Hypoglycaemia
The brain can neither make nor store glucose, so when circulating insulin outlasts the available carbohydrate the counter-regulatory response cannot keep up and neuroglycopenia follows within minutes.
Definition
Hypoglycaemia in an adult with diabetes is a blood glucose below 4.0 mmol/L, with or without symptoms; the guideline mnemonic is that 4.0 is the floor. Severity is defined by who treated it, not by the number: non-severe episodes are self-treated and severe episodes require assistance from a third party. In a person without diabetes the diagnosis requires Whipple's triad, which is symptoms consistent with hypoglycaemia, a low measured glucose, and relief of the symptoms when glucose is raised.
First principles
The brain has no fuel store, so counter-regulation is the only defence
Neurons cannot make or store glucose and depend on continuous delivery. As glucose falls, endogenous insulin secretion should switch off while glucagon and adrenaline mobilise hepatic glucose, with cortisol and growth hormone following later. Injected insulin cannot be switched off, and a sulfonylurea keeps stimulating insulin secretion, so glucose falls on into neuroglycopenia.
This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.