Paget's disease, also called osteitis deformans, is a chronic bone disorder in which there is abnormal bone remodelling (the process of bone breakdown and rebuilding). With Paget's disease, bone is broken down more quickly and is then replaced with softer, more porous bone. As a result, affected bones become thicker, weaker, and deformed. It usually affects the bones of the skull, pelvis, collarbone, spine, and legs.

Paget's disease affects about 3% of people over the age of 40, but the exact number is not known since many people don't know they have the condition. It seems to affect more men than women. Although this disease is found around the world, it appears more commonly in Europe and Australia.

Although the exact cause of Paget's disease is unknown, it appears to run in families. Up to 30% of people with the disease have other family members who also have it. Some researchers believe that a virus may be involved, but a specific virus has not been identified.

Whatever the cause, Paget's disease results in abnormal bone remodelling (the process of bone breakdown and rebuilding). In Paget's disease, when bone is broken down, it is replaced with bone that is less dense and more brittle, more prone to fracture or bowing, and enlarged.

Most people with Paget's disease have no symptoms. When symptoms do occur, the most common symptom is bone pain. When Paget's only involves one area, it is called monoostotic. When it involves more than one area of the skeleton it is called polyostotic. Other symptoms associated with Paget's disease include:

• joint pain (if Pagetic bone is connected to the joint)
• bone fractures
• bone deformities (e.g., enlarged head, bowed legs, reduced height)
• pain or numbness (from pressure on nerves)
• headache
• hearing loss (when abnormally formed skull bones press on the nerve)
• warmth over an affected area
• fatigue

Unless Paget's disease shows up on an X-ray taken for some other reason, people often don't know they have it.

Later on, chronic bone pain becomes the most common symptom of the disease. The bones are tender to touch, and become large and deformed. They also break easily, require a long time to heal, and are malformed when they do heal.

Enlarged spinal bones may press on sensory nerves, causing pain and sometimes cutting off all feeling in the legs. Enlarged bones in the head can put pressure on nerves leading to tinnitus (ringing or noises in the ear), dizziness, and headaches.

Paget's disease usually progresses slowly. Complications from this disorder can include:

• osteoarthritis (a degenerative joint disease)
• heart failure (only in cases of advanced polyostotic Paget's disease due to the increased blood flow putting extra demand on the heart and blood circulation)
• bone cancer (rarely)
• kidney stones
• hypercalcemia (high levels of calcium in the blood; this usually occurs in severe cases and if a person is immobilized for a long time)

People with Paget's disease may be first diagnosed from blood work, X-rays, or bone scans that are done for other reasons. This is because in the early stages of Paget's disease, there are usually no symptoms. X-rays will show areas of thick, porous bone and other areas with thinner bone, depending on the stage of the disease. The blood tests will show high levels of alkaline phosphatase, an enzyme used in forming bones.

Noticing this condition early is important to help reduce the number and severity of complications that are associated with this condition.