Magnetic resonance imaging changes of thigh muscles in dermatomyositis patients with anti-Mi2 antibody.

BACKGROUND

Anti-complex nucleosomal remodelling histone deacetylase (Mi2)-positive dermatomyositis (DM) is characterized by severe muscle damage. This study investigated muscle magnetic resonance imaging (MRI) features in anti-Mi2-positive DM, compared them with other DM subtypes, and evaluated their associations with clinical features.

METHODS

DM patients who underwent thigh MRI were enrolled, including 20 anti-Mi2-positive, 20 anti-TIF1γ-positive, 15 anti-NXP2-positive, and 19 anti-MDA5-positive cases. Clinical and imaging data were collected and compared across subtypes. The relationship between MRI features and clinical characteristics was analysed in the anti-Mi2-positive subgroup.

RESULTS

Among the four subtypes, anti-Mi2-positive DM had the highest myofascial oedema frequency in the sartorius and the lowest in the biceps femoris (both p < 0.05). All anti-Mi2-positive patients had muscle oedema - most severe in the quadriceps femoris with relative sparing of the semimembranosus and biceps femoris - and the highest total muscle oedema scores (p < 0.001). Eleven (55%) patients had fatty infiltration, predominantly in the gluteus maximus and hamstrings with sparing of the rectus femoris and adductor longus. In this subgroup, treatment-naïve patients had a higher prevalence of diffuse distribution than treatment-experienced (p = 0.042). Compared with anti-TIF1γ-positive patients, the anti-Mi2-positive subgroup had lower total fatty infiltration scores and milder gluteus maximus, adductor longus, and hamstrings involvement (all p < 0.05). In anti-Mi2-positive DM, total muscle oedema scores correlated negatively with Manual Muscle Testing-8 scores and positively with serum creatine kinase levels (both p < 0.05). Preliminary observations in six anti-Mi2-positive patients followed longitudinally showed that muscle oedema decreased after treatment, resolved during remission, and recurred with relapse.

CONCLUSION

This study delineated baseline thigh MRI patterns in anti-Mi2-positive DM and demonstrated that baseline muscle oedema severity correlated with serological and clinical indicators of muscle damage. Additionally, findings from a small longitudinal cohort suggested a potential role for thigh MRI in assessing and monitoring muscle disease activity in this subtype, although these results are preliminary and require validation in larger studies.