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a Division of Clinical Epidemiology, Hennepin County Medical Center, Minneapolis, Minnesota
b Division of Allergy and Infectious Diseases, School of Medicine, University of Washington, Seattle
c Division of Epidemiology, School of Public Health, University of Minnesota, Minneapolis
d Department of Epidemiology, Graduate School of Public Health, University of Pittsburgh, Pennsylvania
e Center for Chronic Disease Outcomes Research, Minneapolis VA Medical Center, Minnesota
Karen L. Margolis, Division of Clinical Epidemiology, Hennepin County Medical Center (865B), 701 Park Avenue South, Minneapolis, MN 55415 E-mail: margo006{at}tc.umn.edu.
| Abstract |
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Methods. Subjects were 1416 women aged 65 to 84 enrolled in the Portland, Ore. site of the Study of Osteoporotic Fractures. Motor vehicle crash information for the years 19861995 for each participant was obtained from the Oregon State Department of Transportation. Items from questionnaires, interviews, and physical examinations were tested prospectively for associations with the occurrence of motor vehicle crashes.
Results. About one third of participants (415 of 1416) had a motor vehicle crash during a mean follow-up time of 5.7 years. After adjustment for age and weekly driving mileage, risk factors significantly associated with motor vehicle crashes were a fall in the previous year [hazard ratio (HR) 1.53, 95% confidence interval (CI) 1.261.86], a greater orthostatic systolic blood pressure drop (HR 1.11 per 12.5 mm Hg, 95% CI 1.011.22), and increased foot reaction time (HR 1.10 per 0.06 second, 95% CI 1.001.22). Other neuromuscular tests, functional status, medical diagnoses, vision tests, and cognitive tests did not predict motor vehicle crashes in this study population.
Conclusions. This prospective study with extended follow-up of a large cohort of elderly women has identified crash risk factors that can be measured in the clinical setting. Further study is needed to determine if interventions aimed at these risk factors can decrease the risk of motor vehicle crashes.
BY the year 2020, it is estimated that there will be 50 million elderly persons eligible to drive in the United States (1). Motor vehicle crash rates adjusted for miles driven are higher for elderly drivers, with an exponential increase above age 75 (1). A similar pattern is observed with the driver fatality rate (1). Identifying elderly drivers who are at high risk for automobile crashes may help direct interventions to reduce the high rate of injuries and deaths.
The health status of older drivers has been the central focus of previous studies on motor vehicle crashes among the elderly. Age-related changes in processing of sensory input, cognitive capacity, and neuromuscular function may all affect an elderly driver's ability to operate a motor vehicle safely. Reduced visual acuity, poor contrast sensitivity, glaucoma, cataracts, and hearing impairment have been related inconsistently to motor vehicle crashes (2)(3)(4)(5)(6)(7)(8). The Useful Field of View, a test of visual processing speed, has a stronger and more consistent relationship with motor vehicle crashes (6)(8)(9). Some studies have shown associations between cognitive impairment (6)(10) or depression (10)(11), and the risk of automobile crashes, but results have been inconsistent (6)(8)(9)(10)(11). Associations between the use of medications with central nervous system effects, medical conditions, and functional impairments and motor vehicle crashes are also controversial (6)(10)(12)(13)(14).
Most previous studies have been limited by cross-sectional or retrospective study designs, small sample sizes, and limited information on potential risk factors. The Study of Osteoporotic Fractures (SOF) overcomes these limitations with its prospective design, detailed clinical examinations, extended follow-up, and large sample of elderly community-dwelling women. The purpose of this study was to describe the motor vehicle crash risk over a 10-year period and to identify risk factors for motor vehicle crashes in elderly women.
| Methods |
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Data Collection
Data from the baseline exam (19861988), Visit 2 (1988 1989), and Visit 4 (19921994) are included in this analysis. Unless noted otherwise, all data used in this analysis were collected at the baseline visit. Driving exposure was measured at Visit 2 and Visit 4 by asking participants if they had driven a car in the past 12 months, and if so, on average, how many miles per week they had driven.
At baseline, the participants were asked the number of falls they had experienced in the previous year. Physical activity was measured using a modified Paffenbarger questionnaire, whereby an estimate of total kilocalories expended per week in recreational activities was calculated (16)(17). Participants were also asked, "Do you walk for exercise?" and "At least once per week, do you engage in any regular activity long enough to work up a sweat?"
Functional status was assessed by self-report of any difficulty performing each of six activities of daily living: walking two to three blocks on level ground, climbing up 10 steps without stopping, walking down 10 steps, preparing meals, doing heavy housework, and shopping for groceries or clothes (18)(19). At Visit 2, participants completed the Geriatric Depression Scale (20). The presence of depression was defined as a positive response to six or more symptoms (21). Participants were asked whether they had ever been diagnosed by a physician with diabetes, stroke, osteoarthritis, or cataracts. Use of medications to aid sleep in the past 12 months was assessed, and the names of the medications were recorded. Participants were asked to report the average number of alcoholic drinks they consumed per week during the past year.
