Updated 08/13/12 – 5:57 p.m.

CHICAGO (CBS) — The Mayo Clinic in Minnesota has provided a new update on their treatment of U.S. Rep. Jesse Jackson Jr. (D-IL), revealing the congressman is being treated for bipolar disorder.

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“Following extensive evaluation, Congressman Jesse Jackson, Jr. is undergoing treatment for Bipolar II depression,” Mayo Clinic spokeswoman Traci Klein said in an email on Monday. “Bipolar II Disorder is a treatable condition that affects parts of the brain controlling emotion, thought and drive and is most likely caused by a complex set of genetic and environmental factors.”

Dr. Mark Pollack, chairman of the Psychiatry Department at Rush University Medical Center, said while Bipolar I is characterized by dramatic mood swings, Bipolar II is different.

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“People with Bipolar II Disorder refers specifically to those individuals who have recurrent depressive periods, and periods of hypomania — that is, not full manic episodes. So their mood may be a little elevated, for … relatively brief periods of time.”

WBBM Newsradio’s Lisa Fielding reports an estimated 5.7 million American adults suffer from some form of bipolar disorder, which can sometimes be a lifelong disease.

Experts say many people with bipolar disorder are able to work and function in their daily lives with the help of medication and therapy.

Dr. Benjamin Shain, a psychiatrist with NorthShore University HealthSystem, said, “With Bipolar II, the manic side never gets past a milder form of mania, called hypomania. And, typically, the depression part is the severity feature of Bipolar II Disorder.”

Jackson underwent a duodenal switch — a type of weight loss surgery — in 2004, but Shain said there’s no evidence that type of surgery can cause bipolar disorder. However, stress can be a trigger.

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The congressman has been on medical leave since June 10. His aides initially said his absence was due to “exhaustion,” then later explained he was suffering unspecified “physical and emotional ailments.”

About a month ago, his office said he was suffering an unspecified “mood disorder.” Bipolar disorder is one of many mood disorders.

Late last month, the Mayo Clinic confirmed the congressman had been undergoing treatment for depression and “gastrointestinal issues.”

No timetable has been provided for Jackson’s return to Congress or for when he will resume campaigning for re-election.

Last week, his wife, Ald. Sandi Jackson (7th) said she expects her husband to return to work after he completes his treatment at the Mayo Clinic, but she declined to speculate when.

“My hope is that he will come back. All of that is dependent on the doctors. I’m encouraged by what they’ve been saying to me thus far,” she said. “At this point, we’re taking every day one day at a time, but we here on the ground are preparing for his eventual return. We don’t know when that’s going to be, but we want his constituents to know that they’re very much on his mind. He loves them all very much, and is very grateful for their support, and he wants them to know that he doesn’t take their support for granted.”

Congressman Jackson’s aide Rick Bryant recently said he expects Jackson to return to work in a few weeks. Bryant said Jackson wanted him to call mayors in his district that stretches south of Chicago to get updates on projects in the district.

It’s not clear what the political fallout will be.

“Maybe it’s time to focus on his health, focus on his family and let the district go,” Jackson’s Republican opponent in the general election, Brian Woodworth, tells CBS 2’s Dana Kozlov. “There’s not an issue of pride here.”

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Political consultant Don Rose of Chicago says Jackson Jr.’s newly disclosed medical status should play better with voters, compared to the air of secrecy that previously surrounded the congressman’s absence.