A long QT syndrome characterized by perinatal onset of markedly prolonged corrected QT (QTc) interval, 2:1 atrioventricular (AV) block, and bradycardia or ventricular tachycardia (torsades de pointes) that has_material_basis_in heterozygous mutation in the CALM3 gene on chromosome 19q13.32. Syncope, cardiac arrest, and sudden death are common.