Paget’s Disease and Other Dysplasias of Bone¶
Chapter 424 | Part 12: Endocrinology and Metabolism
KEY CLINICAL POINTS¶
- Paget’s disease is a localized bone-remodeling disorder characterized by excessive osteoclastic resorption and disorganized bone formation, leading to structural weakness and increased fracture risk.
- Glucocorticoid-induced osteoporosis (GCIOP) is a major complication of long-term corticosteroid use, with fractures occurring within 3 months of therapy due to rapid trabecular bone loss.
- Diagnosis of Paget’s disease relies on elevated alkaline phosphatase (ALP), radiographic findings (e.g., 'cotton wool' skull lesions, cortical thickening), and bone scans. Biochemical markers like ALP and P1NP are critical for monitoring treatment response.
- Bisphosphonates (e.g., zoledronic acid) are first-line therapy for Paget’s disease, with denosumab and calcitonin as alternatives. Surgical intervention is required for complications like fractures or nerve compression.
- Genetic mutations (e.g., SQSTM1, TNFRSF11A) and viral etiologies (e.g., measles) contribute to Paget’s disease, while other dysplasias like osteopetrosis and fibrous dysplasia involve distinct pathogenic mechanisms.
1. DEFINITION & OVERVIEW¶
Paget’s disease is a chronic bone remodeling disorder characterized by excessive osteoclastic resorption and disorganized osteoblastic formation, leading to structurally abnormal bone. It is distinct from other dysplasias like osteopetrosis and fibrous dysplasia, which involve different pathogenic mechanisms and clinical presentations.
Table 424-1: Pharmacologic Agents Approved for Treatment of Paget’s Disease¶
| NAME | DOSE AND MODE OF DELIVERY | NORMALIZATION OF ALKALINE PHOSPHATASE (ALP) |
|---|---|---|
| Zoledronic acid | 5 mg IV over 15 min | 90% of patients at 6 mo |
| Pamidronate | 30 mg/d IV over 4 h on 3 days | ~50% of patients |
| Risedronate | 30 mg/d PO for 2 mo | 73% of patients |
| Alendronate | 40 mg/d PO for 6 mo | 63% of patients |
| Tiludronate | 800 mg/d PO for 3 mo | 35% of patients |
| Etidronate | 200–400 mg/d PO × 6 mo | 15% of patients |
| Calcitonin (Miacalcin) | 100 U SC daily for 6–18 mo (may reduce to 50 U 3× per week) | (Reduction of ALP by up to 50%) |
Table 424-2: Diseases and Conditions Associated with Ectopic Calcification and Ossification¶
| Metastatic calcification | Dystrophic calcification | Ectopic ossification |
|---|---|---|
| Hypercalcemic states | Inflammatory disorders | Myositis ossificans |
| Primary hyperparathyroidism | Scleroderma | Postsurgery |
| Sarcoidosis | Dermatomyositis | Burns |
| Vitamin D intoxication | Systemic lupus erythematosus | Neurologic injury |
| Milk-alkali syndrome | Trauma-induced | Other trauma |
| Renal failure | Cell lysis following chemotherapy | Fibrodysplasia ossificans progressiva |
| Hyperphosphatemia | Therapy with vitamin D and phosphate |
1.1 Paget’s Disease¶
A localized bone-remodeling disorder with increased osteoclastic activity and disorganized bone formation. Affected bones are expanded, less dense, and more vascular, leading to deformities and fractures.
1.2 Other Dysplasias¶
Includes osteopetrosis (impaired osteoclast function), fibrous dysplasia (mesenchymal proliferation), and genetic disorders like hypophosphatasia. These conditions involve distinct pathophysiology and require tailored management.
2. EPIDEMIOLOGY¶
Paget’s disease shows marked geographic variation, with high prevalence in Western Europe (e.g., UK, France, Germany) and among immigrants to Australia, New Zealand, and South Africa. It is rare in native populations of the Americas, Africa, Asia, and the Middle East. The disease is more common in males and increases with age, with autopsy series showing ~3% prevalence in individuals over 40.
2.1 Risk Factors¶
Age, male sex, family history (15–25% of patients), and geographic location. Immunosuppression and viral infections (e.g., measles) may contribute to pathogenesis.
2.2 Demographics¶
Incidence peaks in 50–70 years. Prevalence in men: 2.5% (>55 years), women: 1.6%. Elevated ALP levels in asymptomatic patients correlate with age-adjusted incidence of 12.7/100,000 in men and 7/100,000 in women.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Paget’s disease has both genetic and viral etiologies. Mutations in SQSTM1, TNFRSF11A, and other genes contribute to pathogenesis. Viral infections (e.g., measles) may trigger osteoclast activation. Osteopetrosis involves impaired osteoclast function due to genetic defects (e.g., CA II deficiency).
3.1 Genetic Factors¶
Mutations in SQSTM1 (sequestosome-1), TNFRSF11A (RANK), and other genes. Autosomal dominant inheritance in some forms (e.g., fibrous dysplasia).
