Paget Disease of Bone

Meet the patient

A 68-year-old retired engineer asks his GP for a hearing-aid referral. His wife adds, almost as an aside, that his hats no longer fit, his forehead has grown prominent, and his left shin has ached every night for a year. Routine bloods return an alkaline phosphatase of 480 — but the calcium, phosphate, renal function and gamma-GT are all normal.^[1][2]

Two questions frame the whole topic, and this vignette holds both: why is bone turnover roaring while the minerals stay perfectly calm? and which drug, given once into a vein, switches it off? Hold those two and Paget disease answers itself.^[1]

Thick but weak — the Paget paradox

Paget disease of bone (historically osteitis deformans, described by Sir James Paget in 1877) is a chronic, focal — not systemic — disorder of adult bone remodeling. At an affected site, the normal coupled cycle of osteoclastic resorption followed by osteoblastic formation becomes pathologically accelerated and disordered: osteoclasts are abnormally large and multinucleated, resorb bone excessively, and the resulting osteoblast repair is chaotic, laying down coarse, disorganised woven bone in a characteristic "mosaic" pattern. The paradox of Paget disease is that the bone becomes bulkier, thicker and hypervascular yet is mechanically weak, prone to bowing, deformity and fracture.^[1][2]

Because turnover is markedly increased at affected sites but confined to them, the biochemical signature is distinctive: alkaline phosphatase (a bone-formation marker) is markedly raised while serum calcium, phosphate and parathyroid hormone remain normal. Most patients are asymptomatic; the disease is discovered incidentally on blood tests or imaging. When symptoms occur, they reflect either the bone itself (pain, deformity), compression of adjacent structures (deafness, cranial-nerve or cord compression), or a complication (pathological fracture, osteosarcoma, high-output cardiac failure). Treatment with a potent bisphosphonate — most often a single zoledronic acid infusion — suppresses the disordered remodeling for months to years.^[1][3]

Etymology for viva gold: osteitis deformans — Latin and Greek for "deforming bone inflammation" — was Sir James Paget's own 1877 name, and it tells the whole story in two words: the bone is remodelled, and it changes shape. The eponym stuck because Paget's clinical description, drawn from a single coachman watched across twenty years, has never been bettered.^[1]

Clinical distinctions and naming pitfalls

Paget disease of bone is entirely unrelated to Paget disease of the breast or vulva (an intra-epidermal adenocarcinoma sharing only the eponym). Juvenile Paget disease is a separate, genetically distinct entity caused by loss-of-function mutations in TNFRSF11B (encoding osteoprotegerin, OPG); it produces massive, widespread juvenile bone turnover with deafness and retinopathy and is not adult Paget disease. Another distinct entity, familial expansile osteolysis, and the IBMPFD syndrome (inclusion-body myopathy with Paget disease and frontotemporal dementia, due to VCP mutations) overlap genetically but are multisystem — examiners test the distinction.^[2][4]

Classification

Paget disease is classified along three axes that the examiner will probe: ^[1]

1. By number of sites — monostotic (single bone, ~one-third) versus polyostotic (multiple sites, ~two-thirds). Polyostotic disease with the classic pelvis–spine–skull pattern carries a higher complication burden.
2. By pathological phase at a given site — every lesion evolves through three phases: an early lytic (osteoclastic, "hot") phase, an intermediate mixed phase, and a late sclerotic ("burnt-out") phase.
3. By aetiology — sporadic (the vast majority) versus familial (autosomal dominant, SQSTM1 mutations, ~10–30 percent of familial cases and a smaller fraction of sporadic).^[2][4]

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Epidemiology & Risk Factors

Paget disease is a disease of older adults and of specific populations. The key epidemiological facts the examiner rewards: ^[1]

• Age — rare under 40; prevalence rises steeply with age, reaching roughly 2–3 percent of individuals over 55 in high-prevalence regions and higher still in the very elderly. Onset is essentially never in childhood.
• Geography and ancestry — highest prevalence in the United Kingdom, Western Europe, North America (especially among those of British descent), Australia and New Zealand. It is notably rare in Asia, Africa, Scandinavia and the Indian subcontinent, supporting a strong interplay of genetic susceptibility with an environmental trigger.
• Sex — slight male predominance (roughly 1.5:1).
• Familial clustering — a family history is obtained in 15–30 percent of cases; SQSTM1 mutations account for a substantial proportion of familial disease, with autosomal dominant inheritance and variable penetrance.^[4]
• Declining incidence and severity — over recent decades the prevalence, severity and diagnostic age at onset have all fallen markedly in traditional high-prevalence countries. The decline is too rapid to be genetic and is attributed to environmental changes (improved nutrition, reduced childhood infections, declining domestic coal/air pollution), supporting a viral/environmental contribution to aetiology.^[5]

