People with a history of fainting spells or blackouts may be more likely to get into car crashes than the typical driver, a Danish study suggests.

Researchers focused on people who experience syncope - a sudden loss of consciousness unrelated to a head injury - and found that affected individuals had almost twice the risk of motor vehicle accidents compared with the general population.

"This risk was small in absolute terms, yet raises important questions about policies toward driving," said lead study author Dr. Anna-Karin Nume of Copenhagen University Gentofte Hospital in Denmark.

"This is one particular instance of drivers who develop medical conditions that could affect their driving ability, and there are tradeoffs between restrictions on driving and the ability of the patients to work, shop, etc.," Nume added by email.

Roughly one in three people will experience syncope at least once in their lifetime, and about one-third of patients who have one episode will experience another one within three years of the first event, Nume and colleagues report in JAMA Internal Medicine.

Syncope may be caused by a variety of circumstances including dehydration, overheating, emotional stress, pooling of blood in the legs, or exhaustion, as well as by medical conditions that affect the nervous system or the heart.

To assess how syncope influenced the odds of car crashes, researchers examined data on almost 4.3 million Danish adults from 2008 through 2012, including about 41,000 individuals with a syncope diagnosis.

About one-third of the patients with syncope had cardiovascular disease, and half of them were at least 66 years old.

Researchers followed half of the syncope patients for at least two years and found 1,791 of them had a motor vehicle crash during the study.

Roughly four in five crashes resulted in injuries; just six of the accidents were fatal.

After adjusting for age, sex, socioeconomic status, other medical conditions and drug regimens, people with syncope were 83 percent more likely to have a crash, the study found. The odds of crashes were higher for men with syncope than for women.

The five-year crash risk for people with a history of syncope was 8.2 percent among the population aged 18 to 69 years old, compared with 5.1 percent in the general population.

One shortcoming of the study is its focus on observational data, which made it impossible for researchers to say whether syncope directly caused crashes or what types of circumstances contributed to episodes of fainting or blackouts, the authors note. Researchers also lacked crash data on alcohol or drug use, seat belts or road conditions.

Even so, the findings provide a reminder to clinicians to consider traffic safety when managing patients with syncope, Dr. Donald Redelmeier of the University of Toronto and Sunnybrook Health Sciences Center wrote in an accompanying editorial.

While road rules for people with syncope vary, eight U.S. states require doctors to notify vehicle licensing authorities of episodes involving a loss of consciousness, Redelmeier said by email.

"Each case will be different, depending on the specific medical diagnosis," Redelmeier added. "A uniform policy of license suspension, therefore, would be overly draconian and counterproductive."

Other things like young or old age, drowsiness, speeding, drinking or texting contribute to crashes and are also bigger causes of crashes than syncope, said Carol Chen-Scarabelli, a researcher at the Birmingham VA Medical Center in Alabama who wasn't involved in the study.

"Despite the concern of syncope-related driving accidents, there are many other conditions or factors which contribute to motor vehicle accidents to a greater degree," Chen-Scarabelli said by email.

That's because with syncope, drivers often have several seconds or minutes of symptoms like dizziness or heart palpitations before losing consciousness, said Dr. Dan Sorajja, a researcher at Mayo Clinic in Arizona who wasn't involved in the study.

"This duration usually gives people enough time to pull over and avoid an accident," Sorajja said by email. "There are a subset of people who have no warning prior to losing consciousness, and these persons may be at increased risk of accidents."

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