Summary by Willem De Ridder

Publication

Refining MRI criteria in Inclusion Body Myositis and expanding the pattern beyond the distal quadriceps.

Pubmed link

PMID: 41549703

DOI: 10.1002/jcsm.70173

Eleonora Torchia, Matteo Lucchini, José Verdu‐Diaz, Sara Bortolani, Beatrice Ravera, Vincenzo Carlomagno, Alessandra Cicia, Daniela Bernardo, Mauro Monforte, Robert Rehmann, Rudolf Andre Kley, Mario Sabatelli, Jordi Díaz‐Manera, Enzo Ricci, Massimiliano Mirabella, Giorgio Tasca

Summary

Inclusion body myositis (IBM) remains one of the most enigmatic late-onset acquired myopathies. Its slow progression and striking selectivity of muscle involvement, particularly of finger flexors and quadriceps, form the cornerstone of clinical recognition. Yet diagnosis is frequently delayed, especially in patients with so-called “uncommon” presentations as defined by the 2024 ENMC criteria. Selective axial weakness (e.g. neck extensors or paraspinal muscles), isolated dysphagia related to cricopharyngeal dysfunction, or the absence of overt finger flexor or quadriceps weakness may obscure the diagnosis. In this context, muscle MRI – now recognised as a supportive investigation in the 2024 ENMC criteria – has become an increasingly valuable tool.

In this study, the authors refine MRI criteria for IBM and systematically explore muscle involvement beyond the traditionally emphasised compartments. Building on established lower limb patterns, the revised criteria retain a focus on distal quadriceps involvement (fatty replacement and oedema) and characteristic lower leg changes, particularly in the medial gastrocnemius. Additional features include sartorius involvement and relative sparing of pelvic musculature, aiding differentiation from limb-girdle muscular dystrophies. Importantly, the revised pattern showed high sensitivity, including in patients classified as clinically defined or probable IBM according to the 2011 ENMC criteria, confirming its robustness even when key clinical or histopathological features are incomplete.

A key strength of this work is its comprehensive evaluation of axial and upper body musculature, thereby expanding the imaging framework beyond the “typical” MRI pattern incorporated in the 2024 ENMC criteria. Cranial muscles were spared in all patients – even in advanced disease – providing useful discriminatory value in the differential diagnosis with conditions such as FSHD. In contrast, the authors document frequent and striking involvement of paraspinal muscles (up to 90%), exceeding what would be expected from age-related change and displaying a selective pattern highly suggestive of disease-specific pathology. Imaging of the shoulder girdle revealed marked generalised muscle wasting accompanied by relatively mild fatty infiltration, yet with frequent involvement of periscapular muscles such as the serratus anterior and subscapularis. While a distinct or pathognomonic pattern was not identified, these findings suggest that clinically apparent periscapular weakness should not be considered incompatible with IBM.

Notably, the sensitivity of the revised MRI criteria did not differ across clinical subgroups, including patients with “uncommon” presentations. This underscores a key message: MRI patterns in IBM appear consistent across heterogeneous clinical phenotypes, paralleling the imaging signatures observed in certain hereditary myopathies despite marked phenotypical variability. Although the study focuses primarily on sensitivity rather than specificity, and prospective multicentre studies will be needed to define performance in IBM-mimicking conditions, the practical implications are considerable. In diagnostically challenging scenarios, such as atypically early onset, cases initially labelled as “PM-mito,” or complex differentials including chronic sarcoid myopathy, recognition of a characteristic MRI pattern may substantially enhance diagnostic confidence. Taken together, these refined criteria represent an important step toward earlier recognition and more accurate diagnosis across the IBM spectrum.

About the author

Eleonora Torchia is a neurologist and final-year PhD candidate at Fondazione Policlinico Universitario A. Gemelli IRCCS, Università Cattolica del Sacro Cuore in Rome. Her research focuses on neuromuscular disorders, with a particular interest in FSHD and myositis. She has a strong interest in muscle imaging and histology and works with her team to integrate imaging data with clinical outcome measures to better characterise disease severity and monitor progression over time. She combines daily clinical practice with research activity in myology.