Aug. 18, 2000 — Maybe this has happened to you: You go into the ophthalmologist’s office for a checkup, and about 45 minutes later, walk out with a new prescription for glasses and hopelessly dilated pupils. After donning a pair of dark glasses to ward off the sun’s cruelly bright rays, you get behind the wheel of your car — without thinking twice about it– and drive home.

Should you be more cautious about driving with dilated pupils? Should you arrange for a friend or spouse to take you home? Should your doctor have warned you against driving? An eye-opening study, published in Eye, The Scientific Journal of the Royal College of Ophthalmologists, examines exactly how much, and in what ways, driving is affected by dilated pupils. It found that for some people, in some situations, it may be safe to drive while dilated.

The main reason to have the pupils dilated is so the eye doctor can better see the nerves and blood vessels in the back of the eye, Rohit Varma, MD, tells WebMD. “It is by evaluating both that one can tell whether or not an individual has blinding eye diseases, such as tumors in the eye, diabetes in the eye, or glaucoma,” says Varma, an associate professor of ophthalmology at the Keck School of Medicine at University of Southern California, in Los Angeles. Varma was not involved in the study.

When dilated, the pupil is unable to get smaller, and thus block out light in bright areas, explains Varma. This causes glare in your vision “so you can’t [see] things clearly,” he says. The drops also make it difficult to focus on things up close, Varma says, but usually don’t affect distance vision. So do the drops affect driving?

The researchers, led by Theo Potamitis, recruited 12 healthy volunteers to test several aspects of vision while in a driving simulator, both before and after having their pupils dilated. Potamitis is from the Academic Unit of Ophthalmology at the Birmingham and Midland Eye Center (U.K.).

The researchers found that, while dilating the pupils did cause vision to deteriorate in at least half of the patients, the changes were, for the most part, not severe enough to make a big difference. And performance on the driving simulator did not appear to be affected by pupil dilation.

This could be due to the fact that over half of the drivers appeared to compensate for the deterioration in their vision by adjusting their driving, including reducing their speed, which improved steering accuracy.

“They did the same on the driving test, which, in essence, is what is important,” says Varma. “I mean, who cares about the vision, if ultimately they are doing the tasks as accurately as they were doing them before? That is what’s important.”

“The quality of the vision may not be quite as sharp” after dilation, Andrew Iwach, MD, tells WebMD. “The issue is, will you see the dog? Will you see the child run out? Those types of large objects you should be able to see.” Iwach, an assistant clinical professor of ophthalmology at the University of California, San Francisco, is also a spokesperson for the American Academy of Ophthalmology.

The study’s authors conclude that, while deterioration of vision and driving ability does not necessarily occur when dilating drops are used in young and healthy adults, the drops could have an important impact in older people.

“The bottom line is ‘safety first,’ and common sense usually gets you through things,” Iwach says.

“What most people would like to know is, can I drive or can I not drive? And it is not quite that simple to answer that question,” he says. “The reason is the effect of dilation will be different … depending on the [vision] of that individual patient.” And an older patient might have glaucoma or cataracts, two common conditions that could compound the effects of the dilation drops.

Other factors, besides a person’s eye health, that Iwach says are important include: driving conditions (time of day, amount of traffic, weather, and distance), how the drops affect the individual, what kinds of tests were performed during the exam (which can also lead to temporary problems), and how comfortable a patient is.

“The reality is, there are multiple factors that come into play,” says Iwach. “In most cases – certainly in healthy patients — they are safe [driving], but when in doubt, it is safest to have someone else do the driving.

“The ultimate responsibility of the decision has to lie with that patient, because they have the perception of where they are.”