicians may look for a wider genetic or developmental diagnosis.

== Research and Future Perspectives ==
Further research is needed to better understand the underlying causes and genetic mechanisms of megalocornea. Studying the specific genes associated with genetic megalocornea can provide insights into the normal development and function of the cornea. Animal models and cellular studies can contribute to understanding the molecular pathways involved in corneal growth and the impact of gene mutations on corneal development.
	== Diagnosis ==
	Diagnosis is clinical and is made by eye examination. The key findings are enlarged corneas and a deep anterior chamber without the raised intraocular pressure expected in congenital glaucoma.

Advancements in genetic technologies, such as next-generation sequencing, may facilitate the identification of additional genes and mutations associated with megalocornea. Understanding the genetic basis of megalocornea can lead to improved diagnostic techniques, genetic counseling, and potential future therapeutic approaches.
	Assessment may include:

Collaboration among researchers, ophthalmologists, and geneticists is essential to further unravel the complexities of megalocornea, improve management strategies, and enhance the quality of life for individuals affected by this condition.
	* corneal diameter measurement
	* intraocular pressure measurement
	* slit-lamp examination
	* optic nerve assessment
	* refraction testing
	* imaging of the front of the eye
	* genetic testing when an inherited form is suspected

	Family history can help, especially when an X-linked pattern is possible.

	== Management ==
	Management depends on symptoms and complications. Some people need monitoring only, while others need treatment for refractive error, cataract, glaucoma or lens problems.

	Care may include glasses or contact lenses, regular eye-pressure checks, cataract monitoring, glaucoma surveillance and advice before eye surgery. Surgery can be more complicated because the front part of the eye has unusual dimensions.

	== Differential Diagnosis ==
	The main condition to distinguish from megalocornea is primary congenital glaucoma. Both can involve large corneas, but congenital glaucoma is usually associated with raised intraocular pressure and may cause corneal clouding, tearing and sensitivity to light.

	Other conditions that may be considered include anterior segment dysgenesis, corneal enlargement linked to systemic syndromes, Marfan syndrome, Down syndrome and other inherited eye disorders.

	== See Also ==
	* [[CHRDL1]]
	* [[Chromosome_3p25.3]]
	* [[Genetic_Disorder]]
	* [[Glaucoma]]

== References ==	== References ==
	* [https://rarediseases.info.nih.gov/diseases/12648/isolated-congenital-megalocornea GARD: Isolated congenital megalocornea]
	* [https://www.ncbi.nlm.nih.gov/books/NBK554374/ StatPearls: Megalocornea]
	* [https://my.clevelandclinic.org/health/diseases/25078-megalocornea Cleveland Clinic: Megalocornea]
	* [https://panelapp.genomicsengland.co.uk/panels/250/gene/CHRDL1/ Genomics England PanelApp: CHRDL1]
	* [https://www.aao.org/eyenet/article/fundamentals-of-megalocornea American Academy of Ophthalmology: Fundamentals of Megalocornea]

# Aldahmesh MA, Nowilaty SR, Alkuraya H, et al. Megalocornea associated with zonular weakness and childhood lens-related secondary glaucoma--a distinct phenotype caused by recessive LTBP2 mutations. Mol Vis. 2010;16:2572-2579.
# Abu-Amero KK, Hellani AM, Salih MA, Alorainy IA. Megalocornea associated with multiple skeletal anomalies: a new autosomal recessive syndrome. Ophthalmic Genet. 2005;26(2):71-76.
# Héon E, Mathers WD, Alward WL, et al. Linkage of posterior polymorphous corneal dystrophy to 20q11. Hum Mol Genet. 1995;4(3):485-488.
# Khan AO, Aldahmesh MA, Alkuraya FS. Genetic and genomic analysis of classic cases of anterior megalophthalmos. Ophthalmology. 2011;118(1):85-92.
	[[Category:Genetics]]
	[[Category:Medicine]]
	[[Category:Ophthalmology]]