Multiple hereditary exostoses (multiple osteochondromas): symptoms, genetics and care

Multiple hereditary exostoses (MHE) is the older name for what is now called hereditary multiple osteochondromas (HMO). An osteochondroma is a benign bony growth, covered in a cap of cartilage, which grows outward from a bone near its growth plate. In this condition many such growths develop, mainly on the long bones of the arms and legs. MedlinePlus notes that they can also form on the vertebrae and on flat bones such as the hips and shoulder blades.

The number and the position of the growths vary greatly from person to person, even within one family. Some people have a few large lumps and others have several small ones. They appear during childhood and stop forming once growth ends, and NORD says they usually continue to grow until shortly after puberty.

A single osteochondroma is common and is usually not inherited. HMO is the term used when there are multiple growths and a genetic cause. MedlinePlus says hereditary multiple osteochondromas account for 15 percent of all cases of osteochondromas.

Many growths cause nothing more than a hard lump that can be felt under the skin. Problems come from where the growth is and how big it becomes. NORD lists deformities of the forearm, knees, ankles, spine or pelvis; pressure on nerves, tendons or blood vessels that can cause pain or interfere with movement and circulation; and, rarely, spinal cord compression if the vertebrae are affected.

MedlinePlus adds that the condition can disrupt bone growth, so affected people may be shorter than others in their family, may have uneven limb lengths, mild curving of the spine, forearms or ankles, hip dysplasia and early joint disease (premature osteoarthritis). Growths on the vertebrae or ribs can be life-threatening if they press on nerves, blood vessels or the spinal cord, or interfere with lung function, although this is uncommon.

NORD says that bowing of the forearm and ankle are the problems that most often require surgical correction, and that around 40 percent of affected people have mild short stature as a result of shortened or bowed legs.

Changes (pathogenic variants) in the EXT1 and EXT2 genes cause HMO. These genes make proteins that help to process heparan sulfate, which is involved in bone formation and in the specialisation of cartilage-forming cells. MedlinePlus says EXT1 variants account for roughly 65 to 70 percent of cases and tend to cause more severe symptoms than EXT2 variants.

It is an autosomal dominant condition: one altered copy of the gene is enough to cause it. About 90 percent of affected people inherit the variant from an affected parent, and about 10 percent have a new variant, so there may be no family history. That means a child of an affected person has a 1 in 2 chance of inheriting the condition. Genetic counselling is recommended for affected individuals and families by NORD.

NORD says diagnosis is based on clinical features, X-ray findings and family history. Molecular genetic testing of the EXT1 and EXT2 genes is available to confirm the diagnosis. In the UK, a GP is the usual first contact, with referral to specialist orthopaedic or genetics services as needed.

X-ray findings are one part of the diagnosis according to NORD. The exact referral pathway varies by region.

There is no medicine that stops the growths forming, so care is supportive and focused on problems that arise. NORD says that monitoring the size of affected bones by annual scans to screen for malignant change is sometimes recommended, and that other treatment is symptomatic and supportive.

See your GP, or your specialist team if you have one, if a growth becomes rapidly bigger, becomes more painful, especially after growth has finished, or if new numbness, weakness, tingling, or changes in circulation appear. NORD identifies rapid growth and increased pain as signs of possible malignant change. Sudden loss of power or sensation in the legs or loss of bladder or bowel control needs emergency care.

NORD says the growths stop forming after puberty, and that many people need no treatment. Where pain or reduced movement limits activity, your specialist team can advise on surgery and on supportive care.

Because the condition is inherited, other family members may also be affected, and some may not yet know. A genetics service can discuss testing and options for family planning. A clinician can point you to patient organisations for multiple osteochondromas.