Focal disorder of accelerated, disorganized bone remodeling producing enlarged, weakened bone.
Also known as: Paget disease of bone, osteitis deformans, Paget disease
Overview
A focal disorder of bone metabolism characterized by accelerated and disorganized osteoclastic resorption followed by exuberant osteoblastic formation, producing structurally abnormal, enlarged, and mechanically weakened bone. May involve a single bone (monostotic) or multiple bones (polyostotic) but does not spread to new sites once established.
Epidemiology
Most common in adults of Northern European descent older than 55; prevalence rises with age. Male-to-female ratio approximately 3:2. Incidence has declined globally over recent decades.
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Question 1MusculoskeletalEasy
A 72-year-old woman is evaluated for several months of right hip pain. Routine labs show an isolated alkaline phosphatase of 520 U/L with normal serum calcium, phosphorus, and renal function. Pelvic radiographs demonstrate coarse trabeculation and cortical thickening of the right ilium. She has no contraindications to oral or intravenous therapy. Which of the following is the most appropriate first-line pharmacologic treatment?
ATeriparatide
BSalmon calcitonin
CZoledronic acid
DDenosumab
Reveal answer & full explanation
Correct answer: C — Zoledronic acid
ATeriparatide
BSalmon calcitonin
CZoledronic acid✓
DDenosumab
Why zoledronic acid is correct
An isolated elevation of alkaline phosphatase with normal calcium and phosphorus, plus radiographic coarse trabeculation and cortical thickening, indicates Paget disease of bone
Aminobisphosphonates are first-line for symptomatic Paget disease and for active disease at high-risk sites; a single intravenous infusion of zoledronic acid 5 mg produces durable biochemical remission in most patients
It suppresses the overactive osteoclasts driving disorganized remodeling, normalizes alkaline phosphatase, relieves bone pain, and lowers the risk of fracture and deformity
Calcium and vitamin D should be repleted before infusion to prevent hypocalcemia
Why the others are wrong
Teriparatide — a recombinant PTH analog that stimulates osteoblastic activity to treat osteoporosis; it would worsen the already excessive bone turnover of Paget disease (right-diagnosis-wrong-drug: it treats low bone mass, not high turnover)
Salmon calcitonin — modestly reduces pain and turnover but is far weaker than bisphosphonates and is now reserved for patients who cannot take a bisphosphonate (premature closure on an outdated agent)
Denosumab — a RANKL inhibitor used off-label in selected cases; not first-line and carries rebound bone turnover after discontinuation (buzzword-matching an antiresorptive without checking the guideline first-line)
Question 2MusculoskeletalEasy
A 72-year-old man is referred for evaluation of an isolated alkaline phosphatase of 412 U/L found on routine labs. He reports mild dull right hip pain for the past year and notes his hats have become tighter. Calcium and phosphorus are normal. Pelvic radiograph shows cortical thickening, coarsened trabeculae, and a mixed lytic-sclerotic lesion of the right hemipelvis. He is started on zoledronic acid. Which of the following is the most likely long-term complication of his underlying bone disease?
AHigh-output heart failure
BHypercalcemic crisis
CPathologic fracture
DSecondary osteoporosis
Reveal answer & full explanation
Correct answer: C — Pathologic fracture
AHigh-output heart failure
BHypercalcemic crisis
CPathologic fracture✓
DSecondary osteoporosis
Why Pathologic fracture is correct
Paget disease of bone is a disorder of accelerated, disorganized bone remodeling characterized by increased osteoclastic resorption followed by exuberant but structurally abnormal osteoblastic bone formation
The resulting woven bone is thickened but mechanically weak
Classic biochemical signature is isolated elevation of alkaline phosphatase with normal calcium and phosphorus
Radiographs show cortical thickening, coarse trabeculae, and mixed lytic-sclerotic lesions
Pathologic fracture, particularly of weight-bearing long bones such as the femur, is the most common skeletal complication and the leading cause of morbidity
Bowing deformities and incomplete cortical fissure fractures are also characteristic
The abnormal pagetic bone is structurally weak and fractures with minor trauma or even physiologic loading
Why the others are wrong
A) High-output heart failure — can occur due to hypervascular pagetic bone acting as an arteriovenous shunt, but is uncommon and requires extensive polyostotic involvement
B) Hypercalcemic crisis — calcium is typically normal in Paget disease because resorption and formation are coupled; hypercalcemia only occurs with immobilization
D) Secondary osteoporosis — Paget disease produces dense, disorganized bone rather than osteoporotic bone, so generalized low bone mass is not a feature
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Northern European (especially British, French, Australian, New Zealand) ancestry
Family history (15-40 percent have an affected first-degree relative)
SQSTM1/p62 gene mutations in familial disease
Possible environmental trigger — historically associated with paramyxovirus exposure (controversial)
Pathophysiology
Disordered osteoclast activity (giant, multinucleated osteoclasts with numerous nuclei and viral-like inclusions) produces excessive resorption (lytic phase), followed by rapid but disorganized osteoblastic activity (mixed and sclerotic phases). The resulting woven bone is denser but mechanically weaker and more vascular than normal lamellar bone.
