Influence of alcohol on cognitive performance during mild hypoglycaemia; implications for Type 1 diabetes

Alcohol and hypoglycaemia independently affect cognitive function. This may be relevant for insulin-treated diabetic patients who drive motor vehicles. The aim of this study was to examine the effect of mild hypoglycaemia (2.8 mmol/l) with modest alcohol intoxication (levels below UK driving limits) on intellectual performance in patients with Type 1 diabetes.

A hyperinsulinaemic glucose clamp (60 mU/m2) was used to study 17 subjects [age 35 +/- 8 years, HbA1c 8.1 +/- 1.4% (mean +/- sd)] on four occasions: (A) euglycaemia (4.5 mmol/l) with placebo, (B) euglycaemia with alcohol, (C) hypoglycaemia (2.8 mmol/l) with placebo, and (D) hypoglycaemia with alcohol. Cognitive performance was assessed using four-choice reaction time (4CRT, primary outcome), measurements of general intellectual skills [trail making B (TMB) and digit symbol substitution (DSST)], and visual information processing [visual change detection (VCD)]. A test related to driving performance (hazard perception) was also used.

In experiments B and D the average blood alcohol level was 43 mg/dl. This was associated with deterioration in 4CRT [+ 35 ms [95% confidence interval (CI) 20, 50]] and TMB, whereas hypoglycaemia without alcohol increased 4CRT only [+ 39 ms (95% CI 5, 73)]. However, when alcohol was combined with hypoglycaemia, there was marked deterioration in all the cognitive function tests [4CRT 74 ms (95% CI 35, 113), TMB, DSST and VCD]. Hazard perception was not affected. The effect of alcohol was no different in euglycaemia than in hypoglycaemia, i.e. there was no interaction. Whereas hypoglycaemia did not reduce the likelihood that the subjects would drive, alcohol did.

The cumulative effect of alcohol and hypoglycaemia on cognitive function together has implications for driving in patients with Type 1 diabetes. Both independently impair cognitive function and together the effects are additive. Patients with Type 1 diabetes should be educated about hypoglycaemia and driving and should avoid alcohol completely if planning to drive.