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).
The finding of a clot in veins of the lower limbs (deep vein thrombosis, DVT), increases the probability of embo-lization through the PFO. A DVT has been observed in approximately 33% of individuals with cryptogenic stroke and PFO. It is extremely difficult to be certain of the diagnosis in individuals without DVTs or where predis¬posing factors for DVT are absent. These predisposing factors include postsurgical immobilization, fractures of the lower limbs, plaster casts applied to the lower limbs, immobilization for several days, and the postpartum state.
A PFO may play a role in other situations such as venous-to-arterial gas embolism in serious forms of severe decompression sickness in underwater divers and high altitude aviators and astronauts. It may also occur in the platypnea-orthodeoxia syndrome. In this syndrome signif¬icant right-to-left shunting of venous blood through a PFO occurs precipitated by postural and other undeter¬mined mechanisms.

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