Diabetic hypoglycemia produces cognitive-motor slowing, which is assumed to increase risk of automobile crashes. This study investigated driving decrements during and after hypoglycemia, and the patients' awareness of driving decrements. We used a randomized, single-blind, crossover design and conducted the study at the University of Virginia's General Clinical Research Center. We studied a volunteer sample of 27 consecutive adult type I diabetic patients who responded to newspaper ads. Two dropped out (final n = 25). Mean age (± SD) was 35.9 ± 14 yr. Diabetes history was 14.6 ± 10.5 yr, with HbA1 of 10.8 ± 2.9%. Driving experience was 19 ± 13.2 yr. Participants drove a state-of-the-art driving simulator on two consecutive days: Control day involved four euglycemia (mean blood glucose, 6.3 ± 0.89 mM) driving tests; experimental day involved testing at euglycemia, mild hypoglycemia (mean blood glucose, 3.6 ± 0.33 mM), moderate hypoglycemia (mean blood glucose, 2.6 ± 0.28 mM), and again at euglycemia. Patients were blind to blood glucose manipulations and levels. Driving performance was not disrupted at mild hypoglycemia nor after recovery from moderate hypoglycemia. Moderate hypoglycemia disrupted steering, causing more swerving (P < 0.03), spinning (P < 0.03), time over midline (P < 0.05), and time off road (P < 0.01). It also resulted in an apparent compensatory slowing, with more very slow driving (P < 0.04). Global driving performance decrements were observed in 35% of the patients, only 50% of whom stated they would not drive under similar conditions. Driving decrements were unrelated to demographic, disease, or driving history variables. Between 3.6 and 2.6 mM, driving performance was disrupted and not reliably recognized by our patients.

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