Paget Disease of Bone

• Epidemiology: uncommon before age 40, rising with age, with familial clustering and SQSTM1 mutations implicated.
• Phases: an early lytic phase (osteoclast-dominant), a mixed phase, and a late sclerotic/blastic phase. Histology shows the characteristic mosaic pattern of lamellar bone with irregular cement lines.
• Most patients are asymptomatic, discovered through an isolated raised alkaline phosphatase with normal calcium and phosphate — the classic biochemical signature.
• When symptomatic: bone pain, skull enlargement (hats no longer fit), conductive or sensorineural hearing loss from involvement of the temporal bone and ossicles, bowing of the tibia and femur, and pathological fractures.
• High-output cardiac failure results from the markedly increased vascularity of pagetic bone.
• Malignant transformation to osteosarcoma is rare but devastating, and should be suspected when pain changes character or a soft-tissue mass appears in an affected bone.
• Imaging: cortical thickening, coarse trabeculae, "cotton wool" skull; bone scan defines the extent.
• Treatment: bisphosphonates for symptomatic disease, with adequate calcium and vitamin D.

Genetic mechanisms (non-modifiable)

• ***SQSTM1*/p62 mutations: the best-established genetic cause, found in a substantial minority of familial cases and some sporadic ones. p62 is a scaffold protein in RANK–NF-κB signalling**; loss-of-function in its ubiquitin-associated domain leaves NF-κB constitutively active in osteoclast precursors, lowering the threshold for osteoclastogenesis.
• Family history: first-degree relatives of affected patients have substantially increased risk; inheritance in familial pedigrees is autosomal dominant with incomplete penetrance.
• Rare related disorders: familial expansile osteolysis and juvenile Paget disease (*TNFRSF11A*/RANK and *TNFRSF11B*/osteoprotegerin defects) prove the RANKL–RANK–OPG axis is the final common pathway.

Environmental/acquired mechanisms

• Paramyxovirus hypothesis: nuclear and cytoplasmic inclusion bodies resembling paramyxovirus nucleocapsids are seen in pagetic osteoclasts, and measles/canine distemper virus transcripts have been reported. Causation remains unproven — treat this as an association examiners like, not a settled fact.
• Environmental exposures proposed but not confirmed: rural residence, cattle/dog contact, low dietary calcium and vitamin D in youth. The falling incidence in Western countries supports some environmental contribution.

Demographic risk factors the stem plants

• Age: rare before 40, prevalence climbs steeply thereafter — the stem almost always names a patient in the sixth decade or older.
• Ancestry and geography: highest in people of Northern European descent (UK, and countries of British emigration); distinctly uncommon in Asia and Scandinavia.
• Sex: slight male predominance.

The initiating lesion is the osteoclast

• Pagetic osteoclasts are abnormally large, with dozens of nuclei, hypersensitive to RANKL and to 1,25-dihydroxyvitamin D, and they oversecrete IL-6, which recruits still more precursors — an autocrine amplification loop confined to focal skeletal sites.
• Constitutive NF-κB activation (as with *SQSTM1*/p62 mutation) lowers the osteoclastogenic threshold, explaining why the disease is focal and often monostotic or oligostotic rather than a generalised metabolic bone disease, and why it does not spread to new bones over time.

The three-phase sequence

• Lytic phase: resorption dramatically outpaces formation. Radiographically this is the advancing lytic front — osteoporosis circumscripta in the skull, the blade-of-grass or flame-shaped lucency in a long bone.
• Mixed phase: osteoblasts, coupled to resorption, are recruited in massive numbers and lay down bone rapidly and disorganised. Fibrovascular marrow replaces fatty marrow, and the lesion becomes intensely hypervascular.
• Sclerotic phase: net bone accumulation. Hurried remodelling produces woven bone converted piecemeal to lamellar bone with irregular, jigsaw-like cement lines — the mosaic pattern on histology.

