It is actually called an AVM, or an arteriovenous malformation. Think about it this way, it is a tangle of blood vessels that shouldnt be there. It can generate seizures or worse strokes or bleeds. Typically, they are treated with staged embolization of the feeder vessels and eventually resected, but if symptomatic, they need immediate resection.
When I moonlight, I take care of a lot of patient's post-neurosurgery and a few patients with AVM resections. The entire prognosis is based upon...
A. the location. As with anything, the location of the brain that you will resect will determine the deficit that the patient will have post-procedure
B. The complications from the procedure. Sometimes they bleed in surgery or the wrong vessel gets embolized and you have a stroke
Regardless, it is not a good thing for anyone to have, let alone a top notch athlete. Here is to hoping that he at the very least gets to live a full and long life, as this is not guaranteed with patients who have this disorder