There are various types and degrees of thoracic outlet syndrome, which, using the simplest explanation, is caused when blood vessels or nerves are compressed in the lower neck and upper chest area.
Surgery is scheduled for July 19 and will be performed by Dr. Gregory Pearl in Dallas. According to general manager John Mozeliak, the usual recovery time for such a procedure is three to six months. That leaves the Cardinals confident that Carpenter, who is in the first season of a two-year, $21 million contract, will be at full strength at the start of Spring Training in 2013.
Carpenter said on Tuesday that he has every intention of pitching next season.
"I want to pitch again, and this is the way for me to pitch again," Carpenter said. "We tried since Spring Training to get this going, and every time I tried to come back, unfortunately, it just doesn't allow me to do that."
Carpenter was first shut down with what was described as a nerve-related issue in March, when he began feeling weakness and numbness in his arm, shoulder and face following his throwing sessions. He sat out for nearly three months before resuming a throwing program in mid-June.
Though Carpenter maintained in recent days that his strength was where it needed to be, it became clear over the past week that there were still underlying problems. He skipped a scheduled batting practice session on June 25, which prompted a visit to Dr. Pearl's office on Thursday.
At that time, Dr. Pearl laid out the possibility of addressing the issue with surgery. That option was further solidified when Carpenter was unable to properly bounce back following a throwing session at Busch Stadium on Friday.
"I think everybody in there is disappointed that we'll lose the year," Carpenter said. "I think they're disappointed, including myself, that I have to [have surgery] again. But this is also the first time that there are some answers and we can get this taken care of. Hopefully, I can get back out there and get better than I have been in the past."
The procedure will involve removing the top rib on Carpenter's right side, as well as taking out two muscles in his neck.
Because Carpenter's condition is caused by a nerve issue and is not vascular-related, the operation is considered relatively simple, Mozeliak said.
Other Major League players who have undergone thoracic outlet surgery are Kenny Rogers, Jeremy Bonderman, Matt Harrison, Aaron Cook and Kip Wells. Rogers, who was 36 at the time of his procedure, returned to pitch another seven seasons in the Majors.
"I asked [Dr. Pearl] one simple question," said Carpenter. "I said, 'I'm 36 years old. I've had a nice career. Is this worth getting this done?' And his comment to me was that he has seen me pitch. He saw me pitch last year. He knows that I can still pitch. There's no question that he believes that I can come back and be as strong as ever and continue to be successful."
Before getting those assurances, Carpenter admitted, he had considered retirement. Now such a consideration is no longer on the table.
"It's my obligation to do everything I can to get out there," Carpenter said. "I'm past the 'woe is me' part. That was the last few days, when we finally made the decision that I was going to do it. I'm past that. I'm ready to work. I'm excited about the outcome."
The diagnosis of thoracic outlet syndrome is actually one that Carpenter first received in 2008, when he was having shoulder trouble while recovering from surgery on his right elbow.
He was limited to 15 1/3 innings that season but returned at full strength in 2009, and finished second in the National League Cy Young Award voting that year.
Nerve-related issues bothered Carpenter during Spring Training 2010, but he was able to work through them and pitch for the next two seasons.
This year he couldn't.
"It's been tough to watch him go through the roller coaster of thinking he's going to be there and all of a sudden have some relapses," manager Mike Matheny said. "I think right now he's got as good of a view and perspective as he's had in a long time. He's got a chance to really put this thing behind him. I don't think people realize how long and how much he's dealt with and how successful he's been being much less than 100 percent."
This marks the second straight year in which the Cardinals will be down a staff ace for the entirety of a season, as Adam Wainwright spent all of 2011 on the mend after undergoing Tommy John surgery. But led largely by Carpenter, who pitched 273 1/3 innings in the regular season and postseason, the Cardinals found a way to capture the franchise's 11th World Series title.
Such perspective should eventually help this club, but it didn't mitigate the initial reaction to Tuesday's news.
"I can't imagine a bigger blow than missing Chris Carpenter," Skip Schumaker said. "We knew there was a chance for it, just because of what he was dealing with day in and day out since Spring Training. As far as a leader, not only of the staff but of this clubhouse, you can't take a much bigger hit than that."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.