PFO size and length tunnel increase with each decade of life [6]. The PFO size has a mean diameter of 5.1 mm and ranges from 3.4 mm in the first decade to 5.8 mm in the tenth decade. Furthermore, PFO diameter is larger in women than men (5.6 mm vs 4.7 mm, p = 0.028). PFOs larger than 4 mm in size may convey greater risk of recurrent stroke [8]. The extent of overlap of the septa determines the PFO length; this can be short resulting in a slit-like opening, or there can be significant overlap forming a tunnel (Figs. 3.2 and 3.3 respectively). This tunnel length ranges between 3 and 18 mm with an average length of 8 mm. A tunnel length more than 8 mm is considered to be a complex PFO [6, 9]. Another morphological variant which is the most difficult to cross is the tunnel-in-tunnel variant (Fig. 3.4).

As previously reported [7, 10, 11], PFO anatomy is classified based on a separate analysis of the PFO and atrial septal morphology (Table 3.1).

A simple PFO is defined as a short tunnel (up to 8 mm), not associated with an atrial septal aneurysm (ASA) or prominent Eustachian valve, and the thickness of the muscular septum is up to 6 mm. These characteristics of a simple PFO are present in approximately 45 % of the cases submitted to catheter closure [12].

In the presence of any of the features described in Table 3.1, a PFO is defined as complex.

Atrial septal aneurysm (ASA) is defined as a redundant and mobile septum primum with phasic excursion into the right and/or left atrium of at least 10–15 mm during the cardiorespiratory cycle [13]. The aneurysm may either bulge persistently into the right or left atrium or exhibit striking oscillations from right atrium to left atrium during respiration (Fig. 3.6). TEE is more sensitive than TTE for diagnosis of ASA since the inter-atrial septum is visualized more consistently. According to one report, 47 % of ASA are missed by TTE [14].

The prevalence of atrial septal aneurysm varies with the method of identification and varies from 1 % in necropsies to 2.2 % on TEE [15, 16]. PFOs are frequently present in up to 78 % of patients with ASA [14, 15, 17]. ASA have been associated with increased cerebral ischemic events in 7.9–15 % of patients with cryptogenic stroke [15, 16]. Two mechanisms have been proposed to explain the association between atrial septal aneurysm and cryptogenic stroke. Since atrial septal aneurysm is commonly associated with PFO and atrial septal defect, paradoxical embolism may occur via the septal defect. In patients with atrial septal aneurysm without an intracardiac shunt, it has been hypothesized that fibrin-platelet particles adhere to the left atrial side of the aneurysm and are dislodged by oscillations of the aneurysm, causing systemic embolism. The presence of ASA is not associated with an increased rate of complications or decreased success rate of PFO closure [18]; an ASA is present in 25 % of cases referred for catheter closure.

A long-tunnel PFO is defined by an overlap between septum primum and secundum of more than 8 mm [6, 9]. Recently, it has been suggested to further characterize the overlap by taking into consideration the so-called functional tunnel length [19]: type I, when septum primum and secundum overlapping is greater than or equal to 4 mm in the absence of redundant or aneurysmal septum primum (this is present in 80 % of cases) (Fig. 3.7a); type II, in the presence of PFO with aneurysmal septum primum and tunnel length at maximal excursion of at least 4 mm (15 % of cases) (Fig. 3.7b); and type III where the septum primum is aneurysmal and the attachment of septum secundum is a point of contact as opposed to a length (Fig. 3.7c). A long tunnel is present in approximately 10 % of the cases submitted to catheter closure. In the presence of these anatomic variants, the use of a sizing balloon is an important technique to help in the assessment of the shape of the defect, especially in the presence of a tunnel, when a longer indentation is visualized on the inferior edge of the balloon. By using a compliant balloon with static inflation, a waist is delineated and it may be easier to measure the tunnel length.

LHAS is histologically characterized by a non-encapsulated mass of adipocytes interspersed with atypical and hypertrophied myocytes in the atrial septum [20]. The incidence has been reported to be between 1 and 8 % depending on the series and methods used to detect the lesion (autopsy 1 %, TTE 8 %) [21]. Hypertrophy of the atrial septum has been defined as a thickness between 6 and 14 mm, whereas LHAS is characterized by massive fatty deposits in the secundum septum and the atrial thickening tends to be greater than 15 mm. The thickened septum typically spares the fossa ovalis, thus it appears as a classical dumbbell or hourglass shape (Fig. 3.8). Furthermore, the tissue is isodense to the surrounding subcutaneous fatty tissue [21, 22]. In a large prospective study, among 1,292 patients undergoing multislice CT, 29 patients (2.2 %) were found to have LHAS [23]. The exact cause of LHAS is unknown but it has been associated with obesity and aging. Although it is a benign tumor remaining asymptomatic in most people, it rarely may be associated with obstruction or atrial arrhythmias that require antiarrhythmic agents. Only in rare cases of very thick septums, which can cause circulatory obstruction, are patients required to undergo surgical resection and septal reconstruction.