Jan. 1, 2013 -- Cody Lehe's headache following a helmet-to-helmet hit in a 2006 high school football game was so bad that he asked his mother to take him to the doctor. His CT scan a few days later came back normal, but the decision to go to practice the day after that would change his life forever.
After taking a shoulder hit during practice, Cody, now 23, became a victim of what doctors call Second Impact Syndrome. His teammates thought he was having a seizure, but he was actually falling into a coma, with massive brain swelling and an irregular heartbeat. It's been six years since that day, and Cody lives with his parents outside Lafayette, Ind., where he struggles to recall things that happened hours earlier and can no longer walk on his own.
"I think the biggest thing is we have got to get the kids to understand to listen to their bodies," Cody's mother, Becky Lehe, told ABCNews.com. "I think he had this discussion with himself, saying, 'I feel like crap, but my CT scan is fine, so I'll play.'"
Second Impact Syndrome, or SIS, has a 90 percent mortality rate, said Dr. Michael Turner, who led a case study on Cody that was published today in the Journal of Neurosurgery. Of the 30 SIS cases in medical literature, Turner said it always happens to high-school or college-aged people. He's never seen SIS in a professional football player.
"It's probably something to do with the immature brain, because our brain really keeps growing until you're 18, 19 years old," Turner told ABCNews.com.
For years, doctors hypothesized that teens who suffered from SIS actually had blood clots from their first concussions that went unnoticed and ultimately triggered brain swelling and further bleeding after the second impact, Turner said. Cody's CT scan, which is the first to exist for an SIS patient between hits, proved the hypothesis wrong.
"We've looked at it. It was flat-out normal," Turner said. "Our patient had many tiny blood clots, but they weren't the cause of his illness."
Instead, Cody's brain sent a message to the rest of his body for more blood, which filled the veins and arteries in his head and caused his brain to swell against the skull, Turner said. The pressure caused further injury and damage.
His mother recalled the first hit, though neither she nor her husband saw it. Both players "had their bells rung," Becky said, but they kept playing. When Cody talked to Becky afterward, he told her he had a had a headache, but it was nothing he hadn't experienced before. No one thought it was a concussion because he hadn't passed out or thrown up.
The following weekend, Cody tailgated a college football game and visited a college campus with his girlfriend. By Monday, he asked to go to the doctor.
"Mr. Tough Guy never asks to go to the doctor," Becky said.
But everything was fine.
After returning to practice and taking the second hit, he walked over to the water fountain, and then fell forward into another player, Becky said. The ambulance technicians told her later that they didn't think he would make it to the hospital.
After 55 days in the intensive care unit -- his football jersey number -- Cody was transferred to rehabilitation section of the hospital, where he would stay but was not well enough to participate in any activities. He left the hospital after 98 days, having suffered temporary cardiac arrest, hypotension, pneumonia, renal failure and sepsis. He could not walk or talk.
Since then, Cody has regained the ability to speak, but still lacks short-term memory. He can walk on a treadmill for six minutes at a time with his mother or father at his side, but he can't balance on his own and depends on a wheelchair to get around.
"For someone who wasn't supposed to make it through, he's beaten all the odds," Becky said, adding that she's able to see a different side to her son that had previously been reserved only for his school friends. "He was a class clown. He was hilarious, but we didn't see that much at home."
She said now they call him their entertainer, and they're encouraged by his progress.
Cody's case shows that athletes need to get off the playing field if they've had head injuries, Turner said. The Centers for Disease Control and Prevention has released guidelines suggesting that players with signs of concussion should be removed from play and not return until they have permission from their doctor and show no symptoms with exertion.
In August 2011, 22-year-old Derek Sheely died one week after collapsing at football practice at Frostburg State University, where he was a starting fullback. His uncontrollable brain swelling led doctors and his father to believe it was SIS.
An estimated 1.7 million people receive a traumatic brain injury, which includes concussions, each year, according to the CDC.
"We have this gladiator mentality out there," Becky said. "'Be tough.' 'Hang in there.' 'Take one for the team.' We've got to change."