People with untreated sleep apnea may be less alert behind the wheel, and more vulnerable to the effects of sleep loss and alcohol, than healthy drivers, a study published Monday suggests.

The findings, say researchers, suggest that people with the nighttime breathing disorder should be especially careful about avoiding alcohol and getting enough rest before hitting the road.

Obstructive sleep apnea, or OSA, is a disorder in which the tissues at the back of the throat temporarily collapse during sleep, causing repeated stops and starts in breathing during the night. This leads to poor-quality sleep and, often, daytime drowsiness.

It's estimated that traffic accidents related to OSA cause 1,400 fatalities in the U.S. each year.

In the new study, published in the Annals of Internal Medicine, Australian researchers used driving-simulation tests to see how 38 people with untreated OSA compared with 20 healthy people in their driving skills.

They also looked at whether drivers with OSA were any more vulnerable to two factors that can impair anyone's driving: sleep deprivation and alcohol.

As a group, the study found, the drivers with OSA had more trouble staying in their driving lane and were more likely to crash during the 90-minute virtual trip than their healthy counterparts.

What's more, the OSA group's driving performance deteriorated to a greater degree when participants were tested after getting no more than 4 hours of sleep the night before.

The same was true when they took the tests after a modest amount of alcohol -- just enough to raise their blood alcohol level to 0.05, which is below the legal limit.

After the sleep-deprived night, 12 people with sleep apnea crashed during the test, while eight crashed after drinking. In contrast, none of the healthy drivers had an accident.

Under normal conditions, four drivers with sleep apnea crashed, versus one in the healthy group.

It is risky for anyone to drive when sleep-deprived or after drinking. The current findings suggest that people with OSA should be especially cautious, particularly when they will have to drive for more than an hour, according to lead researcher Andrew Vakulin, of the Adelaide Institute for Sleep Health.

And while this study included only people with untreated OSA, it is not yet clear whether people with adequately managed OSA are less vulnerable to driving impairments, Vakulin told Reuters Health in an email.

Some previous studies have shown improvements in relatively short driving tasks following OSA treatment, the researcher noted.

However, he added, it is not yet known if treating OSA improves performance and reduces vulnerability to the effects of sleep loss and alcohol during long, monotonous driving tasks. 

Vakulin said he and his colleagues are currently conducting a study aimed at answering this question.

SOURCE: Annals of Internal Medicine, October 6, 2009.