The neurosurgeon treating Mark Kirk said in a statement Thursday that the U.S. senator has undergone a second surgery to relieve pressure on his brain. Kirk suffered a stroke last weekend and remains in “serious but stable condition.”

Doctors Wednesday night removed “two small pieces of tissue” from Kirk’s brain that were already destroyed by the stroke, according to the statement from Dr. Richard Fessler of Northwestern Memorial Hospital. This was done to “accommodate the expected peaking of swelling.”

Fessler’s statement called the procedure “common,” but other experts wouldn’t phrase it like that.

“It’s not very common, but it can happen,” said Dr. Fadi Charbel, chief of neurosurgery at the University of Illinois. “And, clearly, I think the idea is to stay ahead of the curve here and not to let the pressure go up for a long period of time.”

Previously, a 4-inch-by-8-inch portion of Kirk’s skull was removed, also to relieve pressure from swelling. Fessler earlier this week said damage to the right side of Kirk’s brain could result in some paralysis and possibly memory problems.

Fessler’s statement said the senator on Thursday morning gave a “thumbs up” to doctors when asked to do so, a sign Dr. Charbel calls that “very encouraging.”