The practical consequence of this epidemiology is twofold. First, in an older patient of Anglo-Saxon ancestry, Paget disease is the leading explanation for an isolated raised ALP with normal calcium and phosphate. Second, a young patient, or one of non-European ancestry, with apparent Pagetoid change warrants careful thought about mimics (fibrous dysplasia, metastases) and the rare genetic syndromes above. ^[1]

Pathophysiology

Paget disease is best understood as a two-hit process: a genetically susceptible host (germline SQSTM1/p62 or related mutations lowering the threshold for osteoclast activation) encounters an environmental trigger (a paramyxovirus-like infection of osteoclast precursors) that ignites focal, disordered remodeling. ^[1]

The osteoclast: the cell that drives the disease

Pagetic osteoclasts are abnormally large and hypernucleated (often containing 20–100 nuclei versus the normal 3–10) and frequently contain nuclear inclusion bodies with microcylindrical filaments resembling paramyxovirus nucleocapsids (measles and canine distemper have been implicated by in-situ hybridisation). These giant osteoclasts resorb bone rapidly and excessively through an upregulated RANK–RANKL pathway: RANKL (receptor activator of NF-κB ligand), expressed by osteoblasts and stromal cells, binds RANK on osteoclast precursors, driving their fusion and activation; osteoprotegerin (OPG), the decoy receptor that normally restrains this axis, is functionally impaired. SQSTM1 (sequestosome-1/p62) mutations — which encode an adaptor protein in the NF-κB signalling cascade — further sensitise osteoclasts to RANKL, lowering their activation threshold.^[2][4]

The three pathological phases

At any affected site the disease evolves through three overlapping phases: ^[1]

1. Lytic ("hot", osteoclastic) phase — the dominant activity is aggressive osteoclastic resorption. On X-ray this produces sharply demarcated lytic lesions: osteoporosis circumscripta of the skull vault, and the V-shaped "blade of grass" (or "flame-shaped") advancing lytic wedge in a long bone. Bone scan uptake is intense. The bone may be subtly expanded and hypervascular.
2. Mixed phase — compensatory osteoblast overdrive kicks in to replace the resorbed bone, but the repair is disorganised and excessive. Bone is laid down as coarse woven bone rather than normal lamellar bone, producing the characteristic "mosaic" pattern of irregular cement lines under the microscope. X-rays show a mixture of lytic and sclerotic areas with cortical thickening, coarsened trabeculae and bone expansion.
3. Sclerotic ("burnt-out") phase — turnover slows; the lesion is dominated by dense, thick, disordered sclerotic bone. X-rays show a thickened, expanded, sclerotic bone (e.g., a "cotton-wool" skull, an "ivory" vertebra) that is bulky but mechanically weak.^[1][2]

Why the bone is "thick but weak" — and why ALP rises

The rapid, chaotic osteoblastic repair produces woven bone that lacks the organised lamellar architecture giving normal bone its tensile and compressive strength. The result is bone that is bulky, sclerotic and hypervascular on imaging, yet brittle — hence the characteristic bowing deformity of weight-bearing long bones and the propensity to pathological fracture. Because osteoblast activity (and therefore alkaline phosphatase) is markedly increased while the process is confined to focal sites, ALP rises but calcium and phosphate remain normal — the resorbed calcium is promptly redeposited by the overactive osteoblasts, so there is no net systemic mineral disturbance. This is the central biochemical insight that distinguishes Paget disease from systemic metabolic bone disease.^[1][5]

The RANK–RANKL–OPG axis and the genetic basis of susceptibility

Normal bone remodeling is governed by a tightly coupled cytokine triad. RANKL, expressed on the surface of osteoblasts and marrow stromal cells (and also secreted in soluble form), binds its receptor RANK on osteoclast precursors, committing them to fusion, differentiation and bone-resorbing activation. The decoy receptor osteoprotegerin (OPG), encoded by TNFRSF11B, binds RANKL and prevents it from engaging RANK, braking osteoclast formation. In Paget disease this balance is tipped decisively toward RANK activation: pagetic osteoclasts and their precursors show exaggerated RANK responsiveness, and the SQSTM1/p62 mutations carried by familial cases encode an adaptor protein in the NF-κB signalling cascade downstream of RANK. By altering p62's sequestosome/ubiquitin-binding function, these mutations (the recurrent P392L substitution being the prototypical example) lower the threshold at which RANKL triggers osteoclastogenesis and reduce apoptosis of mature osteoclasts — producing the giant, hypernucleated, long-lived cells that define the disease histologically.^[4]

