Hyperosmolar Hyperglycemic State (HHS) in Pets: ICU Fluid and Insulin Management

Last updated: May 16, 2026

What Hyperosmolar Hyperglycemic State Is

Hyperosmolar hyperglycemic state — HHS — is a severe complication of diabetes mellitus characterized by extreme blood glucose elevation, profound dehydration, and altered mentation, without the marked ketoacidosis seen in diabetic ketoacidosis (DKA). It is less common than DKA but carries a substantially higher mortality, even with prompt ICU care. Hyperosmolar hyperglycemia in pets is most often recognized in cats with concurrent kidney or liver disease, although dogs can develop it as well.

The clinical picture is one of a profoundly sick animal: severely depressed mentation, weakness, sunken eyes, dry mucous membranes, and sometimes seizures or coma. Blood glucose is dramatically elevated — often two to three times the threshold that defines diabetes — yet ketones are absent or minimal. The mismatch between extreme hyperglycemia and absent ketones is what makes HHS its own diagnostic and therapeutic category.

If you are reading this because your diabetic pet is acting unwell, do not finish the article first. Call your veterinarian or the nearest 24-hour emergency hospital. The ASPCA Pet Loss Hotline (877-474-3310) is available if you and your family need support around end-of-life decisions; HHS prognosis is guarded, and being prepared emotionally is part of being prepared as an owner.

How HHS Differs From Diabetic Ketoacidosis

Both DKA and HHS occur when insulin deficiency overwhelms the body’s ability to regulate glucose, but the metabolic pathways diverge.

In DKA, severe insulin deficiency forces the body to burn fat as fuel, producing ketones. The ketones cause metabolic acidosis, which drives the classic DKA picture: nausea, vomiting, panting, abdominal pain, dehydration, weakness. Blood glucose is high but typically not extreme.

In HHS, there is enough residual insulin activity to suppress fat breakdown and ketone production, but not enough to control glucose. Glucose climbs to extreme levels, drags water out of cells (osmotic diuresis), and produces severe dehydration. The brain dehydrates too, which is why neurologic signs — obtundation, ataxia, seizures, coma — are prominent. Acidosis is mild or absent.

Some pets present with a mixed picture (HHS plus mild ketoacidosis). Your veterinarian will use blood glucose, ketone testing, blood gas analysis, and serum osmolality calculations to sort out where on the spectrum your pet sits — because the treatment differs.

Who Develops HHS

HHS is more commonly seen in cats than dogs. It is particularly associated with concurrent illness that limits the pet’s ability to drink or excrete glucose normally. Common scenarios include:

• Diabetic cats with underlying chronic kidney disease — impaired urinary glucose loss accelerates the climb to extreme hyperglycemia.
• Diabetic dogs with chronic kidney disease or acute kidney injury from any cause.
• Newly diagnosed diabetics whose disease was missed for weeks or months.
• Diabetic pets that stopped drinking water adequately — from anorexia, vomiting, or being trapped without access.
• Pets recently treated with glucocorticoids, which exacerbate hyperglycemia.

A diabetic pet with acute kidney injury superimposed is at especially high risk because the kidneys can no longer compensate by excreting excess glucose.

Clinical Signs That Should Trigger an ER Visit

HHS does not present subtly. By the time it is recognizable, the pet is gravely ill. Watch for any of these signs in a known diabetic — or in a previously undiagnosed pet with the classic diabetes history of excessive drinking, urination, and weight loss:

• Profound lethargy or unresponsiveness — the pet barely lifts its head.
• Stumbling, weakness, or inability to stand.
• Seizures or twitching.
• Sunken eyes, dry mouth, skin that does not snap back when tented.
• Refusal to eat or drink for more than 24 hours.
• Cool extremities and weak pulses.

Any of these in a diabetic pet is an emergency. Do not wait for your regular vet to open in the morning — go to a 24-hour facility immediately.

Diagnostic Workup at the Emergency Hospital

Once your pet is in the hospital, the workup happens fast. Blood is drawn for a complete blood count, chemistry, electrolytes, blood gas, lactate, and a venous or arterial pH. Glucose is measured immediately on a glucometer; severe readings often max out the device. Urine is collected for glucose, ketones, specific gravity, and culture.

Calculated serum osmolality — derived from sodium, glucose, and BUN — confirms the hyperosmolar state. The classic HHS pattern is markedly elevated osmolality, severe hyperglycemia, minimal or absent ketones, and a relatively preserved blood pH.

Imaging (abdominal ultrasound, chest radiographs) often follows once the pet is stabilized — your vet is looking for the underlying trigger: pancreatitis, urinary tract infection, neoplasia, pyometra, or an unrecognized endocrine disorder like Cushing’s syndrome.

ICU Treatment: Slow and Careful

HHS is a slow-correction emergency. Aggressive fluids and rapid insulin — appropriate for some other crises — can cause cerebral edema and worsen the neurologic picture. Treatment proceeds in deliberate steps.

