Insulinoma

Signalment

More common in dogs than cats.
Middle-aged to older dogs.
No sex predilection.
Breed predisposition, more common in medium to large-breed dogs:

Description

Insulinomas are slow growing, well-encapsulated, functional tumours of the beta cells of the pancreatic islet cells. They secrete inappropriately high amount of insulin irrespective of the serum glucose level. They are predominantly malignant (90% of canine insulinomas), with a high metastatic rate to regional lymph nodes, liver and omentum. 60% of isulinomas are carcinomas, which are more likely to be endocrinologically active, the others being adenomas.

Diagnosis

Clinical Signs

The following signs are related to hypoglycaemia:

collapse
seizures
muscle tremor
muscle weakness
ataxia
lethagy and depression
exercise intolerance

These signs may be intermittent at the beginning of the disease, but they become more often with time. In between hypoglycaemic episodes, the animals are generally normal. A presumptive diagnosis can be made on the demonstration of the Wipple's triad. This includes the presence of clinical signs associated with hypoglycaemia, fasting hypoglycaemia and amelioration of clinical signs with the administration of glucose. Insulinoma should always be considered when clinical signs are associated with exercise, fasting, excitement or feeding.

Laboratory Tests

Biochemistry

Hypoglycemia

Other Tests

A tentative diagnosis can be made if an inappropriately high serum insulin level in the presence of a hypoglycaemia is shown.

Histopathology

This is needed for confirmation the diagnosis.

For pathology see Insulinoma

Diagnostic Imaging

Radiography

A thoracic radiograph may be used to identify any pulmonary metastases, but it is uncommon for insulinomas to metastasise to the lungs.

Ultrasonography

Occasionally, it may be possible to visualise the location of the tumour on the pancreas. However, this may not always be possible, especially if the tumour is too small. Metastases to lymph nodes and liver can sometimes be seen.

Treatment

Emergency

In the event of a hypoglycaemic episode, a dextrose bolus should be given immdediately.
This should be followed by intravenous fluid therapy with 2.5% dextrose.
Alternatively, if the patient is able to eat, frequent feedings can be used instead of dextrose fluid therapy. This may be preferred to avoid the risk of rebound hypoglycaemia.

Medical

This is more suitable for patients in which surgery has been declined or when surgery is inappropriate or fails due to the presence of metastasis.

Small and frequent meals (3-6 times/day) of complex carbohydrate content.
Exercise restriction.
Glucocorticoid such as prednisolone to increase hepatic glucose production and decrease cellular glucose uptake.
Diazoxide, an oral hyperglycaemic drug, to inhibit pancreatic insulin secretion and tissue glucose uptake.

Surgery

A partial pancreatectomy is suitable for patients with a solitary tumour, provided there is no metastasis. Complications such as persistent hypoglycaemia, pancreatitis, diabetes mellitus, epilepsy and diffuse polyneuropathy can occur.

Prognosis

This is dependent on the WHO staging of the tumour. A stage I and II can expect a median survival time of 18 months whereas it is only 6 months for a stage III. Patients suitable for surgical excision has better prognosis than those treated medically.

References

Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition) W.B. Saunders Company.
Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition) Mosby Elsevier
Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition) Mosby Elsevier.
Polton, G. A., White, R. N., Brearley, M. J. and Eastwood, J. M. (2007) Improved survival in a retrospective cohort of 28 dogs with insulinoma Journal of Small Animal Practice 48:151-156 [1]