Insulinoma in Cats: Causes, Symptoms, & Treatment

Weakness, dullness and collapse are common signs of insulinoma. Lili-OK / Shutterstock

An insulinoma is a tumor of the pancreas, a serious condition which is thankfully only seen rarely in cats.

The pancreas is made up of cells that produce enzymes, which enter the intestines to digest food, and hormones, which control glucose levels in the blood. These hormones include insulin and glucagon. An insulinoma is an insulin-secreting tumor: it involves proliferation of certain pancreatic cells that produce insulin. An insulinoma creates high levels of insulin in the blood, which in turn causes low levels of blood glucose, and this then causes the signs of the illness.

Other Names: Pancreatic tumor, pancreatic cancer

Common Symptoms: Weakness, dullness, incoordination, ataxia, collapse, twitching, trembling, seizures

Requires Ongoing Medication: In some cases

Vaccine Available: No

Treatment Options: Medication, diet, surgery

Home Treatment: Specialized diet and feeding regime

Causes of Insulinoma

The cause of insulinoma, as for most tumors, remains unknown, but it is likely to be a combination of genetic, physiological and environmental factors.

Insulinoma is a rare condition in cats, with only a very small number of cases reported in the literature. When it does happen, it’s more likely to affect older cats, aged twelve years or over. It is just as common in the domestic shorthair cat as in pedigree breeds.

Symptoms of Insulinoma

Clinical signs of insulinoma are linked to the main consequence of the tumor on the pancreas: low blood glucose (hypoglycemia, or a “hypo”) caused by the increased levels of insulin produced by the tumor cells.

The signs include:

• Weakness
• Dullness
• Incoordination
• Ataxia (wobbling, unsteady on legs)
• Collapse
• Seizures
• Muscle twitching
• Trembling
• Confusion
• Blindness
• Gastrointestinal upset (e.g. vomiting or diarrhea)
• Weight loss

It’s important to note that other causes of low blood glucose could cause similar signs. These include conditions which cause increased glucose use in the body, such as sepsis, seizures and certain paraneoplastic syndromes, and any condition that causes decreased production of glucose, such as liver disease, neonatal hypoglycemia and hypoadrenocorticism (Addison’s Disease).

Diagnosis of Insulinoma

A blood sample is the fastest way to identify low blood glucose in a collapsed cat. Deyan Georgiev / Shutterstock

If you take a cat that’s weak, trembling, twitching or with a history of seizures to a DVM veterinarian, they will first discuss your cat’s condition and relevant medical history. Any possible exposure to toxins (e.g. flea treatments, garden chemicals, toxic plants, medications) should be mentioned. If you have any videos of your cat showing neurologic signs such as trembling, twitching or having seizures, it will help to show these to your vet. The timing of these episodes may also be important: low blood glucose (and therefore the signs of illness) are more likely to happen after a long gap after a meal, or shortly before a meal.

Your veterinarian will then check your cat over carefully, taking the body temperature, the heart rate, and listening to the heart and lungs with a stethoscope. The body will be palpated all over. A full neurological examination will normally be carried out, checking different reflexes and other aspects of nerve and muscle function. Your veterinarian may carry out blood tests, including the usual panel of diagnostic tests, such as hematology (blood count) and biochemistry profiles (including liver, kidney and other parameters).

The key finding for diagnosis of insulinoma is a consistently low blood sugar (glucose). Your cat may be admitted to the veterinary clinic for a day, so that a series of blood sugar levels can be measured (usually hourly) to confirm that the low blood glucose is a consistent, repeatable finding.

The blood glucose concentration should normally be above 60 mg/dL (3.33 mmol/L). If the glucose level is lower than this, this is known as hypoglycemia, and it is one of the key indicators of insulinoma.

A fructosamine test may be done: this is indicative of your cat’s longer term blood glucose level, and if the fructosamine level is lower than normal, this will support the diagnosis of insulinoma. An insulin assay may also be useful: if the insulin level is higher than normal when the blood glucose is low, this is strongly supportive of a diagnosis of insulinoma.

Other tests may be suggested, such as thyroid hormone levels to rule out hyperthyroidism, tests for serious viruses such as FeLV and FIV and urinalysis. Radiography (x-rays) may be suggested, and abdominal ultrasound may also be recommended, to search for any tumor-like enlargement in the pancreas, as well as to check for any other cause of a low blood glucose, and to check for the presence of any metastasis (spread of tumors). Rarely, a physical biopsy of a mass on the pancreas may be suggested.

Treatments for Insulinoma

Surgical removal of the pancreatic tumor is the only curative treatment for insulinoma. Andrii Medvednikov / Shutterstock

Initially, conservative treatment for insulinoma may be attempted, with frequent meals (including diets with complex carbohydrates and higher levels of protein), and steroids (e.g. prednisone) to try to boost the blood glucose level.

Insulinoma is more common in dogs, and medications such as diazoxide, octreotide and streptozotocin have been used to try to raise blood glucose levels, with limited success. However these medications have not been used widely in cats, and dosing of the medications is not straight forward.

The only curative treatment is surgery to remove the tumor from the pancreas, but this is a complicated procedure that needs to be undertaken with careful planning. Surgical excision of insulinoma has a risk of complications that include pancreatitis, hyperglycemia (blood glucose too high), and treatment failure (i.e. persistent hypoglycemia).

Before surgery is considered, accurate staging needs to be undertaken, as for any animal with a malignant tumor. The five stages of cancer are:

• Stage 0: No cancer, but there are abnormal cells with the potential to become cancerous.
• Stage I: The cancer is small and just in one area.
• Stage II and III: The cancer is larger and has grown into nearby tissues or regional lymph nodes.
• Stage IV: Cancer has spread to other parts of the body (metastases).

Once staging has been carried out, you can make a decision as to how you wish to proceed, and you may, or may not, prefer to be referred to a specialist oncologist service for treatment. If the tumor has already spread to elsewhere in the body (stage four) then surgery is less likely to be successful. If there is no sign of metastatic spread (i.e. stage 1 – 3), and analysis of the removed tumor shows no sign of invasion into adjacent tissues, then these prognostic factors suggest a higher chance of a cure.

In one report of twenty cats surgically treated for insulinoma, the 1-, 2- and 3-year survival rates were 75%, 51%, and 10%, respectively. Your cat will be re-examined regularly, as recommended by your veterinarian, to monitor their progress, with blood glucose measurement a key guide, aiming for euglycemia (normal blood glucose levels).