HHS, or hyperosmolar hyperglycemic syndrome, is a serious and potentially life-threatening complication of diabetes in which blood sugar climbs so high that the blood becomes dangerously concentrated and the body loses massive amounts of fluid. It occurs most often in people with type 2 diabetes, typically older adults, and it develops over days to weeks rather than hours. Mortality rates have historically ranged from 10 to 50%, though hospital death rates have dropped significantly in recent years, falling from about 1.4% in 2008 to under 1% in 2018.
How HHS Develops in the Body
The underlying problem in HHS is that the body doesn’t have enough working insulin to move sugar from the bloodstream into cells. Without that signal, cells can’t use glucose for energy, and sugar accumulates in the blood. As blood sugar rises, it pulls water out of tissues and into the bloodstream through osmosis. The kidneys then try to flush the excess sugar out by producing large volumes of urine, dragging water and electrolytes along with it. Over time, this cycle creates severe dehydration.
What makes HHS different from the other major diabetic emergency, diabetic ketoacidosis (DKA), is that the pancreas is still producing some insulin. That small amount isn’t enough to control blood sugar, but it is enough to prevent the body from breaking down fat into acidic compounds called ketones. In DKA, the near-total absence of insulin leads to a dangerous buildup of those ketones, making the blood acidic. In HHS, ketone production stays minimal and blood acidity remains closer to normal. The trade-off is that because there’s no dramatic acid shift to trigger early symptoms, HHS often goes unnoticed longer, allowing blood sugar and dehydration to reach extreme levels before anyone realizes something is wrong.
HHS vs. DKA
Both HHS and DKA involve dangerously high blood sugar and insufficient insulin, but they differ in important ways:
- Blood sugar levels: HHS typically produces much higher blood sugar, often above 600 mg/dL, compared to DKA where levels are usually above 250 mg/dL.
- Ketones: DKA is defined by significant ketone production and metabolic acidosis. HHS produces little to no ketones because enough insulin remains in circulation to block that process.
- Dehydration: HHS causes more severe dehydration because it develops over a longer period.
- Acid levels: In DKA, blood pH drops below 7.3 (dangerously acidic). In HHS, pH typically stays above 7.30.
- Who it affects: DKA is more common in type 1 diabetes, while HHS occurs predominantly in type 2 diabetes.
Some people develop features of both conditions simultaneously, which clinicians refer to as overlap. This is particularly common in people with poorly controlled type 2 diabetes.
Symptoms to Recognize
HHS doesn’t come on suddenly. Early symptoms are easy to dismiss as general unwellness or simply being under the weather, especially in older adults. The first signs are usually increased thirst and frequent urination, the body’s attempt to dilute and flush out excess sugar. As dehydration worsens over days, additional symptoms appear: dry mouth and tongue, weakness, nausea, weight loss, and fever.
The hallmark of HHS, and what makes it particularly dangerous, is neurological impairment. As the blood becomes more concentrated, it affects brain function. This can range from mild confusion and decreased alertness to seizures, loss of muscle function, speech difficulty, and eventually coma. These neurological symptoms are what typically prompt emergency medical attention, but by that point the condition has usually been building for days.
Common Triggers
HHS rarely appears out of nowhere. It’s almost always set off by another event that either raises blood sugar or impairs the body’s ability to stay hydrated. Infections are the most common trigger, particularly pneumonia and urinary tract infections. When the body fights an infection, it releases stress hormones that raise blood sugar, and fever increases fluid loss.
Other common triggers include illness that causes vomiting or diarrhea (accelerating dehydration), certain medications like corticosteroids that raise blood sugar, and simply not taking diabetes medications as prescribed. In some cases, HHS is the first sign that someone has diabetes at all. Older adults living alone are at particular risk because they may not drink enough fluids, especially if they have reduced thirst sensation or limited mobility.
What Treatment Looks Like
HHS is a medical emergency that requires hospital treatment, often in an intensive care setting. The immediate priority is replacing lost fluid. Patients typically receive intravenous saline solution, with the goal of replacing roughly half the estimated fluid deficit in the first 12 hours and the rest over the following 12 hours. This alone often begins to bring blood sugar down.
Potassium and other electrolytes are monitored closely and replaced as needed, since both the dehydration itself and the treatment can shift electrolyte levels in dangerous ways. Insulin is given intravenously but usually not right away. Current guidelines recommend starting fluids first and adding insulin only once blood sugar has stopped dropping from rehydration alone, or if significant ketones are present. This cautious approach prevents blood sugar from falling too fast, which can cause dangerous fluid shifts in the brain.
Recovery depends on the severity of dehydration, whether there’s an underlying infection or other trigger, and the patient’s overall health. Neurological symptoms like confusion typically improve as hydration is restored and blood sugar normalizes, but the process can take 24 to 48 hours or longer.
Who Is Most at Risk
HHS disproportionately affects older adults with type 2 diabetes, particularly those who have other chronic conditions, limited access to fluids, or cognitive impairment that makes it harder to recognize early symptoms. People in nursing homes or those living alone are especially vulnerable because the early warning signs of excessive thirst and frequent urination can go unnoticed or be attributed to other causes.
The single most important prevention strategy is staying hydrated, particularly during illness, hot weather, or any period when blood sugar may be running higher than usual. Monitoring blood sugar more frequently when you’re sick, maintaining your diabetes medications, and having a plan for sick days can help catch a rising trend before it spirals into HHS. If blood sugar readings are consistently above 300 mg/dL and you’re experiencing increased thirst, frequent urination, or any confusion, that combination warrants urgent medical evaluation.