3.2 Viral Hypothesis¶
Evidence for measles virus involvement includes cytoplasmic inclusions in osteoclasts and viral mRNA in precursor cells. However, no live virus has been isolated from pagetic bone.
3.3 Pathophysiology¶
Excessive osteoclastic resorption and disorganized bone formation. Osteoclasts are hyperresponsive to RANKL, leading to increased resorption and bone turnover. Osteoblasts produce abnormal bone matrix, resulting in structural weakness.
4. CLINICAL FEATURES¶
Symptoms include bone pain, fractures, deformities, and neurological complications (e.g., hearing loss, cranial nerve palsies). Radiographic findings show cortical thickening, trabecular coarsening, and 'cotton wool' skull lesions. Complications include cardiovascular issues, hypercalcemia, and sarcomatous degeneration.
4.1 Common Symptoms¶
Bone pain, fractures, deformities (e.g., bowing of femur/tibia), and secondary arthritis. Neurological complications include hearing loss, cranial nerve palsies, and spinal cord compression.
4.2 Radiographic Findings¶
Enlarged bones with cortical thickening, trabecular coarsening, and 'cotton wool' lesions. 'Ivory vertebra' and 'picture frame' vertebrae are characteristic. Long bones show bowing and periosteal new bone formation.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include osteoporosis, osteosarcoma, and other bone tumors. Key distinguishing features include elevated ALP, radiographic patterns, and biochemical markers. Fibrous dysplasia and osteopetrosis require genetic testing for confirmation.
5.1 Bone Tumors¶
Osteosarcoma, giant cell tumors, and metastases must be excluded via biopsy. Pagetic bone has a low risk of malignancy (<0.5%).
5.2 Other Bone Disorders¶
Osteopetrosis (radiopaque bones), fibrous dysplasia (ground-glass appearance), and hypophosphatasia (rachitic deformities) require distinct diagnostic approaches.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis combines biochemical markers (ALP, P1NP, N-telopeptide), imaging (X-ray, bone scan, CT/MRI), and bone biopsy. ALP levels >2.5x ULN are diagnostic. Bone scans are more sensitive than X-rays for detecting active lesions.
6.1 Biochemical Tests¶
Elevated ALP (most sensitive), P1NP (bone formation marker), and decreased N-telopeptide (resorption marker). Serum calcium and phosphate are normal in Paget’s disease.
6.2 Imaging¶
X-rays show cortical thickening, trabecular coarsening, and lytic lesions. Bone scans detect active lesions. CT/MRI are used for surgical planning and assessing soft tissue involvement.
7. MANAGEMENT & TREATMENT¶
First-line therapy includes bisphosphonates (zoledronic acid, pamidronate). Denosumab and calcitonin are alternatives. Surgical intervention is required for fractures, nerve compression, or sarcomatous degeneration. Monitoring includes ALP levels and bone density scans.
7.1 Pharmacologic Therapy¶
Bisphosphonates suppress osteoclastic activity. Zoledronic acid (5 mg IV) is preferred for severe cases. Denosumab (60 mg subcut) is effective but not FDA-approved for GCIOP. Calcitonin is reserved for patients intolerant of bisphosphonates.
7.2 Surgical Management¶
Indicated for fractures, nerve compression, or sarcomatous degeneration. Intramedullary rods are used for fracture fixation. Bone marrow transplantation is considered for severe osteopetrosis.
8. PROGNOSIS & COMPLICATIONS¶
Most patients have a favorable prognosis with treatment. Complications include cardiovascular issues (e.g., calcific aortic stenosis), hypercalcemia, and sarcomatous transformation. Untreated Paget’s disease can lead to severe deformities and mortality in infants.
8.1 Complications¶
Cardiovascular (calcific aortic stenosis), neurological (cranial nerve palsies), and skeletal (fractures, deformities). Hypercalcemia and hypercalciuria may occur with immobilization.
8.2 Prognosis¶
Spontaneous remission is rare. Early treatment prevents deformities and complications. Mortality is highest in untreated infants with severe osteopetrosis.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Bisphosphonates are contraindicated in the third trimester. Pediatrics: Early treatment prevents skeletal deformities. Elderly: Monitor for renal impairment with zoledronic acid. Genetic counseling is recommended for familial cases.
9.1 Pregnancy¶
Avoid bisphosphonates in the third trimester. Monitor for fetal skeletal abnormalities. Denosumab is contraindicated during pregnancy.
9.2 Pediatrics¶
Early treatment prevents growth retardation and deformities. Genetic testing is critical for familial cases of osteopetrosis or fibrous dysplasia.
10. KEY POINTS & CLINICAL PEARLS¶
- Paget’s disease is diagnosed by elevated ALP, radiographic findings, and bone scans. 2. Bisphosphonates are first-line therapy, with zoledronic acid preferred for severe cases. 3. Genetic mutations (e.g., SQSTM1) and viral factors (e.g., measles) contribute to pathogenesis. 4. Monitor for hypercalcemia and renal function during treatment. 5. Early intervention prevents complications like fractures and deformities.