Glycaemic threshold for changes in electroencephalograms during hypoglycaemia in patients with insulin dependent diabetes.

The relation between blood glucose concentration, the symptoms and signs of hypoglycaemia, and electroencephalographic changes in diabetic patients is not known. The effect of hypoglycaemia on brain function was studied in 13 patients with insulin dependent diabetes. During a gradual fall in blood glucose concentration induced by a bolus injection of insulin followed by an intravenous infusion of insulin, during 60 minutes of biochemical hypoglycaemia, and after restoration of normoglycaemia with intravenous glucose electroencephalograms were evaluated continuously by period-amplitude analysis; blood samples were taken every 10 minutes throughout. No changes were seen in electroencephalograms when the blood glucose concentration was above 3 mmol/l. At a median blood glucose concentration of 2.0 (95% confidence interval 1.7 to 2.3) mmol/l alpha activity decreased abruptly in the electroencephalograms concomitant with an increase in theta activity, reflecting neuronal dysfunction in the cortex. When the blood glucose concentration was further lowered changes were observed in the electroencephalograms indicating that deeper brain structures were affected. A normal electroencephalogram was re-established at a blood glucose concentration of 2.0 (1.8 to 2.1) mmol/l. There was no significant correlation between the blood glucose concentration at the onset of changes in the electroencephalograms and age, duration of diabetes, insulin dose, haemoglobin A1c concentration, initial blood glucose concentration, rate of fall in blood glucose concentration, and appearance of symptoms and signs of hypoglycaemia. Changes in electroencephalograms during hypoglycaemia appear and disappear at such a narrow range of blood glucose concentrations that the term threshold blood glucose concentration for the onset of such changes seems justified.