Background We evaluated the functional consequences of diaphragm involvement in patients with inclusion body myositis (IBM). Methods Ultrasound diaphragm thickening fraction (TFdi), lung function and dyspnea levels were compared between IBM patients and matched controls. Patients with IBM were grouped into “low” and “high” diaphragm activity based on TFdi values (with cutoff value being the lowest observed TFdi in the control group), and clinical characteristics were compared between groups. Results 20 IBM patients were included. TFdi was significantly lower in patients and correlated with time since symptom onset (rho = 0.74, P < .001). Patients had significantly lower forced vital capacity and higher dyspnea scores than controls. IBM patients with “low” diaphragm activity (n = 9) had lower 6‐min walking distance, higher resting and exertional dyspnea and a larger positional decrease in vital capacity (all P ≤ .03) than patients with ‘high’ activity. Timed Up and Go time and St. George's Respiratory Questionnaire were not different between groups. Conclusions Diaphragm involvement in IBM is related to disease duration and has detrimental effects on lung function, dyspnea and exercise capacity. Further studies are required to investigate its potential as a therapeutic target.

A 63-year-old man was referred for slowly progressive dyspnea on exertion that had developed over 7 years. Dyspnea was initially only present during high-intensity physical activity, but was now present while walking rapidly on a flat surface. Symptoms were accentuated while supine and when bending forward. He reported respiratory difficulties when submerged in water and a recent onset of slight symmetric lower limb weakness that was only apparent during strenuous physical activity. He also had OSA, which was adequately controlled with continuous positive airway pressure therapy. Neurologic and rheumatologic histories were otherwise unremarkable. He denied any impact accidents or trauma to the cervical spine and prior neck or thoracic surgeries.