Clinical presentation
Symptoms
Asymptomatic in 70-90 percent — discovered incidentally on imaging or alkaline phosphatase elevation
Bone pain in the affected region (deep, dull, often worse at night)
Bony enlargement or deformity (frontal bossing, hat size increase, bowed tibia)
Hearing loss from skull involvement (cochlear and ossicular changes)
Headache, vertigo, cranial nerve palsies from skull base involvement
Pathologic fracture
Symptoms of high-output heart failure from vascular shunting (severe polyostotic disease)
Signs / physical exam
Bony deformity — enlarged skull, kyphosis, anterolateral bowing of the tibia
Warmth over involved bone (hypervascularity)
Decreased range of motion in affected joints
Conductive or mixed hearing loss
Cranial nerve deficits in advanced skull base disease
Classic findings
Older man with isolated elevated alkaline phosphatase, frontal bossing, and bowed tibia, with characteristic mixed lytic and sclerotic appearance on radiographs.
Differential diagnosis
Metastatic bone disease — Prostate, breast, lung, kidney, thyroid primary; multiple lesions without bone enlargement; PSA, mammography, CT
Fibrous dysplasia — Younger patients, ground-glass appearance, McCune-Albright syndrome features
Osteosarcoma — Aggressive lytic and sclerotic lesion with soft tissue mass; consider in older Paget patients with new pain (Paget sarcoma)
Hyperparathyroidism — Generalized osteopenia, brown tumors, elevated PTH and calcium
Osteomyelitis — Localized infection with constitutional symptoms and elevated inflammatory markers
Diagnostic workup
Diagnostic criteria
Diagnosis is based on characteristic radiographic findings in the appropriate clinical context. Elevated total or bone-specific alkaline phosphatase in the absence of liver disease supports the diagnosis. Bone biopsy is rarely needed.
Bone turnover markers: serum P1NP and urinary N-telopeptide are alternative markers
Serum calcium and phosphate — usually normal (calcium may rise during immobilization)
25-hydroxyvitamin D and PTH
If alkaline phosphatase elevated, isolate to bone with gamma-glutamyl transferase (normal) or bone-specific alkaline phosphatase
Imaging
Plain radiographs of affected bone — classic findings include cortical thickening, coarsened trabeculae, lytic lesions (osteoporosis circumscripta in skull), V-shaped advancing lytic front in long bones (blade-of-grass appearance), bony enlargement and deformity
Bone scintigraphy (technetium-99m bisphosphonate) to identify and map all affected sites — the most sensitive imaging study
CT and MRI for complications, surgical planning, or suspected malignant transformation
Audiometry if skull involvement
Diagnostic algorithm
Stage
Bone Activity
Imaging Findings
Lytic (incipient-active)
Osteoclast-predominant
Sharply defined lucent lesions (osteoporosis circumscripta in skull; blade-of-grass front in long bone)
Marked sclerosis, persistent enlargement and deformity
Radiographic stages of Paget disease of bone.
Treatment
First-line
Nitrogen-containing bisphosphonates — zoledronic acid 5 mg IV as a single infusion is preferred (HORIZON-Paget trial)
Oral alternatives: alendronate, risedronate (less potent and require longer courses)
Treatment indications: bone pain attributable to Paget, planned surgery on Paget bone, hypercalcemia in immobilized patients, neurologic compromise, involvement of weight-bearing bones, active disease near major joints, asymptomatic disease with markedly elevated bone turnover
Adjunctive analgesia (NSAIDs, acetaminophen)
Calcium and vitamin D supplementation before and during bisphosphonate therapy to prevent hypocalcemia
Second-line / adjunct
Calcitonin (subcutaneous or nasal) for patients unable to tolerate bisphosphonates
Orthopedic surgery for fractures, severe deformity, joint replacement, or decompression of neurologic structures
Hearing rehabilitation for skull base involvement
Long-term follow-up with alkaline phosphatase to assess response — normalization or near-normalization within 6 months indicates effective therapy
Complications
Pathologic fracture (most often femur and tibia)
Osteoarthritis adjacent to Paget bone (hip, knee, spine)
Hearing loss and cranial neuropathies from skull involvement
Spinal stenosis from vertebral enlargement
High-output heart failure (rare, in extensive polyostotic disease)
Hypercalcemia during immobilization
Osteosarcoma (Paget sarcoma) — rare (<1 percent) but devastating; should be suspected with new pain or rapidly growing mass in Paget bone
PANCE pearls
An isolated elevation of alkaline phosphatase in an older patient with normal liver enzymes should prompt evaluation for Paget disease.
Bone scintigraphy is the most sensitive imaging modality and is essential to map disease extent and identify silent sites.
Zoledronic acid 5 mg IV produces durable suppression of disease activity — often years of remission from a single infusion.
New or progressive bone pain in a patient with known Paget disease must raise concern for sarcomatous transformation.
References
Endocrine Society 2014 — Singer FR et al., Paget's Disease of Bone: An Endocrine Society Clinical Practice Guideline (J Clin Endocrinol Metab 2014)
HORIZON-PFT Paget — Reid IR et al., Comparison of a single infusion of zoledronic acid with risedronate for Paget's disease (NEJM 2005)
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