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Abstract   Aims  Although increasingly recognized as a distinct pathological entity, left bundle branch block‐induced cardiomyopathy (LBBB‐ICMP) is not included among the possible aetiologies of acquired dilated cardiomyopathies (DCM). While diagnostic criteria have been proposed, its recognition remains principally retrospective, in the presence of clinical and instrumental red flags. We aimed to assess the prevalence and clinical and instrumental features of LBBB‐ICMP in a large cohort of patients with DCM.    Methods and results  We analysed a cohort of 242 DCM patients from a two‐centre registry. Inclusion criteria were age &gt; 18, non‐ischaemic or non‐valvular DCM, and LBBB on electrocardiogram. LBBB‐ICMP was defined according to previously proposed diagnostic criteria: (i) neither family history nor clinically identifiable potential causes for DCM; (ii) negative genetic testing; (iii) echocardiographic features including non‐severe chamber dilation, normal absolute and relative wall thickness, marked dyssynchrony, and normal right ventricular function; and (iv) absence of late gadolinium enhancement (LGE) on cardiac magnetic resonance (CMR). From the entire cohort, we identified 30 subjects (similar in terms of New York Heart Association class I or II in 80% vs. 75%,  P  = 0.56; QRS width of 150 ± 22 vs. 151 ± 24 ms,  P  = 0.82; and cardiac remodelling of baseline end‐diastolic diameter 66 ± 8 vs. 65 ± 10 mm,  P  = 0.53) with a comprehensive dataset including CMR and genetic testing, required to verify the presence of the diagnostic criteria proposed for LBBB‐ICMP. The main characteristics of this subgroup were 73% males, age 45 ± 13 years, left ventricular ejection fraction (LVEF) 30 ± 10%, LGE in 38% of patients, and QRS complex of 150 ± 22 ms. Patients were under guideline‐directed medical therapy, and 57% of them were treated with cardiac resynchronization therapy (CRT). Two patients (6.67%, 50% males, age 53 ± 13 years) fulfilled the diagnostic criteria proposed for LBBB‐ICMP. After a follow‐up of 44 (12–76) months, LVEF was normal and QRS width significantly reduced (from 154 ± 25 to 116 ± 52 ms) in patients with LBBB‐ICMP. Both patients were under optimal medical treatment, and one was implanted with CRT‐D. Neither of the two patients experienced death, malignant ventricular arrhythmia, or heart failure hospitalization at follow‐up.    Conclusions  Left bundle branch block‐induced cardiomyopathy emerges as a distinct pathological entity, promptly identifiable in a minority but not negligible proportion of patients with newly diagnosed DCM and LBBB, using a series of diagnostic criteria including CMR and genetic testing. Further studies are needed to better elucidate the clinical course of LBBB‐ICMP.

Prevalence, clinical and instrumental features of left bundle branch block‐induced cardiomyopathy: the CLIMB registry