Hyperosmolar hyperglycaemic state (HHS)
Overview
HHS (formerly hyperosmolar non‑ketotic coma, HONK) is an acute metabolic emergency seen mainly in people with type 2 diabetes. It is characterised by very high plasma glucose, marked hyperosmolality and severe dehydration with little or no ketoacidosis.
It usually develops over days to weeks and carries substantially higher mortality than DKA (reported 5-20%). HHS is commonly precipitated by intercurrent illness (infection, myocardial infarction, stroke), reduced oral intake or medication effects.
Key diagnostic features
- Severe hyperglycaemia - commonly ≥ 30 mmol/L.
- Minimal or no ketosis - ketones < 3 mmol/L or urine ketones < 2+; pH
> 7.3 and bicarbonate
> 15 mmol/L.
- Elevated plasma osmolality - commonly ≥ 320 mOsm/kg (effective osmolality increased).
- Clinical hypovolaemia and dehydration.
Pathophysiology
- Excess plasma glucose exceeds renal reabsorption → osmotic diuresis → large losses of water and electrolytes → extracellular hyperosmolality.
- Water shifts out of cells produce intracellular dehydration (notably cerebral cells), causing altered mental state.
- Insulin levels are generally sufficient to suppress ketogenesis, so ketoacidosis is absent or mild.