Insulinoma in dogs – symptoms, tests and treatment

Overview

Insulinoma is an endocrine pancreatic tumour: its cells produce a hormone that acts elsewhere in the body. In dogs, it is considered malignant because it readily spreads beyond the pancreas.

Diagnosis is rare: in Britain, the annual proportion of new diagnoses was 0.003% in 2019. All cases diagnosed up to that point accounted for 0.004% of that year's dog population. Recognition is difficult because low blood sugar can be intermittent and resemble neurological or circulatory disease.

The tumour usually appears as a small pancreatic nodule around 1–3 cm across. Up to 80% of patients have a single tumour mass. Multiple pancreatic nodules can also occur.

Causes

Excess insulin removes glucose from the blood

The tumour develops from beta cells in the pancreatic islets of Langerhans, which normally produce insulin. An insulinoma secretes insulin even when the blood glucose level does not require it. Insulin moves glucose from blood into tissues for use, so blood sugar can fall dangerously low.

The brain needs glucose for energy, so a shortage particularly affects nervous-system function. Without glucose, the brain's oxidative energy metabolism declines.

The body responds by releasing hormones that counter insulin, such as glucagon and stress hormones. Their effect may temporarily correct blood sugar, partly explaining the episodic signs.

Why can an episode start during a walk or around a meal?

Exertion can trigger low blood sugar because active tissues use more glucose. Fasting reduces available glucose. Eating can also trigger an episode because food may stimulate the tumour's insulin secretion.

Risk factors

The disease mainly affects older dogs. Median age at diagnosis, the middle value, has been 10.4 years. Affected dogs ranged from 3.2–16.2 years old.

Higher odds of diagnosis have been identified in Dogues de Bordeaux, Flat Coated Retrievers and Boxers. West Highland White Terriers and English Springer Spaniels are also predisposed.

Spayed females had higher odds of diagnosis than entire females. Dogs heavier than their breed-and-sex mean weight also had higher odds. However, greater weight may indicate a larger frame rather than obesity. Insulinoma itself can also cause weight gain through insulin's metabolic effects.

Dogs with adult weights from 20 to under 30 kg had higher odds of diagnosis than those under 10 kg.

The greater odds in spayed females do not demonstrate that spaying itself causes the tumour.

Typical symptoms

Initially, episodes may be infrequent, so the disease may not appear as continuous illness. As it progresses, episodes generally become more frequent and last longer.

• General weakness may appear as a clear loss of stamina.

• The hind legs may weaken and become unreliable at supporting the dog.

• Your dog may collapse or have a faint-like episode.

• Seizures may occur intermittently.

• Small involuntary muscle twitches may be visible.

• Your dog may be restless or more easily excited than usual.

• Hunger may increase abnormally because of the stress-hormone response to low blood sugar.

An episode may look like epilepsy even though low blood sugar is responsible. Similar signs can also occur with hypoglycaemia, excessively low blood sugar, from other causes.

Complications and consequences

Prolonged, severe low blood sugar can permanently damage brain nerve cells. Poor coordination or weakness may then remain after treatment.

In around 40–50% of dogs, the tumour has already spread to nearby lymph nodes or the liver before diagnosis. Regrowth and metastases can bring hypoglycaemia signs back after surgery.

After successful removal of a single tumour, survival has ranged from around 1½ years to slightly over 2 years. Some dogs can live with good quality of life for more than 2 years when all visible tumours are removed and blood sugar normalises. In disease already spread at diagnosis, survival has been around 7–9 months with combined surgery and medication.

Diagnosis at the vet

Glucose and insulin are tested together

Repeated weakness, collapse or seizures in an older dog are reasons to measure blood glucose. Low glucose combined with inappropriately high insulin at the same time suggests insulinoma. Insulin may fall within the laboratory reference range yet still be inappropriately high for the dog's low blood sugar.

Diagnostic low-glucose thresholds include values below 3 or below 3.5 mmol/l alongside inappropriately high insulin.

Lower-than-usual fructosamine may also support suspicion. Red blood cell breakdown in the sample can reduce measured insulin.

Routine clinical examination may be entirely normal between episodes. A blood count, other biochemistry and urinalysis may also be normal. A continuous glucose sensor can help demonstrate intermittent low blood sugar.

If blood glucose is normal during examination, the vet may need a supervised fasting test. Glucose is monitored hourly because it can fall before visible signs develop. Do not therefore fast your dog at home yourself for testing when insulinoma is suspected.

Using insulin-to-glucose ratios alone is not recommended because they may miss affected dogs.

Other causes of low blood sugar include Addison's disease, liver failure, sepsis and excessive insulin medication. Measurement error can also explain an abnormal result.

Imaging locates the tumour and investigates spread

Reported ultrasound sensitivity for finding insulinoma includes 36% and around 56%, so a normal scan does not rule it out.

