Applied Radiology

CASE SUMMARY

A 68-year-old white man presented with worsening lower-back pain. An abnormality on the thoracolumbar spine radiographic study (Figure 1) prompted further imaging evaluation, which included a skull radiograph (Figure 2). Upon further questioning, the patient reported occasional throbbing left-sided headaches, which he said were relieved temporarily with over-the-counter analgesics. He denied any recent head trauma or known malignancy and had been in relatively good health recently. Physical examination of the head, neck, and back was unremarkable.

DIAGNOSIS

Paget's disease

IMAGING FINDINGS

The lateral skull radiograph shows a large radiolucent lesion with geographic margins that involves a large portion of the frontoparietal and temporal skull calvarium (Figure 2). This appearance is known as osteoporosis circumscripta and is consistent with Paget's disease. It is seen in the early or lytic phase, when osteoclastic resorption overwhelms bone production. The initial spine study demonstrates another characteristic finding of Paget's disease, the picture-frame vertebral body, which is secondary to severe osteoporosis centrally and a thickened, sclerotic cortex (Figure 1). As part of the work-up, skeletal scintigraphy was performed subsequently, showing diffuse, intense uptake at the frontal calvarium (Figure 3), corresponding to the lytic lesion on the skull radiograph. This scintigraphic finding is consistent with increasing osteoblastic activity, suggesting that the disease has progressed to a more advanced phase.

DISCUSSION

Paget's disease is a condition of uncertain pathogenesis that is characterized by disorderly organization of bone and osseous overgrowth. Predominantly affecting the Caucasian or Northern European population, Paget's is characterized by three phases of marrow cellular activity. ^1,2 The lytic or early phase is marked by high levels of osteoclastic activity that overwhelm the activity of osteoblasts, resulting in the appearance of increasing radiolucent osseous lesions. Disease during this phase is responsible for the flame appearance or the blade-of-grass sign. The geographic, lytic skull lesion depicted in this patient is a manifestation of the lytic phase of Paget's disease. The lytic phase is followed by the mixed blastic-lytic phase, in which increasing levels of osteoblastic activity (evidenced by increasing sclerosis) coexist with the ongoing osteoclastic activity, resulting in a mixed sclerotic and lytic appearance to the affected bone. It is during this particular phase that trabecular coarsening, cortical thickening, and generalized osseous en-largement occur. The third or late phase is char-acterized by gradual diminishment of both osteoclastic and osteoblastic activity, with residual smoldering osteoblastic activity. The affected long bones are weak and can bow during this phase. At any point in time, different involved bones may be in different cellular stages. Most patients are asymptomatic at detection. ³ Laboratory markers of disease activity are serum alkaline phosphatase and urinary hydroxy-proline, which are usually elevated when bone destruction is occurring. The major complications of Paget's disease include full-thickness long bone fractures, insufficiency fractures, and development of bone sarcomas. ⁴ The goal of treatment is to reverse negative bone balance, using agents that inhibit bone resorption and stimulate growth, such as calcitonin or the bisphosphonates.

CONCLUSION

Paget's disease of bone is a chronic osseous disease characterized by bony enlargement or deformity and bone fragility. It may involve one or more regions of the skeleton and commonly presents as pain. Various imaging appearances are possible and often suggest the phase of cellular activity at the time of imaging. Most cases are diagnosed by conventional radiography.

Prepared by Justin Q. Ly, MD at the Department of Radiology, Wilford Hall Medical Center, San Antonio, TX.