Objective: The purpose of this study is to describe MRI findings of muscle involvement in patients with polyarteritis nodosa (PAN), with an emphasis on the contrast enhancement characteristics.
Materials and methods: MRI studies of eight patients with PAN were retrospectively reviewed (four male and four female patients; mean age at presentation, 34.3 years; age range, 7-64 years). MR images were evaluated focusing on the following features: affected structures (muscle, fascia, or periosteum), lesion signal intensity and characteristics on T1-and T2-weighted images, and contrast enhancement patterns.
Results: Seven patients had lower leg involvement, and one had thigh involvement. In the seven patients with lower leg involvement, the main lesions were within the muscles, whereas in one patient with thigh involvement, the investing fascia was mainly involved. In the seven patients with predominantly muscle involvement, T2-weighted images showed either diffuse (n = 3) or patchy (n = 4) hyperintensity of the affected muscles. On contrast-enhanced images, small fluffy enhancing lesions centered on vessels (i.e., the "cotton-wool appearance") were noted within the affected muscles in six patients. In the one patient with mainly fascial lesions, the investing fascia showed diffuse enhancement. Periosteal enhancement was found along the tibial cortex in 50% (4/8) of our patients.
Conclusion: PAN should be considered a differential diagnosis in cases where MR images show patchy or diffuse muscle signal changes. Muscle involvement in PAN may show fluffy enhancing lesions centered on vessels on contrast-enhanced images and may accompany fascial or periosteal enhancement.
Keywords: MRI; fascia; muscles; polyarteritis nodosa; vasculitis.