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Osteopetrosis

A hereditary bone disorder

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Updated October 08, 2013

Dense, brittle bones
Osteopetrosis is a congenital disorder, meaning a person is born with the condition. There are several forms of the disease, ranging from mild to severe. The basics of the disease is that special bone cells called osteoclasts do not function normally. These osteoclasts usually break down old bone tissue as new bone tissue grows. In osteopetrosis, since the old bone tissue is not broken down, the bones in the body overgrow. In the head and spine, this overgrowth puts pressure on nerves and causes neurological problems. In the bones where bone marrow usually forms, the overgrowth crowds out the bone marrow. The overgrown, dense bones are very brittle and break easily.

Who is affected
The adult form of osteopetrosis affects approximately 1,250 people in the United States. The most severe form, malignant infantile osteopetrosis, is very rare, affecting only 8 to 40 children born in the United States each year. It is difficult to estimate how often the different forms of osteopetrosis occur worldwide.

Malignant infantile osteopetrosis
This severe form begins to affect the baby even before he or she is born. The infant's bones become very brittle (they have been compared to sticks of chalk) and break easily. During the birth process, the baby's shoulder bones may break.

Malignant infantile osteopetrosis is usually apparent at birth, and during the first few months of life the infant has:

  • low calcium (which can cause seizures)
  • pressure on the optic nerve in the brain (leading to visual impairment or blindness)
  • frequent bone fractures
During the first year of life about 75% of children with malignant infantile osteopetrosis will develop blood problems such as anemia (low number of red blood cells) and thrombocytopenia (low number of platelets in the blood). Untreated children with malignant infantile osteoporosis have difficulty surviving beyond age 10.

Malignant infantile osteopetrosis occurs when both parents have an abnormal gene that is passed to the child (called "autosomal recessive" inheritance). The parents do not have any symptoms of the disorder even though they carry the gene. Each child they have has a 1 in 4 chance of having the disorder.

Adult (benign) osteopetrosis
Originally called Albers-Schonberg disease after the German radiologist who first described it, this form of osteopetrosis is mild and is usually found in adults between 20 and 40 years old. Adults with osteopetrosis have frequent bone fractures that don't heal well. Bone infections (osteomyelitis), pain, degenerative arthritis, and headaches may also occur.

In benign osteopetrosis, a child can inherit the disorder if he/she receives only one defective gene (called "autosomal dominant" inheritance). The children of an adult diagnosed with osteopetrosis have a 50% chance that they, too, have the gene for the disorder.

Intermediate osteopetrosis
There seems to be a third type of osteopetrosis that appears in childhood before age 10. It has some of the same symptoms as the malignant infantile form, and appears to be inherited the same way, but is less severe.

Other possible forms of osteopetrosis
Some researchers believe there is another form, called transient infantile osteopetrosis, which is diagnosed early in life but resolves on its own without treatment. Other researchers consider carbon anhydrase Type II deficiency (CAII) to be yet another form of osteopetrosis. CAII affects the bones, kidneys, and brain, and causes the blood chemistry to change. This disorder usually appears in the first few years of life in children of Mediterranean and Arab descent. Children affected by CAII develop calcium deposits in the brain and mental retardation, and about 3/5 of children will develop blindness and/or hearing loss due to bone pressure on the nerves in the skull.

Treatment
Both the adult and childhood forms of osteopetrosis may benefit from Actimmune injections. Actimmune (interferon gamma-1b) delays the progression of malignant infantile osteopetrosis because it causes an increase in bone resorption (breakdown of old bone tissue) and in red blood cell production.

Bone marrow transplant is the only complete cure available for malignant infantile osteopetrosis. Bone marrow transplant has many risks, but if it is successful it saves the life of a child who would otherwise die from complications of the disorder.

Other treatments include nutrition, prednisone (helps improve the blood cell count), and physical and occupational therapy.

Source:
- National Institutes of Health. Information for Patients about Osteopetrosis

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