Effects of previous glycaemic control on the onset and magnitude of cognitive dysfunction during hypoglycaemia in Type 1 (insulin-dependent) diabetic patients

Abstract

To determine whether the degree of previous glycaemic control may modify cognitive responses to hypoglycaemia, the glycaemic thresholds for, and magnitude of cognitive dysfunction as assessed by P300 event-related potentials as well as subjective and hormonal responses during hypoglycaemia were evaluated. Hypoglycaemia was induced by intravenous insulin infusion in 18 Type 1 (insulin-dependent) diabetic patients, 7 of whom were strictly controlled (HbA1c: 6.3±0.3%; mean±SEM; Group 1) and 11 of whom were poorly controlled (HbA1c: 9.1±0.4%; Group 2). Within 60 min, mean blood glucose declined from 5.6 and 5.7 mmol/l (baseline) to a nadir of 1.6 and 1.8 mmol/l followed by an increase to 5.6 and 4.3 mmol/l after 120 min in Group 1 and 2, respectively. There was no significant difference between the groups in regard to P300 latency at baseline, but between 50 and 70 min a significant prolongation of this component was noted in Group 2 as compared with Group 1 at blood glucose levels between 1.6 and 2.3 mmol/l (p<0.05). The glycaemic thresholds at which a significant increase of P300 latency over baseline was first noted were 1.6±0.2 mmol/l in Group 1 and 3.5±0.2 mmol/l in Group 2 (p<0.05). The glucose thresholds at which this prolongation was no longer demonstrable were 1.9±0.1 mmol/l in Group 1 and 3.8±1.4 mmol/l in Group 2, respectively (p<0.05). The glycaemic threshold at which the P300 amplitude was first significantly reduced was 2.2 mmol/l in Group 2, whereas no such reduction was observed in Group 1. The glycaemic thresholds for the perception of subjective symptoms were 1.7±0.2 mmol/l in Group 1 and 2.5±0.2 mmol/l in Group 2 (p<0.05), and those for the first significant rise of the counter-regulatory hormones were 2.3±0.1 and 1.6±0.2 mmol/l in Group 1 as well as 2.8±0.1 mmol/l in Group 2 (p<0.05). Thus, the glycaemic threshold for and magnitude of, cognitive dysfunction during hypoglycaemia are reduced in strictly-controlled as compared with poorly-controlled Type 1 diabetic patients. In the latter group, cognitive impairment may precede the onset of counter-regulatory hormone responses and symptom awareness. These findings support the concept of cerebral adaptation to previous low blood glucose levels. © 1992 Springer-Verlag.