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.

In about half of people with mild OI a useful sign is seen in the skull where there may be additional small bones in the sutures known as wormian bones.

Bone density measurements are usually unhelpful for the diagnosis of OI. They frequently give normal results in bones that have not previously been fractured.

In the USA two specialised tests are sometimes used for the diagnosis of OI. One involves taking a small piece of skin, culturing the cells and chemically examining the collagen produced. The other uses a blood sample and searches for mutations of the genes coding for the collagen of bone. Both tests are labour-intensive and neither test is more than 85 per cent accurate in identifying cases of OI.

What treatment can be given?

The mainstay of treatment is competent orthopaedic care at the time of fractures, to ensure that each fracture heals in a good position. Patients should be mobilised as early as possible to minimise the loss of bone due to immobilisation. In some circumstances 'rodding' operations, in which fixed or telescopic metal rods are inserted into the shafts of bones, are very helpful, particularly in children with very frequent fractures or appreciable deformity.

Help in the form of competent occupational therapy may be invaluable in ensuring that parents are given good advice in handling of a young child, in prescribing the most appropriate seating or wheelchairs, in advising on adaptations to the home and on practical ways of ensuring a good education.

There is no drug treatment for OI itself. Trials of growth hormone have been disappointing. Trials of various bisphosphonate drugs are in progress and have given encouraging results in some patients with the more severe types of OI.

What about older women with OI?

We know that after the menopause women with OI lose bone like anybody else. This bone loss can be prevented with hormone replacement therapy (HRT).

Because people with OI already have bones that are prone to fractures, women with OI should consider taking HRT from approximately the time of the menopause.

Like any drug treatment HRT has advantages and disadvantages. On the positive side, HRT preserves the bones and is also helpful in preventing blood vessel problems such as heart attacks. On the negative side, there is a very small increase in the likelihood of breast cancer. Another disadvantage is the fact that with some preparations regular monthly bleeds continue. However, for most people, this can be avoided by modern forms of HRT (continuous combined HRT).

For most people with OI the advantages of HRT greatly outweigh the disadvantages - without it the likelihood of fractures increases quite quickly after the menopause.

Stopping smoking is vital; smoking diminishes the bone by up to 5 per cent - a loss which people with OI can ill afford. Apart from this there is evidence that smoking diminishes the effectiveness of HRT. Some women cannot or should not take HRT. For some, other drugs may have a place depending on a detailed specialist review. These include the selective oestrogen receptor modulators, such as raloxifene, or the bisphosphonates, such as etidronate and risedronate.

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