Tennis fans worldwide were stunned when top player Venus Williams abruptly dropped out of the U.S. Open this week. The reason for her sudden withdrawal was a diagnosis of Sjögren's syndrome, a little-known disease with a wide range of symptoms. Here, a brief guide to this condition:

What is Sjögren's syndrome?

Sjögren's (pronounced show-grens) syndrome is an autoimmune disease — in other words, the body's immune defenses attack healthy tissue. The disease targets the tear ducts and salivary glands, causing dryness of the eyes and mouth. In advanced cases of the disease, it can cause debilitating joint pain, fatigue, and severe rheumatoid arthritis. Other symptoms include acid reflux, sleep disturbances, and numbness in the legs and feet.

Who is most at risk for Sjögren's syndrome?

The disease affects some four million Americans — 90 percent of whom are women. Symptoms usually appear when people are in their 40s. There's no known cause, though Sjögren's syndrome seems to have a genetic component, since it and other autoimmune diseases tend to run in families.

Will Williams return to professional tennis?

Hopefully. But there's no cure for Sjögren's. "How the disease will play out for Williams is unclear," says Margaret Steele in USA Today. "Symptoms can remain mild, worsen or, occasionally, go into remission." Patients usually manage the symptoms with drops for dry eyes, anti-inflammatory drugs, and immunosuppressive agents. But let's not worry too much, suggests Dr. Robert Spiera of the Hospital for Special Surgery in New York City, as quoted by MedPage Today. "Most patients with the disorder live very productive lives."

Sources: MedPage Today, NY Times, USA Today, WebMD