Mary Beth Wighton of Southampton, Ont., remembers the day four years ago when her doctor delivered her a devastating one-two combination of bad news. 

"She said, 'I am sorry to tell you but you have probable frontotemporal dementia.' She explained what it was and then she said, 'and there is another thing that I need to do immediately, which is to revoke your driver's licence ... effective immediately.'" 

Wighton, who was then 44, was still in the early stages of dementia.

But research is increasingly showing that even mild cognitive impairments (MCI), short of dementia, have the potential to be a problem on the road.

Megan Hird, a researcher at St. Michael's Hospital in Toronto, carried out tests on 22 patients with MCI and 17 healthy individuals, using driving simulators and brain scans.

"The results of our study showed that patients with mild cognitive impairment ... exhibited increased risky driving errors, such as collisions and lane deviations, compared to cognitively healthy drivers," she told CBC News.

"This was particularly the case during more cognitively demanding aspects of driving, such as left-hand turns at a busy intersection."  

3 times more driving errors

Her study, presented Thursday at the Alzheimer's Association International Conference in Toronto, suggests that drivers with mild impairment in their cognitive ability — but who haven't been diagnosed with Alzheimer's disease or any other kind of dementia — were much more likely to commit major driving mistakes than healthy drivers. In some cases, the error rate was triple. 

While people with MCI are at an increased risk of later developing Alzheimer's disease or another form of dementia, their current impairments in daily behaviour are more subtle — so people with MCI often continue to carry out all of their routine daily activities, like working, managing their finances, cooking ... and driving.

The problem, researchers say, is that driving is an especially complex task, involving attention, memory, executive functioning and the processing of visual information about where objects are. 

Currently, doctors are required in many provinces to report the medical condition of someone they think shouldn't be driving to the relevant transportation ministry. But if the cognitive deficits are minor, that may not happen.  

Hird says many doctors don't feel comfortable assessing the driving fitness of their patients. "It's a very difficult conversation to have with someone," she acknowledges.

Senior drivers 

Most provinces require senior drivers, usually those 75 or 80 or over, to undergo some kind of vision and/or written test every two years to assess their driving ability. Depending on the outcome of that initial assessment, a road test may be required.

While driving is not a right, losing one's licence can be a major life-changer. For instance, someone with a mild cognitive impairment or in the early stages of Alzheimer's disease may not qualify for door-to-door municipal transit services, because they're not recognized as disabled.

Screening tools

The Canadian Medical Association publishes a "driver's guide" for its members that doctors can use to detect and assess health conditions that can affect their patients' ability to drive, including dementia and mild cognitive impairment.

Hird says the ultimate goal of her research is to help develop tools that doctors can use to screen people who may be at risk, because she says there are currently "no valid tools" to help them assess the driving fitness of patients with mild cognitive impairment.

"Because [driving] is such an important source of independence, you don't want to be taking away someone's licence when they are still able to drive safely," Hird says. 

"We need to achieve a balance between maintaining patient independence [and] the safety of the driver and the general public."

With files from the CBC's Kas Roussy and Melanie Glanz