Background and aims: The prediction of the first major arrhythmic event (MAE) is still an unmet need in the recently defined scenario of non-dilated left ventricular cardiomyopathy (NDLVC). Methods: A cohort of 337 patients with NDLVC and no history of MAE was retrospectively identified at two large centres. Patient-tailored diagnostic workup included cardiac magnetic resonance (CMR), endomyocardial biopsy, and genetic testing. The primary endpoint was the occurrence of the first MAE, including sustained ventricular tachycardia (VT), ventricular fibrillation, or appropriate implantable cardioverter-defibrillator therapy, by 60-month follow-up. A pool of 216 NDLVC patients from 11 European centres served as a validation cohort. Results: In the study cohort (mean age 37 ± 15 years, 62% male), the mean left ventricular ejection fraction (LVEF) was 52 ± 8%, and 79% of patients had late gadolinium enhancement (LGE) at baseline CMR. By 60-month follow-up, 51 patients (15%) experienced a MAE. The primary endpoint was predicted by male sex [hazard ratio (HR) 2.4, 95% confidence interval (CI) 1.3-4.4, P = .007], baseline non-sustained VT (HR 3.1, 95% CI 1.7-5.6, P < .001), LVEF < 45% (HR 5.5, 95% CI 2.7-11.0, P < .001), septal (HR 2.0, 95% CI 1.0-4.0, P = .046) and ring-like pattern of LGE (HR 1.3, 95% CI .6-2.6, P = .54), pathogenic/likely pathogenic variants in guideline-defined high-risk genes (HR 4.6, 95% CI 2.3-9.1, P < .001), and biopsy/CMR-proven myocardial inflammation (HR 15.7, 95% CI 6.1-40.3, P < .001). The results were confirmed in the validation cohort (Uno's C-index 0.81, 95% CI .75-.88). A novel risk score was subsequently derived. Conclusions: In NDLVC, male sex, non-sustained VT, LVEF < 45%, septal and ring-like LGE, high-risk genotypes, and myocardial inflammation predicted the first episode of MAE by 60 months.
