Paget’s disease is a chronic bone disorder where the body’s normal process of breaking down and rebuilding bone goes haywire, producing enlarged, misshapen bones that are structurally weaker than healthy bone. It’s the second most common bone remodeling disorder after osteoporosis, and it most often affects the pelvis, skull, spine, and leg bones. Many people with Paget’s disease have no symptoms at all and only discover the condition incidentally, through blood work or imaging done for another reason.
How Normal Bone Remodeling Goes Wrong
Your skeleton is constantly renewing itself. Specialized cells called osteoclasts dissolve old bone, and another type called osteoblasts lay down fresh bone in its place. In Paget’s disease, the osteoclasts become abnormally large and overactive, containing dozens of nuclei instead of the usual handful. They chew through bone far faster than normal.
The osteoblasts try to keep up, rushing to fill in the gaps, but the new bone they produce is disorganized, fragmented, and irregular. Instead of the tightly packed structure of healthy bone, pagetic bone is fibrous, coarse, and arranged in a chaotic mosaic pattern. This bone is also unusually vascular, meaning it has more blood vessels running through it than normal bone. The end result is bone that looks thicker on an X-ray but is actually mechanically weaker and more prone to fractures.
The disease progresses through distinct phases. It starts with a wave of aggressive bone destruction, moves through a mixed phase where both breakdown and rebuilding happen simultaneously, then shifts to a phase dominated by excessive but poorly organized bone formation. In rare cases, a final stage involves malignant transformation of the affected bone.
What Causes It
The exact cause remains unclear, but both genetics and environmental factors play a role. Paget’s disease runs in families. The most commonly implicated gene is SQSTM1, which carries mutations in roughly 24% of people with a family history of the disease and about 7 to 10% of those with no known family connection. In some populations the numbers are even higher: among French-Canadians, 46% of familial cases carry the SQSTM1 mutation.
A long-standing theory points to a slow viral infection, specifically from paramyxoviruses like measles. Researchers have found measles virus genetic material inside pagetic osteoclasts, and viral proteins from measles and related viruses have been detected in affected bone cells. Interestingly, the viral genetic material also appears in nearby non-osteoclast cells, but only osteoclasts seem to actually express it. Whether the virus directly causes the disease or simply triggers it in genetically susceptible people is still an open question.
Symptoms and Where It Strikes
Bone pain is the hallmark symptom, and it’s often mistaken for arthritis. The pelvis is the most frequently affected site, followed by the skull, spine, and the long bones of the legs (the femur and tibia). When Paget’s disease affects the pelvis or thighbone, you may feel a deep, aching hip pain that doesn’t respond to typical over-the-counter treatments the way you’d expect.
Beyond pain, the disease can cause visible changes. Affected bones may bow or enlarge. A leg bone that bows outward can alter your gait and put stress on nearby joints, accelerating arthritis. Skull involvement can cause the head to appear larger, and in severe cases the skull base can soften and settle downward, potentially compressing the spinal cord or brainstem.
Hearing Loss and Other Complications
When Paget’s disease involves the skull, hearing loss is a prominent complication. Research using auditory brainstem testing has pinpointed the cochlea, the spiral-shaped structure of the inner ear, as the primary site of damage rather than the auditory nerve. The mechanism involves loss of bone mineral density in the cochlear capsule, the bony shell that houses the cochlea. As this bone becomes pagetic, it alters the acoustic mechanics of the ear, producing both a high-frequency hearing loss and a characteristic low-frequency gap in hearing tests. This means the hearing loss in Paget’s disease is not caused by problems with the tiny bones of the middle ear, as was once thought, but by changes in how sound vibrates through the diseased bone surrounding the cochlea.
Fractures are another significant risk. Because pagetic bone is structurally disorganized, it can break under stresses that healthy bone would easily absorb. The most concerning complication is malignant transformation: approximately 1% of Paget’s patients develop osteosarcoma, a bone cancer. While 1% sounds small, this represents a risk several thousand times higher than the general population. A sudden increase in pain or rapid swelling in a previously stable area of Paget’s disease warrants immediate evaluation.
How It’s Diagnosed
Paget’s disease is often first suspected from a routine blood test showing elevated alkaline phosphatase, an enzyme that rises when bone formation is unusually active. In one study of 57 patients with confirmed disease, the average bone-specific alkaline phosphatase level was roughly 15 times higher than that of healthy controls. This blood marker is also useful for tracking whether treatment is working, since levels drop as disease activity slows.
X-rays can confirm the diagnosis and show the characteristic features at each stage. Early on, you see areas of bone loss where osteoclasts have been most active. In the mixed phase, the bone takes on a mosaic appearance with patches of thickening and destruction side by side. Late-stage disease shows dense, sclerotic bone with coarse internal texture and thickened outer walls. Bone scans, which highlight areas of increased metabolic activity throughout the skeleton, are particularly helpful for mapping the full extent of the disease since multiple bones can be affected at once without causing symptoms.
Treatment and What to Expect
Not everyone with Paget’s disease needs treatment. If the condition is discovered incidentally and isn’t causing pain, isn’t in a high-risk location (like the skull or a weight-bearing bone), and blood markers are only mildly elevated, monitoring alone may be appropriate.
When treatment is needed, the goal is to shut down the overactive osteoclasts. The most effective option is a single intravenous infusion of a bisphosphonate, a class of drugs that slows bone breakdown. In a head-to-head trial, a single infusion normalized alkaline phosphatase levels in nearly 89% of patients, compared to about 58% of those taking a daily oral bisphosphonate for two months. More striking was the durability: at 18 months of follow-up, 98% of patients who received the infusion maintained a sustained response, versus 57% on the oral medication. Fewer than 1% of infusion-treated patients showed signs of recurrent disease activity during the study period.
For most people, a single treatment provides years of disease control. Some experience flu-like symptoms for a day or two after the infusion, but this is temporary. The treatment doesn’t reverse bone that’s already been remodeled in a pagetic pattern, but it stops the cycle of chaotic remodeling from continuing and can substantially reduce pain.
Living With Paget’s Disease
Because Paget’s disease is typically diagnosed in people over 50 and often progresses slowly, many people live with it for years with minimal impact on daily life. Periodic blood tests to monitor alkaline phosphatase levels help track disease activity. If you have skull involvement, baseline hearing tests are useful so any changes can be caught early. Weight-bearing exercise and adequate calcium and vitamin D intake support overall bone health, though they don’t directly treat the pagetic process itself.
Pain that was previously well controlled but suddenly worsens deserves attention, as it can signal a new fracture, joint involvement, or in rare cases the development of osteosarcoma. The vast majority of people with Paget’s disease, however, respond well to treatment and experience significant pain relief once the abnormal bone remodeling is brought under control.