Fragile X syndrome (FXS) is a problem of the X chromosome. It is a condition that is inherited from the parents. FXS is the most common cause of inherited intellectual disability.
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FXS is caused by problems with the FMR1 gene. These problems prevent fragile X mental retardation protein (FMRP) from developing. The protein helps to make connections in the brain. Without the protein, certain connections in the brain cannot be made. This causes developmental problems and prevents children from fully developing some higher cognitive functions.
The main risk factor for FXS is having a parent with an FMR1 mutation. Most people who inherit a minor mutation, which is sometimes called a premutation, do not develop the symptoms and signs of FXS. A few premutated children may show signs that resemble autism. Others, primarily males, may develop a set of neurological symptoms called fragile X tremor ataxia syndrome in later adult life.
Children of mothers with FMR1 premutations are at risk of inheriting a fully mutated FMR1 gene severe enough to cause symptomatic FXS. If a woman does not have symptoms and is a carrier of an FMR1 mutation or premutation, each child of hers has a 50% chance of inheriting that gene. Of the children that inherit that gene, boys are much more likely to develop symptoms than girls. The severity of the disorder may vary between different individuals.
In general, each generation tends to have worse mutations and a higher risk of FXS than the previous one.
The number and severity of symptoms varies widely. Symptoms tend to be less frequent and milder in females. Symptoms can include:
- Mental impairment
ranging from learning disabilities to intellectual disability
Behavioral difficulties, including:
Physical problems and abnormalities, including:
- Long face with protruding jaw
- Large, protruding ears
- Flat feet
- Hyperextended joints
- High-pitched voice and enlarged testes in males after puberty
- Mitral valve prolapse
- Low muscle tone and delayed motor skills such as sitting and walking
The doctor will ask about symptoms and medical history. A physical exam will be done. FXS may be suspected if a child has:
- Physical, cognitive, or emotional symptoms of FXS
- Unexplained developmental delays or intellectual disability
The diagnosis can be confirmed by genetic testing with a DNA blood test.
There is no cure for FXS. Treatment is aimed at controlling symptoms. Treatment may include:
Medications used to treat symptoms of FXS include:
- Stimulants and other medications to treat attention deficit/hyperactive behavior
Psychotherapeutic drugs such as fluoxetine to help control:
- Emotional outbursts
- Anticonvulsants to control seizures
Specific educational strategies depend on the degree of developmental delays and/or intellectual disability. Educational strategies include setting a stable educational environment for the patient that includes:
- School work that only requires a short attention span
- Minimal distractions
- Predictable activities and classroom routine
- Close communication between parent and school
- Emphasis on functional life skills
There are no known ways to prevent FXS once a person is born. If you have FXS or risk factors for FXS, you can talk to a genetic counselor when deciding to have children.
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http://www.ebscohost.com/dynamed. Updated February 19, 2013. Accessed June 24, 2013.
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Updated April 3, 2013. Accessed June 24, 2013.
What is fragile X?
FRAXA Research Foundation website. Available at:
Accessed June 24, 2013.
Willemsen R, Anar B, et al.
Noninvasive test for fragile X syndrome, using hair root analysis.
Am J Hum Genet. 1999;65(1):98-103.
Last reviewed May 2014 by Michael Woods, MD
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.
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