Illinois Sen. Mark Kirk underwent successful surgery on Monday after having suffered a stroke on Saturday night, his office announced Monday.

Kirk underwent surgery to remove a 4 inch by 8 inch piece of his skull to relieve swelling in the brain, Dr. John Kessler of Northwestern Memorial Hospital told reporters late Monday morning in Chicago.

The 52-year-old senator suffered his stroke on the right side of his brain, an area that threatens Kirk’s physical function on the left side of his body. Kessler said that Kirk’s future use of his left arm will be “very difficult,” and that the stroke could mean difficulty in his left leg, and in the left side of his face.

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