Magnetic resonance imaging criteria for distinguishing between inclusion body myositis and polymyositis.

OBJECTIVE

To develop diagnostic imaging criteria for polymyositis (PM) and sporadic inclusion body myositis (sIBM).

METHODS

We investigated 220 patients with suspected inflammatory myopathies by magnetic resonance imaging (MRI). Findings were compared with the results of clinical and biological examinations and muscle biopsy. PM and IBM were diagnosed in 25 patients each. Quantitative and qualitative MRI analysis of the 3 muscle groups of the 2 thighs included fatty infiltration, atrophy, inflammation, and the type and distribution of the lesions.

RESULTS

MRI was abnormal in all patients. Fatty infiltration and atrophy were more frequent in patients with sIBM (p < 0.05). Inflammation as the sole abnormality was preferentially encountered in PM (p = 0.05). Widespread abnormalities were more frequent in sIBM (p < 0.01). Abnormalities in PM tended to be distributed along the fascia. Involvement of the anterior group, an asymmetrical distribution, and a distal predominance were all more frequent in sIBM (p < 0.001).

CONCLUSION

Despite some overlap in MRI findings between the 2 diseases, MRI was useful for distinguishing PM from sIBM.