Like HowStuffWorks on Facebook!

How Osteogenesis Imperfecta Works


Types of Osteogenesis Imperfecta
©iStockphoto.com/Mostly Mozart

The exact symptoms of osteogenesis imperfecta can vary greatly from person to person, even among people with the same variety of the disorder. Some patients experience just a few fractured limbs in the course of a lifetime, while others may experience hundreds. More extreme types can cause bone deformity or even death.

Doctors classify the eight types of OI based on when each type was discovered -- type I being the first and type VIII being the most recent. Types I-IV constitute the vast majority of diagnosed OI cases. Doctors link all four to dominant mutations in genes responsible for producing type-1 collagen.

Type I osteogenesis imperfecta is the mildest and most common variety. In addition to fragile bones, type I is frequently accompanied by blue sclarae, hearing loss, thin skin, loose joints, low muscle tone and brittle teeth. Type I patients may also develop scoliosis, a condition in which spine curvature leads to chronic back pain and difficulty breathing. Triangular faces are also common. Type I patients typically experience most of their fractures, about 40 breaks, before they enter puberty [source: Cleveland Clinic Center for Consumer Health Information].

On the other end of the spectrum, type II osteogenesis imperfecta is the most severe form of the disease. It occurs in 10 percent of OI patients and is frequently lethal. Most patients are stillborn or die shortly after birth due to respiratory problems. Type II usually occurs due to spontaneous gene mutation. Newborns typically have a small birth weight, underdeveloped lungs, fractures and bone deformities.

Patients with type III osteogenesis imperfecta tend to be short in stature with poor dental development and barrel-shaped ribcages. They also experience many of the symptoms associated with type I, including colored sclera, loose joints, poor muscle development, triangular faces and hearing loss. In addition, these people live with bone deformities that grow progressively worse with age. Spinal deformities and respiratory problems are also possible.

Type IV osteogenesis imperfecta is very similar, only less severe. Type IV patients tend to experience most of their fractures before puberty.

Four additional, less common varieties of the disease also exist. Type V and type VI are both similar to type IV; except it's unknown which collagen mutation causes these types. Researchers are also unsure if the gene is dominant with type VI, as with types I-V, or if it's actually recessive, meaning that it must be inherited from both parents to be passed on. Type V doesn't affect dental health, but does cause large calluses at the sites of fractures or surgeries. This type can also cause calcification of connective tissue in the forearm, restricting arm movement.

Doctors know of two exceedingly rare forms of recessive OI: type VII and type VIII. A mutation in the genes responsible for producing a cartilage-associated protein causes the condition. Type VIII patients frequently have small bodies, round faces and short limbs. Typically lethal, type VIII results in severe growth deficiency and very low mineral counts in an infant's bones.

Currently, there is no cure for OI. However, there are a number of treatments aimed at making life easier for those born with the condition. Learn about them on the next page.