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PAGET’S DISEASE OF BONE
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Paget’s disease is a localized bone-remodeling disorder that affects widespread, noncontiguous areas of the skeleton. The pathologic process is initiated by overactive osteoclastic bone resorption followed by a compensatory increase in osteoblastic new bone formation, resulting in a structurally disorganized mosaic of woven and lamellar bone. Pagetic bone is expanded, less compact, and more vascular; thus, it is more susceptible to deformities and fractures. Although most patients are asymptomatic, symptoms resulting directly from bony involvement (bone pain, secondary arthritis, fractures) or secondarily from the expansion of bone causing compression of surrounding neural tissue are not uncommon.
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There is a marked geographic variation in the frequency of Paget’s disease, with high prevalence in Western Europe (Great Britain, France, and Germany, but not Switzerland or Scandinavia) and among those who have immigrated to Australia, New Zealand, South Africa, and North and South America. The disease is rare in native populations of the Americas, Africa, Asia, and the Middle East; when it does occur, the affected subjects usually have evidence of European ancestry, supporting the migration theory. For unclear reasons, the prevalence and severity of Paget’s disease are decreasing, and the age of diagnosis is increasing.
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The prevalence is greater in males and increases with age. Autopsy series reveal Paget’s disease in about 3% of those over age 40. Prevalence of positive skeletal radiographs in patients over age 55 is 2.5% for men and 1.6% for women. Elevated alkaline phosphatase (ALP) levels in asymptomatic patients have an age-adjusted incidence of 12.7 and 7 per 100,000 person-years in men and women, respectively.
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The etiology of Paget’s disease of bone remains unknown, but evidence supports both genetic and viral etiologies. A positive family history is found in 15–25% of patients and, when present, raises the prevalence of the disease seven- to tenfold among first-degree relatives.
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A clear genetic basis has been established for several rare familial bone disorders that clinically and radiographically resemble Paget’s disease but have more severe presentation and earlier onset. A homozygous deletion of the TNFRSF11B gene, which encodes osteoprotegrin (Fig. 36-1), causes juvenile Paget’s disease, also known as familial idiopathic hyperphosphatasia, a disorder characterized by uncontrolled osteoclastic differentiation and resorption. Familial patterns of disease in several large kindred are consistent with an autosomal dominant pattern of inheritance with variable penetrance. Familial expansile osteolysis, expansile skeletal hyperphosphatasia, and early-onset Paget’s disease are associated with mutations in TNFRSF11A gene, which encodes RANK (receptor activator of nuclear factor-κB), a member of the tumor necrosis factor superfamily critical for osteoclast differentiation (Fig. 36-1). Finally, mutations in the gene for valosin-containing protein cause a rare syndrome with autosomal dominant inheritance and variable penetrance known as inclusion body myopathy with Paget’s disease and frontotemporal dementia (IBMPFD). The role of genetic factors is less clear ...