Fluid Resuscitation First

Intravenous fluids — typically a balanced isotonic crystalloid such as Lactated Ringer’s, Plasmalyte, or Normosol-R — restore circulating volume. The first phase is moderately aggressive to address shock, then the rate is reduced. Some clinicians transition to a hypotonic fluid (such as 0.45% saline) for slow free-water replacement. The goal is to bring osmolality down gradually over 24 to 48 hours, not within hours.

Insulin Therapy Comes After

Insulin is introduced once intravascular volume is restored and electrolytes are addressed. Regular insulin as a continuous low-rate infusion (or intermittent intramuscular doses in some protocols) lowers glucose smoothly. Glucose is monitored every one to two hours; the target is a gradual decline, not a rapid drop, because rapid shifts worsen cerebral edema.

Electrolyte Management

Potassium falls during treatment as insulin drives it into cells. Phosphorus often falls too. Both must be supplemented in the IV fluids. Sodium and chloride are tracked closely. This is why HHS belongs in an ICU with a clinician at the bedside — the numbers change quickly.

Addressing the Underlying Trigger

HHS rarely strikes a healthy diabetic out of nowhere. Identifying and treating the precipitating cause is critical to recovery and to preventing a repeat episode. Common triggers include:

• Pancreatitis (especially in cats).
• Urinary tract infection — common in diabetics and often subclinical.
• Renal disease, particularly when superimposed on diabetes.
• Liver disease such as feline hepatic lipidosis or cholangitis.
• Concurrent endocrine disease — Cushing’s, acromegaly.
• Recent corticosteroid administration.
• Sepsis from any source.

Without addressing the trigger, glucose control will not stabilize and the pet may relapse soon after discharge.

Prognosis and Realistic Conversations

Even with aggressive ICU care, HHS prognosis is guarded. Mortality is significantly higher than for DKA — both because the metabolic derangement is more extreme and because the pets who develop HHS often have severe concurrent disease. Pets who survive the first 48 to 72 hours have a better outlook, but recovery can take a week or more in the hospital.

Your veterinarian and the ICU team will have honest conversations with you about prognosis, cost, and quality of life at multiple points. Decisions to continue, pause, or transition to comfort care are deeply personal. Resources such as the ASPCA Pet Loss Hotline (877-474-3310) and the pet loss support resources page can help if you are navigating end-of-life questions.

Recovery and Long-Term Management

Pets who recover from HHS go home with a renewed diabetes management plan, often more conservative than what they had before. That may include a different insulin, a stricter feeding schedule, more frequent glucose curves, and possibly home monitoring with a continuous glucose monitor.

If your pet was newly diagnosed at the HHS crisis, the path forward will include all the standard diabetic care decisions. For dogs, that means committing to lifelong insulin under an insulin protocol. For cats, it means evaluating whether the remission-oriented protocol is still realistic once the kidneys and liver have stabilized.

Preventing the Next Crisis

The most important prevention step is consistent, accurate insulin dosing paired with reliable feeding. Beyond that:

• Recheck bloodwork on the schedule your vet recommends — usually every three to six months for a stable diabetic, more often for one with concurrent disease.
• Treat urinary tract infections promptly; submit a urine culture, not just a dipstick.
• Monitor for new clinical signs and call early. A diabetic pet that “isn’t quite right” today is much easier to stabilize than one that is collapsed tomorrow.
• Keep a written record of insulin doses, feedings, and at-home glucose values.
• Have a 24-hour emergency hospital identified before you need one. Driving 20 minutes longer than necessary while your pet declines is a preventable problem.

When to Involve a Specialist

Internal medicine specialists (ACVIM Small Animal Internal Medicine) are central to HHS care and to managing the underlying disease afterward. Critical care specialists (ACVECC) often run the ICU phase. If your pet survives an HHS event, ask your primary veterinarian about a referral consult — it can clarify the trigger, refine the insulin plan, and reduce the chance of a repeat crisis.

Frequently Asked Questions

Is HHS more dangerous than DKA?

Generally yes. HHS carries higher mortality even with aggressive treatment, partly because the metabolic derangement is more severe and partly because the pets who develop it often have significant concurrent disease.

Can my pet recover fully from HHS?

Some pets recover and go on to live well-managed diabetic lives for years. Others do not survive the initial crisis or develop complications during treatment. Your veterinary team can give you a realistic outlook based on your pet’s specific labs and concurrent conditions.

How long does HHS treatment usually last in the hospital?

Several days at minimum, often a week or more. Glucose, osmolality, and electrolytes must be normalized slowly. Pets are not discharged until they are eating, drinking, urinating normally, and stable on a subcutaneous insulin protocol.

How much does ICU treatment cost?

HHS care frequently runs into thousands of dollars because of the prolonged ICU stay, continuous monitoring, multiple blood panels, and supportive care. Discuss costs with the hospital early; many offer estimates and payment arrangements.

What if I cannot afford ICU treatment?

This is a painful but common situation. Your veterinarian can help you weigh options — including humane euthanasia when intensive care is not feasible and the pet’s suffering would be prolonged. The at-home euthanasia overview describes one option for families seeking a peaceful end-of-life experience.