Osteopetroses Health Article

Definition

Osteopetrosis (plural osteopetroses) is a rare inherited disorder that makes bones increase in both size (mass) and fragility. It is a potentially fatal condition that can deform bone structure and distort the appearance. Osteopetrosis is also called chalk bones, ivory bones, or marble bones.

Description

In healthy individuals, bones are constantly being broken down (resorption) by cells called osteoclasts, and new bone material is constantly being formed by cells called osteoblasts. Osteopetrosis occurs when there is a failure in bone resorption. The mass of bone increases, but the new bone material that is added is porous, weak, and brittle.

There are three types of osteopetrosis. In some children with osteoporosis, bone mass begins to increase at birth, but symptoms may not become evident until adulthood. In mild cases, bone mass increases at gradual, irregular intervals until full adult height is attained. Some bones are not affected. Other forms of osteopetrosis progress at a more rapid pace and destroy bone structure, which can involve bones throughout the body, although the lower jaw is never affected.

Types of osteopetroses

Severe malignant infantile osteopetrosis (early-onset osteopetrosis) is the most severe form of osteopetrosis. It results from a child inheriting defective genes from both parents (autosomal recessive pattern of inheritance). It is most often discovered soon after birth. The ends of the long bones of the arms and legs appear widened and thickened (clubbed). Bone mass continues to increase rapidly, often filling in the hollow middle of the bone where the bone marrow, which produces red blood cells, is found. Early-onset osteopetrosis can be a fatal condition, with death occurring before the age of two. About one-third of all children with malignant infantile osteopetroses die before age ten. Although this form of osteopetrosis is called "malignant," it is not a type of cancer.

Intermediate osteopetrosis generally appears in children under age ten. This condition is usually less severe than early-onset or malignant infantile osteopetrosis and is not normally life-threatening.

Adult (delayed-onset) osteopetrosis may not become evident until after age 20. Albers-Schönberg disease is a mild form of this condition. People who have this disease are born with normal bone structure. Bone mass increases as they age but does not affect appearance, health, intelligence, or life span. Many people with adult osteopetrosis are diagnosed only when abnormalities are discovered on x rays taken for other purposes. There are two distinct types of adult osteopetrosis (types I and II). These types have different biochemical features. Individuals with type II disease have a higher risk of sustaining bone fractures.

Demographics

The incidence of osteopetroses is not known, although it is estimated that adult osteopetrosis occurs in about one of every 1,250 individuals. About 20,000 people in the United States have this form of the disease. Worldwide malignant infantile osteopetrosis occurs in about one in 100,000 to 500,000 births, making it exceedingly rare. Only eight to 40 children are born with this disease in the United States each year. Males and females appear to be equally affected.

Causes and symptoms

Osteopetrosis is the result of a genetic defect that causes the body to add new bone more rapidly than existing bone disintegrates. When fibrous or bony tissue invades bone marrow where red blood cells are made, the individual may develop anemia. Infection results when excess bone impairs the immune system, and hemorrhage can occur when platelet production is disrupted. When the skeleton grows so thick that nerves are unable to pass between bones, the individual may have a nerve damage, paralysis, or become blind or deaf.

Other symptoms associated with osteopetrosis include:

• bones that break easily and do not heal properly
• bruising
• bone pain
• carpal tunnel syndrome
• osteoarthritis
• convulsions
• enlargement of the liver, lymph glands, or spleen
• failure to thrive (delayed growth, weight gain, and development)
• hydrocephalus (fluid on the brain)
• macrocephaly (abnormal enlargement of the head)
• paralysis or loss of control of muscles in the face or eyes

When to call the doctor

A doctor should be consulted if the child has frequent broken bones, bone pain, or failure of normal growth and development.

Diagnosis

Osteopetrosis is usually diagnosed when x rays reveal abnormalities or increases in bone density. Bone biopsy can confirm the diagnosis. Additional tests may be done to look for associated problems in vision, hearing, blood composition, and so forth.