This molecular framework explains both the focal nature of the disease (the genetic susceptibility is necessary but not sufficient; a local environmental trigger — most plausibly a paramyxovirus infection of an osteoclast precursor — must ignite the focus) and its response to therapy: nitrogen-containing bisphosphonates such as zoledronic acid are internalised by osteoclasts and inhibit farnesyl pyrophosphate synthase in the mevalonate pathway, preventing prenylation of small GTPases essential for osteoclast function, so the cells undergo apoptosis and stop resorbing. The result is a rapid fall in osteoblast-stimulating signals, a fall in ALP, and a switch from active lytic/mixed disease toward a quiescent, mechanically competent state.^[1][3]

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The many faces — silent bloods to a changing hat size

The clinical hallmark of Paget disease is its heterogeneity: most patients are entirely asymptomatic, while a minority present with bone pain, deformity, deafness, a complication, or — very rarely — osteosarcoma. ^[1]

Asymptomatic (the majority)

Most cases are detected incidentally — either through routine blood tests showing an isolated raised ALP with otherwise normal chemistry (the single most common presentation in modern practice), or on an X-ray performed for another reason (e.g., a pelvis film for hip osteoarthritis revealing pagetoid change). The introduction of routine ALP measurement on biochemical panels means Paget disease is now most often biochemically, not clinically, diagnosed.^[1]

Symptomatic bone disease

• Bone pain — classically a deep, aching pain at the affected site, often worse at night and unrelated to activity. Pain may arise from the pagetoid bone itself, from periosteal stretching, from microfractures, or from secondary osteoarthritis of an adjacent joint (e.g., hip or knee) caused by altered mechanical loading.
• Deformity — slow, progressive bowing of long bones (the classic sabre shin / sabre tibia from anterior tibial bowing, or a bowed femur); enlargement of the skull (the patient may report needing a progressively larger hat size, or frontal bossing); progressive kyphosis; and an asymmetric, broadened long-bone contour. Long-bone bowing produces a characteristic antalgic, broad-based gait.
• Warmth over the bone — the skin over an affected tibia or skull can feel noticeably warm because pagetoid bone is hypervascular; this is a useful bedside sign.
• Hearing loss — sensorineural deafness (and occasionally tinnitus or vertigo) from cochlear and eighth-nerve involvement at the pagetoid skull base. It develops slowly and is one of the most common symptomatic complications of skull disease.
• Dental and cranial-nerve features — rarely, cranial-nerve palsies (II, V, VII) from skull-base foramina, and hypercementosis / loosening of teeth with ill-fitting dentures. ^[1]

Complication-led presentation (the acute forms)

A subset present through a complication rather than chronic symptoms: ^[1]

• Pathological fracture — through weak pagetoid bone, classically of the femoral neck or subtrochanteric femur, the tibia, or the humerus; a long bone that was bowed and painful suddenly gives way with minimal trauma.
• New neurological deficit — spinal cord compression (paraparesis, sensory level, sphincter disturbance) from pagetoid vertebrae; basilar invagination / platybasia causing brainstem compression, lower cranial-nerve palsies, ataxia, or obstructive hydrocephalus.
• High-output cardiac failure — only with extensive polyostotic disease: the large hypervascular bone bed acts as a chronic arteriovenous shunt, raising cardiac output and precipitating heart failure in those with limited cardiac reserve.
• Sudden new or rapidly worsening pain at a pagetoid site — the red flag for osteosarcoma transformation (see Complications). ^[1]

Differential Diagnosis

The differential depends on whether the presentation is biochemical (isolated raised ALP), radiological (sclerotic or lytic bone lesions), or clinical (bone pain, deformity, deafness). Examiners test the discriminator for each. ^[1]

