MRI patterns of thigh muscle involvement in immune-mediated necrotizing myopathy and dermatomyositis

Abstract

Background: Immune-mediated necrotizing myopathy (IMNM) and dermatomyositis (DM) are characterized by weakness, hyperCKemia, associated autoantibodies, and varying extramuscular manifestations. Muscle MRI, currently subordinate to histopathology and serology in idiopathic inflammatory myopathy (IIM) classification, has an evolving role. Our study aims to define thigh muscle MRI involvement in IMNM and DM by direct comparison.

Methods: This single-center, retrospective, cross-sectional study included 25 participants, who met IIM classification criteria (14 IMNM, 11 DM) and had available thigh MRI. Clinical and paraclinical data were available and reviewed. 11 muscles were graded for edema on MRI using a semi-quantitative scale (0: normal, 1: <30% of muscle involvement, 2: 31-75%, 3: > 75%). For 3 participants with no significant muscle edema, muscle fatty infiltration was scored according to the same scale. Using linear mixed-effects models, muscle scores were compared between the two groups and a secondary analysis was performed of only edema scores, excluding the 3 participants with fatty infiltration scores.

Results: The most affected muscles in IMNM were the semimembranosus (3.0 [2.7-3.0] {median [IQR]}), biceps femoris-long head (BF-LH) (2.7 [2.0-3.0]), and adductors (2.5 [2.0-3.0]). In DM, the most affected muscles were the vastus lateralis (2.7 [2.3-3.0]), vastus intermedius (2.9 [2.2-3.0]), vastus medialis (2.3 [1.7-2.7]), semitendinosus (2.2 [1.0-2.7]), rectus femoris (RF) (2.0 [1.0-2.8]), biceps femoris-short head (BF-SH) (1.9 [1.0-2.7]), gracilis, and sartorius. Intergroup statistical difference of scores was significant (p < 0.01) for 10/11 thigh muscles excluding the RF (p = 0.19), supporting an inverse relationship of muscle involvement for DM and IMNM. The secondary comparative analysis of only muscle edema scores was significant (p < 0.05) for the same 10/11 muscles with a consistent direction for all comparisons.

Conclusion: DM and IMNM affect disparate thigh muscles on MRI. DM preferentially affects the anterior thigh, semitendinosus and BF-SH in the posterior thigh, and gracilis in the medial thigh, whereas IMNM preferentially affects the posterior thigh (semimembranosus and BF-LH) and adductors in the medial thigh.

Keywords: Classification; Dermatomyositis; Immune-mediated necrotizing myopathy; Inflammatory myopathy; MRI.

Fig. 1

Muscle grading of thigh MRIs in dermatomyositis (DM) and immune-mediated necrotizing myopathy (IMNM). Vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), rectus femoris (RF), biceps femoris-short head (BF-SH), biceps femoris-long head (BF-LH), semimembranosus (SM), semitendinosus (ST), adductors (Add), gracilis (Gr), and sartorius (Sa) MRI scores (0–3, y-axis) demonstrated as boxplots (vermilion, DM; teal, IMNM), which include median, interquartile range, and outliers (dots). Linear mixed effects model comparison of pooled individual thigh muscles scores between DM and IMNM indicated by significance bars. ***, p-value < 0.001. **, p-value < 0.01. NS, not significant

Fig. 2

Representative thigh MRIs of immune-mediated necrotizing myopathy (IMNM) and dermatomyositis (DM). (A, B) Muscle edema of selective posterior compartment muscles (semimembranosus [arrow] and biceps femoris-long head [BF-LH] [arrowhead], sparing the semitendinosus [open arrow] and biceps femoris-short head [BF-SH] [open arrowhead]) on short tau inversion recovery (STIR) images in IMNM cases with shorter disease duration. (C, D) On T1-weighted (T1W) images, complete fatty replacement of the semimembranosus (arrow) and BF-LH (arrowhead) with persistent sparing of the BF-SH (open arrowhead) in IMNM cases with prolonged disease duration. The semitendinosus is relatively spared (open arrow, C) or has undergone significant fatty replacement (black arrow, D). (E-H) Muscle and fascial edema of anterior compartment muscles, semitendinosus (arrow), and BF-SH (arrowhead) on T2-weighted fat-saturated (T2FS) images in DM