Case Report: persistent diaphragmatic and cardiac involvement despite creatine kinase normalization in anti-mitochondrial M2 antibody-positive inflammatory myopathy with primary biliary cholangitis

Anti-mitochondrial M2 antibodies (AMA-M2) are a serologic hallmark of primary biliary cholangitis (PBC), but in inflammatory myopathy they have also been associated with chronic muscle disease, cardiac involvement, and respiratory muscle weakness. We report a 44-year-old woman who developed progressive proximal lower-limb weakness followed by palpitations, exertional dyspnea, and severe hypercapnic respiratory failure. Serum creatine kinase (CK) was only moderately elevated (peak, 789 U/L). AMA-M2 remained strongly positive with anti-gp210 and anti-sp100 antibodies. Low-level anti-HMGCR IgG was detected on two occasions (0.35 and 0.27 U/mL; positivity cutoff >0.15 U/mL), whereas anti-SRP was negative. Electromyography showed myopathic changes with abundant myotonic discharges, and thigh magnetic resonance imaging demonstrated inflammatory signal abnormalities with muscle atrophy and fatty infiltration. An outside muscle biopsy was recorded as favoring dermatomyositis, but the original report and slides could not be retrieved and repeat biopsy was declined. Without independently reviewable biopsy data, the 2017 EULAR/ACR score was 7.0 (estimated IIM probability, 62.5%), meeting the threshold for probable IIM. Arterial carbon dioxide tension reached 94.6 mmHg. Chest imaging showed no extensive parenchymal disease sufficient to explain the degree of hypoventilation, whereas serial diaphragm ultrasonography demonstrated marked dysfunction. Cardiac evaluation revealed conduction abnormalities, atrial and ventricular arrhythmias, elevated high-sensitivity cardiac troponin I (hs-cTnI) and B-type natriuretic peptide (BNP), regional wall-motion abnormality, and reduced global longitudinal strain despite preserved left ventricular ejection fraction. After glucocorticoids, intravenous immunoglobulin, noninvasive ventilation, and individualized immunotherapy, CK normalized earlier than the cardiac and respiratory abnormalities. At approximately 4 months, CK and hs-cTnI were normal, but FVC remained 51% predicted, TLC 57.5% predicted, bilateral diaphragm thickening fractions 10%-11%, and mild hypercapnia persisted. At approximately 7 months, cardiac abnormalities remained evident. These asynchronous trajectories indicate that CK normalization may underestimate ongoing organ burden and support organ-specific surveillance during follow-up.