This study demonstrated that type 1 diabetic drivers with a history of recurrent hypoglycemia-related driving mishaps during the previous year differed on several basic levels from drivers with no such history. However, it is important to point out that these groups did not differ in terms of general demographic variables (e.g., age, education, and BMI), diabetes parameters (e.g., duration of disease, A1C, insulin regimens, hypoglycemia unawareness, and long-term complications), or driving parameters (e.g., driving history or miles driven) (). The exception was that the +history subjects reported three times more episodes of severe hypoglycemia during the previous year.
Although the +history group demonstrated equivalent driving performance during euglycemia, relative to the −history group, their overall driving performance during the 30-min induction of hypoglycemia from 3.9 to 2.5 mmol/l was worse. Our design did not allow us to determine at what blood glucose level this impairment first manifested itself. In contrast, our −history group did not demonstrate a decay.
Drivers with a positive history of mishaps tended to require more infused dextrose to maintain euglycemia during similar insulin challenges, suggesting that these individuals may be more vulnerable to hypoglycemia due to increased glucose utilization. When exposed to progressive mild hypoglycemia, they released less epinephrine, possibly making them more likely to slip into deeper hypoglycemia. Further, when they were experiencing progressive mild hypoglycemia, they demonstrated greater neuroglycopenia as suggested by a significant worsening of driving performance by 2.5 SD.
Drivers with and without a history of hypoglycemia-related driving mishaps did differ significantly in symptom perception during euglycemia but were symptomatically equivalent during progressive hypoglycemia. Detection of autonomic and neuroglycopenic symptoms is a key way for individuals with type 1 diabetes to recognize hypoglycemia during routine functioning (23
). Not only did +history drivers fail to detect an increase in symptoms during the induction of hypoglycemia, but they also actually reported more such symptoms during euglycemia than −History drivers. It is as if the former group has to deal with symptom “noise,” i.e., a background of symptoms occurring during euglycemia that may make it difficult to detect the “signal” of hypoglycemia, in other words a poor symptom-to-noise ratio. It is not clear from the present study whether this is a general condition for these individuals or if there is something unique to driving that triggers this inversion of symptom perception. Despite these differences in epinephrine release and perceived symptoms, −history (59%) and +history (44%) subjects were similarly likely to self-treat. This may be because self-treatment of hypoglycemia seems to be related to detected difficulties driving and not classic symptoms of hypoglycemia (24
). Further, this relatively low rate of self-treatment while hypoglycemic is consistent with the subjects' self-report that they seldom carried fast-acting glucose in their car, along with previously reported data indicating that drivers are willing to drive with low blood glucose (10
Because a recent history of hypoglycemia-related driving mishaps heralds the likelihood of future driving mishaps (1
), these findings have several clinical implications: Such high-risk drivers 1
) may require more robust carbohydrate dosing to prevent or to treat hypoglycemia, 2
) should be counseled in terms of an appropriate blood glucose threshold when not to begin driving, e.g., 5 mmol/l, which would vary depending on the length of the drive and whether their blood glucose will be rising or falling during the course of the drive, and 3
) should be encouraged to immediately stop driving if blood glucose falls to <4 mmol/l, treat themselves with sufficient fast-acting carbohydrates, and not resume driving until blood glucose is >5 mmol/l.
Limitations of this study should be considered. First, like most insulin clamp studies, these data represent a single observation in a laboratory setting. Therefore, the external validity of these findings cannot be confirmed. Second, this study was partially based on driving a simulator, not an actual car with real-life traffic and driving demands/risks. Third, this was a relatively small sample of only 38 adult drivers with type 1 diabetes. This small sample size may not have had sufficient power to identify small but potentially important differences between these two groups, such as differences in sex (−history = 34% women as compared with 62% for +history group). Finally, although this crossover design controlled for effects of antecedent hypoglycemia, an alternative design would have been to separate testing days by 2 weeks while rigorously avoiding hypoglycemia for 2 weeks before each testing. However, these limitations are offset by the fact that these a priori findings replicate previous post hoc analyses with an independent sample and different research staff but using similar methodologies and technologies (14
). In addition, the simulator used in this study has been found to predict on-road driving behaviors (21
) and predict future collisions (20
). Given the potential gravity of the consequences of hypoglycemia-related collisions (9
), it would seem clinically prudent to use these findings as a guide when working with individuals who are at a higher risk for hypoglycemia while driving, despite these methodological limitations.