Osteopetrosis is a group of rare diseases in which bone is not remodeled, causing the bone to become denser but more prone to fracture. St. Jude's Children's Research Hospital estimates the most severe form of osteoporosis affects only 20 babies each year in the United States, almost all of whom will die by the age of 10. The more common osteoporosis diseases are less severe.
Bone Metabolism
Bone is being continually remodeled by two type of cells: osteoclasts form bone and osteoclasts resorb bone. Low-bone density, or osteoporosis, is due to failure of bone formation or more bone being resorbed than made. In osteopetrosis, there is a defect in bone removal with normal bone formation. Dense bones result.
Inheritance
The more severe forms of osteopetrosis are autosomal recessive. Symptomatic children have two defective copies of the same gene. Erna Cleiren and other genetic researchers from the University of Antwerp have found the parents of these children do not have signs of osteoporosis. The less severe forms of brittle bone disease, osteopetrosis tarda, are autosomal dominant. Cleiren reports that the same osteopetrosis gene mutation has risen spontaneously several times throughout the world. Cleiren estimates that this form of osteopetrosis might affect 5 people out of 100,000. The American Academy of Family Physicians reports that half of the patients with the autosomal dominant form of osteopetrosis have no symptoms.
Malignant Infantile Osteopetrosis
Malignant infantile osteopetrosis, also called osteoporosis congenita, is the most severe variant of the disease. In early infancy, the babies develop large heads but fail to put on weight and grow normally. As they grow older, the bones are getting thicker, hoarding more calcium. In early childhood, the children might have seizures due to low calcium. The liver and spleen enlarge to make more blood cells since the bone marrow is not contributing the usual number of cells. Patients might be anemic, and without normal blood cell manufacturing the body has difficulty fighting infection. Children with this form of osteopetrosis have spontaneous bleeding and bruise easily. As the skull bones continue to grow, hearing loss and dental problems occur. Patients do not grow to normal height.
St. Jude's Children's Research Hospital states that the only curative therapy is stem cell or bone marrow transplant, preferably from a sibling. If the child does not have a sibling, marrow from a matched donor is used. A delay in finding a donor can result in irreversible disease progression, such as hearing loss.
Osteopetrosis with Renal Tubular Acidosis
A less severe form of osteopetrosis affecting children is associated with renal tubular disease, mental retardation and cerebral calcifications. These children also might have hearing loss. Merck Manual says the treatment for this condition is bone marrow transplant, but these individuals need continued treatment of the associated kidney disease afterward.
Pyknodysostosis
Merck Manual describes pyknodysostosis as a form of osteopetrosis. These patients have a characteristic appearance. Their height is less than 5 feet and they have short, stubby hands and feet. They have a large head with a small face and receding chin. The major health problem they face is easily fractured bones.
Osteopetrosis Tarda
Albers-Schönberg disease is another name for osteopetrosis tarda. Patients might not realize that they have osteopetrosis until it is seen incidentally on X-rays or they have a fracture. The American Academy of Family Physicians reports that a fracture brings the disease to the attention of the physician.in 40 percent of patients. The fractured bones are brittle but heal normally. These patients have an increased incidence of bone infection and degenerative arthritis. Osteopetrosis tarda also might cause anemia, enlarged spleens, facial palsy and deafness.
References
- St. Jude's Children's Research Hospital: Rare Diseases: Osteopetrosis (malignant infantile form)
- American Academy of Family Physicians: Osteopetrosis;
- Albers-Schönberg Disease (Autosomal Dominant Osteopetrosis, type II) Results from Mutations in the ClCN7 Chloride Channel Gene; Erna Cleiren, Olivier Bénichou1, Els Van Hul, Jeppe Gram, Jens Bollerslev, Frederick R. Singer, Katherine Beaverson, Alexander Aledo, Michael P. Whyte, Tatsuo Yoneyama, Marie-Christine deVernejoul and Wim Van Hul: 2001
- NIH: Clinical Trials; Osteopetrosis
- Merck Manual Online; Osteopetroses: Bone and Connective Tissue Disorders in Children
