Muscle Ultrasound in Inclusion Body Myositis: Integrating Qualitative and Quantitative Approaches with Clinical and MRI Findings

Background Inclusion Body Myositis (IBM) is a distinct, slowly progressive inflammatory myopathy characterized by a complex clinical presentation and diagnostic delay. Imaging techniques such as MRI and muscle ultrasound (Muscle US) can aid diagnosis by identifying characteristic patterns of muscle involvement and differentiating IBM from other myopathies. Muscle US offers a rapid, accessible and cost-effective tool, though its diagnostic role in IBM remains under investigation. This study aimed to evaluate the diagnostic and clinical value of qualitative and quantitative muscle ultrasound in IBM by examining their correlation with clinical and MRI findings. Methods Thirteen patients with histologically confirmed IBM were prospectively enrolled. All underwent comprehensive clinical evaluation, including the Inclusion Body Myositis Functional Rating Scale (IBM-FRS) and the Medical Research Council (MRC) muscle strength grading. Qualitative muscle US was performed using the Heckmatt scale, while quantitative US assessed muscle echo-intensity (EI) and muscle thickness (MT) z-scores. MRI of the lower limbs (T1 Mercuri and STIR scores) was performed in nine of the thirteen patients. Inter-rater reproducibility for US grading and correlations between US, MRI, and clinical scores (IBM-FRS, muscle MRC) were analysed. 2 Results Qualitative muscle US analysis showed increased echogenicity predominantly in the flexor digitorum profundus (FDP), vastus lateralis (VL), rectus femoris (RF), tibialis anterior (TA) and medial gastrocnemius (GM). Quantitative US showed increased EI predominantly in the FDP, biceps brachii (BB), VL, RF and GM, with milder involvement of the TA and lateral gastrocnemius (GL). Quantitative EI correlated strongly with Heckmatt grading and MRI T1 Mercuri scores (p < 0.001). Muscle thickness analysis also confirmed atrophy in the most affected muscles. Furthermore, EI in FDP and TA showed an inverse correlation with muscle strength (MRC scores). Conclusion Qualitative and quantitative muscle US provide complementary and reproducible measures of muscle damage in IBM, correlating with both clinical and MRI parameters. These findings support the integration of muscle US as a bedside, non- invasive tool for diagnosis and disease monitoring in IBM, consistent with the 2024 ENMC framework promoting multimodal, imaging-supported diagnostic approaches.