"Awareness about sudden cardiac death is critical," Fowler said, adding that 350,000 people die suddenly each year in the United States. "The people at most risk for sudden cardiac death often have the least amount of symptoms."
When detected, dilated cardiomyopathy can be treated using drugs that lower blood pressure or dilate the blood vessels. But screening tests that effectively detect so-called "silent" heart problems are limited. One reason for the dearth is the rarity of sudden death from all cardiovascular causes among young athletes. The prevalence, according to the NEJM review, is only 0.5 percent -- an infrequency that carries significant cost-benefit considerations.
Currently, most U.S. high school and college athletic programs require athletes to complete a health history questionnaire and undergo a physical exam before they can participate. But the quality of both has been the topic of scrutiny, according to the NEJM review.
"The guidelines examiners are given for screening high school athletes are inadequate in 40 percent of the states when measured against the recommendations of the American Heart Association," wrote Dr. Barry Maron, director of the Hypertrophic Cardiomyopathy Center at the Minneapolis Heart Institute Foundation. "Improvements in the screening process related to history taking and physical examination would undoubtedly result in the identification of greater numbers of athletes with previously undiagnosed but clinically relevant cardiovascular abnormalities."
Dr. Paul Thompson, director of cardiology and the athletes' heart program at Hartford Hospital in Connecticut, said it's unlikely that a routine physical exam would have picked up Leonard's condition.
"If there's a family history, it's detectable. If not, it's not," Thompson said. "We see people who felt totally fine, and then all of a sudden they get sick and their heart dilates up. It can happen very, very quickly. It comes out of the blue."
On Wednesday night, Leonard wrote on Facebook: "Got a good long shower ...ready for bed and game tomorrow!!!!!"
Last week Leonard had the flu and was recovering, his cousin Krys Leonard told ABC News.
"That's the classic story. Kid gets a bad case of the flu, they come back and they don't feel quite the same," Thompson said. "If he was screened at the start of season it might not have been detected. It can happen within two weeks."
Thompson said the flu can cause inflammation of the heart, called myocarditis, which can weaken the muscle walls leading to dilated cardiomyopathy.
Sudden death is not always the result of heart defects. Head and spine trauma, asthma and aneurysm are also reported. Last October, 26-year-old Fran Crippen died during a swimming race off the coast of Abu Dhabi, possibly due to heat stroke.
Another Fennville athlete, 14-year-old wrestler Nathaniel Hernandez, died in Jan. 2010 following a seizure.
Leonard was also quarterback for the Fennville football team, which won the Southwestern Athletic Conference North Division championship this season. Leonard threw seven touchdowns in the winning game, according to the Holland Sentinel.
Larry Gilbert, coach of one of Fennville's biggest competitors, Gobles High School, said Leonard's goal was to be a Division 1 athlete.