The single highest-yield discriminator is the biochemical signature: Paget disease is the only common cause of a markedly raised ALP with simultaneously normal calcium, phosphate, renal function and (usually) liver function. If the gamma-GT is normal, the ALP is of bone origin and Paget moves straight to the top of the differential in an older patient. A normal ALP with a destructive skull or spine lesion points away from Paget toward myeloma, metastasis or a primary bone tumour. Any change in a known pagetoid lesion — new pain, cortical destruction, soft-tissue mass, rising ALP after a period of stability — mandates exclusion of osteosarcoma by MRI and biopsy.^[1][6]

Clinical & Bedside Assessment

The focused examination in suspected or known Paget disease looks for deformity, hypervascularity, neurological compromise and cardiac involvement, and surveys the sites the disease favours. ^[1]

• General inspection — frontal bossing and skull enlargement (measure head circumference; ask about hat size); kyphosis; antalgic gait; sabre (anteriorly bowed) tibia; bowing of the femur; apparent shortening of a limb from deformity.
• The bone — palpate for bony enlargement, warmth and tenderness over the skull, spine, pelvis, femur and tibia. Compress the affected tibia for warmth. Listen for a bruit over a hypervascular lesion (rare but classical).
• Skull-base and cranial nerves — audiometry for sensorineural hearing loss (CN VIII is the most commonly affected); test the other cranial nerves for basal invagination signs (lower cranial-nerve palsies, ataxia, long-tract signs).
• Spine and nervous system — a full neurological examination looking for spinal cord or nerve-root compression (sensory level, paraparesis, sphincter disturbance) from pagetoid vertebrae.
• Joints — assess for secondary osteoarthritis of the hip, knee or sacroiliac joint adjacent to a pagetoid segment.
• Cardiovascular — examine for high-output cardiac failure (raised JVP, basal crackles, third heart sound, flow murmurs) in patients with extensive polyostotic disease.
• Functional impact — gait, mobility, hearing, and activities of daily living; weight loss or new systemic symptoms suggesting malignancy. ^[1]

Targeted history

Ask specifically about family history of Paget disease or unexplained fractures (suggests SQSTM1 familial disease), a change in hat or shoe size, progressive hearing decline, previous fractures, and any new or worsening pain in a known pagetoid site (the osteosarcoma red flag). Establish ethnic ancestry and age of onset.^[1]

Raised ALP, normal calcium — the signature and the scan

The diagnosis of Paget disease is made by combining the biochemical signature with characteristic imaging; biopsy is reserved for suspected malignancy. ^[1]

Biochemistry

• Alkaline phosphatase (ALP) — markedly raised (often 3–10 times the upper limit of normal in active polyostotic disease); it is the principal marker of disease activity and treatment response. Importantly, the rise is proportionate to the burden of active disease, so monostotic or burnt-out disease may have only a mildly raised or even normal ALP.
• Calcium, phosphate, renal function, parathyroid hormone — all normal. This combination — raised ALP with normal Ca, PO4, eGFR and PTH — is the biochemical signature of Paget disease. (Calcium can rise with prolonged immobilisation, e.g., after a fracture, but is otherwise normal.)
• Liver function, especially gamma-GT — to confirm the ALP is of bone rather than liver origin; a normal gamma-GT with raised ALP points to bone.
• Bone turnover markers — bone-specific ALP and P1NP (total procollagen type 1 N-terminal propeptide, a formation marker) and urinary NTX or serum CTX (resorption markers) quantify turnover and are more sensitive than total ALP in mild disease, though they are not required for routine diagnosis.
• 25-hydroxyvitamin D — measure before bisphosphonate therapy; vitamin D deficiency must be corrected first to avoid treatment-induced hypocalcaemia.^[1][5]

Interpreting the raised ALP — the discriminator table

An isolated raised ALP is one of the commonest abnormal results in adult medicine, and Paget disease is only one of several explanations. The fastest discriminator is to fractionate the ALP (or measure gamma-GT in parallel): a normal gamma-GT means the ALP is of bone origin, after which the calcium and phosphate decide whether the cause is focal (Paget) or systemic. The table below is the reasoning an examiner expects: ^[1]

The clinical rule the examiner rewards: in an older patient of Anglo-Saxon ancestry with an isolated bone-source ALP and normal calcium and phosphate, Paget disease heads the differential — order a pelvic and skull X-ray and a bone scan. A raised gamma-GT redirects the work-up to the liver; a disturbed calcium or phosphate redirects to metabolic bone disease or malignancy.^[1][5]

