Obviously, I have no way of knowing the actual cause of this unfortunate runner's death, and I will not speculate on it. However, while it took local race officials off the hook, the Chicago medical examiner's confident pronouncement also may have created at least some degree of panic among up to 75 million Americans (by some estimates) who might also have MVP.
So, the question bears asking: Does MVP really cause sudden death?
The answer is that nobody really knows, but if it does, the excess risk posed by typical MVP is exceedingly small.
Evidence that MVP may be associated with sudden death comes mainly from autopsy series. When the heart is carefully examined in people who have died suddenly, something like 10% of them will turn out to have MVP. What is not mentioned in these studies is that when you carefully look for MVP in living people (using echocardiography), then depending on the diagnostic criteria used, at least 2% and up to 35% of the general population will receive the diagnosis of MVP.
The bottom line is that it is not at all clear that MVP - at least, the mild form of MVP that the vast majority of people diagnosed with this condition have - is even associated with sudden death, let alone a cause of sudden death. A small number of patients with MVP will eventually develop significant mitral regurgitation (valve leakage), which in turn can produce heart muscle weakness - and these patients clearly do have an increased risk of life threatening arrhythmias, as do any patients with cardiac muscle disease. It is not clear from news reports whether the Chicago marathoner may have had this severe form of MVP.
But the vast majority of patients with MVP have never been shown to have any higher risk of sudden death than the general population, and the recent tragedy in Chicago should not cause undue concern. Read more about MVP here.