Hyperglycemic hyperosmolar syndrome (HHS) is a potentially life-threatening complication of diabetes. It involves very high blood sugar, usually over 600 milligrams per deciliter, and blood pH levels of 7.3 or higher.

Symptoms include severe dehydration and neurologic symptoms that may lead to a coma. Without prompt treatment, it can result in a heart attack or stroke. In some cases, it can be fatal.

It is crucial for people with diabetes to know the symptoms of HHS so they can seek prompt medical attention and treatment.

This article discusses what HHS is, its symptoms, treatment, and more.

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HHS happens when a person’s blood sugar levels are very high. When a person has HHS, their body will try to lower its blood sugar by passing sugar through the urine.

If a person does not drink enough fluids to make up for the fluids they are losing, it can lead to a further spike in blood sugar levels. It also leads to hyperosmolarity, which is when the blood is more concentrated and less hydrated than usual.

HHS normally does not involve ketosis.

It has also been called:

  • hyperosmolar nonketotic coma (HONK)
  • nonketotic hyperglycemic coma
  • hyperglycemic hyperosmolar nonketotic syndrome (HHNS)

HHS rarely affects people with type 1 diabetes. Of all HHS cases, 90–95% occur in people with type 2 diabetes. It is more likely to occur in people who have obesity.

HHS can happen due to an infection or other illness, such as a heart attack or stroke. It can also be brought on by:

  • taking medications that increase fluid loss
  • taking medications that decrease the effect of insulin in the body
  • running out of, or not taking, prescribed diabetes medications

According to a 2021 article, symptoms of HHS include:

People with HHS may also have symptoms of dehydration, which can include dry mouth, lips, and eyes.

People with HHS are often severely dehydrated, which may alter their brain function. This can cause cognitive symptoms, including:

A person with HHS may have other symptoms depending on how they developed it. Often, people develop HHS following an infection or cardiac event.

A stroke or heart attack can lead to high stress levels, known as a stress response. This can cause an increase in adrenaline and cortisol, which raises blood sugar levels.

If a person develops HHS after an infection, they may have the following additional symptoms:

If a person develops HHS after a cardiac event, they may have the following additional symptoms:


Another serious complication of diabetes is diabetic ketoacidosis (DKA). DKA happens when the body starts breaking down fat too quickly. This can lead to a high level of ketones, or acidic substances, in the blood.

Although they are two distinct conditions, HHS and DKA have similar symptoms. For example, both conditions present with high blood sugar and frequent urination.

According to a 2021 article, over 30% of diabetic patients have features of both conditions.

The following chart from the same article highlights some of the differences between HHS and DKA:

ConditionPrimarily affectsBlood sugar levelsArterial pH levelsMortality rateDifference in symptoms
HHSpeople with type 2 diabetesmore than 600 milligrams per deciliter (mg/dl)more than 7.310–50%• fast heart rate
• symptoms present over days or weeks
DKApeople with type 1 diabetesmore than 250 mg/dlless than 7.31.2–9%• fruity-smelling breath
• high ketone levels in urine
• symptoms present within a few hours

Illness or infection accounts for the majority of HHS occurrences, causing over 50% of cases.

Other factors that may cause HHS include:

  • a cardiac event such as a stroke or heart attack
  • certain medications that can make a person more prone to dehydration, such as diuretics
  • undiagnosed type 2 diabetes
  • not managing blood sugar levels appropriately through the use of diet, exercise, and medication

Some people may have other factors that raise their risk of developing HHS. Risk factors for developing HHS include:

  • type 2 diabetes
  • dehydrating medications like diuretics, beta-blockers, and certain antipsychotics
  • morbid obesity
  • a family history of diabetes

HHS and ethnicity

HHS affects certain groups of people more than others. These groups include:

  • African Americans
  • Native Americans
  • Hispanics

Research suggests the disproportionately high number of people affected by HHS in these groups may be due to a high prevalence of type 2 diabetes.

This could be because they are more likely to experience barriers to methods that could lower their risk of developing it.

Learn more about why diabetes is more common in African Americans.

Find out more about racial discrimination in healthcare.

If a person has HSS, receiving a prompt diagnosis and medical care is crucial.

Diagnosis generally happens in an emergency room, where a person will undergo a physical exam and several lab tests.

Lab tests to diagnose HHS include testing blood sugar levels, urine, kidney function, and more.

A doctor will likely diagnose a person with HHS if they meet the following criteria:

  • blood sugar levels of over 600 mg/dl
  • blood pH levels of 7.3 or higher
  • mild metabolic anion-gap acidosis (present in half of patients)

Find out more about kidney function tests.

Treatment is crucial to a successful outcome for people with HHS. Prompt emergency treatment can remedy HHS quickly.

Treatment involves intravenous therapy. This means a healthcare professional will administer medication straight into a person’s vein through an IV drip.

Intravenous treatment of HHS involves giving the person:

  • fluids to treat dehydration
  • insulin to lower blood sugar
  • electrolytes to replace those lost through dehydration

Throughout treatment, a doctor will monitor a person’s electrolyte levels and heart rhythm.

If an infection or another health condition triggers a person’s HHS, a doctor will also treat the underlying condition.

Additionally, after treatment ends, a person who has recovered from HHS should discuss their blood sugar management and diabetes treatment plan with a healthcare professional to prevent HHS in the future.

Treatment for HHS can lead to side effects.

Taking too much insulin and fluid too quickly can cause a person’s blood sugar to drop rapidly. This can lead to cerebral edema. Cerebral edema is a rare complication of HHS that occurs when the brain swells due to rapid rehydration and a drop in blood sugar levels.

Other complications from treatment can include acute respiratory distress syndrome.

HHS can cause serious complications and is fatal in up to 20% of cases. Complications of HHS can include:

  • abnormal electrolyte levels
  • stroke
  • heart attack
  • seizure
  • coma

Contacting a doctor

HHS is a serious medical emergency. If a person believes they or a loved one may have HHS, they should seek immediate medical attention.

With prompt treatment, a person is likely to recover from HHS.

A person’s age and underlying conditions generally affect their outlook. Older adults who are in a coma and have low blood pressure due to HHS have a less favorable outlook than younger adults with easily treatable underlying illnesses.

HHS is a medical emergency that is a complication of type 2 diabetes. It often occurs after an illness or other medical event, such as a heart attack.

Outcomes are better with prompt, careful treatment.

A person with diabetes should work with their care team to manage their blood sugar to prevent HHS and other serious complications of diabetes.