Paget’s Disease




Keywords

Paget’s disease, Osteitis deformans

 




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.




Figure 48.1


Normal bone remodeling. Normal bone remodeling is conducted by osteoclasts and osteoblasts. Their activity differs in cortical bone (where they tunnel through tissue) and cancellous bone (where they move across the trabecular surface). Normal bone remodeling is initiated by the recruitment of osteoclast precursors, which eventually form multinucleated active osteoclasts that carry out bone resorption. Osteoblasts move in to start the process of new bone formation by secreting osteoid, which is eventually mineralized into new bone. Paget’s disease is a chronic disorder characterized by increased bone turnover and breakdown with formation of abnormal, weak bone.



Figure 48.2


Common sites of Paget’s disease. Paget’s disease predominantly involves the axial skeleton and proximal long bones. Normal bone scan image highlighting (red circles) the most common sites of Pagetic involvement. These include the skull, the humerus, ribs, spine, iliac bone, femur, and tibia. The pelvis is the most common site of involvement, followed by spinal involvement. Percentage-wise, the most frequent sites of involvement are the pelvis (70%), the spine (53%), the skull (25% to 65%), and proximal long bones (25% to 30%).



Figure 48.3


Monostotic and polyostotic Paget’s disease. Paget’s disease is monostotic in approximately 34% of the cases and monostotic 66% of the time. Bone scan images demonstrate markedly increased uptake in areas affected by Paget’s. Bone scan images in two different patients demonstrate monostotic (A) and polyostotic (B) Pagetic involvement (arrows) . Bone scintigraphy is a sensitive but not specific examination in Paget’s disease.




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 ).




Figure 48.4


Classic imaging characteristics of Paget’s disease. Anteroposterior radiograph (A) and axial computed tomography (B) images of the pelvis in the patient depicted in Fig. 48.3A demonstrate prominent left iliac wing cortical thickening (red arrows) , coarsened/thickened bone trabeculations (blue arrows) , and osseous expansion (compare the left iliac bone with the normal right iliac wing). In conjunction, these features are referred to as “a caricature of bone.”



Figure 48.5


Classic imaging characteristics of Paget’s disease. Lateral (A) and anteroposterior (B) radiographs of the tibia demonstrate massive osseous expansion and diffuse coarsened/thickened internal trabeculation of the entire tibia in a patient with a long-standing history of Paget’s disease. Note the marked anterior bowing of the softened bone. “Banana fractures” refer to small horizontal cortical insufficiency fractures through this type of bowed, softened bone, which is unable to withstand normal stresses. Anteroposterior radiograph (C) in a different patient with a known history of Paget’s disease demonstrates lytic long bone involvement, referred to as the characteristic “blade-of-grass” or “flame-shaped” appearance. The advancing wedge of osteolysis (arrows) is clearly evident as a sharply defined wedge-shaped lucency without sclerosis. It begins in the epiphyseal subchondral bone and extends into the metaphysis and diaphysis at a rate of about 1 cm per year.


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.

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Jun 26, 2019 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Paget’s Disease
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