Osteogenesis Imperfecta (OI) – also known as brittle bone disease – is a genetic disorder that affects the bones and bone formation. The cause of OI is a gene defect that affects how the body makes collagen, a protein needed to make bones strong. Either the Type 1 collagen gene isn’t producing enough collagen or the collagen produced does not work properly.
Children born with OI have bones that are less dense and break easily for no known reason. OI can also cause weak muscles, brittle teeth, a curved spine and hearing loss.
Most children inherit the defective gene from one or both parents. In some cases, neither parent passes the gene or the gene stops working properly soon after conception.
There are four main types of OI:
Type I: Type 1 OI is the mildest and most common form of OI; in fact, 50 percent of the OI population have this type. There is mild bone fragility and your child may not fracture a bone until they are learning to walk. OI Type 1 is inherited and can be contracted via an affected parent, and is caused by a spontaneous mutation, or change, in the type 1 collagen gene. A child with OI 1 may have a smaller stature than their peers, and there is a strong likelihood of hearing loss.
Type II: Type II, also known as neonatal lethal OI congenita, is the severest form of OI in newborns. OI Type II is caused by a dominant mutation, or change, in the Type 1 collagen gene. Infants with Type II OI are born with short limbs, small chests and soft skulls, and frequently their legs are in a frog-leg position. The child’s lungs are usually underdeveloped and they have low birth weights. Respiratory and swallowing difficulties are common. Newborns with this type of OI generally do not survive past several weeks of delivery due to respiratory and cardiac complications.
Type III: Type III OI is the the most severe form of the disorder in newborns who survive the neonatal period, and the type of bone fragility and fracture rate varies greatly among patients. The cause of Type III OI is from a structurally defective Type 1 collagen gene. Type III OI patients have short stature, spinal curvature and multiple recurrent fractures.
Type IV: Type IV OI is classified as intermediate in terms of severity. The cause of Type IV OI is a new mutation, which is a defect in Type 1 collagen. Children with Type IV OI may be diagnosed at birth but diagnosis can also occur later in their childhood. Patients with this type of OI may not experience a bone fracture until they begin walking and it is common for their shoulder, upper arm and thigh areas to be shorter than average. Their height may also be shorter than other children in their age group.
Over the past decade, additional types of OI have been identified – a Type V and Type VI that are not caused by defects with the Type 1 collagen gene. In 2006 Types VII and VIII were identified and are inherited in a different way than the other OI types.
Symptoms of OI
In most cases, all patients with OI have fragile bones that break easily. Hearing loss occurs in 50 to 65 percent of patients with all OI types. The major symptoms found in each of the four main types of OI are:
- The whites of the child's eyes are blue, also called sclera
- Bone fractures are common during infancy and decrease after puberty
- Bruises easily
- High rate of hearing loss
- Bone fractures occur or are present before birth, often in the skull, long bones or vertebrae
- The whites of newborn's eyes are dark blue or gray
- Bones are malformed
- Lungs are underdeveloped causing breathing difficulties
- Highly susceptible to respiratory infections
- 50 percent of patients have bone fractures prior to birth
- The other 50 percent experience bone fractures after birth
- Arms and legs are shorter or deformed
- Facial abnormalities are present, such as a triangular face
- Short stature
- Bone fractures begin in infancy
- Children have difficulty growing and have a short stature
Diagnosing OI is usually based on the number of fractures found in a child’s arms, legs and ribs. For some children with milder forms of OI, the diagnosis may not be found until they are older. If you are concerned that your child may have OI, your child’s doctor will investigate their medical history and current symptoms, and will perform a physical examination for any visible signs of OI.
The types of testing for diagnosing OI are:
X-rays: X-rays can find fractures that may not be visibly obvious. It is important to note that child abuse can also lead to broken bones but the ways in which the bones are fractured are usually different in OI patients.
Genetic Testing: Both the collagen biopsy test and a DNA test detect nearly 90 percent of all collagen Type 1 mutations. A positive collagen Type 1 result confirms the OI diagnosis, yet a negative diagnosis does lead to the possibility that either the collagen Type 1 mutation is present but not detected, or the patient may have a form of OI not associated with the gene mutation.
Bone Densitometry: Commonly known as a bone density test or scan, this test alone does not confirm an OI diagnosis. When the bone density test is combined with family medical history, any findings on a physical examination of your child, and results of X-rays and genetic testing, it can offer important information supporting an OI diagnosis. In some mild cases of OI, the bone density may be normal, and in other types, the density is greatly reduced.
Treatment for OI
Treatment for OI ranges from prescribed medications such as Fosamax, to increase bone density, to infusions of medications such as pamidronate.
Shriners Hospitals for Children® is leading OI treatment with the use of pamidronate infusions. Pamidronate – also known as pamidronate disodium – is a bone resorption inhibitor, meaning that it works to prevent bone loss. The medication is given via an intravenous infusion that last about three hours. To keep kids mobile and distract them from the infusion therapy, OI patients at Shriners have the IV bag placed into a backpack that can be worn on the back or hung on a wheelchair. This allows the child to play a video game, read a book or work on homework while the drug is being administered.
Side effects of pamidronate may include bone pain, nausea and dizziness. Pamidronate infusions may not be right for every child with OI. Your medical team will discuss with you the pros and cons of this treatment, and whether it’s the right option for your child based on your child’s condition and any other heatlh issues he or she may have.