A rare idiopathic congenital thoracic malformation characterized by a sternal fusion defect, that can be complete or partial (either superior or inferior), that is usually asymptomatic in the neonatal period (apart from a paradoxical midline thoracic bulging) but that can lead to dyspnea, cough, frequent respiratory infections and increased risk of trauma-related injury to the heart, lungs and major vessels if left untreated.