P.06Muscle ultrasound in patients with inclusion body myositis: differentiating from mimics

Abstract

Inclusion body myositis (IBM) is the most common inflammatory myopathy in individuals over the age of 50 years. When typical features of IBM are missing, its presentation can easily be confused with other forms of myositis or neuromuscular disorders. Recent studies have shown that muscle ultrasound (US) has the potential to differentiate between IBM and its mimicking diseases. In this study we aimed to further evaluate the ability of US to differentiate between IBM and Polymyositis/Dermatomyositis (PM/DM) and neuromuscular disorders using two separate cohorts. Patients were included from two centers: the Johns Hopkins Myositis Center and the Muscle Center Radboudumc. We compared echo intensity and muscle thickness of the flexor digitorum profundus (FDP), gastrocnemius, rectus femoris and vastus lateralis muscles in patients with IBM (n=41), PM/DM (n=37), other neuromuscular disorders (n=11) and healthy controls (n=88). All patients with IBM, PM and DM met the EULAR/ACR classification criteria for each disease. The neuromuscular disorders consisted of five myopathies, four motor neuron diseases (ALS or PSMA), one polyneuropathy and one LEMS. Echo intensity was higher and muscle thickness lower in all four muscles in IBM compared to PM/DM. When comparing IBM to the neuromuscular disorders, only the FDP showed a significantly higher echo intensity in IBM. The vastus lateralis was the only muscle with a significantly lower muscle thickness than the neuromuscular disorders. When evaluating the diagnostic potential per muscle group, logistic regression models using both ultrasound parameters showed that the FDP and vastus lateralis were the most discriminatory for IBM. However, the FDP proved to be the best candidate with a mean sensitivity/specificity of 94,9%/78,9% versus PM/DM and 93,8%/70,0% versus other neuromuscular disorders. Among several characteristically involved muscles, the FDP and quadriceps muscles are the most discriminating for IBM on US when comparing to mimicking diseases (other myositis and neuromuscular controls). This may have value as a diagnostic tool for IBM.