Section 5. Bone, Joint, and Muscle Disorders

Chapter 61. Paget's Disease of Bone

Topic: Paget's Disease of Bone

Paget's Disease of Bone

Paget's disease of bone is a chronic disorder of the skeleton in which areas of bone undergo abnormal turnover, resulting in areas of enlarged and softened bone.

Paget's disease can affect any bone, but the most commonly affected bones are the pelvis, thighbone (femur), skull, shin (tibia), spine (vertebrae), collarbone (clavicle), and upper arm bone (humerus).

Paget's disease rarely occurs in people younger than 40. In the United States, about 1% of people older than 40 have the disorder; the prevalence increases with age. Men are 50% more likely than women to develop it. Paget's disease is more common in Europe (excluding Scandinavia), Australia, and New Zealand than in the Americas, Africa, and Asia. It is particularly common in England.

Normally, cells that break down old bone (osteoclasts) and cells that form new bone (osteoblasts) work in balance to maintain bone structure and integrity. In Paget's disease, both osteoclasts and osteoblasts become overactive in some areas of bone, and the rate at which bone is broken down and rebuilt (bone remodeling (see Section 5, Chapter 60)) in these areas increases tremendously. The overactive areas enlarge but are structurally abnormal and therefore weaker than normal areas.

The cause of Paget's disease is unknown. Although the disorder tends to run in families, no specific genetic pattern has been discovered. Some evidence suggests that a virus (the paramyxovirus) is involved. Even if it is, there is no evidence that the disorder is contagious.

Symptoms

Paget's disease usually produces no symptoms, although bone pain, bone enlargement, or bone deformity may occur. Bone pain may be deep, aching, and occasionally severe and may worsen at night. The enlarging bones may compress nerves, adding to the pain. Sometimes Paget's disease distorts the adjacent joint structure and leads to the development of painful osteoarthritis (see Section 5, Chapter 66). Stiff joints and fatigue may develop slowly and subtly.

Symptoms vary, depending on which bones are affected. The skull may enlarge, and the brow and forehead may look more prominent. A person may notice this enlargement when a larger-size hat is needed. Enlarged skull bones can cause hearing loss and dizziness, by damaging the inner ear (cochlea) or compressing the nerve that connects the ear to the brain. The enlarged skull bones can also cause headaches, by compressing nerves. The veins on the scalp may bulge, possibly because of the increased blood flow through the skull bones. The vertebrae may enlarge, weaken, and buckle, resulting in a loss of height. Damaged vertebrae may pinch the nerves of the spinal cord, causing pain, numbness, tingling, weakness, or even paralysis in the legs. People whose hip or leg bones are affected may have bowed legs and take short, unsteady steps. Affected bones are more likely to break.

Rarely, heart failure develops because the increased blood flow through the affected bone puts extra stress on the heart. In fewer than 1% of people who have Paget's disease, the affected bone becomes cancerous.

High blood levels of calcium (hypercalcemia (see Section 12, Chapter 155)) occasionally occur in bedridden older people with Paget's disease or in anyone with severe Paget's disease who becomes immobilized or dehydrated. These high levels of calcium can result in many symptoms, such as high blood pressure, muscle weakness, mild bowel disturbances, and stones in the urine.

Diagnosis and Treatment

Paget's disease is often discovered accidentally when x-rays or laboratory tests are performed for other reasons. Otherwise, the diagnosis may be suspected on the basis of the symptoms and physical examination. The diagnosis can be confirmed by x-rays showing abnormalities characteristic of Paget's disease and by a laboratory test measuring blood levels of alkaline phosphatase, an enzyme involved in bone cell formation. A bone scan (a radionuclide test using technetium) shows which bones are affected.

A person who has Paget's disease needs treatment if the symptoms cause discomfort or if there is a significant risk or suggestion of complications, such as hearing loss, osteoarthritis, and deformity.

Aspirin, other nonsteroidal anti-inflammatory drugs, and commonly used analgesics such as acetaminophen may reduce bone pain. If one leg becomes bowed, heel lifts can help make walking easier. Sometimes surgery is needed to relieve pinched nerves or to replace a joint that has become arthritic from Paget's disease.

One of several bisphosphonates--alendronate, etidronate, pamidronate, risedronate, or tiludronate--can be used to slow the progression of Paget's disease. Except for pamidronate, which is usually given intravenously, these drugs are given by mouth. These drugs are given before surgery to prevent or reduce bleeding during surgery; they are also given to treat pain caused by Paget's disease, to prevent or slow the progression of weakness or paralysis in people who cannot have surgery, and to attempt to prevent arthritis, further hearing loss, or further bone deformity.

Calcitonin is occasionally used as an injection under the skin or in muscle. It is not as effective as the bisphosphonates and is used only when the other drugs cannot be given.

Bed rest (except for sleeping at night) should be avoided, if possible, to prevent hypercalcemia. If hypercalcemia does develop, intravenous fluids and diuretics such as furosemide are given.

Dietary intake of calcium and vitamin D (necessary for calcium uptake) should be sufficient to ensure that the incorporation of calcium into bone (bone mineralization) is adequate in bone that is being remodeled rapidly (see Section 5, Chapter 58). Otherwise, poor mineralization (osteomalacia (see Section 12, Chapter 154)) may occur.

Prognosis

The prognosis for people with Paget's disease is most often very good. However, the few people who develop bone cancer (osteosarcoma, fibrosarcoma, or chondrosarcoma (see Section 5, Chapter 63)) have a poor prognosis. People who develop other complications, such as heart failure, compression of the spinal cord, or hypercalcemia may also have a poor prognosis, unless treatment of these complications is timely and successful.