What Causes Marfan Syndrome?

Marfan syndrome is caused by a defect (mutation) in the gene that determines the structure of fibrillin, a protein that is an important part of connective tissue. A person with Marfan syndrome is born with the disorder, even though it may not be diagnosed until later in life. Although everyone with Marfan syndrome has a defect in the same gene, the mutation is specific to each family and not everyone experiences the same characteristics to the same degree. This is called variable expression, meaning that the defective gene expresses itself in different ways in different people. Scientists do not yet understand why variable expression occurs in people with Marfan syndrome.

The defective gene can be inherited: The child of a person who has Marfan syndrome has a 50 percent chance of inheriting the disease. Sometimes a new gene defect occurs during the formation of sperm or egg cells, but two unaffected parents have only a 1 in 10,000 chance of having a child with Marfan syndrome. Possibly 25 percent of cases are due to a spontaneous mutation at the time of conception.

How Is Marfan Syndrome Diagnosed?

There is no specific laboratory test, such as a blood test or skin biopsy, to diagnose Marfan syndrome. The doctor and/or geneticist (a doctor with special knowledge about inherited diseases) relies on observation and a complete medical history, including.

• information about any family members who may have the disorder or who had an early, unexplained heart-related death

• a thorough physical examination, including an evaluation of the skeletal frame for the ratio of arm/leg size to trunk size

• an eye examination, including a "slit lamp" evaluation

• heart tests such as an echocardiogram (a test that uses ultrasound waves to examine the heart and aorta).

The doctor may diagnose Marfan syndrome if the patient has a family history of the disease and there are specific problems in at least two of the body systems known to be affected. For a patient with no family history of the disease, at least three body systems must be affected before a diagnosis is made. Moreover, two of the systems must show clear signs that are relatively specific for Marfan syndrome.

In some cases, a genetic analysis may be useful, but such analyses are often time consuming and may not provide any additional helpful information. Family members of a person diagnosed with Marfan syndrome should not assume they are not affected if there is no knowledge that the disorder existed in previous generations of the family. After a clinical diagnosis of a family member, a genetic study might identify the specific mutation for which a test can be performed to determine if other family members are affected.

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