is derived from the primary atrial septum If development proceeds normally, the floor of the fossa is usually of sufficient dimension to overlap the infolded superior rim, often described as the septum secundum The floor and rim, however, do not always fuse with one another, and failure of such fusion produces the probe-patent oval foramen (Fig 29.5) FIG 29.5 Normal heart sectioned along its short axis The flap valve of the oval fossa, derived from the primary atrial septum, has failed to fuse with the superior rim, even though it is of sufficient dimensions to close the septal deficiency This constitutes persistent patency of the oval fossa Even if the flap valve fails to fuse with the superior interatrial fold, there will be no interatrial shunting as long as the left atrial pressure is higher than right, which is the normal postnatal situation Nonetheless, probe patency is known to be responsible for paradoxic embolism and can be an important finding in those undertaking deep sea diving The suggested association with migraine,2 however, has still to be proven The simplest true defect within the oval fossa is found when the flap valve is minimally deficient, so that it fails to entirely overlap the left atrial margin of the rim of the fossa With increasing deficiency of the flap valve or in presence of a perforated valve, the defect becomes larger (Fig 29.6, left) In extreme cases there may be no floor to the fossa (see Fig 29.6, right) FIG 29.6 Examples of the lesions that can be found within the confines of the oval fossa Left, Shown in a flap valve that is not only of insufficient size to overlap the margins of the fossa but is also perforate posteroinferiorly and anteroinferiorly Right, Example in which the flap valve is deficient over the full extent of the fossa in combination with a perimembranous inlet ventricular septal defect (VSD) Note that the defect extends to the mouth of the inferior caval vein When the deficiency is marked, the hole can extend toward the mouth of the inferior caval vein, which may straddle the persisting rim to open in part to the left atrium (see Fig 29.6, right) Although these defects may exist in isolation and do not disturb the location of the conduction tissues, they often occur in combination with many other congenital cardiac malformations, as shown at right in Fig 29.6 Vestibular Defect The heart shown in Fig 29.7, in addition to the multiple perforations in the floor of the oval fossa, has an additional defect in the anteroinferior buttress This lesion is the second type of true atrial septal defect.7 FIG 29.7 The right atrium has been opened to show the perforated floor of the oval fossa In addition, there is a relatively large defect within the anteroinferior buttress of the fossa This is a vestibular defect Sinus Venosus Defects The essence of the sinus venosus defects is that they exist outside the confines of the oval fossa.8 Found most frequently in the mouth of the superior caval vein, they can also be found at the orifice of the inferior caval vein9 or draining to the midpoint of the systemic venous sinus In the case of the superior sinus venosus defect, the orifice of the superior caval vein typically overrides a defect, which has the superior rim of the fossa as its floor (Fig 29.8, left)