Why the presentation looks the way it does

• Bone is bigger and denser but mechanically inferior, because collagen lamellae are not aligned along stress lines → bowing of tibia and femur, fissure (chalk-stick) fractures on the convex cortex, and secondary osteoarthritis when a deformed bone alters joint mechanics.
• Skull and vertebral expansion encroaches on foramina and canals → hearing loss (cochlear capsule and ossicular involvement), other cranial neuropathies, spinal stenosis, and basilar invagination.
• Hypervascularity creates a low-resistance arteriovenous shunt; with extensive skeletal involvement this raises cardiac output and can precipitate high-output heart failure, and it makes surgery on pagetic bone bloody.
• Coupled remodelling keeps serum calcium and phosphate normal; only the turnover markers rise. Hypercalcaemia appears when the coupling is broken by immobilisation or a fracture.

Most common presentations

• Asymptomatic (the majority): discovered as an isolated elevated alkaline phosphatase on routine chemistry, or as sclerosis/cortical thickening on a film obtained for another reason.
• Bone pain: deep, boring, worse at rest and at night — distinguishing it from mechanical/arthritic pain, which worsens with use. Mechanism is periosteal stretching by expanding, hypervascular bone; overlying skin may feel warm from increased blood flow.
• Skull involvement: increasing hat size, frontal bossing, headache. Leontiasis ossea with facial bone involvement.
• Hearing loss: mixed conductive (ossicular chain) and sensorineural (cochlear capsule, CN VIII) — the temporal bone is a classic site.

Skeletal deformity and mechanical consequences

• Anterior/lateral bowing of the tibia (sabre shin) and lateral bowing of the femur, from weight-bearing on mechanically inferior bone.
• Kyphosis and loss of height with vertebral involvement; secondary osteoarthritis of hip and knee from altered joint loading.
• Pathological fracture, sometimes the presenting event — transverse chalk-stick fractures of a long bone.
• Most commonly involved sites: pelvis, spine, femur, skull, tibia. Involvement of the hands and feet is unusual.

Neurological and cardiac features

• Cranial neuropathies and spinal stenosis from foraminal/canal narrowing; basilar invagination may cause brainstem compression or hydrocephalus.
• High-output heart failure: warm extremities, wide pulse pressure, bounding pulses, elevated JVP — seen with extensive polyostotic disease, particularly if there is underlying cardiac disease.

Step 1 — biochemistry

• Total serum alkaline phosphatase is the initial and best screening test: elevated, often several-fold above normal in active polyostotic disease, and it correlates with disease activity and extent.
• Calcium, phosphate and PTH are normal — this triad with a high ALP is the exam signature. Hypercalcaemia should prompt a search for immobilisation, a coincident fracture, or concurrent primary hyperparathyroidism.
• Confirm the ALP is skeletal, not hepatic: a normal GGT (or a normal 5′-nucleotidase) points to bone; bone-specific ALP can be measured directly if liver disease coexists.
• If ALP is normal but disease is suspected (monostotic or burnt-out disease), a resorption marker such as serum CTX or urinary NTX, or the formation marker P1NP, may be elevated. The Endocrine Society favours total ALP for routine diagnosis and monitoring.
• Check 25-hydroxyvitamin D and renal function before treating.

Step 2 — plain radiographs of the symptomatic site (the confirmatory study in most cases)

• Cortical thickening, coarsened/disorganised trabeculae, and bone expansion — enlargement of the bone is the finding that distinguishes Paget from blastic metastases.
• Osteoporosis circumscripta: sharply demarcated skull lucency of the lytic phase.
• Cotton-wool skull: patchy sclerosis of the later phase, with diploic widening.
• ***Blade-of-grass*/flame-shaped lytic front** advancing along a long bone.
• Picture-frame vertebra (thickened cortical rim) and the ivory vertebra.
• Pseudofractures/fissure fractures on the convex cortex of a bowed long bone.

Who to treat (Endocrine Society 2014)

• Symptomatic disease — bone pain attributable to pagetic activity, headache, neurological compromise — is the core indication.
• Asymptomatic active disease at sites where complications would be costly: skull, vertebrae, weight-bearing long bones, bone adjacent to a major joint. Suppressing turnover here is intended to prevent deformity and neurological injury.
• Before elective surgery on pagetic bone, to reduce the intense intraoperative bleeding from hypervascular bone.
• Prophylactic treatment of asymptomatic disease purely to normalise ALP is debated; the guideline permits it when a biochemical marker is used to follow the patient.