Plain radiographs — the diagnostic X-ray features

Plain X-rays remain the primary imaging modality and carry named, examinable signs specific to the phase and site: ^[1]

• Skull — early osteoporosis circumscripta (a well-defined lytic defect, usually frontoparietal); later the "cotton-wool" appearance of mixed sclerotic patches on a lytic background; thickening of the vault and base; basilar invagination.
• Long bones (femur, tibia) — the advancing V-shaped "blade of grass" (also called a "flame-shaped") lytic wedge marking the active resorption front; cortical thickening, coarsened trabeculae, bone expansion and anterolateral bowing (sabre deformity).
• Pelvis — the most commonly affected site; thickening of the iliopectineal line, sclerosis, coarse trabeculae and asymmetry ("whiskering").
• Spine — the "picture-frame" vertebra (a square vertebral body with a thickened sclerotic rim and relatively lucent centre) and, in the burnt-out phase, the "ivory" vertebra (a uniformly dense, enlarged vertebral body).
• General — cortical thickening, bone expansion, loss of the corticomedullary distinction, and the mixed lytic-sclerotic texture that is the radiological hallmark.^[1][2]

Isotope bone scan (technetium-99m)

The Tc-99m diphosphonate bone scan is the most sensitive test for defining the extent and number of sites — it identifies polyostotic disease that plain films miss and is performed at diagnosis to map the skeleton. Active pagetoid lesions appear as intensely avid "hot spots" reflecting high osteoblastic turnover. Bone scan does not characterise the lesion (lytic vs sclerotic, cortical destruction, soft-tissue mass) — that is the job of plain films and cross-sectional imaging.^[1]

Cross-sectional imaging and biopsy

• MRI / CT — reserved for a pagetoid site with suspected osteosarcoma (new or escalating pain, cortical destruction, an extraosseous soft-tissue mass), for spinal disease with neurological compromise, and for basilar invagination with brainstem signs. MRI defines cord compression and tumour extent; CT defines cortical destruction.
• Bone biopsy — when imaging raises suspicion of sarcomatous transformation; histology confirms osteosarcoma, fibrosarcoma or chondrosarcoma arising in pagetoid bone.
• Audiometry — at baseline and for any symptomatic hearing loss in skull disease.
• ECG and echocardiogram — in extensive polyostotic disease to screen for high-output cardiac failure.^[1][6]

Management — Resuscitation & Emergencies

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Uncomplicated Paget disease is chronic and requires no acute resuscitation. Three scenarios, however, are time-critical emergencies that every candidate must be able to manage: ^[1]

1. Spinal cord compression from pagetoid vertebrae — urgent MRI of the spine, IV dexamethasone to reduce oedema, and an immediate neurosurgical / spinal surgical opinion for decompression; bisphosphonate is started once the acute situation is addressed.
2. Pathological or impending fracture — orthopaedic assessment for internal fixation, with pre-operative bisphosphonate therapy (where feasible) to reduce the marked hypervascularity and intra-operative bleeding typical of pagetoid bone.
3. Suspected osteosarcoma transformation (new, severe, rapidly escalating pain, a palpable mass, or a sharp ALP rise after remission) — urgent MRI and biopsy; refer to a sarcoma / orthopaedic oncology service.
4. High-output cardiac failure from extensive polyostotic disease — standard guideline-directed heart-failure therapy plus prompt bisphosphonate suppression of the hypervascular bone bed that is driving the high-output state.^[1][6]

Management — Definitive & Stepwise

Who to treat — and who to watch

Treatment is not automatic at diagnosis. The decision balances symptom burden against complication risk: ^[1]

• Treat if — symptomatic bone pain; deformity or its imminent risk; complication risk (deafness, cord or nerve compression, basilar invagination, high-output failure); a lesion at a weight-bearing or fracture-prone site (femur, tibia); polyostotic / extensive active disease with a markedly raised ALP; young patients (who have decades of disease ahead); before elective orthopaedic surgery at a pagetoid site; and to suppress disease before joint replacement.
• Observe (do not treat) if — the disease is asymptomatic, monostotic, in a non-critical site (e.g., a single scapular lesion), biochemically quiescent, and there is no impending complication.^[1][3]

