QUESTION: What is hyperosmolar hyperglycemic nonketotic syndrome?
Hyperosmolar hyperglycemic state (HHS) is a life-threatening diabetes emergency caused by extremely high blood glucose, severe dehydration, and very concentrated blood. It requires immediate hospital treatment. Call 911 or go to the emergency department for confusion, extreme sleepiness, seizures, fainting, severe weakness, inability to drink, repeated vomiting, or very high glucose with worsening illness.
Answer by Dr. Albana Greca, MD, MMedSc
Hi there,
The condition once commonly called hyperosmolar hyperglycemic nonketotic syndrome (HHNS) is now usually called hyperosmolar hyperglycemic state (HHS). It most often affects people with type 2 diabetes, particularly older adults, but it can also occur in younger people or be the first sign of previously undiagnosed diabetes.
When blood glucose becomes extremely high, the kidneys try to remove some of the excess glucose through urine. Glucose pulls water with it, causing frequent urination and major fluid loss. As dehydration worsens, the kidneys become less able to remove glucose, so the blood glucose and blood concentration rise further.
This cycle can cause profound dehydration, loss of electrolytes such as sodium and potassium, reduced kidney function, and changes in brain function. Without rapid treatment, HHS may lead to seizures, coma, blood clots, shock, or death.
The older word “nonketotic” can be misleading. In HHS, the body usually has enough insulin activity to limit major ketone production, but not enough to control blood glucose. Small amounts of ketones may still be present, and some patients have a mixed HHS and diabetic ketoacidosis picture. HHS cannot be confirmed or excluded by a home ketone test alone.
HHS often involves a laboratory blood glucose of 600 mg/dL (33.3 mmol/L) or higher, but the diagnosis is not based on glucose alone. Doctors also evaluate blood osmolality, hydration, electrolytes, kidney function, ketones, acid-base balance, and mental status.
A person should not wait for glucose to reach 600 mg/dL before seeking help. Repeated readings above 400 mg/dL, a meter displaying “HI,” or very high glucose accompanied by dehydration, confusion, vomiting, weakness, or illness requires urgent medical assessment. Learn more about dangerous blood sugar levels and emergency warning signs.
HHS usually develops over several days or sometimes longer, but it can deteriorate quickly once severe dehydration and mental-status changes appear.
Common triggers include pneumonia or urinary infection, missed diabetes medicine or insulin, newly diagnosed diabetes, heart attack, stroke, surgery, severe illness, dehydration, and medicines that raise glucose or fluid loss, including corticosteroids and some diuretics.
HHS must be treated in a hospital. Treatment usually includes carefully monitored intravenous fluids, insulin, electrolyte replacement, frequent laboratory testing, and treatment of the underlying trigger. Fluid and glucose correction must be controlled because changing blood osmolality too rapidly can cause serious complications.
Do not try to treat suspected HHS by exercising, taking extra medicine without instructions, or using another person’s insulin. Exercise can worsen severe hyperglycemia when dehydration or insulin deficiency is present.
Seek emergency care immediately for very high blood glucose with confusion, severe drowsiness, seizures, fainting, repeated vomiting, inability to keep fluids down, breathing difficulty, chest pain, signs of stroke, or severe dehydration. Do not drive yourself if you feel confused, faint, or extremely weak.
HHS is not simply “high sugar for a week or two.” It is an emergency defined by a combination of severe hyperglycemia, hyperosmolality, dehydration, and laboratory findings. A person with persistent severe readings should seek urgent care rather than trying to lower them alone at home.
Educational safety note: This answer is for general diabetes education only. It does not replace personal medical advice, diagnosis, or treatment. Do not start, stop, or change medicines, supplements, diet, or exercise plans without speaking with your healthcare provider.
Last reviewed: July 2026.
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