First-line therapy

• Nitrogen-containing bisphosphonates (inhibit farnesyl pyrophosphate synthase → osteoclast apoptosis). Zoledronic acid 5 mg IV as a single infusion is the agent of choice per the Endocrine Society, producing more durable remissions than oral agents.
• Oral alternatives: risedronate or alendronate courses, if IV therapy is unavailable or declined.

Second-line / escalation

• Salmon calcitonin (subcutaneous/intramuscular) for patients in whom bisphosphonates are contraindicated or not tolerated — less potent, with tachyphylaxis.
• Analgesia with NSAIDs or acetaminophen for pain that persists after turnover is suppressed (often osteoarthritic, not pagetic).

Skeletal

• Pathological fracture: transverse chalk-stick fracture through mechanically inferior woven bone; incomplete fissure fractures on the convex cortex of a bowed femur or tibia are precursors. Signalled by abrupt focal pain and inability to bear weight.
• Secondary osteoarthritis: deformity alters joint loading; pain worse with activity, unlike pagetic pain.
• Progressive bowing and limb-length discrepancy with gait disturbance.

Neurological (several are emergencies)

• Hearing loss from temporal bone and ossicular involvement — the most common neurological complication and largely irreversible.
• Basilar invagination: the softened skull base settles around the odontoid → brainstem compression, hydrocephalus, lower cranial neuropathies. Long-tract signs or gait ataxia in a patient with skull Paget is a neurosurgical emergency.
• Spinal stenosis / cord compression: from vertebral expansion, and additionally from vascular steal, where hypervascular bone diverts blood from the cord. New bowel/bladder dysfunction or a sensory level demands emergent MRI.

Cardiovascular and metabolic

• High-output heart failure from arteriovenous shunting in extensive polyostotic disease — bounding pulses, wide pulse pressure, warm extremities.
• Hypercalcaemia when the patient is immobilised or fractured, breaking the resorption–formation coupling; also consider coexisting primary hyperparathyroidism. Hypercalciuria and nephrolithiasis may follow.

Malignant

• Osteosarcoma — rare (well under 1% of patients) but the feared complication. Presents as new unrelenting pain, a soft-tissue mass, or rapid ALP rise in a known pagetic bone (pelvis, femur, humerus). Prognosis is far worse than in adolescent osteosarcoma. Giant cell tumour may also arise in pagetic bone.

Treatment-related

• Acute-phase reaction after IV zoledronic acid: transient fever, myalgia, flu-like illness within days.
• Hypocalcaemia, especially with unrecognised vitamin D deficiency.
• Osteonecrosis of the jaw and atypical subtrochanteric femoral fracture with prolonged bisphosphonate exposure; renal deterioration with IV infusion; pill oesophagitis with oral agents.

• The biochemical fingerprint: markedly elevated alkaline phosphatase with normal calcium, phosphate and PTH in an older adult. If calcium is high, look for immobilisation, fracture, or coexisting primary hyperparathyroidism — not Paget alone.
• Confirm the ALP is skeletal: a normal GGT points to bone rather than liver. This is a favourite one-line discriminator.
• Buzzwords by phase: osteoporosis circumscripta (lytic skull), cotton-wool skull (sclerotic), blade-of-grass/flame-shaped lytic front in a long bone, picture-frame vertebra, and the histologic mosaic pattern of lamellar bone with irregular cement lines.
• Best next step after plain films: a radionuclide bone scan to map the extent of skeletal involvement — it changes whether the disease qualifies as "at-risk" for treatment. Biopsy is not the next step unless sarcoma is suspected.
• The single association examiners test: increasing hat size plus hearing loss. Temporal bone involvement gives mixed conductive and sensorineural loss.
• First-line drug: IV zoledronic acid (a nitrogen-containing bisphosphonate) per the Endocrine Society, after correcting vitamin D and calcium — otherwise the answer choice "symptomatic hypocalcaemia" becomes the complication.
• New unrelenting pain or a soft-tissue mass in a pagetic bone = osteosarcoma until excluded; it is rare but the highest-yield malignancy association in this disease.
• Distractors to avoid: osteomalacia also raises ALP but shows low or low-normal calcium and phosphate with pseudofractures; blastic prostate metastases raise ALP but the bone is not enlarged and PSA is high — bone expansion with cortical thickening is what makes it Paget. And remember osteoporosis has a normal ALP unless there has been a recent fracture.