Paget Disease of Bone
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Direct answer
Paget disease of bone is a focal disorder of bone remodelling in which excessive resorption is followed by disorganised, enlarged and weakened new bone. Alkaline phosphatase is typically raised while serum calcium and phosphate stay normal, and it usually presents after the age of 40. Its complications include pathological fracture, secondary osteoarthritis, deafness and, rarely, osteosarcoma.
What you must remember
- Three phases: osteolytic (active resorption), mixed (lytic and sclerotic change together) and sclerotic (burnt-out bone that is dense but weak).
- Biochemistry: alkaline phosphatase is raised, reflecting increased osteoblastic activity, with normal serum calcium and phosphate.
- Radiology: a flame-shaped or blade-of-grass lytic front in long bones, thickened cortex and coarse trabeculae, and a cotton-wool appearance in the skull.
- Distribution: the pelvis, lumbar spine, skull, femur and tibia are most often involved, and the disease is frequently polyostotic.
- Complications: pathological fracture, secondary osteoarthritis of adjacent joints, deafness from skull or cranial nerve involvement and high-output cardiac failure in extensive disease.
- Sarcomatous change: osteosarcoma is the commonest sarcoma arising in pagetic bone; it is uncommon but serious.
- Treatment: bisphosphonates such as zoledronic acid, alendronate or risedronate suppress bone turnover and relieve pain, with calcium and vitamin D given alongside.
Recognising a hat that no longer fits
A 62-year-old man has a dull ache in his thigh and a rising hat size, and his X-ray shows a thickened femoral cortex with a coarsened skull. The reasoning moves from recognising the pattern, to confirming the diagnosis, and then to screening for complications.
- Recognise the pattern: enlarged, thickened bones with coarse trabeculae in an older adult, often with bone pain and an enlarging skull, point towards Paget disease.
- Confirm biochemically: alkaline phosphatase is raised with normal calcium and phosphate, and a bone scan shows the extent of disease through increased uptake in active lesions.
- Stage the disease: identify the lytic, mixed or sclerotic phase, because the phase guides treatment urgency and the risk of fracture.
- Screen for complications: check for pathological fracture, joint degeneration, hearing loss and features of high-output failure in extensive disease.
- Treat active disease: a bisphosphonate is given to reduce turnover and pain, after ensuring that calcium and vitamin D status is adequate.
- Watch for sarcomatous change: a new painful lesion, especially with a soft-tissue mass, raises the possibility of osteosarcoma and needs biopsy.
Viva angles on phases and complications
Examiners test whether you can read the biochemistry and the radiograph together, and recognise the serious complication.
- Trap: calling a raised alkaline phosphatase with normal calcium a sign of bone metastasis. Correct: in Paget disease the raised enzyme reflects increased osteoblastic activity with normal calcium, and metastasis is a separate diagnosis to exclude by history and imaging.
- Trap: regarding the sclerotic phase as healthy bone. Correct: burnt-out bone is dense but disorganised and weak, so the fracture risk persists.
- Trap: attributing skull thickening to hydrocephalus or generalised hyperostosis. Correct: the cotton-wool appearance and enlarged calvaria are typical of Paget disease, and the coarse trabeculae help distinguish them.
Frequently asked questions
What are the phases of Paget disease of bone?
Paget disease passes through an osteolytic phase of active resorption, a mixed phase with both lytic and sclerotic change, and a sclerotic phase in which the bone is dense but disorganised. The phase helps determine treatment urgency and the risk of fracture.
Why is alkaline phosphatase raised in Paget disease of bone?
Alkaline phosphatase is raised because osteoblasts are highly active in pagetic bone, producing a large amount of new, disorganised matrix. Serum calcium and phosphate usually remain normal, and the enzyme level is used to monitor disease activity.
What is the most serious complication of Paget disease of bone?
Sarcomatous transformation into osteosarcoma is the most serious complication of Paget disease of bone, although it is uncommon. Suspicion arises with a new painful lesion or a soft-tissue mass, and biopsy is needed to confirm the diagnosis.
What radiological appearance suggests Paget disease of the skull?
A cotton-wool appearance of the skull, with mixed areas of lysis and sclerosis and an enlarged calvaria, is the typical radiological picture of Paget disease. The skull changes are often seen alongside lesions in long bones and the pelvis.
Which drugs are used to treat Paget disease of bone?
Bisphosphonates such as zoledronic acid, alendronate and risedronate are the main drugs used to suppress bone turnover and relieve pain in Paget disease of bone. Calcitonin is an older alternative that is used less often, and calcium and vitamin D are given alongside treatment.