Megalocornea
Patient Information
Characteristics
Background and History
Numerous congenital malformations of the cornea (“the windshield of the eye”) have been reported, many associated with other ocular and systemic abnormalities. Isolated megalocornea is one of these in which the cornea is larger than normal and usually occurs only in males.
Clinical Correlations
Megalocornea is a rare congenital malformation of the cornea which can cause blurred vision. The cornea is larger than normal and sometimes the cornea loses some of its transparency later in life. Cataracts may occur and the iris may be thinned. Usually vision can be corrected to normal or near normal with glasses. Megalocornea may occur as an isolated problem or in association with more extensive disease.
Genetics
Inheritance
Isolated megalocornea is usually found only in males as the gene found in most families is located on the X chromosome. Affected males cannot pass this mutation to their sons and it is not known to cause disease in carrier daughters. However, males can have affected grandsons among their daughters’ children.
Pedigree
X-linked recessive, father affected
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked recessive, carrier mother
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
Diagnosis and Prognosis
Simple inspection of the eye can reveal an enlarged cornea. Eye professionals can measure the diameter of the cornea to document enlargement. A complete medical eye examination is important for other signs such as mild opacification may occur along with cataracts. Megalocornea can occur by itself but it is also sometimes seen in other syndromes so that complete medical examinations are recommended.
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