Hyperosmolar hyperglycemic syndrome (HHS) is a clinical condition that arises from a complication of diabetes mellitus. This problem is most commonly seen in type 2 diabetes. Won Frerichs and Dreschfeld first described the disorder around 1880. They described patients with diabetes mellitus with profound hyperglycemia and glycosuria without the classic Kussmaul breathing or acetone in the urine seen in diabetic ketoacidosis. This clinical condition was formerly called non-ketotic hyperglycemic coma, hyperosmolar hyperglycemic non-ketotic syndrome, and hyperosmolar non-ketotic coma (HONK).

Diabetes mellitus is a clinical condition associated with hyperglycemia as the main metabolic disorder. This is a result of an absolute or relative deficiency of insulin. Insulin is an anabolic hormone produced by the beta cells of the islets of Langerhans in the pancreas. The main function of this hormone is to lower the level of glucose in the blood by promoting the uptake of glucose by the adipose tissue and skeletal muscle, known as glycogenesis. Insulin also inhibits the breakdown of fat in the adipose tissue, known as lipolysis. The metabolic effect of insulin is countered by hormones such as glucagon and catecholamines.

In type 1 diabetes, there is the autoimmune destruction of the beta cells in the pancreas. Only about 5% to 10% of all diabetes falls into this category. The most common complication of type 1 diabetes is diabetic ketoacidosis (DKA).

Type 2 diabetes accounts for about 90% to 95% of diabetes cases. It is most commonly seen in patients with obesity. As a consequence of obesity and high body mass index (BMI), there is resistance of the peripheral tissue to the action of insulin. The beta-cell in the pancreas continues to produce insulin, but the amount is not enough to counter the effect of the resistance of the end organ to its effect.

HHS is a serious and potentially fatal complication of type 2 diabetes.

The mortality rate in HHS can be as high as 20%, which is about 10 times higher than the mortality seen in diabetic ketoacidosis. Clinical outcome and prognosis in HHS are determined by several factors: age, the degree of dehydration, and the presence or lack of other comorbidities.