The injury-plagued 31-year-old announced this week that she has an autoimmune disorder that causes dry mouth, joint pain, and fatigue.

The injury-plagued 31-year-old announced this week that she has an autoimmune disorder that causes dry mouth, joint pain, and fatigue.

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After playing just one match, Venus Williams announced yesterday that she is withdrawing from the U.S. Open. The 31-year-old has had nearly every injury in the book, but she offered an unusual reason this time around: Sjögren's syndrome, a poorly understood autoimmune disorder that causes joint pain and can deplete energy levels.

Williams had played only 11 matches this season due to injuries and illness. "I am thankful I finally have a diagnosis and am now focused on getting better and returning to the court soon," Williams said in a statement.

As many as 4 million Americans have Sjögren's syndrome (pronounced SHOW-grens), according to the Sjögren's Syndrome Foundation, making it the second most common autoimmune disease after rheumatoid arthritis, and ahead of lupus. Ninety percent of the people with the syndrome are women.

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It's "a major women's health problem," says Frederick B. Vivino, MD, associate professor of clinical medicine at the University of Pennsylvania and chief of rheumatology at Penn Presbyterian Medical Center, in Philadelphia. "A lot of patients look a lot better than they feel."

In Sjögren's, the white blood cells attack moisture-producing glands. The most common symptoms are persistent dry eyes and dry mouth, but the syndrome can lead to complications including extreme fatigue, joint pain, and problems with the kidneys, lungs, liver, pancreas, and central nervous system.

Because the symptoms are similar to those of many other conditions, Sjögren's is difficult for even the most experienced doctors to diagnose. It often takes years for a patient to receive a diagnosis after first experiencing symptoms.

Elite athletes like Williams are no more or less likely to come down with the syndrome, but they may be more likely to receive earlier an diagnosis.

"An elite athlete might notice symptoms sooner," Vivino says. "When you're running around huffing and puffing, your mouth is more likely to dry out sooner. Another important symptom—fatigue—will slow them down and affect their performance. And if they develop joint and muscle pain [that] persists, that leads to further testing to try to figure out what's wrong."

There is no known cure for Sjögren's syndrome, but symptoms can be managed with both over-the-counter products like artificial tears and sugar-free lozenges and prescription immunosuppressive drugs. The condition disappears suddenly in about 5% of cases, but the majority of people have lifelong problems.

That doesn't necessarily mean Williams is done with tennis for good.

"Most manifestations are treatable and in most cases we can restore people to healthy and productive lives," Vivino says. "Whether we can return an elite athlete to competition, that's another story, but we do feel the treatment makes a big difference."