Hypoglycaemia unawareness in type 1 diabetes increases the risk of severe hypoglycaemia and impairs quality of life for people with diabetes. To explore the central mechanisms of hypoglycaemia awareness, we used [¹¹C]-3-O-methyl-d-glucose (CMG) positron emission tomography (PET) to measure changes in global and regional brain glucose metabolism between euglycaemia and hypoglycaemia in aware and unaware diabetic subjects.

Twelve men with type 1 diabetes, of whom six were characterised as aware and six as unaware of hypoglycaemia, underwent two CMG-PET brain scans while plasma glucose was controlled by insulin and glucose infusion either at euglycaemia (5 mmol/l) or at hypoglycaemia (2.6 mmol/l) in random order.

With hypoglycaemia, symptoms and sweating occurred only in the aware group. Brain glucose content fell in both groups (p=0.0002; aware, 1.18±0.45 to 0.02±0.2 mmol/l; unaware, 1.07±0.46 to 0.19±0.23 mmol/l), with a relative increase in tracer uptake in prefrontal cortical regions, including the anterior cingulate. No detectable differences were found between groups in global brain glucose transport parameters (K ₁, k ₂). The cerebral metabolic rate for glucose (CMRglc) showed a relative rise in the aware subjects (11.839±2.432 to 13.958±2.372) and a fall in the unaware subjects (from 12.457±1.938 to 10.16±0.801 μmol 100 g⁻¹ min⁻¹, p=0.043).

Hypoglycaemia is associated with reduced brain glucose content in aware and unaware subjects, with a relative preservation of metabolism in areas associated with sympathetic activation. The relative rise in global glucose metabolic rate seen in aware subjects during hypoglycaemia contrasted with the relative fall in the unaware subjects and suggests that cortical neuronal activation is a necessary correlate of the state of hypoglycaemia awareness.