Those details about Kirk's illness were among many revealed by the senator's medical team, which met with reporters after his much-heralded return to the Senate.
Fessler was among hundreds of people on hand — including dozens of lawmakers, both Republicans and Democrats — to welcome Kirk, a Republican, back.
Kirk's ischemic stroke, caused by a blood clot in his right carotid artery, left the lawmaker's cognitive functions intact, Fessler said. The senator had 10 to 11 months of intensive therapy — likened to boot camp — after he was stricken.
Fessler said Kirk has normal use of his right hand and arm but not his left arm. His left leg is strong near his hip and weaker at his foot, so he wears a leg brace. Kirk's speech, while hesitant, is "vastly improved … and will continue to improve," the surgeon said.
Other medical professionals, while raving about Kirk's progress, said his rehabilitation would be a lifelong process with additional progress, sometimes spotty, ahead.
Kirk was greeted by several Illinoisans, including Democrat Dick Durbin, the state's senior U.S. senator, and Terry Gainer, the Senate sergeant-at-arms. Also in attendance were former Rep. Bob Dold, R-Ill., who had Kirk's old House seat for two years and left office Thursday, and incoming Rep. Tammy Duckworth, D-Ill., a double amputee from the Iraq War who called it a "fantastic day" for people with disabilities and for the nation.
They joined hundreds of other well-wishers who endured temperatures in the 30s while applauding Kirk.
Vice President Joe Biden, addressing Kirk as he began the climb, remarked: "You got all day, pal. It took me seven months to make these steps." The vice president in 1988 was absent from the Senate for months because of surgeries for brain aneurysms.
Biden grasped Kirk's right upper arm, and Sen. Joe Manchin, D-W.Va, on the left, kept his hand around Kirk's waist to assist and guide him on the way up.
Kirk paused at times, giving a hearty wave or a thumbs-up to the crowd. He got a hug and kiss from Sen. Kay Hagan, D-N.C.
"Bravo," he was cheered when he alighted the last step, and was met by Senate Minority Leader Mitch McConnell, R-Ky., and Senate Majority Leader Harry Reid, D-Nev.
"Nice to see you guys," Kirk told reporters when he entered the Capitol en route to the Senate chamber. When a reporter asked him what it was like to be back, he had two words: "Feels great."
Kirk took his regular desk on the Senate floor and, while seated, enthusiastically greeted colleagues. McConnell, addressing the chamber where new members were sworn in Thursday, had a special welcome for the first-term senator.
"The fact that Mark is here today says a lot about his tenacity, his dedication and his commitment to the people of Illinois," McConnell said.
Reid, likewise, paid tribute to Kirk from the Senate floor, saying everyone was grateful for his recovery and proud. "Today, on the east front of the Capitol, to see him walk up those steps … said it all."
Durbin issued a statement, saying: "It's a historic comeback for a senator who has worked hard to come back and show that those who have suffered strokes can survive and prosper and return to work. It's also evidence that a lot of us, regardless of party affiliation, can come together to show the human side of politics."
Kirk, from Highland Park, is up for re-election in 2016. He will continue to undergo rehabilitation in Washington, where he has new, handicapped-accessible living quarters on Capitol Hill, said spokesman Lance Trover.
Fessler, the neurosurgeon, said in an interview that he had three "very simple jobs" after Kirk was stricken: to keep him alive, to prevent additional damage to his brain after the stroke and to prevent any major complications during his hospitalization.
In Kirk's first surgery, a section of his skull was removed to accommodate brain swelling. Soon thereafter Kirk underwent a second operation to remove more of his skull as well as dead brain tissue. In a third surgery, the excised portions of his skull were replaced.
Fessler, looking back, said that if six more hours had passed before he and others began the second procedure, Kirk "might have experienced a much worse outcome" than he did.
Kirk's return came on a day when Duckworth, from Hoffman Estates, and five other freshmen were sworn in as House members for the 113th Congress. The others are Democratic Reps. Bill Foster of Naperville, Brad Schneider of Deerfield, Cheri Bustos of East Moline, Bill Enyart of Belleville and Republican Rep. Rodney Davis of Taylorville. Foster is a returning member of Congress after serving from 2008 until 2011.
Tribune reporter Patrick Svitek contributed.