Sen. Mark Kirk suffered a stroke and underwent surgery on Monday, after which doctors indicated his prognosis is good for recovery though he will need rehabilitation and likely the use of his left arm "will be very difficult." 

The 52-year-old Kirk checked himself into Lake Forest Hospital in Illinois on Saturday. He was transferred on Sunday to Northwestern Memorial Hospital in Chicago, where tests showed that he had suffered an ischemic stroke.

"Early this morning the senator underwent surgery to relieve swelling around his brain stemming from the stroke," a spokesperson from his office said. "The surgery was successful. Due to his young age, good health and the nature of the stroke, doctors are very confident in the senator's recovery over the weeks ahead."

Dr. Richard Fessler, a neurosurgeon who performed the surgery, told reporters that Kirk was having "dizziness and perhaps a headache," but he wasn't at the hospital where Kirk first checked in so doesn't know the exact symptoms Kirk presented when he first arrived for help. 

At some point, however, Fessler said Kirk began to "deteriorate neurologically." He said Kirk's condition evolved rapidly and "he was becoming less responsive." Fessler said he doesn't think the medical team could have done anything to stop the rapid progression.

Kirk is sedated and in intensive care, where he will spend several days. Fessler said they were removing a piece of skull approximately four inches by eight inches to allow the brain to swell, and when the swelling goes down, they will put back the bone shell. 

Fessler said Kirk's artery was completely blocked, which mean surgery was necessary despite the fact there was no bleeding. 

The stroke occurred on the right side of brain is where stroke took place, which is good Fessler said, because cognitive function is primarily on the left side. He said prospects for a full mental recovery were "very good," but a full physical recovery on his left side were "not very good"

"I believe his left side of the brain will be fine," Fessler said, adding that he is hopeful Kirk will be able to use his left leg.

"He's got a pretty good chance of living a very vibrant life," he said, adding that recovery time is "impossible to predict" but "it's not going to be days."

Kirk's stroke is similar to the type of illness that struck Senate Majority Leader Harry Reid in 2005, at the age of 66. Reid was not hospitalized and made a full recovery.

Kirk was elected to the Senate in 2010, winning the seat formerly held by President Obama. He had previously agreed to sit with Democratic Sen. Joe Manchin of West Virginia this year for the president's State of the Union Tuesday night. Last year, he crossed the aisle to sit with Sen. Dick Durbin, D-Ill., the No. 2 in the Senate.

"I was stunned to learn that Mark suffered a stroke," Durbin said in a statement. "He is young and in good physical condition and I have no doubt he will make a speedy recovery. I have reached out to his staff and offered to do anything I can to help with his Senate duties. Loretta and I will keep Mark and his family in our prayers."

Manchin said that while he will miss Kirk at the address, "I promised Mark that I will keep his seat warm."