Contrast-enhanced computed tomography, or CT, is the preferred method for more detailed imaging. Ultrasound-guided fine-needle samples may be taken from suspicious liver or lymph node changes.

Final confirmation comes from histopathology of a biopsy collected during surgery. Tumour location within the pancreas affects the surgical technique. Stage helps assess the outlook and choose between surgery and medication.

Chest and abdominal imaging look for metastases beyond the pancreas before treatment is selected.

Treatment

Emergency care stabilises blood sugar

Severe hypoglycaemia may require round-the-clock hospital care and monitoring. Giving glucose can rapidly improve signs caused by low blood sugar. A glucagon infusion may also be used in hospital to increase liver glucose production.

Glucose administration is monitored closely because an excessive insulin response can lower blood sugar again afterwards.

Tumour surgery

Surgery generally offers the best chance of controlling signs and extending survival, particularly with a single localised tumour. Part of the pancreas is usually removed with the tumour. The whole pancreas is examined during surgery for other nodules.

Visible lymph node and liver deposits may also be removed to reduce the overall tumour burden. Metastases left behind may sustain hypoglycaemia. Hospitalisation usually lasts 1–3 days after surgery.

After removal, blood sugar may temporarily rise because normal insulin secretion takes time to recover. Handling the pancreas may also cause pancreatitis. Its signs include vomiting, poor appetite and abdominal pain.

After partial pancreatic removal, around 10–19% of dogs have developed diabetes requiring its own ongoing treatment.

Medication for remaining symptoms or when surgery is unsuitable

The vet chooses medication to raise or stabilise blood sugar. Diazoxide reduces pancreatic insulin secretion. It also increases liver glucose production. It additionally reduces tissue glucose uptake. Liver, kidney or heart failure may prevent its use.

Possible diazoxide adverse effects include increased drooling, reduced appetite and vomiting.

A glucocorticoid, or corticosteroid medicine, such as prednisolone may be used instead. Prednisolone increases liver glucose production. It also opposes insulin's effects in tissues.

Octreotide mimics the body's somatostatin hormone and can inhibit insulin secretion. It may also suppress secretion of glucagon and growth hormone, which raise blood sugar. Hypoglycaemia may therefore worsen during treatment in some dogs. Possible adverse effects include injection-site pain, vomiting and constipation.

The chemotherapy drug streptozotocin has been used for insulinoma but carries a high risk of serious adverse effects. It can damage the kidneys, limiting its suitability for dogs. Toceranib has also been investigated, but its independent effect on long-term glucose control remains unclear.

Prevention

Everyday management aims to prevent dangerous falls in blood sugar. The daily food allowance may be divided into 4–6 small meals rather than leaving long gaps. The diet includes protein, fat and more slowly digested carbohydrates.

Fibre-rich food may slow sugar absorption. Exercise is limited to short lead walks rather than strenuous activity. Long fasts are avoided because lack of food can trigger hypoglycaemia.

Keep meal size and composition as consistent as possible. Rapidly absorbed sugars are not added to the routine diet because they can abruptly stimulate insulin secretion.

Important for dog owners

Record each episode's start time, duration, behaviour and relationship to meals or exercise. Take a short video safely if this does not delay leaving for treatment. Agree with the treating vet on a written plan for future episodes and a method of glucose monitoring. Keep instructions and emergency contact details readily available to other carers too.

After surgery, monitor eating and comfort particularly closely at home. Rest is central to recovery. Persistently low blood sugar after surgery may indicate microscopic tumour remaining.

Frequently asked questions

Is insulinoma the same as diabetes?

No: in insulinoma, tumour beta cells produce excessive insulin, lowering blood sugar. Insulin-deficiency diabetes has a different starting point because the pancreas's own insulin secretion is absent. Medication for insulinoma aims to raise or stabilise blood sugar.

Can insulin be normal even when a dog has insulinoma?

Yes: with low blood sugar, an insulin level within the reference range can still be abnormal. Measuring blood glucose and insulin simultaneously is therefore central. Insulin secretion should normally be very low when blood glucose is low. An insulin result labelled normal does not rule out the tumour.

Does a normal ultrasound rule out insulinoma?

No: ultrasound sensitivity for pancreatic insulinoma has been only 36%. Contrast-enhanced CT is better suited to locating the tumour. Tumours are often small pancreatic nodules around 1–3 cm across. Examination of tissue collected surgically confirms the final diagnosis.

Can insulinoma look like epilepsy?

Yes: low blood sugar can cause seizures. Insulinoma can therefore be confused with epilepsy or another brain disease. An episode may occur during physical exertion, for example. Repeated seizures, weakness or collapse, particularly in an older dog, also require investigation of blood glucose.

Can signs return after successful surgery?

Yes: a tumour may regrow or metastases may bring hypoglycaemia signs back. If blood sugar remains low immediately after surgery, microscopic tumour tissue may remain. Nutritional management and medication can still control signs.