Isolated left bundle branch block progressing to complete heart block and asystole: A novel presentation of a desmin mutation

Key Teaching Points•An isolated left bundle branch block is a rare conduction abnormality in the pediatric population and when present is typically pathologic.•Desminopathies are genetic mutations that encode a protein essential for the structural integrity of skeletal, cardiac, and smooth muscle. Clinically, desminopathies present with a wide range of variability and severity, with almost a quarter of cases presenting with an isolated cardiomyopathy or conduction disturbance without skeletal muscle involvement.•The main cause of death in all desminopathies is a cardiac arrhythmia.•The 2012 updated guidelines for device-based therapy for cardiac rhythm abnormalities from the American Heart Association, Heart Rhythm Society, and American College of Cardiology suggest the consideration of permanent pacemaker placement for neuromuscular diseases with any degree of atrioventricular block, with or without symptoms, due to the progressive nature of the disease.•The clinician should have a high index of suspicion for a genetic defect associated with conduction abnormalities and order an appropriate genetic workup early on because recognition of certain mutations, such as a desminopathy, may sway the provider to place a pacemaker sooner.•When placing a pacemaker, the clinician should select a magnetic resonance imaging (MRI)-compatible device, as this is the imaging modality most sensitive for detecting disease progression. If the patient already has an MRI-incompatible pacemaker, echocardiography may be used for surveillance. •An isolated left bundle branch block is a rare conduction abnormality in the pediatric population and when present is typically pathologic.•Desminopathies are genetic mutations that encode a protein essential for the structural integrity of skeletal, cardiac, and smooth muscle. Clinically, desminopathies present with a wide range of variability and severity, with almost a quarter of cases presenting with an isolated cardiomyopathy or conduction disturbance without skeletal muscle involvement.•The main cause of death in all desminopathies is a cardiac arrhythmia.•The 2012 updated guidelines for device-based therapy for cardiac rhythm abnormalities from the American Heart Association, Heart Rhythm Society, and American College of Cardiology suggest the consideration of permanent pacemaker placement for neuromuscular diseases with any degree of atrioventricular block, with or without symptoms, due to the progressive nature of the disease.•The clinician should have a high index of suspicion for a genetic defect associated with conduction abnormalities and order an appropriate genetic workup early on because recognition of certain mutations, such as a desminopathy, may sway the provider to place a pacemaker sooner.•When placing a pacemaker, the clinician should select a magnetic resonance imaging (MRI)-compatible device, as this is the imaging modality most sensitive for detecting disease progression. If the patient already has an MRI-incompatible pacemaker, echocardiography may be used for surveillance.