Nomura was apparently diagnosed with a dissociative aortic aneurysm (解離性大動脈瘤). At this point, it doesn't appear to be severe enough to require a surgery.
It seems Katsuya Nomura is a little under-the-weather and visited a hospital for treatment on Wednesday. Despite reports that Nomura was hospitalized (a Daily Sports article mentions a possible 10-day stay), Nomura's wife (Sachio) has told the press differently.
"He does have a cold," said Sachio. "But he wasn't hospitalized."
Regardless of the mixed reports, what does seem certain is that Nomura will likely take some time away from work (at least about a week) to recuperate.