Blood pressure was measured both in the supine position after a 5-minute rest period and after standing for 1 minute. Neuromuscular function tests included ability to rise from a chair five times without using the arms, number of step-ups completed in 10 seconds, average of right and left grip strength using a hand-held dynamometer (Preston Grip Dynamometer, Takei Kikikogyo, Tokyo, Japan), walking speed over a 6-meter course, time to complete a walk in a tandem position, and the number of seconds to complete 10 taps with the dominant foot. At Visit 2, dominant hand and foot reaction times were measured by depressing a pad in response to a light cue. The results of 10 trials were averaged.
Corrected visual acuity was measured by the letter charts using the Bailey and Lovie method (22), and acuity was examined as both a dichotomous "impaired acuity" variable (visual acuity of 20/40 or worse) and as a continuous variable [log minimum angle of resolution (logMAR) (23)]. Contrast sensitivity was measured using the Vistech Vision Contrast Test System 6500 (24) at 10 feet for spatial frequencies ranging from 1.5 to 18 cycles per degree. The scores for low and high spatial frequencies were averaged separately (25). Near depth perception was measured with the Randot Stereotest Contoured Circles test (26). Distance depth perception was measured using the Howard Dolman apparatus and was scored as the standard deviation of four trials (27). The Mini-Mental Status Examination (28) was administered at Visit 1, and the Trail Making Part B (29) and Digit Symbol Substitution (30) tests were administered at Visit 2 to assess cognitive function.
All participants were asked to provide their drivers' license numbers during Visit 2. Motor vehicle crash information for the years 19861995 for each participant was obtained from the Driver and Motor Vehicle Services, division of the Oregon State Department of Transportation. The date of any crash that resulted in a police report was recorded. Participants who had any motor vehicle accident from enrollment in SOF until the end of July 1995 were considered to have the outcome of interest.
Data Analysis
Bivariable relationships were examined with chi-square and t tests. Alcohol consumption was highly skewed, with a large proportion of nondrinkers. Therefore, drinks per week were categorized into five levels, with the first level representing those who reported never drinking, the second level representing former drinkers, and the next three levels representing the remaining participants divided into tertiles based on reported drinks per week. Cox proportional hazards regression was utilized for multivariable analyses. The hazard ratio was determined before and after adjustment both for age in 5-year increments and for weekly miles driven in 50-mile increments. For all other continuous variables, it was expressed per one standard deviation. Independent variables that had a relationship at a significance level of p < .10 with the outcome variable after adjustment for age and miles driven per week were included in a multivariable model. We tested for interactions between variables that entered the final multivariable model by adding an interaction term to the model.
There were 772 participants who reported that they had continued driving through Visit 4 and had no record of a motor vehicle accident before July 31, 1995 (the latest date for which motor vehicle information was available). These individuals were considered to be at risk from their date of enrollment until July 31, 1995. Persons who had a motor vehicle accident (n = 415) were considered to be at risk from their time of enrollment until the date of their first accident. Participants who reported that they had stopped driving between visits 2 and 4 (n = 89) were censored halfway between their Visit 2 and Visit 4 dates, as the specific date on which they stopped driving was not available. Nonattenders at Visit 4 (n = 51) were censored halfway between their Visit 2 dates and the midway point of Visit 4 (July 31, 1993), as information regarding whether they were still driving was not available. Participants who died at any time after Visit 2 (n = 89) were censored on their date of death.
| Results |
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| Discussion |
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Three studies have examined the association between a history of falls and motor vehicle crashes, but results were inconsistent (6)(11)(31). In the case-control study by Sims and colleagues (6), older drivers with at least one at-fault crash in the previous 6 years and a history of a fall in the previous 2 years had more than twice the odds of a motor vehicle crash. In a later prospective study, the same authors (11) also found a doubled motor vehicle crash risk in people who reported frequent falling or tripping. Marottoli and coworkers (32) found that people who reported not walking had a two-fold increased risk of motor vehicle crashes, but did not report on whether any association with falls was found. Our data support the limited evidence that falls are associated with motor vehicle crashes and further suggest that the motor vehicle crash risk in women who fall is confined to those who are sedentary.
The mechanism of the association of falls and motor vehicle crashes may be mediated by the association of falls with other risk factors. In the SOF study, falls are associated with increasing age, poorer health status, the presence of one or more medical conditions, not walking for exercise, functional impairment, trouble with dizziness, depression, weight loss, slow gait speed, inability to stand from a chair, poor visual acuity, benzodiazepine use, anticonvulsant use, and antidepressant use (33). Although most of these variables were not individually associated with motor vehicle crashes in our study sample, we speculate that falling may be a marker for elderly women in whom these risk factors combine to produce impairments that also impede driving ability.
