A defect that occurs at or below the linea semicircularis but above the point at which the inferior epigastric vessels cross the lateral border of the rectus abdominis muscle. This type of hernia may contain preperitoneal fat or may be a peritoneal sac containing intraperitoneal contents. It is rare and difficult to diagnose unless large, because it is typically not palpable when small. Large Spigelian hernias may be mistaken for sarcomas of the abdominal wall. Ultrasonography or computed tomography scans are often used in diagnosis.