I. Developmental Features
II. Clinical Features and Investigations
III. Proof of PFO Involvement in Stroke
IV. Perspective and Research Implications

GLOSSARY
cryptogenic of obscure or doubtful origin.
embolism, embolus a blood clot that forms in an artery, a vein, or the heart and breaks off and is carried by the circulating blood, finally lodging and blocking the artery that supplies an organ with blood; for example, pulmonary embolism is an embolus blocking an artery in the lung.
TIA transient ischemic attack; transient lack of blood supply (ischemia) to the brain causing symptoms of mild stroke that recover within 24 h.
DURING THE PAST 15 YEARS, BECAUSE OF echocardiographic visualization of the left atrium and the septum, it became apparent that a patent foramen ovale (PFO) is present in approximately 25% percent of a randomly selected population. Because the transesophageal echocardiogram (TEE) gives excellent clarity of this region of the heart, it is the preferred method of study for diagnosis. There has been increasing interest in PFO and its role in systemic embolization especially to the brain which causes stroke of undetermined etiology (cryptogenic stroke).

• I. DEVELOPMENTAL FEATURES
  During fetal development PFO allows the necessary shunting of oxygenated blood from the right atrium to the left atrium; thus, oxygenated blood flows into the sys¬temic circulation to organs and tissues. This right-to-left shunting of blood is crucial for fetal development, but it is also vital that no shunt should occur after birth, because the right side of the heart is no longer filled with oxygenated blood. A right-to-left shunt in individuals with congenital heart disease (CHD) causes cyanotic congenital heart disease which is life-threatening (see chapter entitled Congenital Heart Disease).
• II. CLINICAL FEATURES AND INVESTIGATIONS
  Cryptogenic stroke is a nonhemorrhagic stroke (cerebral infarcts) that occurs in younger individuals without identifiable risk factors such as hypertension, diabetes, paroxysmal atrial fibrillation, atheroma of the ascending aorta, hyperlipidemia, and lacunar strokes from small vessel disease.
• III. PROOF OF PFO INVOLVEMENT IN STROKE
  Illustrations depicting PFOs are Figs. 1, 4, and 5 and atrial septal defects, are given in Fig. 2. It is extremely difficult to be certain if a PFO observed by TEE is the cause of stroke in a given individual. There are very few credible reports on this subject. Findings of a thrombus in transit through a PFO have been reported in a few autopsy and echocardio-graphic reports (see Fig. 4).
• IV. PERSPECTIVE AND RESEARCH IMPLICATIONS
  PFOs are very common, but not all PFOs are identical or carry the same risk. Individuals who have PFOs can be grouped according to many PFO characteristics such as large PFOs 0.6–1 cm, versus small PFOs less than 0.6 cm and associated anatomic structures.
• BIBLIOGRAPHY
  Braun, M. U., Fassbender, D., Schoen, S. P. et al. Transcatheter closure or