What is fragile X syndrome?

Fragile X syndrome (FXS) is an inherited genetic disease passed down from parents to children that causes intellectual and developmental disabilities. It’s also known as Martin-Bell syndrome.

FXS is the most common hereditary cause of mental disability in boys. It affects 1 in 4,000 boys. It’s less common in girls, affecting about 1 in every 8,000. Boys usually have more severe symptoms than girls.

People with FXS usually experience a range of developmental and learning problems.

The disease is a chronic or lifelong condition. Only some people with FXS are able to live independently.

FXS can cause learning disabilities, developmental delays, and social or behavioral problems. Disabilities vary in severity. Boys with FXS usually have some level of intellectual disability. Girls may have some intellectual disability or learning disability, or both, but many with fragile X syndrome will have normal intelligence. They might only be diagnosed with FXS if another family member is also diagnosed.

People with FXS may show a combination of the following symptoms as children and throughout life:

  • developmental delays, such as taking longer than normal to sit, walk, or talk compared with other children of the same age
  • stuttering
  • intellectual and learning disabilities, like having trouble learning new skills
  • general or social anxiety
  • autism
  • impulsiveness
  • attention difficulties
  • social issues, like not making eye contact with other people, disliking being touched, and trouble understanding body language
  • hyperactivity
  • seizures
  • depression
  • difficulty sleeping

Some people with FXS have physical abnormalities. These may include:

  • a large forehead or ears, with a prominent jaw
  • an elongated face
  • protruding ears, forehead, and chin
  • loose or flexible joints
  • flat feet

FXS is caused by a defect in the FMR1 gene located on the X chromosome. The X chromosome is one of two types of sex chromosomes. The other is the Y chromosome. Women have two X chromosomes while men have one X chromosome and one Y chromosome.

The defect, or mutation, on the FMR1 gene prevents the gene from properly making a protein called the fragile X mental retardation 1 protein. This protein plays a role in the functioning of the nervous system. The exact function of the protein is not fully understood. A lack or shortage of this protein causes the symptoms characteristic of FXS.

Being a fragile X permutation carrier can increase your risk for various medical conditions. Let your doctor know if you think you may be a carrier or if you have a child with FXS. That will help your doctor manage your care.

Women who are carriers are at an increased risk for premature menopause, or menopause that starts before the age of 40. Men who are carriers are at increased risk for a condition known as fragile X tremor ataxia syndrome (FXTAS). FXTAS causes a tremor that gets increasingly worse. It also can lead to difficulty with balance and walking. Male carriers may also be at an increased risk for dementia.

Children who show signs of developmental delays or other outward symptoms of FXS, such as a large head circumference or subtle differences in facial features at a young age, may be tested for FXS. Your child may also be tested if there’s a family history of FXS.

The average age of diagnosis in boys is 35 to 37 months. In girls, the average age of diagnosis is 41.6 months.

FXS can be diagnosed using a DNA blood test called the FMR1 DNA test. The test looks for changes in the FMR1 gene that are associated with FXS. Depending on the results, your doctor may choose to do additional testing to determine the severity of the condition.

FXS cannot be cured. Treatment is aimed at helping people with the condition learn key language and social skills. This may involve receiving extra help from teachers, therapists, family members, doctors, and coaches.

There may be services and other resources available in your community to help children learn important skills for proper development. If you’re in the United States, you can contact the National Fragile X Foundation at 800-688-8765 for more information regarding specialized therapy and educational plans.

Medications that are typically prescribed for behavior disorders, like attention deficit disorder (ADD) or anxiety, may be prescribed to treat the symptoms of FXS. Medications include:

  • methylphenidate (Ritalin)
  • guanfacine (Intuniv)
  • clonidine (Catapres)
  • selective serotonin reuptake inhibitor (SSRI), such as sertraline (Zoloft), escitalopram (Lexapro), duloxetine (Cymbalta), and paroxetine (Pail, Pexeva)

FXS is a lifelong condition and can affect all aspects of life, including a person’s schooling, work, and social life.

Findings from a national survey indicate that about 44 percent of women and 9 percent of men with FXS reach a high level of independence as adults. The majority of women achieved at least a high school diploma and about half succeeded in holding a full-time job. The majority of men with fragile X syndrome required assistance with daily activities as adults. Few men in the survey achieved a high school diploma or succeeded in finding full-time work.

If you’re concerned that you may be a carrier for fragile X permutation, ask your doctor about genetic testing. They can help you understand your risks and what being a carrier means for you.