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Several retrospective studies have found that people with dementia are involved in more accidents than age-matched control subjects, and that many drivers with dementia continue to drive despite having had crashes.^3-5 Notably, these studies do not agree on issues such as whether the duration of the dementia is an accurate predictor of driving ability, and many rely on reports by caregivers of driving history and crashes, the reliability of which are uncertain.

Recent neuropathological findings in 98 older drivers killed in traffic accidents showed that 33% had neuritic plaque scores indicating certain Alzheimer's disease (AD) and, in a further 20%, findings were suggestive of AD.⁶ This raises the possibility that more accidents are attributable to AD than previously thought. In contrast, another study of Michigan State records showed that road crash and violation rates among AD patients did not differ significantly from those of matched controls;⁷ this study did not control for mileage driven, and reduced driving exposure of AD patients may have kept their crash rate equal to that of control subjects. Drachman and Swearer investigated crash rates for patients with AD over a 10-year period.⁸ They also did not control for mileage driven, but found that, although the AD patients had fewer reported crashes than 16-24 year old drivers, they had more than twice as many in the years after the onset of their AD, than matched control subjects.

There are few reports on actual driving performance of people with dementia. Fitten and colleagues examined the performance of patients with mild AD and patients with mild vascular dementia.⁹ Compared with control groups, the groups with AD and vascular dementia had lower mean scores on the driving test and made more errors in the complex stages of the course. In addition, a retrospective analysis of crashes and driving violations for these patients was consistent with road test results. However, Hunt and colleagues found that, while 40% of drivers with mild dementia of the Alzheimer type (DAT) were unsafe, some others may drive safely. The driving competency of individuals with DAT could not be determined reliably from self report.¹⁰ Fox and colleagues found that 63% of licensed drivers diagnosed with probable AD failed a standardised on-road evaluation.¹¹ Conversely, 37% passed this evaluation, suggesting that a diagnosis of AD alone may be insufficient justification for stopping people from driving. These studies of on-road driving behaviour of patients with dementia indicate that older drivers with a range of cognitive abilities can be safely and reliably evaluated by a road test, with validity equal to that of driver licence tests.

As noted by Lipski,¹ while data increasingly show risks to individuals and the community associated with driving by people with dementia, there are few guidelines for helping doctors determine who can or cannot drive. This assessment may be further complicated if any of the new drugs currently undergoing clinical trial for AD are found to enhance performance on cognitive or driving tasks. Doctors cannot reliably predict driving competence or increased crash risk in drivers with AD on the basis of a clinical examination.¹⁰ There is also a lack of consensus about the predictive validity of neuropsychological assessment for driving competence among patients with dementia, partly because of the different neuropsychological tests employed in different studies. While the Mini Mental State Examination has been proved to significantly predict driving competence in studies of patients with dementia, its specificity and sensitivity were not sufficient for efficient prediction of driving safety.¹¹

In patients for whom unsafe driving behaviours have not been reported, an on-road assessment is currently the most valid means of determining driver competence and safety. The driving test should be standardised, designed for neurologically impaired people, include some complex traffic situations, and, ideally, should be available in both urban and rural areas. It has been suggested that patients with dementia who drive with the assistance of a passenger or "copilot" should be assessed with, and subsequently permitted to drive with, the "copilot".¹² However, several logistic and legal problems (surrounding such questions as: Who is licensed to drive? How can the presence of the "copilot" be ensured? Can the cognitive status of the "copilot" be monitored?) render this proposal impractical.

For patients whose driving tests indicate safe and competent driving, driving performance must be reviewed regu larly (e.g., six-monthly), or after a noticeable increase in dementia severity. Criteria for driving competence and licence cancellation should be discussed with the patient and family. If assessment indicates that the patient should stop driving, the patient and family should be involved in discussion of transport alternatives which may be available from family or friends, or through community transport options. Counselling of the patient and family about lifestyle changes and future planning of transportation may be critical to compliance as well as to psychological wellbeing, as driving cessation may be associated with depressive symptoms.

As a society, we need to devote more planning and resources to provision of safe, convenient and affordable transportation alternatives for those unable to drive.

Gillian K Fox
Clinical Neuropsychologist, Rehabilitation Studies Unit, University of Sydney, NSW.

Guy M Bashford
Staff Specialist, Department of Rehabilitation and Geriatrics, Illawarra Area Health Service, Warrawong, NSW

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