Paget’s disease of the bone is a chronic condition where the normal process of breaking down and rebuilding bone goes haywire, producing enlarged, misshapen bones that are structurally weaker than healthy bone. It most commonly affects the pelvis, spine, skull, and leg bones (particularly the femur and tibia), and it’s the second most common bone remodeling disorder after osteoporosis. Most people with Paget’s disease have no symptoms at all, and the condition is often discovered incidentally through a blood test or X-ray done for another reason.
How Normal Bone Remodeling Goes Wrong
Your skeleton is constantly renewing itself. Specialized cells called osteoclasts dissolve small areas of old bone, and other cells called osteoblasts fill those gaps with fresh, well-organized bone tissue. In Paget’s disease, osteoclasts become abnormally overactive and far larger than normal, containing dozens of nuclei instead of the usual handful. They chew through bone much faster than they should.
The body senses this accelerated bone loss and responds by ramping up bone-building activity to compensate. But the new bone laid down in this frantic repair effort is disorganized, fibrous, and coarse rather than the tightly packed, orderly structure of healthy bone. The result is bone that’s larger and denser-looking on an X-ray but paradoxically weaker, more porous, more prone to fracture, and laced with extra blood vessels.
The disease progresses through roughly four stages: an initial phase of rapid bone breakdown, a mixed phase where destruction and rebuilding happen simultaneously, a phase dominated by excessive new bone formation, and finally a quiet phase where the bone has been permanently remodeled into its abnormal architecture. Not every bone progresses through all stages at the same speed, and the disease can remain confined to just one or two bones for years.
Who Gets Paget’s Disease
Paget’s disease is rare before age 40 and becomes increasingly common with age. It affects men slightly more often than women. There’s a strong genetic component: 15 to 40 percent of patients have a family member with the condition, and the inheritance pattern is autosomal dominant, meaning a single copy of a mutated gene from one parent can be enough. Mutations in a gene called SQSTM1 account for about 40 percent of familial cases, though the genes responsible for the rest remain unidentified.
Interestingly, the incidence and severity of Paget’s disease have been declining in multiple countries over recent decades. A large Canadian study tracking cases over 20 years found that while the total number of people living with the diagnosis remained relatively stable, new cases appeared less frequently and tended to be milder. The reasons for this decline aren’t fully understood but may involve changing environmental exposures interacting with genetic susceptibility.
Symptoms and How It Feels
Most people with Paget’s disease never notice anything wrong. When symptoms do appear, they depend entirely on which bones are affected.
- Bone pain: A deep, persistent ache in the affected area, often worse at night or at rest. This is the most common symptom.
- Skull involvement: Gradual head enlargement over years, headaches, or hearing loss. The hearing loss results from changes in bone mineral density around the inner ear’s cochlear capsule, which disrupts how sound vibrations travel through it.
- Spine involvement: Back pain, stiffness, or in more advanced cases, nerve compression from enlarged vertebrae pressing on the spinal cord or nerve roots. This can cause numbness, tingling, or weakness in the legs.
- Leg bones: Bowing of the legs, altered gait, and joint pain when abnormal bone changes the alignment of the hip or knee.
Because pagetic bone has extra blood vessels running through it, the skin over an affected bone sometimes feels warm to the touch. In cases with extensive skeletal involvement, the heart has to work harder to pump blood through all that extra vasculature, though this is uncommon.
How Paget’s Disease Is Diagnosed
The diagnosis often starts with an unexpectedly high result on a routine blood test measuring alkaline phosphatase (ALP), an enzyme released during active bone formation. Normal ALP levels fall between roughly 30 and 120 U/L; in active Paget’s disease, levels can climb well above that range, sometimes two to three times normal or higher. ALP levels also serve as a useful marker for tracking whether the disease is active or responding to treatment.
X-rays show characteristic changes depending on the stage. In long bones, early disease produces a wedge-shaped area of bone loss that radiologists describe as a “blade of grass” or flame-shaped lesion, creeping along the bone shaft. In later stages when excessive bone formation dominates, the skull develops a distinctive “cotton wool” appearance with patchy, fluffy areas of increased density scattered throughout. Bones may appear enlarged, thickened, or bowed. A bone scan using a small amount of radioactive tracer can reveal all affected areas in the skeleton at once, which is useful because the disease can be present in bones that aren’t yet causing symptoms.
Treatment Options
Not everyone with Paget’s disease needs treatment. If the condition is limited to one or two bones, causing no symptoms, and ALP is only mildly elevated, monitoring with yearly blood tests is a reasonable approach. Treatment becomes important when there’s bone pain, risk of complications (such as disease near a joint or the skull base), or before planned surgery on an affected bone to reduce its heavy blood supply.
The mainstay of treatment is a class of drugs called bisphosphonates, which work by slowing down the overactive osteoclasts. The most effective option is a single intravenous infusion given over 15 minutes. Clinical trials have shown this approach to be highly effective, and many patients achieve a sustained remission lasting years from just one treatment session. For those who can’t receive an infusion, a daily oral bisphosphonate taken for about two months is an alternative, though it requires more commitment and can cause stomach irritation.
After treatment, ALP levels are monitored periodically. A drop back toward normal indicates the disease has quieted. Some patients never need retreatment, while others may need another round years later if ALP starts climbing again or symptoms return.
Complications Worth Knowing About
The most feared complication is a rare transformation into bone cancer (osteosarcoma), which occurs in less than 1 percent of patients. This tends to develop in the pelvis, femur, upper arm, or skull rather than the spine. A sudden increase in pain, rapid swelling, or a new soft tissue mass in an area known to have Paget’s disease warrants urgent evaluation. Pagetic osteosarcoma is aggressive, but its rarity means most people with Paget’s disease will never face it.
More common complications include osteoarthritis in joints adjacent to affected bone, fractures through weakened pagetic bone, and the neurological issues mentioned earlier. Hearing loss from skull involvement affects the inner ear itself rather than the auditory nerve, which means hearing aids can help. Spinal stenosis, where overgrown vertebrae narrow the spinal canal, can sometimes require surgical decompression if nerve symptoms become severe.
Fractures in pagetic bone can occur through areas of active bone resorption and often heal slowly because the underlying remodeling process is abnormal. Treatment with bisphosphonates before and after a fracture can improve healing outcomes by calming the chaotic cellular activity.