One of the thorniest decisions facing older adults is when to give up their keys and stop driving. A new study by researchers at Washington University School of Medicine in St. Louis could provide guidance in helping seniors plan ahead. The researchers found that impaired cognitive function foreshadows the decision for many seniors to stop driving -- more so than age or molecular signs of Alzheimer's disease. Even very slight cognitive changes are a sign that retirement from driving is imminent. Further, women are more likely to stop driving than men, the study showed.

The findings suggest that routine cognitive testing -- in particular, the kind of screening designed to pick up the earliest, most subtle decline -- could help older adults and their physicians make decisions about driving to maximize safety while preserving independence as long as possible.

"Many older drivers are aware of changes occurring as they age, including subjective cognitive decline," said corresponding author Ganesh M. Babulal, PhD, OTD, an associate professor of neurology. "Doctors should discuss such changes with their older patients. If risk is identified early, there is more time to support the remaining capacity and skills, extending the time they can drive safely, and to plan for a transition to alternative transportation options to maintain their independence when the time comes to stop driving."

The study is published May 22 online in Neurology, the medical journal of the American Academy of Neurology.

Adults over age 65 are the most careful drivers on the road. They are less likely than drivers in any other age group to speed or to drive in bad weather, at night or under the influence of substances. Despite these precautions, age-related changes such as slower reaction time, impaired vision and cognitive decline still put older drivers at risk of crashes, and when such crashes happen, older drivers are more likely to be killed or seriously injured than younger drivers are. At the same time, giving up driving is not without its own risks. People who stop driving are more likely to develop depression and become isolated.

The American Academy of Neurology concluded in 2010 that cognitive impairment, as measured by a score greater than zero on the Clinical Dementia Rating (CDR) scale, was the best predictor of stopping driving. The CDR scale goes from zero, indicating normal cognitive function, to three, indicating severe dementia. But the CDR, which was developed at Washington University in 1982, is designed to detect impairments significant enough to affect daily life. Studies have shown that a person's cognitive skills can deteriorate for years before a CDR score indicates trouble.

Babulal and colleagues set out to determine the role of other factors, including subtle cognitive changes, in the decision to step away from the wheel. They studied 283 people with an average age of 72 who drove at least once a week and who had no cognitive impairments at the start of the study. The researchers were primarily interested in determining when and why each participant stopped driving.

The participants underwent cognitive tests at the start and then every year for an average of 5.6 years. The cognitive testing included the CDR and a preclinical Alzheimer's cognitive composite (PACC) score, which is designed to detect subtle cognitive changes in people who score as unimpaired on the CDR. The participants also underwent brain scans and donated cerebrospinal fluid at the start of the study and then every two to three years, so the researchers could look for molecular signs of Alzheimer's disease. At baseline, about one-third of the people met the criteria for preclinical Alzheimer's disease based on levels of biomarkers for the disease -- amyloid plaques and tau tangles -- in the brain and cerebrospinal fluid.

During the study, 24 people stopped driving, 15 people died, and 46 people developed cognitive impairment as measured by a CDR score greater than zero.

Analysis showed that three factors predicted who would stop driving during the study: cognitive impairment, worsening PACC scores, and being a woman. People who met the criteria for cognitive impairment by scoring 0.5 or greater on the CDR were 3.5 times more likely to stop driving than were those who remained at zero, and people with lower scores on the PACC were 30% more likely to stop driving than were those with higher scores. Age and the presence of biomarkers of Alzheimer's disease were not tied to the decision to stop driving.

The biggest effect was seen regarding gender, with women four times more likely to stop driving during the course of the study than men were.

"We know from past studies that there isn't a difference in driving ability between men and women," Babulal said. "What we have shown in prior work is that women are often more aware of their abilities, are more willing to admit that they are no longer able to safely drive, and plan more in advance to transition out of driving compared to their male counterparts. It is highly recommended that older male drivers talk with their providers about driving and consider stopping driving earlier."

Doctors do not routinely counsel older patients on driving cessation, a fact that Babulal sees as a missed opportunity to promote healthy aging.

"There are things we can do to help people adapt to age-related changes," Babulal said. "Driver rehabilitation programs, often led by occupational therapists, can provide specialized training and strategies for older drivers to adjust to physical and cognitive changes to maintain driving capacity. Community support programs provide a forum for older adults to share experiences and learn from each other about safe driving practices and alternative transportation options. Ultimately, most people will need to stop driving, but by starting the conversation early, we can better support older adults' independence and quality of life."