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Osteopetrosis is a disorder in which an imbalance in the formation and breakdown of bone causes bones to be overly dense, yet weak and prone to break easily. It is a congenital condition, which means people are born with it.
Excessive bone density can interfere with vital tissues and structures, causing serious problems throughout the body. Compression of nerves in the skull, for example, can lead to problems such as vision loss, hearing loss, or paralysis of facial muscles. It can also cause crowding of the bone marrow, which is the tissue inside the bones that produces blood cells and platelets. This can lead to low levels of cells needed to fight infection, carry oxygen to the body’s cells, or control bleeding. In the most severe cases, these problems can be life-threatening. The specific complications and their severity depend largely on the type of osteopetrosis.
There are two major types of osteopetrosis: malignant infantile and adult. The malignant infantile form is evident at or shortly after birth and can greatly reduce life expectancy. Despite its name, it is not related to cancer. The formal name for this is autosomal recessive osteopetrosis.
The adult form is milder and may not be diagnosed until adolescence or adulthood. The formal name for this is autosomal dominant osteopetrosis.
Because some cases of osteopetrosis do not fit clearly into these two categories, some scientists recognize a third type called intermediate osteopetrosis. Found in children younger than age 10, this form is more severe than the adult form, but less severe than the malignant infantile form. The vast majority of the patients falling into this category have a severe form of autosomal dominant osteopetrosis (the “adult” disease).
Osteopetrosis is caused by defects in one or more genes involved in the formation, development, and function of cells called osteoclasts. These cells break down bone tissue during bone remodeling. Remodeling is a normal process in which old bone is removed and new bone is created to replace it. The malignant infantile form of osteopetrosis occurs when a child inherits a copy of the defective gene from both parents.
Only one copy of the defective gene is necessary for the adult form. This can be passed from one parent. However, in many cases, the abnormal gene does not come from either parent, but may be the result of a new gene mutation that happens at conception.
The severity of symptoms differs for the three forms of osteopetrosis. The most common symptoms are bone fractures, low blood cell levels, impaired vision and hearing, and dental problems related to infection.
The bones in people with osteopetrosis appear unusually dense and chalky white on x rays. A bone biopsy is generally not recommended.
Doctors may use other tests to diagnose and gain additional information about specific problems related to osteopetrosis. These may include hearing and vision tests, blood tests, computerized axial tomography (CAT) scans, and magnetic resonance imaging (MRI).
Treatment for osteopetrosis depends, in part, on the form. For children who are severely affected, a bone marrow transplant may be performed. In this procedure, abnormal osteoclasts are replaced with normal ones. This can completely stop bone and bone marrow abnormalities. Unfortunately, it cannot reverse damage that has already occurred. Furthermore, finding an appropriate bone marrow donor can be difficult, and the procedure itself is very risky.
Other treatments for children or adults include:
Several different types of doctors may be involved in the diagnosis and treatment of osteopetrosis. Often patients see a team of doctors who work together to provide the best treatment. These doctors may include:
Some researchers supported by the National Institutes of Health (NIH) are examining the genetic and molecular mechanisms of abnormal bone formation and breakdown. Their goal is to find new targets for therapy. Others are studying the surgical management of complications. Elsewhere, scientists are looking at modifications to bone marrow transplantation that would make the procedure more readily accessible to children shortly after diagnosis, with the hope of stopping the disease and preventing its complications.
For more information on Osteopetrosis, contact the:
The National Institutes of Health Osteoporosis and Related Bone Diseases ~
National Resource Center gratefully acknowledges the assistance of Michael
J. Econs, M.D., in the preparation and review of this publication.
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