Scimitar syndrome is a form of partial anomalous pulmonary venous drainage which can go undiagnosed for some time. The resulting left to right shunt can lead to pulmonary hypertension later in life. It is associated with sinus venosus type atrial septal defect (ASD), which results in a larger shunt.
Scimitar syndrome is also associated with pulmonary sequestration (commonly right lower lobe) and horseshoe lung. When these findings occur in the same patient, they are known as congenital pulmonary venolobar syndrome. Detection of the sequestration is important prior to corrective surgery to avoid morbidity and mortality caused by intraoperative injury to the systemic vessel(s).