Hereditary Multiple Osteochondromas (Multiple Hereditary Exostoses)

Multiple hereditary exostoses, or hereditary multiple osteochondromas, is a genetic condition that causes benign bone tumors to grow in children and adolescents. Healthcare providers use surgery, physical therapy and pain management to treat symptoms that may arise.


What is hereditary multiple osteochondromas?

Hereditary multiple osteochondromas (HMO) — formerly known as multiple hereditary exostoses — is a genetic condition caused by a change, or mutation, in one of your genes. This change causes many noncancerous (benign) tumors, known as osteochondromas, to form in certain bones.


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Does HMO have another name?

Yes, you might hear this condition called:

  • Hereditary multiple osteochondromas (HMO).
  • Multiple hereditary exostoses.
  • Diaphyseal aclasis.

What are osteochondromas?

Osteochondromas are benign bone tumors. These tumors form when cells in the bones grow in ways that aren’t typical.

Osteochondromas form on the flat surface of bones or the growth plates (cartilage at the ends of bones). A piece of cartilage (flexible tissue that is part of the skeletal system) known as a cartilage cap covers these tumors. These tumors are not cancer and don’t spread throughout the body.


Who might get hereditary multiple osteochondromas?

HMO is usually diagnosed in young children or during adolescence. Boys and girls can get this condition.

How common is hereditary multiple osteochondromas?

About 1 in 50,000 to 100,000 people develop HMO.


Symptoms and Causes

What causes hereditary multiple osteochondromas?

HMO is a genetic disorder. In 90% of people with HMO, the condition happens when there is a change (mutation) in the EXT1 or EXT2 gene (a piece of DNA that tells your body how to make certain proteins). For the other 10%, the genetic cause is unknown.

Most often, the condition happens when parents pass these genetic mutations to their children. But some people with HMO don’t have a parent with this genetic mutation. Sometimes, this condition can happen even if no one else in the family has HMO.

What are the symptoms of hereditary multiple osteochondromas (previously known as multiple hereditary exostoses)?

HMO causes lumps that might be painful in one or more bones. Other HMO symptoms include:

Some children may feel pain if tumors grow near blood vessels or nerves. Sometimes, bumps may be painless.

Is hereditary multiple osteochondromas (multiple hereditary exostoses) contagious?

Hereditary multiple osteochondromas (previously known as multiple hereditary exostoses) is not contagious. The condition does not spread directly from person to person. But if you have HMO, you have a 50% chance of passing on the gene mutation that causes the condition to your child.

Diagnosis and Tests

How is hereditary multiple osteochondromas diagnosed?

You or your child’s healthcare provider may notice a lump on or near a bone, or other symptoms of HMO. Healthcare providers will ask about your family’s medical history and do a physical examination.

What tests do healthcare providers use to diagnose hereditary multiple osteochondromas?

Healthcare providers may recommend imaging tests to diagnose HMO. They usually begin with an X-ray to see tumors. If they need more information, they may recommend:

Management and Treatment

How is HMO treated?

Treatment for HMO varies according to your symptoms. Depending on how many tumors you have, their location and any other symptoms, healthcare providers might recommend:

  • Observation of the tumor (watchful waiting).
  • Pain management.
  • Physical therapy.
  • Surgery to lengthen limbs that have not grown correctly.
  • Surgery to remove the tumor(s).

Are there complications of treatment for hereditary multiple osteochondromas (previously known as multiple hereditary exostoses)?

Some people who have surgery to remove osteochondromas develop scars.


How can I prevent hereditary multiple osteochondromas?

There aren’t any steps you can take to prevent HMO. If you know there’s a history of HMO in your family, you might consider prenatal genetic testing such as Chorionic Villus Sampling (CVS) or genetic amniocentesis during pregnancy. This can determine if the baby has inherited the condition. In some cases, preimplantation genetic testing may also be an option.

How do I know if my child is at risk for HMO?

If you or someone in your family has HMO, let your healthcare provider know. If there is no family history, the chance is quite low but not zero.

Outlook / Prognosis

What can I expect if my child has hereditary multiple osteochondromas?

Your child may develop just a few osteochondromas or many tumors. Most often, these tumors no longer grow once a child’s growth plates close. But they may come back (recur) in some people, even after the growth plates have closed.

HMO can also cause other symptoms. Some people have fatigue, pain and trouble moving.

Not all people with HMO need treatment, but some need surgery or physical therapy. Healthcare providers may recommend medications or therapies to manage pain.

How long will my child have multiple hereditary exostoses?

HMO is a lifelong condition, but most people with benign osteochondromas have a normal life expectancy. Talk with your healthcare provider about treatment options to help your child stay healthy and thrive.

Can osteochondromas become cancerous?

Yes, in rare instances, osteochondromas in people with HMO become cancerous (malignant). About 2% to 5% of people with HMO develop cancer. When this happens, the benign osteochondromas turn into bone cancers called chondrosarcoma or osteosarcoma.

Your healthcare provider may check to see if osteochondromas are cancer if:

  • New tumors grow after your child goes through puberty.
  • Osteochondromas become painful.
  • The cartilage cap on a tumor is thicker than 2 centimeters.

Healthcare providers can remove many malignant tumors with surgery. Depending on the stage of cancer (whether it has spread or metastasized), your child may not need additional treatment.

Living With

How do I take care of my child with hereditary multiple osteochondromas (previously known as multiple hereditary exostoses)?

Let your healthcare provider know about any new symptoms your child has, especially a sudden change in tumor size or increased pain. Follow your provider’s instructions to help manage these symptoms.

When should I seek care for my child with hereditary multiple osteochondromas?

If your child develops any new tumors or has new symptoms like pain, especially after puberty, let your healthcare provider know right away.

Additional Common Questions

Do children with hereditary multiple osteochondromas also have autism?

Experts know about a few cases of children with HMO and autism spectrum disorder. But they don’t know whether there is a definite link between the two conditions. Researchers continue to study whether the gene mutations that cause HMO are also involved in the development of autism.

A note from Cleveland Clinic

Hereditary multiple osteochondromas (HMO) — sometimes called multiple hereditary exostoses — causes benign tumors to form along the bones and growth plates of children and adolescents. HMO is a lifelong condition that can cause pain and skeletal changes. Surgery, physical therapy and pain management can help ease symptoms and correct bone deformities. Talk to your healthcare provider about needed tests and treatments to help your child stay healthy.

Medically Reviewed

Last reviewed on 07/05/2022.

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