“PAPAHOOD”

Brittle bone disease (osteogenesis imperfecta)
Written by Dr Colin R Paterson, consultant physician

What is osteogenesis imperfecta?

Osteogenesis imperfecta (OI) is the most common disease causing fractures in childhood. It also causes fractures in adults.

OI is a genetic disorder usually resulting from abnormalities of the genes that control the production of a protein called collagen; which is the main protein in bone and essential for its strength. The fragility of bone in OI is due to the collagen problems; it has nothing to do with the calcium part of bone, which is what shows up on X-rays.

How common are fractures?

Some OI children are born with fractures that have taken place in the womb. Others have their first fractures soon after birth or several years later.

Some people with OI have so few fractures in childhood that the correct diagnosis is not made. Fractures are difficult to predict, especially in childhood. Some occur with normal handling. Some occur with so little trauma that the usual signs of a fracture may not be seen and the fracture is not identified till some weeks or months later when an X-ray is done for another reason. The bones do not always behave in a brittle way; fractures may fail to occur when expected from an injury. The reason for these variations is quite unknown.

In both sexes and in almost all types of OI the fracture rate diminishes during the teenage years and remains low in adult life. The reason for this is not known.

What other clinical problems can occur?

Besides fractures there may be problems in other parts of the body; most of these are, like the fractures, the result of the defects of collagen.

* The joints may be lax.

* The whites of the eyes may be blue or grey.

* The teeth may be discoloured and fragile.

* There may be an increased liability to bruising (thought to be due to the defective collagen in small blood vessels).

* Deafness may occur (see below).

* Hernias are more common in people with OI.

* Excessive sweating or intolerance of heat are common complaints; the cause of this is not known.

Is OI inherited?

OI in an individual is present from the time of conception.

In some people, mostly those with milder OI, the disorder passes from one generation to another. In some of these people, and in most with severe OI, it arises without any family history. In most, but not all, of these the cause is a ‘new genetic mutation’ - in other words the responsible change in the person’s genes arises anew, and not because it has been passed on from a parent.

It is important to obtain advice from a specialist in gene problems (clinical geneticist) who may be able to identify the pattern of inheritance and advise on the risk to further children or the risk of passing on the condition.

How is OI diagnosed?

In most people the diagnosis is made from the pattern of fractures and the finding of any of the associated clinical features such as blue or grey whites of the eyes. However, it is important to recognise that none of these signs may be present and that the diagnosis may be very difficult.

In severely affected people X-rays may show characteristic abnormalities - the result of previous fractures. In many people with only mild or moderate OI the X-rays may appear normal at the time of the first few fractures. Later, in bones that have been the site of previous fractures, the bones may appear demineralised (less white on X-ray) and reduced radiation may be needed to obtain satisfactory films for the diagnosis of fractures.