A 55-year-old man with spinal muscular atrophy (SMA) Type 3b suffered from palpitations since the last few months, without concomitant dyspnoea or chest pain. Electrocardiogram (ECG) showed sinus rhythm (90 b.p.m.) and left axis deviation (Panel A). A long-term ECG documented sinus tachycardia, without any arrhythmia and conduction disturbances. Blood tests indicated elevated levels of serum creatinine kinase [268 U/L; normal (N) <190 U/L] and myoglobin (155 µg/L; N: 28–72 µg/L), and normal levels of N-terminal pro-B-type natriuretic peptide (121 ng/L; N < 121 ng/L) and high-sensitive troponin T (11 ng/L; N < 14 ng/L). Echocardiography revealed dilated, non-hypertrophic left ventricle (LV) with severely reduced ejection...