Abstract 4367021: Intermittent Complete Heart Block as a Manifestation of Noncompaction Cardiomyopathy in a Young Adult

Description of Case: A 36-year-old male with a history of intermittent palpitations and shortness of breath was referred for cardiology evaluation after an event monitor captured an episode of complete atrioventricular (AV) block. He was asymptomatic at the time and engaged in routine activities, including driving. The rhythm strip demonstrated AV dissociation with absent QRS complexes despite ongoing atrial activity, without any identifiable vagal or positional triggers. Baseline ECG revealed a known right bundle branch block and left anterior fascicular block. Cardiac magnetic resonance imaging showed prominent trabeculations in the apical and lateral walls of the left ventricle, with a non-compacted to compacted myocardial ratio of 3.5, consistent with noncompaction cardiomyopathy (NCCM). A prior transthoracic echocardiogram showed normal left ventricular systolic function (LVEF 60%) without any structural or regional wall motion abnormalities. As the patient remained asymptomatic without syncope or heart failure, device therapy was deferred. He was referred to cardiomyopathy clinic for outpatient follow-up, including genetic testing and extended rhythm monitoring. Discussion: This case highlights an uncommon presentation of NCCM, in which conduction system disease—specifically intermittent complete heart block—precedes overt systolic dysfunction. NCCM is a phenotypically diverse cardiomyopathy resulting from failed embryologic compaction of the myocardium. While it is often associated with heart failure, arrhythmias, and thromboembolism, early signs may include conduction abnormalities due to disrupted Purkinje architecture or myocardial fibrosis. These abnormalities can be silent and episodic, making diagnosis challenging in young patients without overt structural disease on echocardiogram. Recognition of NCCM in patients with unexplained AV block is critical, particularly when accompanied by fascicular block or a history of palpitations. Multimodality imaging, particularly cardiac MRI, enables accurate diagnosis when standard echocardiography appears normal. Early referral to cardiomyopathy specialists facilitates appropriate genetic testing, family screening, and longitudinal risk stratification, even in the setting of preserved ejection fraction. This case reinforces the importance of structural evaluation in young adults presenting with conduction disease to ensure timely identification and management of cardiomyopathies like NCCM.