Updated

South Dakota Sen. Tim Johnson's health continues to improve more than one month after he underwent brain surgery for a blood-vessel complication but his recuperation could take another "several months," his office said in a statement released Friday.

"Senator Johnson is participating in approximately three hours of physical, occupational and speech therapy a day in our in-patient rehabilitation unit," said Dr. Philip Marion, George Washington University Hospital's physical rehabilitation director, according to the statement.

Until Johnson returns to the Senate, his fellow Democrats will be missing a crucial vote. The Nov. 7 elections gave Democrats slim margin over Republicans. Johnson's illness caused a razor-thin margin of 50-49 Democratic vote majority in the chamber.

Johnson "has been working with parallel bars and participating in speech therapy, including naming objects," his staff said.

His staff also said that Johnson's tracheostomy -- an airway surgically implanted in the patient's throat to assist in breathing -- was removed Thursday.

Marion indicated that Johnson's right-side mobility was an area of concern: "His program includes strengthening exercises to gain mobility on his right side." But there was no mention in the release of the senator's left-side mobility.

Since Johnson's admission to the hospital, all medical information about the senator has been distributed in the form of releases through his office.

In the statement, Johnson's wife, Barbara, sounded upbeat.

"Our family is delighted with Tim's progress. He develops more skills each day and typically exceeds the goals that others set for him. It is obvious to us that his personal goal is complete recovery and he is working hard to make that happen. We know that he continues to be supported in his efforts by the prayers of South Dakotans and many others," Barbara Johnson said.

On Dec. 13 while talking with reporters, Johnson became disoriented and was admitted to George Washington University Hospital after suffering complications from arteriovenous malformation, which causes veins and arteries to grow abnormally large, tangled, and can sometimes burst. He underwent eight hours of surgery.

Johnson was upgraded from critical to fair condition on Jan. 9.