Bisphosphonate treatment of first choice

A single 5 mg intravenous infusion of zoledronic acid, given over 15 minutes, is the bisphosphonate most likely to give a favourable response in Paget disease and the treatment of first choice. In two pooled randomised trials, it outperformed oral risedronate on every endpoint: at six months, 96 percent of zoledronic acid patients had a therapeutic response versus 74 percent on risedronate, ALP normalised in 88.6 versus 57.9 percent, and during post-trial follow-up only 1 of 113 zoledronic acid patients lost their response, against 21 of 82 on risedronate.^[3][1][7]

Oral and alternative bisphosphonate regimens

When oral therapy is preferred or IV access is impractical, oral risedronate is the standard alternative; older or less potent agents are largely historical or second-line: ^[1]

• Zoledronic acid — a single 5 mg IV infusion over 15 minutes; more rapid, more complete and more sustained responses than risedronate, with long-term remission in the majority.^[3][2]
• Risedronate — 30 mg orally daily for 60 days; ALP normalised in 57.9 percent of patients at six months, so it remains an effective oral option where IV therapy is impractical.^[3]

Other bisphosphonates are largely historical; the guideline recommendation is for zoledronic acid when a bisphosphonate is indicated, because it is the agent most likely to relieve pain.^[1]

Adjuncts and supportive care

• Analgesia — bisphosphonate therapy is recommended for bone pain arising from metabolically active disease, and pain scores improve after treatment.^[1][3]
• Orthopaedic surgery — total hip or knee replacement is recommended when Paget patients develop secondary osteoarthritis in which medical treatment is inadequate.^[1]

Monitoring

• Alkaline phosphatase every 6–12 months — the principal marker of response and relapse; a normalised ALP indicates biochemical remission.
• Re-treat on biochemical relapse (a sustained rise in ALP above the patient's remission baseline, typically above 1.25 times the upper limit of normal) or on symptomatic relapse.
• Lifelong surveillance for osteosarcoma: any new, focal, escalating pain, palpable mass, or unexpected ALP rise in a treated patient demands urgent imaging and biopsy.^[1][6]

Worked example — from biochemistry to treatment

A 70-year-old man of British descent is referred for an asymptomatic ALP of 540 U/L with normal liver function tests, calcium and phosphate. A pelvic X-ray shows pagetoid change and a bone scan demonstrates intense uptake in the right pelvis, L3 and the skull vault: polyostotic Paget disease. He has no pain, but the disease is polyostotic and biochemically active, so after discussion he receives a single 5 mg IV zoledronic acid infusion, aimed at his symptoms and complication risk rather than at a target ALP value. If, two years later, his ALP climbs and he develops new tibial pain, the priority is not to re-treat empirically but to image the tibia and biopsy to exclude sarcomatous transformation before considering a second dose.^[1][3][6]

Specific Subtypes & Scenarios

• Monostotic versus polyostotic — about a third of cases involve a single bone (often found incidentally on an X-ray); the remainder involve multiple sites in the classic pelvis–spine–skull–femur distribution. Polyostotic disease carries a higher risk of deafness, deformity, basilar invagination and high-output cardiac failure and is more often an indication for treatment.
• The three phases — every lesion evolves from lytic (active, "hot", osteoporosis circumscripta, blade of grass) through mixed to sclerotic/burnt-out (cotton-wool skull, ivory vertebra). Treatment is most effective against the active, lytic/mixed phase; burnt-out lesions may show little biochemical response because turnover has already declined.
• Familial Paget disease (SQSTM1) — autosomal dominant with variable penetrance; earlier onset, more often polyostotic, and an indication to screen first-degree relatives with ALP in middle age.^[4]
• IBMPFD (VCP mutation) — inclusion-body myopathy with Paget disease of bone and frontotemporal dementia; suspect when a patient with Paget disease also develops progressive myopathy or cognitive decline.
• Juvenile Paget disease (TNFRSF11B/OPG) — a separate entity: massive, widespread childhood bone turnover, deafness, retinopathy and vascular calcification from loss of osteoprotegerin; do not confuse with adult Paget disease.^[2]

