Paget’s Disease

Introduction

Paget’s disease, or osteitis deformans, is a chronic metabolic disorder characterized by increased bone turnover and breakdown secondary to excessive and disorganized osteoclastic and osteoblastic activity ( Fig. 48.1 ). The precise etiology is still unclear. The excessive breakdown of bone is followed by the formation of weak and abnormal bone. Over time this robust and disordered bone metabolism can result in enlarged and/or deformed bones, leading to a variety of complications. An understanding of the evolution of this disease process is essential to accurately identify a variety of imaging appearances. Sources of confusion arise due to the distribution and wide spectrum of manifestations related to the pathologic phase of disease. Involvement may be widespread, monostotic, or polyostotic ( Figs. 48.2 and 48.3 ). Paget’s disease of bone is the second most common bone disease affecting the elderly population of the United States. It is estimated to affect 1% to 3% of the population over 50 years of age, although this has recently been declining.

Temporal Evolution: Overview

Paget’s disease of bone can demonstrate a variety of patterns of appearance related to the pathologic state of disease progression. Progression is generally slow and manifests with subtle changes over time. The cardinal and classic imaging features of the disease have been referred to as a “caricature of bone” and include advancing osteolysis, coarsened/thickened bone trabeculations, cortical thickening, and osseous expansion ( Figs. 48.4 and 48.5 ).

In order to understand the evolution of Paget’s disease, one must appreciate the rate of progression; recognize the presence of three distinct phases, which in fact represent a continuum; and be aware of the marked variability of disease.

1.

Progression: Paget’s disease is characterized by a gradual progression that evolves through various phases of activity. There is considerable variability in disease severity.
2.

Three phases: Three distinct phases of disease progression exist. Although the three phases have been described as discrete, they represent a continuum ( Fig. 48.6 ).
- a.
  
  Lytic phase: characterized by osteoclastic resorption seen as osteolysis on imaging. So-called osteoporosis circumscripta in the skull (described in further detail later).
- b.
  
  Mixed phase: characterized by osteoclastic and osteoblastic hyperplasia, although ostoblastic activity predominates. All the classic features may be seen in this phase, including advancing osteolysis, coarsened/thickened bone trabeculations, cortical thickening, and osseous expansion (described in further detail later).
- c.
  
  Blastic phase: characterized by a gradual decline in osteoblastic activity seen as osteosclerosis (described in further detail later).
Figure 48.6

Three distinct phases of Paget’s disease progression. As evident in the activity-versus-time portion of the diagram, osteoclastic activity diminishes over time and osteoblastic activity increases as the disease progresses from the lytic phase (1) to the mixed phase (2) , with osteoblastic activity predominating in the osteoblastic phase (3) . Although the three phases have been described discretely, they represent a continuum.
3.

Marked variability:
- a.
  
  There is prominent variability in the rate of disease progression in comparing different patients and comparing different sites of involvement within the same patient ( Figs. 48.7 and 48.8 ).
  
  Figure 48.7
  
  Variability of disease at different sites in the same patient. As individual sites may progress at variable rates, different phases may be seen in the same patient. Mixed-phase findings are evident in lateral (A) and anteroposterior (B) images of the skull with cortical thickening and multiple areas of mixed lysis and sclerosis. In particular, the multiple areas of focal sclerosis evident in these images give rise to the typical “cotton-wool” appearance of Paget’s disease involving the skull. Mixed-phase findings in the lateral radiograph of the spine (C) in the same patient demonstrate cortical sclerosis involving all four vertebral body margins of multiple lower thoracic vertebral bodies (arrows) , leading to an encased “picture-frame” appearance. Anteroposterior pelvic (D) and knee (E) radiographs in the same patient demonstrate blastic/sclerotic and expansile changes of both iliac wings as well as the right patella (arrow) . The presence of different phases in the same patient underscores the prominent variability in the rate of disease progression, which is independent and unpredictable from site to site.
  
  Figure 48.8
  
  Variability of disease within the same bone. Even within the same bone, a variable rate of progression may be evident. Despite the fact that the three phases of bone involvement represent a continuum, variability of progression may diverge even within a single site of involvement. Different rates of disease progression are seen in this axial computed tomography image in a patient with Pagetic skull base involvement. Lytic (red arrows) , mixed phase (green arrow) , and blastic (blue arrow) phases of involvement are seen side by side.
- b.
  
  In addition to evolution over time, two differing patterns of bone involvement may be seen: subperiosteal cortical thickening and accentuation/coarsening of trabeculae with bone enlargement or obliteration of trabeculae with a hazy, “washed out” pattern.

Temporal Evolution: Overview

Paget’s disease of bone can demonstrate a variety of patterns of appearance related to the pathologic state of disease progression. Progression is generally slow and manifests with subtle changes over time. The cardinal and classic imaging features of the disease have been referred to as a “caricature of bone” and include advancing osteolysis, coarsened/thickened bone trabeculations, cortical thickening, and osseous expansion ( Figs. 48.4 and 48.5 ).