Potential pitfalls in the differential diagnosis of myositis versus hereditary myopathies

Hereditary myopathies and myositis share similar clinical features, making diagnosis challenging. The 2017 EULAR/ACR classification criteria for idiopathic inflammatory myopathies (myositis) provide a probability score for myositis and its subtypes. This study aims to test the accuracy of these criteria in distinguishing myositis from hereditary myopathies and to identify diagnostic pitfalls.

This retrospective study examined data from 105 patients diagnosed with genetically confirmed myotonic dystrophy type 1 and 2 (DM1 and DM2), facioscapulohumeral muscular dystrophy (FSHD), limb girdle muscular dystrophy (LGMD) and Duchenne and Becker muscular dystrophy (DMD and BMD). The performance of the 2017 EULAR/ACR criteria for myositis was tested on patient data to identify variables leading to misdiagnosis.

Of the 105 patients with hereditary myopathy, 50% of DM1, 47.8% of DM2, 25% of FSHD, 35.3% of LGMD, 0% of BMD, and 36.4% of DMD patients were misclassified according to the 2017 EULAR/ACR criteria as having "possible", "probable" or "definite" myositis, resulting in an overall specificity of 63.8%. The most frequently proposed myositis subtype in misclassified cases was inclusion body myositis. Analysis of the individual criteria items revealed that proximal muscle weakness, increased CK, perifascicular atrophy and endomysial infiltration in the muscle biopsy were particularly misleading factors.

The diagnostic challenge of differentiating between inflammatory and hereditary myopathies is reflected by the relatively low specificity of the classification criteria in this study. Relevant factors leading to misdiagnosis include similarities in clinical presentation and diagnostic findings, as well as late-onset disease manifestations, e.g., in LGMD, DM1, DM2.