Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI), commonly known as brittle bone disease, is a group of genetic disorders that result in bones that fracture easily. The range of symptoms can vary from mild to severe, affecting not only the skeleton but also other organs. The disorder is often diagnosed at birth and lasts a lifetime. It is primarily caused by mutations in the COL1A1 or COL1A2 genes, leading to issues with the production or quality of type I collagen.

Signs and Symptoms

Orthopaedic

The hallmark of OI is fragile bones with low mineral density, leading to frequent fractures. In severe cases, these fractures can even occur in utero. Other orthopaedic signs include bowed long bones, joint hypermobility, and scoliosis, particularly in types III and IV. Fracture rates typically decrease after puberty but increase again in later life. Joint hypermobility is common due to the same genetic mutations that cause some types of Ehlers-Danlos syndrome.

Otologic

By age 50, about 50% of adults with OI experience significant hearing loss, which can be conductive, sensorineural, or mixed. Hearing loss may begin as early as the second decade of life and is most common in type I OI. Balance issues and vertigo have also been associated with OI, though studies are limited.

Systemic

OI can affect virtually all organs due to the presence of type I collagen throughout the body. Cardiovascular complications such as aortic aneurysms and respiratory issues like pulmonary insufficiency are common. Gastrointestinal symptoms, including chronic constipation and recurrent abdominal pain, are also observed, particularly in severe cases.

Diagnosis

Diagnosis is typically based on medical imaging, including X-rays and DEXA scans, and confirmed through DNA or collagen protein analysis. Blue sclerae are a characteristic sign. Genetic testing of genes like COL1A1, COL1A2, and IFITM5 can confirm the diagnosis. Differential diagnosis includes conditions like child abuse, rickets, and various forms of osteoporosis.

Classification

There are two typing systems for OI: clinical and genetic. The clinical system, developed by David Sillence, includes four types ranging from mild (type I) to perinatally lethal (type II). The genetic system has identified 21 types, with many being extremely rare. Most cases are due to mutations in COL1A1 or COL1A2, inherited in an autosomal dominant manner or occurring spontaneously (de novo).

Treatment

There is no cure for OI; treatment focuses on managing symptoms and preventing fractures. This includes maintaining a healthy lifestyle, physical therapy, and using mobility aids. Bisphosphonates like pamidronate are commonly used to increase bone density, especially in children, though their efficacy in reducing fracture rates is unclear.

Acute Bone Fracture Care

Fractures in OI are treated similarly to the general population but with lightweight materials to prevent additional fractures. Nonunion and malunion are greater risks, so all fractures should be immobilised.

Medications

Bisphosphonates are the mainstay treatment to increase bone density, especially in children. Oral bisphosphonates are less effective than intravenous forms. Nutritional supplements, particularly vitamin D, are often recommended.

Surgery

Surgical interventions include rodding to improve bone strength and spinal fusion for scoliosis. Rodding surgery involves inserting metal rods into the long bones, which can be either telescoping or non-telescoping.

Physical Therapy

Physical therapy aims to strengthen muscles and improve mobility while minimising fracture risk. Water aerobics, light resistance exercises, and walking are recommended. Contact sports and activities that stress the joints are contraindicated.

Genetics

OI is highly genetically heterogeneous, with most cases resulting from mutations in COL1A1 and COL1A2. Autosomal recessive forms are also recognised, often involving defects in collagen chaperones or other proteins involved in bone formation.

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Self-assessment MCQs (single best answer)

What is the primary cause of Osteogenesis Imperfecta?

Which type of collagen is most affected in Osteogenesis Imperfecta?

What is a common systemic complication of Osteogenesis Imperfecta?

Which diagnostic method is commonly used to confirm Osteogenesis Imperfecta?

By what age do approximately 50% of adults with OI experience significant hearing loss?

What is the primary goal of physical therapy for individuals with Osteogenesis Imperfecta?

Which of the following is a common medication used to treat Osteogenesis Imperfecta?

What is the purpose of rodding surgery in the treatment of Osteogenesis Imperfecta?

Which clinical sign is characteristic of Osteogenesis Imperfecta?

What is the inheritance pattern for most cases of Osteogenesis Imperfecta?