Scott ThompsonCollege student Scott Thompson was kicking a soccer ball in the school gym when his head started playing tricks on him.

“I felt a click and a cool rush sensation,” he said. “It felt like somebody was pouring cool water on my head.”

He stopped, thinking a drink of water would help. But he soon couldn’t move his left hand or talk. The 22-year-old was having a stroke.

“At the time I didn’t know exactly what a stroke was or how it affects people, so I wasn’t knowledgeable on what was happening,” said Thompson, who lives in Rochester, New York.

Strokes are the No. 5 killer in the U.S. and are most often the result of a clot that blocks blood flow to the brain, called an ischemic stroke. That clot may have been caused by plaque buildup in one of the neck arteries or an irregular heartbeat called atrial fibrillation that allowed blood to pool in the heart, forming a clot that traveled to the brain.

But in about 30 percent of cases, there is no clear cause. Those ischemic strokes fall into a mysterious bucket called cryptogenic.

That was initially the case for Thompson in November 2014. He was rushed to the hospital and had a procedure to remove the clot. Tests to determine the stroke’s cause were inconclusive.

“There was lots of head-scratching,” Thompson said. “The doctors were doing everything they could to find the cause, but it wasn’t immediately clear what was wrong.”

A few months later he received an insertable cardiac monitor – a device implanted under the skin that is one-third the size of an AAA battery. It has two electrodes to monitor the heart’s rhythm and stores a log of any irregularities. A recent study found the device is much more effective than standard monitoring in detecting AFib as the cause of a cryptogenic stroke.

Thompson_rehab2For Thompson, it did the job 1,500 hours and 90,000 beats later, detecting a faint abnormal heartbeat. AFib that is left untreated can increase the risk of stroke five-fold, but it often goes undiagnosed. It prompted Thompson’s cardiologist to change his blood-thinning medication from aspirin to the more powerful Coumadin (warfarin) to help prevent a second stroke.

Thompson’s college graduation plans were delayed as he worked to regain use of his left side.

About 200,000 strokes in the U.S. each year have no known cause, according to the latest statistics.

Joining Thompson in that group is Kaela Gedda, who was 19 when she had a stroke. It was followed by a mini-stroke more than three years later. AFib hasn’t been ruled out, and she hopes the ICM she got at the end of 2014 gives her answers.

Gedda, 26, thinks genetics might offer some clues.

“My maternal grandmother had numerous blood clots and a pulmonary aneurysm, and my mother has battled heart disease for much of my life,” said Gedda, who works for the American Heart Association in Madison, Wisconsin. “While I have seen numerous hematologists, neurologists, cardiologists and other doctors, I don’t seem to match any diagnosis.”

Back in New York, Thompson, now 24, will graduate from the Rochester Institute of Technology on Saturday. He starts a job in Redmond, Washington, in July, but it won’t be a direct cross-country trip – he’s planning stops at Montana’s Glacier National Park and the Alamo in San Antonio.

“My stroke showed me I need to slow down,” he said. “You can’t set everything up perfectly. It’s more important to enjoy what you can.”