Hereditary Multiple Exostoses (HME)

What is hereditary multiple exostoses?

Hereditary multiple exostoses (say: ex-oh-STO-sees) is a genetic condition that affects bone development. Exostoses are bony growths. People with HME develop these bony growths on the ends of long bones and also on some flat bones such as the shoulder blade (scapula) and the pelvis. These growths typically develop during childhood and adolescence.

Bones Affected by HME
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Another name you may hear for this condition is hereditary osteochondromas. It affects about 1 in 50,000 people worldwide.

How HME affects the body

Bone problems

People with HME have many exostoses that grow at the surface of a number of bones, such as the long bones of the arms and legs, ribs, vertebrae, and hipbones. You can sometimes feel them under the skin. The exostoses can sometimes be painful.

The number and the size of the exostoses vary from person to person and can be different even between parent and child. New exostoses do not grow after someone’s bones have matured. However, existing exostoses can grow slowly over the years.

Height and body shape

The exostoses can sometimes lead to shortening and bowing of long bones and joint problems. Shortening and bowing of the long bones can sometimes lead to one leg being shorter than another.

People with HME have normal intelligence.

X-ray of Exostoses
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This X-ray shows a lower leg bone with three exostoses (bony growths). Exostoses mainly grow on the ends of long bones. The number and the size of the exostoses vary from person to person.

Complications of exostoses

Pain

The most common complication in HME is pain associated with the exostoses.

Cancer risk

There is a small risk for an exostosis to turn malignant (cancerous). This risk is less than 1% over someone’s lifetime.

Pressure effects of the exostoses

Depending on the size and location of the exostoses, they may cause one or more of the following effects:

  • compression of peripheral nerves
  • irritation of overlying muscles and tendons, leading to pain, compression of blood vessels and restriction of joint motion

Pregnancy and childbirth

Exostoses of pelvic bones may cause a problem in pregnancy and childbirth. There is a slightly higher chance of a Caesarean section for delivery in mothers with HME.

Course of the disease

The growth of the exostoses begins in early childhood. The average age of diagnosis is 3 years. Nearly all people with HME are diagnosed before 12 years of age.

Growth of the exostoses slows down in adolescence and there are no new exostoses in adulthood.

Most people with HME lead active and healthy lives.

HME is a genetic disorder

Genetic means related to genes. Each of us inherits our genes from our parents. They provide our bodies with instructions that influence our health, looks, and behavior. In general, each person has two copies of every gene. One copy is inherited from the mother and the other from the father.

HME occurs as a result of a mutation (change) in one copy of the EXT1 or the EXT2 gene.

  • In about 10% of people with HME, this is a new mutation, meaning neither parent has the disease.
  • However, in about 90% of people with HME, the mutation is inherited (passed on) from one of the parents.

HME is inherited as an autosomal dominant condition. This means that:

  • Only one copy of the gene (either EXT1 or EXT2) has a mutation. In this case, a person will have HME. The other copy of the gene works normally but this is not enough to prevent a person from having HME.
  • A person with HME has a 50% chance during each pregnancy of passing this genetic condition on to the child.
  • The risk to the sibling (brother or sister) of a person with HME depends if the parent has HME or not.
Inheritance of HME
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HME is caused by a mutation in either the EXT1 or EXT2 gene. Every person has two copies of each of these genes. A person will have HME if one copy of the gene has a mutation. In this example, the parent with HME has one copy of one of the genes with a mutation and the other has none. If the baby does not inherit the mutation then the baby will not have HME. If the baby inherits the copy of the gene with a mutation, then the baby will have HME.

To learn more about genetics, please read the two-part Understanding Genetics series on AboutKidsHealth: Part 1, Part 2.

HME can be diagnosed before or after birth

Before birth, HME can be diagnosed by testing DNA taken from the fetus. This can be obtained in one of the following ways:

  • by amniocentesis after the 15th week of pregnancy
  • by chorionic villus sampling (CVS) between the 11th and 14th week of pregnancy

These methods are used for high-risk pregnancies, for instance if the parents have another child diagnosed with HME or one of the parents has HME.

To be eligible for this testing, the mutations in the gene responsible for causing HME in the family must be known. This means genetic testing (a blood test) must be done on the person in the family with HME.

HME cannot be diagnosed by ultrasound in pregnancy. This is because the exostoses usually do not start growing until early childhood.

After birth, the diagnosis of HME is based on X-ray findings. A diagnosis of HME can be confirmed by DNA testing of the EXT1 and EXT2 genes (a blood test).

Treatment of HME

There are no treatments to prevent the growth of exostoses. At this time, treatment is based on each person’s symptoms. Children with HME lead fulfilling lives if they receive attentive, informed care from their parents and health care providers.

Treatment of complications

Pain management, physical therapy, and surgery are the treatment options available for the various complications.

  • Pain should be managed with medication when needed.
  • Painful exostoses that result in compression of a nerve or vessel may be treated by surgery.
  • If exostoses are large or if bone deformities occur, then surgery may also be an option.

Monitoring for cancer

The most important aspect in the management of HME is monitoring for cancer. Monitoring the size of exostoses in adults, in particular those involving the pelvis or scapula, may help in early detection and treatment. For this reason, it is important to notify a health care provider if one begins to experience rapid growth of exostoses and increasing pain, especially as an adult.

Treatment of malignant transformation (cancer)

There is less than 1% lifetime risk for an exostosis to become cancerous. In these cases, surgery to remove the exostoses is necessary. The use of radiotherapy and chemotherapy is controversial. Speak to your oncology team for more information about treatment.

Genetic counselling for HME

People with HME and their families should consider assessment by a geneticist and genetic counsellor. They can help with the following:

  • confirming the diagnosis
  • discussing the natural history of the disease
  • evaluating the risk that future children will also be affected
  • discussing available options for managing the disease

Coping with HME

In general, children with visible differences are more likely to have difficulties in school and society. Because of the risk of pain associated with exostoses, your child should avoid activities that increase this pain. These activities are different for every child. Encourage your child to participate in activities with other children the same age. As well, try to physically adapt your child's surroundings to support your child and encourage independence.

Children with HME must be followed carefully by a health care team if they have symptoms. Speak to your health care team if you need help or if you are having trouble coping.

Patient resources

The following organizations and sites can offer more information, support, and contact with other affected individuals and their families.

MHE and Me

www.mheandme.com

The MHE Research Foundation

www.MHEResearchfoundation.org

The MHE Coalition

www.mhecoalition.org

Key points

  • HME is a genetic condition where exostoses (bony growths) grow on certain bones during childhood and adolescence.
  • Children with HME need to be followed carefully by a health care team if they have symptoms.
  • There is a less than 1% lifetime risk for an exostosis to become cancerous.
  • Genetic counselling can help families understand HME, the options for managing it, and the chances that other children will also be affected.

Lucie Dupuis, MSc, MS, CGC

Ayeshah Chaudhry, MBBS

Roberto Mendoza-Londono, MD, MS, FACMG, FCCMG

 




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