Hi guys, MD and surgical resident here chiming in.
Briefly:
When you're a baby in utero, there is a hole (foramen ovale) in your heart that allows you to oxygenate your blood without your lungs (the placenta). Usually this closes when you're born. In a fairly common percentage of the population (approx. 30%), it doesn't. If you are to become symptomatic, most of the time it happens as a very young baby. Otherwise, the rest will grow up without ever realizing they have it.
The persistently open hole (patent foramen ovale) creates a mixing of blood that has not yet gone to the lungs and blood that already has. Therefore, you have a mixing of oxygenated and deoxygenated blood, resulting in a lower than average oxygen level. That's important because it explains the potential symptoms.
From not having that normal level of oxygen, symptoms can include headaches and migraines (which it sounds like Letang has had in the past). Strokes happen when your brain doesn't get enough oxygen long enough to create a more lasting effect. This hole puts you at higher risk of stroke for two reasons: 1) it may allow a clot from somewhere else (legs) to climb up your circulation and into your brain, and block circulation to an area or 2) the level of oxygen in the blood may be temporarily insufficient from time to time.
In terms of what to do about it, it's an area of ongoing research and debate. The thousands and thousands of patients that have been studied so far haven't shown us that surgery, medication or even doing nothing is conclusively the best option. It sounds like the physician they've consulted is leaning towards taking a blood thinner (usually Aspirin is the first step), which is quite reasonable and consistent with the field recommendations.
What does it mean for Letang? Well in terms of recovery time, that will depend on the severity of the stroke. It seems to be more on the lower end from what we've heard. As for his career in general, that will be a risk-reward discussion between he, his family, his team and his doctors. As you can imagine, putting someone on blood thinners increases their risk of bleeding. As a hockey player at the highest level, he suffers his fair share of bumps and bruises (which are bleeding from superficial skin vessels), and a blood thinner will make that worse. In addition, his risk of internal bleeding has also increased. We don't typically tell our patients on blood thinners that they are no longer allowed to play contact sports, just that there is an increased risk they have to accept if they want to do so.
Hope that helps,
Alex