Complications & Pitfalls

• Pathological fracture — through mechanically weak pagetoid bone, most often the femoral neck or subtrochanteric femur, tibia or humerus; may be the presenting event.
• Osteosarcoma / sarcomatous transformation — the most feared complication; incidence around 0.7–1 percent of Paget patients (rising with extent and duration of disease). It presents with new, severe, rapidly worsening pain, a palpable soft-tissue mass, cortical destruction on imaging, and sometimes a sharp rise in ALP after a period of biochemical remission. Histology is most often osteosarcoma, occasionally fibrosarcoma or chondrosarcoma. Prognosis is poor; treatment is surgical (limb-salvage or amputation) with chemotherapy. Any change in a known pagetoid lesion mandates imaging and biopsy.^[6]
• Deafness — sensorineural, from cochlear and eighth-nerve involvement; the commonest symptomatic complication of skull disease; rarely other cranial-nerve palsies.
• Secondary osteoarthritis — of the hip, knee or sacroiliac joint adjacent to a pagetoid segment, from altered mechanical loading; a common source of pain and a frequent indication for joint replacement.
• Spinal cord or nerve-root compression — from pagetoid vertebral expansion; an emergency requiring MRI, steroids and surgical decompression.
• Basilar invagination / platybasia — softening and flattening of the skull base allows the odontoid to invaginate, compressing the brainstem and lower cranial nerves and occasionally causing obstructive hydrocephalus.
• High-output cardiac failure — only in extensive polyostotic disease; the hypervascular bone bed acts as a chronic AV shunt.
• Hypercalcaemia of immobilisation — uncommon, but can occur after a fracture or prolonged bed rest when resorbed calcium is no longer redeposited into active bone.
• Nephrolithiasis — from the increased calcium load in active disease or immobilisation.
• Dental complications — hypercementosis, loosening of teeth, ill-fitting dentures; also bleeding risk during dental extraction in hypervascular bone, and osteonecrosis of the jaw as a rare bisphosphonate-related adverse effect.

The classic pitfalls are three: mistaking a raised ALP for liver disease without checking gamma-GT; failing to image and biopsy a changing pagetoid lesion (delaying the diagnosis of osteosarcoma); and giving a bisphosphonate without first repleting calcium and vitamin D, precipitating hypocalcaemia.^[1][6]

Prognosis & Disposition

Paget disease is usually slowly progressive but highly manageable with modern bisphosphonate therapy. The prognosis splits along clinical lines: ^[1]

• Asymptomatic, monostotic disease — normal life expectancy; observation with periodic ALP is appropriate.
• Symptomatic or complication-risk disease — responds well to zoledronic acid, with biochemical remission (normalised ALP) sustained for months to years; quality of life, mobility and hearing are the principal functional outcomes.
• Osteosarcoma transformation — the dominant disease-specific cause of death; rare but carries a poor prognosis.
• Long-term trend — both the incidence and severity of Paget disease have been declining for decades, so contemporary disease tends to be milder and more often monostotic than historical series suggest.^[1][5]

Disposition is outpatient in almost all cases, with shared care between primary care (ALP monitoring, analgesia, falls prevention, hearing aids) and a metabolic bone clinic (bisphosphonate therapy, response assessment). Admit only for fracture, cord compression, sarcoma work-up, or high-output cardiac failure. ^[1]

Follow-up, safety-net and patient education

Every Paget patient leaves the consultation with three explicit safety-net messages. First, an annual review of ALP (more often in the first year after treatment) is the cornerstone of follow-up — a normalised ALP after zoledronic acid predicts a durable remission, while a rising ALP after remission prompts re-imaging, not automatic re-treatment, to exclude sarcoma. Second, any new, focal, escalating pain at a pagetoid site, a palpable mass, or unexplained weight loss is treated as possible osteosarcoma until MRI and biopsy prove otherwise — patients are taught to report such changes urgently rather than waiting for their next routine appointment. Third, bone health measures apply as in any high-turnover bone disease: adequate calcium and vitamin D, weight-bearing exercise within the limits of deformity, smoking cessation, and falls prevention (home hazard reduction, vision and hearing correction, gait aids). Patients with skull disease should have baseline and periodic audiometry, and those with extensive polyostotic disease a baseline echocardiogram to detect early high-output failure. First-degree relatives of patients with SQSTM1-familial disease are offered ALP screening from middle age.^[1][4][6]

Special Populations

• Familial Paget disease (SQSTM1) — genetic factors are important in pathogenesis, and recurrent SQSTM1/p62 mutations (such as P392L) cause PDB in a high proportion of affected families; patients with SQSTM1 mutations tend to have severe disease with high penetrance as age increases, so keep a lower threshold for treating affected relatives.^[4][7]
• Extensive / polyostotic disease — the disease may affect one or several bones; extensive metabolically active disease carries a higher complication burden and is the setting in which deafness, fracture and nerve compression develop.^[2][7]
• Contemporary patients — the disease is becoming less prevalent, patients present at older ages and fewer bones are involved than historically; treatment with potent intravenous bisphosphonates is so effective that many need only once-in-a-lifetime therapy.^[5]

