For all those moms who have ever shouted, "Go outside and play!" at teens who sit for hours in dim rooms in front of the computer or TV, here's some more ammunition:

Blood clots are afflicting chronic computer users who sit immobile for too long.

Researchers say the malady is essentially the same as "economy class syndrome," and the story has the same moral: Exercise your limbs or risk suffering the effects of deep vein thrombosis.

In the first known case of what New Zealand researchers have dubbed "eThrombosis," a 32-year-old man who spent up to 18 hours at his computer suffered a massive blood clot that caused him to black out.

"(The blood clot) broke off and traveled to his lung, which obstructed the blood flow into his lung, which made him very sick, obviously," said Richard Beasley, a professor at the Medical Research Institute of New Zealand, who co-authored a paper on the incident in the European Respiratory Journal. "He collapsed unconscious, but came round."

The only cause of the clot the researchers could find was the man's habit of sitting in front of the computer without moving for long periods of time.

Stephan Moll, director of the thrombophilia program at the University of North Carolina at Chapel Hill, is following two similar cases in the United States.

"It's just immobility, which we know is a risk factor for blood clots," Moll said.

Like air travelers, computer users should do leg and foot exercises while sitting, drink plenty of water, and get up and walk around whenever possible.

The syndrome has appeared throughout history. It was first recognized during the London blitz in World War II. "People sleeping in deck chairs in air raid shelters had a very high incidence of blood clots that were fatal," Beasley said.

In the 1950s, prolonged car trips, airplane flights and even attending the theater were linked to deep vein thrombosis, he said. These new cases are essentially a known problem perpetuated by the prevalence of computers in many people's lives.

Since the New Zealand case was reported, researchers have perused their records looking for similar cases that may not have been linked to computer use at the time.

The New Zealand man suffered a swollen and painful calf six weeks before collapsing -- a typical scenario, Moll said. Initial symptoms, which can include diffuse pain and swelling in the leg or a bruise-like spot, are often incorrectly identified as muscle cramps or a twisted ankle.

If a part of the clot breaks off and travels to the lung, a phenomenon called pulmonary embolism occurs. Symptoms include shortness of breath, chest pain and, sometimes, phlegm tinged with blood.

"It's (also) so often misdiagnosed because people think it's a touch of pneumonia, bronchitis or maybe asthma," Moll said. "Even when they go to the doctor, often the correct diagnosis is not made."

It's not clear, however, whether sitting in front of the computer alone is enough to cause a blood clot. Obesity, smoking, taking birth control pills or hormone replacement therapy can all increase the risk of thrombosis.

The problem also runs in families. About 5 percent of Americans also have a disorder called factor V Leiden, which also increases their chances of developing a blood clot.

"Anybody who sits for a prolonged period of time in any position should get up every so often and stretch the legs," Moll said.