What is Osteogenesis imperfecta (Brittle bone disease)

Osteogenesis imperfecta (OI) is one of the most common inherited bone disorders. The disease typically involves the bones, teeth, ligaments, eyes and skin, and is characterised by fragile bones that break easily. There are 4 variants or subtypes of osteogenesis imperfecta.

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.

When people have OI, all of their bones are abnormally weak. The severity of the abnormality varies enormously — from Type II OI, which is usually lethal in infancy (or even before birth) to Type I OI, which may be so mild that the diagnosis is not made, even in adulthood.

Statistics on Osteogenesis imperfecta (Brittle bone disease)

Frequency:

In the US: Incidence of OI by type is as follows:

  • Type I – One per 30,000 live births
  • Type II – One per 60,000 live births
  • Type III – One per 70,000 live births
  • Type IV – Rare

Internationally:
A higher incidence has been observed in 2 major tribal groups in Zimbabwe.

Sex:
No known differences based on sex exist.

Age:
Age of onset of symptoms (i.e. fractures) varies depending on the type, as follows:

  • Type I – Infancy
  • Type II – In utero
  • Type III – Half the cases in utero, and other half in the neonatal period
  • Type IV – Usually in infancy

Risk Factors for Osteogenesis imperfecta (Brittle bone disease)

OI is actually a connective tissue disorder, resulting from the defective synthesis of a protein called collagen, which is a structual component of bones, tendons, eyes, organs and skin.

OI is genetically inherited. If an affected parent has the disease, generally there is a 50% chance they will pass the condition on to their child. Occasionally a child may be born with OI despite neither parent having the condition.

Progression of Osteogenesis imperfecta (Brittle bone disease)

There are 4 clinical variants of OI, and these differ in presentation and prognosis.

  • Type 1: the most common and mild form of OI. Fractures occur throughout life, but deformity is uncommon. Features include blue sclerae, hypermobile joints, hearing loss and scoliosis.
  • Type 2: is the most severe form and affects approximately 10% of patients with OI. Severe deformity is common, and infants generally do not survive long after birth.
  • Type 3: is a severe form, and accounts for approximately 20% of OI. It is characterised by bones that fracture easily and also bend. By the age of 6 years, the child has usually had multiple fractures and developed severe deformities. Children surviving till adulthood usually have short stature and disability.
  • Type 4: in terns of severity lies somewhere in between types 1 and 3. Inheritance is autosomal dominant, and fractures are common before puberty. Sclerae are pale blue and become normal in colour in adult life.

How is Osteogenesis imperfecta (Brittle bone disease) Diagnosed?

Diagnosis is based on clinical and physical findings, accompanied by relevant tests.

These include:

  • Taking a skin sample to assess collagen production in the body
  • X-rays which may show thinning of bone and past or current fractures
  • An ultrasound may be used during pregnancy to detect limb abnormalities at 15-18 weeks gestation. However, these may not always be accurate.

Prognosis of Osteogenesis imperfecta (Brittle bone disease)

Permanent deformity of the extremities may occur. Brain damage may result from skull fractures. The disorder can be fatal. The disease is grouped by type:

  • Type I: Mild – Compatible with normal life expectancy.
  • Type II: Lethal – Most, but not all, die in early childhood.
  • Type III: Progressive deforming – Decreased life expectancy.
  • Type IV: Moderately severe – Compatible with normal life expectancy.

How is Osteogenesis imperfecta (Brittle bone disease) Treated?

The goals of treatment are to prevent fractures, prevent deformity when fractures do occur, and to correct deformity.

Parents are educated on how to handle their children so as to lessen the risk of fractures. Genetic counselling for couples wishing to have more children is essential.

Surgery may be performed to correct and prevent deformities in children with OI severe enough to prevent walking. Correction of scoliosis may also be helpful, but difficult due to bone fragility.

Exercise is essential in maintaining bone and muscle strength, which may help in the prevention of fractures.

Appropriate pain relief should be discussed, any may include medications and alternative therapies. For patients with severe osteopaenia and excessive fractures, medications may be considered to increase bone density and reduce fractures.

  1. Good nutrition and directed exercise is key in helping to optimize bone and muscle strength. Physical and rehabilitation therapy can be quite beneficial. Swimming is an excellent conditioning exercise for many people with OI.
  2. Surgical procedures, including the placement of metal rods through bones, can help strengthen bones and prevent deformity. Braces and gait aids are helpful for some.
  3. The use of biphosphonates in children with OI is currently being researched with some promising results. Other medical interventions including bone marrow transplant, the use of growth hormone, and gene therapy are also under investigation.

Osteogenesis imperfecta (Brittle bone disease) References

[1] Apley, Solomon. Concise system of orthopaedics and fractures, 2nd ed. Arnold, London (2001).
[2] Cotran, Kumar, Collins. Robbins Pathological Basis of Disease, 6th ed. WB Saunders, United States of America (1999).
[3] Kumar and Clark. Clinical Medicine, 5th ed. WB Saunders, Toronto (2002).
[4] Pattekar M. Osteogenesis Imperfecta. eMedicine 2003.
[available online @ http://www.emedicine.com/ped/topic1674.htm]
[5] Smith R. Editorial: Severe osteogenesis imperfecta: new therapeutic options?
BMJ 2001;322:63-64