Case report of a ventricular fibrillation storm with a cardiac conduction disorder and HCN4 variant 18 years after ablation of atrial flutter.

Background: Genetic abnormalities causing various arrhythmias including atrial arrhythmias, specialized cardiac conduction disorders, and malignant ventricular arrhythmias have been reported. However, it is sometimes difficult to diagnose and treat patients with various arrhythmias. Case summary: A 49-year-old woman who underwent ablation of typical atrial flutter (AFL) at 31 years of age visited the emergency room due to a cardiopulmonary arrest. Her 12-lead electrocardiogram during sinus rhythm after resuscitation exhibited first-degree atrioventricular block with right bundle branch block and right axis deviation. No structural heart disease was evident on standard imaging screening. An implantation of a single-chamber implantable cardioverter defibrillator (ICD) was indicated. After the ICD implantation, she then experienced multiple ventricular fibrillation (VF) episodes. Radiofrequency catheter ablation of triggered ventricular premature contractions (VPCs) was performed but failed because the clinical VPCs could not be induced during the session. Although no pathogenic variants associated with Brugada syndrome or long-QT syndrome were found, a rare HCN4 variant, c.1209+2_1209+3insGAGT (rs786205418), was identified in a gene panel analysis. Because high-frequency clinical pacing was effective for suppressing the VF, the single-chamber ICD was upgraded to a dual-chamber ICD. Thereafter, high-rate pacing successfully prevented any further ventricular arrhythmias during the follow up. Discussion: A clinical course with prominent wide QRS complexes and AFL in one's early 30s followed by sudden onset of a VF storm about 20 years later is extremely rare. Her clinical phenotype expression was possibly associated with a rare HCN4 variant; however, further study is needed to confirm whether this variant was pathological or not.