MR imaging findings of musculoskeletal involvement in microscopic polyangiitis: a comparison with inflammatory myopathy.
OBJECTIVE
The purpose of this study was to evaluate the importance of MR imaging findings of musculoskeletal involvement of the lower limbs in diagnosing microscopic polyangiitis (MPA) vs polymyositis (PM) or dermatomyositis (DM).


MATERIALS AND METHODS
This study included 13 patients diagnosed with MPA clinically and through histologically, and 38 diagnosed with PM/DM, who underwent MR imaging of the lower limbs prior to treatment. Axial and coronal short tau inversion recovery (STIR) images were reviewed retrospectively.


RESULTS
The sites affected by MPA were the lower legs in six (46%) patients and the thighs in seven (54%). Intramuscular hyperintensity and fascial hyperintensity were observed in all cases of MPA (100%). Fascial hyperintensity was more frequently encountered in MPA than in PM/DM (100% vs. 45%, p < 0.01). As the predominantly involved sites, the fascial regions were more frequently affected by MPA than by PM/DM (77% vs. 18%, p < 0.01). Diffuse subcutaneous fat hyperintensity was more frequently observed in MPA than in PM/DM (100% vs. 16%, p < 0.01). However, no significant differences in intramuscular hyperintensity (100% vs. 97%, p = 0.745) and subcutaneous fat hyperintensity (54% vs. 50%, p = 0.533) were found between MPA and PM/DM.


CONCLUSION
Intramuscular hyperintensity and fascial hyperintensity have always been observed in MPA, and the predominantly affected sites were usually the fascial regions. Compared with PM/DM, fascial hyperintensity and diffuse subcutaneous fat hyperintensity were more frequent in MPA.
['Kawaguchi|Masaya|M|http://orcid.org/0000-0002-2807-0524', 'Kato|Hiroki|H|http://orcid.org/0000-0001-5926-1895', 'Nagasawa|Tomoaki|T|', 'Kaneko|Yo|Y|', 'Taguchi|Koichiro|K|', 'Ikeda|Takahide|T|', 'Morita|Hiroyuki|H|', 'Miyazaki|Tatsuhiko|T|http://orcid.org/0000-0002-4750-2046', 'Matsuo|Masayuki|M|http://orcid.org/0000-0001-7615-3247']
Microscopic polyangiitis Inflammatory myopathy STIR MRI Musculoskeletal abnormalities