Insulinoma

Other disorders of the endocrine pancreas result from functional tumors of hormone producing cells in the islets of Langerhans. Insulinomas, tumors of insulin-producing (β) cells, occur occasionally in dogs and rarely in cats. Functional tumors of the pancreatic β cells result in overproduction of insulin and subsequent hypoglycemia. Although there may be immunohistochemical evidence of other hormone production, such as somatostatin, serotonin, gastrin, glucagon, and pancreatic polypeptide, clinical signs of hyperinsulinemia are most common. Most tumors are malignant based on their tendency to metastasize rather than their histologic appearance. A random or fasting blood glucose concentration of <3.3 mmol/L (<60 mg/dL) in dogs and cats should be investigated, assuming in vitro glycolysis due to inadequate sample handling procedures has been ruled out.

Clinical signs of insulinoma may include seizures, weakness, collapse, ataxia, muscle twitching, lethargy, strange behaviour, loss of consciousness, and coma. These signs are usually episodic and can be precipitated by exercise and both fasting and eating. Food consumption may stimulate release of excessive levels of insulin causing signs to develop a few hours after eating. The CBC results may be within RI or may reflect a stress response, depending on the clinical condition of the animal at the time of sample collection. Serum biochemistry often reveals only hypoglycemia, and urinalysis is unremarkable. Most other differential diagnoses for hypoglycemia result in additional clinical signs, additional laboratory abnormalities, or both.

A diagnosis of insulinoma can be made by measuring serum insulin on the same sample that has a glucose concentration of <3.3 mmol/L (<60 mg/dL). In some cases it may be necessary to monitor serum glucose every hour during fasting in order to attain a concentration of <3.3 mmol/L. A diagnosis of insulinoma is supported by a serum insulin concentration above the RI despite concurrent hypoglycemia. Low serum insulin does not support the diagnosis, as this is the expected result in normal animals when serum glucose is low. Serum insulin in the middle to upper RI is suggestive of insulinoma, however, the test should be repeated under these circumstances. Fructosamine concentrations can be measured as an indicator of chronic hypoglycemia. Imaging may also be useful in confirming the presence of a pancreatic mass. Cytology and histopathology can be helpful in diagnosis as well.

Other pancreatic islet tumors

Tumors of pancreatic α cells result in hyperglucagonemia. Amino acid concentrations are low with these tumors probably from accelerated amino acid catabolism due to high glucagon concentrations. Affected animals develop superficial necrolytic dermatitis from protein depletion in the epidermis. DM can also result from the high glucagon concentrations.

Gastrinomas (gastrin-producing tumors) may result from a reversion of pancreatic δ cells to fetal activity. Gastrinomas can also produce other hormones such as glucagon, insulin, and ACTH. Clinical signs and laboratory findings with gastrinomas are discussed in Chapter 9: Digestive System.