Osteogenesis imperfecta

Alternative names

Brittle bone disease

Definition

Osteogenesis imperfecta is a congenital (present from birth) condition of abnormal fragility of the bones.

Causes, incidence, and risk factors

This bone disorder is usually present at birth as an inherited disease. Osteogenesis imperfecta (OI) is classified into four major types (and further subtypes).

All four types of OI are caused by defects in the amount or structure of type 1 collagen, an important part of the bone matrix. The collagen problem usually results from a dominant genetic defect. This defect may be acquired by several different mechanisms:

• The defect may be inherited in an autosomal dominant pattern from an affected parent. This means that a parent with a single gene for OI has a 50% chance of having a child with the disorder. Any child who inherits this gene will be affected.
  
• The defect may be acquired by a spontaneous mutation occurring in the individual egg or sperm that formed the child. In this case, neither parent carries a gene for the disorder or is affected by it. The parents, in this case, are no more at risk than the general population for having another child with the disorder.
  
• The defect may be acquired through a pattern of inheritance called mosaicism. This occurs when neither parent is affected, but one carries a percentage of sperm or eggs which contain the genetic defect. Therefore, though the parents are unaffected, some of their children may have the disorder and others will not. It is estimated that about 2% to 7% of unaffected parents who have had a child with OI will have another child with OI due to the phenomenon of mosaicism.
  

Symptoms

All of the bones are abnormally weak in a person with OI. The severity of the abnormality varies enormously - from type 2 OI, which is usually lethal in infancy (or even before birth) to type 1 OI, which may be so mild that the diagnosis is not made, even in adulthood.

The three classic symptoms of OI includes fragile bones, early hearing loss, and whites of the eyes that appear bluish (blue sclerae). Nevertheless, not all people with OI will have blue sclerae or hearing loss. All do have fragile bones, but not all people with OI will ever break a bone.

A variety of other symptoms may be found in the various types of OI:

• Bone fracture (broken bone)
  o More than one broken bone occurring in a single episode (multiple)
  o Fractures present at birth
  o Occuring after only minor trauma