Our study also found a weak association between orthostatic systolic blood pressure change and motor vehicle crashes. No association was reported in two other studies, in which blood pressure drop upon standing was modeled as a categorical variable (6)(11). We did not find a significant association between systolic blood pressure drop of 20 mm Hg or more and motor vehicle crashes, a characteristic found in 22% of the participants, nor was self-reported dizziness upon standing associated with motor vehicle crashes (data not shown). This study is the first to examine the association of reaction time with motor vehicle crashes. Slower foot reaction time could be predicted to interfere with removing the foot from the accelerator or applying it to the brake pedal, but these results need confirmation.
We did not find an association between other neuromuscular tests or functional status and motor vehicle crashes. This is in general agreement with previous studies (6)(11)(14)(32). Most previous studies have not found an association of cognitive impairment with motor vehicle crashes when standardized screening instruments have been used (9)(11)(32)(34). However, the women in SOF were fairly healthy and cognitively intact when they entered the study; therefore, the lack of association of these variables with motor vehicle crashes is not unexpected.
Our data are consistent with the majority of other studies that have found no association of diabetes (6)(11)(31)(34) and osteoarthritis (6)(11)(31)(34)(35) with motor vehicle crashes. The evidence for stroke as a risk factor for motor vehicle crashes is less consistent, with two positive studies (11)(34) and two negative studies (6)(31). Women in SOF were not asked if they had heart disease until Visit 4, so we were unable to investigate heart disease as a risk factor for motor vehicle crashes. However, one cross-sectional study (35) and two case-control studies (31)(34) have found an age-independent association between heart disease and motor vehicle accidents. Our findings are in accord with other studies that have found no association of visual acuity, contrast sensitivity, or ocular disease with motor vehicle crashes (2)(7)(9)(11)(32).
Previous studies have had inconsistent findings with regard to depression as a motor vehicle crash risk factor, with two studies reporting an association (11)(14) and three studies reporting no association (6)(32)(34). Only 3.5% of the women in SOF scored high on the Geriatric Depression Scale; therefore, our study's power to find an association of depression with motor vehicle crashes is limited.
Although three other prospective studies found that the use of benzodiazepines was not associated with motor vehicle crashes (6)(11)(34), these findings are contradicted by several case-control studies with much greater power due to the large number of subjects studied (36)(37). Two other studies of older drivers found no association of alcohol consumption with motor vehicle crashes. If the association between very light drinking and motor vehicle crashes is due to physically frail individuals giving up drinking, former drinkers should have an elevated motor vehicle crash risk compared to life-long abstainers, which we did not find. It is possible that the finding is due to chance, given the number of associations examined.
A major methodological issue in research on motor vehicle crashes in the elderly is the measurement of driving exposure. This is particularly important given that older drivers voluntarily curtail their driving as they develop progressive sensory, cognitive, and physical impairment (12). Studies that do not adequately account for driving exposure will thus underestimate the impact that these impairments have on motor vehicle crash risk. In our study, weekly driving mileage was self-reported using a single question, and we lacked specific information about when women stopped driving. Data from the National Personal Transportation Survey shows that women over age 65 drove 3308 miles annually in 1983 and 4750 miles annually in 1990 (38). This suggests that there may have been a modest degree of underreporting of miles driven by the women in our sample. Furthermore, driving exposure was not measured until 2 years after the start of motor vehicle crash ascertainment. Changes in driving behavior between Visit 1 and Visit 2, including a reduction in weekly mileage driven in response to a motor vehicle crash, are not captured by our data. This may have biased some associations.
Several of our potential motor vehicle crash risk factors (depression, hand and foot reaction times, Trail Making Part B, and Digit Symbol Substitution) were also measured at Visit 2 in 19881989. Because motor vehicle crashes from 19861995 were included, about one third preceded the measurement of these five variables. However, limiting follow-up from Visit 2 onward would have unacceptably lowered our study power. The lack of true prospective follow-up with respect to these variables may have obscured or weakened any association with motor vehicle crashes.
Our results should be interpreted with caution for a number of other reasons. The study sample consisted of predominantly white women from Oregon. Our outcome data were limited to motor vehicle accidents that resulted in a police report; therefore, near misses, moving violations, and less serious crashes were not ascertained, nor do we know which of the motor vehicle crashes resulted in injuries. We may have underestimated the number of crashes because we did not have the ability to ascertain crashes that occurred outside of Oregon. No information was available on other diseases, in particular cardiovascular disease.
In conclusion, this prospective study identified motor vehicle crash risk factors that can be measured in the clinical setting: increased age, increased weekly mileage driven, a history of falls in sedentary women, increased systolic blood pressure drop, and foot reaction time. Other neuromuscular, vision, or cognitive tests and knowledge of functional status and medical comorbidity appeared to contribute little to predicting motor vehicle crash risk in this relatively healthy, unimpaired group of women. Future research is needed to examine whether change in physical and cognitive functions predicts motor vehicle crashes and whether the risk of motor vehicle crashes can be lowered by interventions aimed at ameliorating modifiable risk factors, such as a history of falls or slow foot reaction time.
| Acknowledgments |
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Received June 18, 2001
Accepted September 25, 2001
| References |
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