Evidence, Guidelines & Regional Differences

• Screening strategy — serum total ALP measured together with liver function tests is the recommended first-line biochemical screening test for metabolically active Paget disease.^[1]
• Bone scan vs X-ray — the isotope bone scan is the most sensitive test for extent and number of sites, whereas X-rays characterise the lesion (lytic vs sclerotic, cortical destruction, deformity); both are needed at diagnosis.^[1]
• Zoledronic acid versus risedronate (Reid 2005) — a single 5 mg zoledronic acid infusion produced more rapid, more complete and more sustained responses than 60 days of oral risedronate, which is why guidelines favour it when a bisphosphonate is indicated.^[3][1]
• Sarcomatous transformation — the Deyrup clinicopathological series confirmed osteosarcoma as the dominant histology, with a poor prognosis; any change in a pagetoid lesion mandates biopsy.^[6]
• Declining epidemiology — prevalence, severity and skeletal extent are all falling, implying an important environmental factor that remains unidentified; mouse-model data keep paramyxoviral infection in play as a possible trigger alongside genetic susceptibility.^[5][2]

^[1]

Exam Pearls

^{[7] [1] [4] [3] [3] [1]}

Ward-round test

Three stems. Cover the answer, say it aloud, then reveal.^[1]

1. A 70-year-old man has an incidental ALP of 520 with normal calcium, phosphate and gamma-GT. What is the diagnosis, and which two tests confirm it and map its extent?^[1]
2. Three months after a single zoledronic acid infusion he reports new, severe, night-waking thigh pain. What must you assume, and what do you do?^[6]
3. A 60-year-old woman has a raised ALP, a raised calcium and a low phosphate. Is this Paget disease?^[1]

Ward-round answersShowHide

1. Paget disease of bone. The signature is a bone-source ALP (normal gamma-GT) with normal calcium and phosphate. Confirm with plain X-rays of pelvis, skull and spine (cortical thickening, mixed lytic-sclerotic change, cotton-wool skull, picture-frame vertebrae) and a Tc-99m bone scan to map every site.^[1]
2. Osteosarcoma transformation until proven otherwise. New or escalating pain in a known pagetoid site is the red flag — get an urgent MRI and biopsy; do not call it a flare.^[6]
3. No. A raised calcium with a low phosphate points to primary hyperparathyroidism, the mirror image of Paget (whose calcium and phosphate are both normal). Check the PTH and look for subperiosteal resorption.^[1]

Quick self-test: a 70-year-old man with ALP 520 U/L and normal gamma-GT — what does this pattern suggest, and what two tests next?ShowHide

A markedly raised ALP with normal liver function tests points to a bone source, and in an older patient that means metabolically active Paget disease of bone until proven otherwise. The two next tests are a radionuclide bone scan — the recommended way to define the full extent of metabolically active disease — plus targeted radiographs of the hot spots to characterise each lesion.^[1]

References7ShowHide

1. [1]Ralston SH, Corral-Gudino L, Cooper C, et al. Diagnosis and Management of Paget's Disease of Bone in Adults: A Clinical Guideline J Bone Miner Res, 2019.PMID 30803025
2. [2]Gennari L, Rendina D, Falchetti A, et al. Paget's Disease of Bone Calcif Tissue Int, 2019.PMID 30671590
3. [3]Reid IR, Miller P, Lyles K, et al. Comparison of a single infusion of zoledronic acid with risedronate for Paget's disease N Engl J Med, 2005.PMID 16135834
4. [4]Laurin N, Brown JP, Morissette J, Raymond V. Recurrent mutation of the gene encoding sequestosome 1 (SQSTM1/p62) in Paget disease of bone Am J Hum Genet, 2002.PMID 11992264
5. [5]Cundy T. Paget's disease of bone Metabolism, 2018.PMID 28780255
6. [6]Deyrup AT, Montag AG, Inwards CY, et al. Sarcomas arising in Paget disease of bone: a clinicopathologic analysis of 70 cases Arch Pathol Lab Med, 2007.PMID 17550323
7. [7]Ralston SH, Langston AL, Reid IR. Pathogenesis and management of Paget's disease of bone Lancet, 2008.PMID 18620951