Signalment
Insulinomas are malignant insulin-secreting tumours
of the pancreatic B-cells. They typically occur in middle aged-elderly dogs
and both males and females are equally affected. Any breed can be affected
although it usually affects large breeds with Labrador/retrievers, standard
poodles, fox terriers, Irish setters and German shepherd dogs possibly being
predisposed.
Clinical Signs
Clinical signs are usually related to hypoglycaemia and include seizures
intermittent weakness, collapse, ataxia/hind limb weakness, muscle tremors,
polyphagia, polyuria / polydipsia and weight gain. The signs may be associated
with periods of exercise or play. The clinical signs are usually short lived
and intermittent. The tumours are usually responsive (secrete insulin)
to increases in blood glucose and so paradoxical hypoglycaemia and associated
clinical signs may occur 2-4 hours after feeding.
Diagnostic Tests
Routine biochemical and haematological evaluation is usually unremarkable
and these profiles should be performed to help exclude some of the other
causes of hypoglycaemia (hypoadrenocorticism, liver disease, septicaemia).
Laboratory diagnosis of an insulinoma relies on demonstration of fasting
hypoglycaemia and inappropriately normal or increased circulating insulin
concentrations. Insulin concentration should be measured on separated serum.
At the same time, whole blood for glucose estimation should be collected.
The serum for insulin determination should be frozen immediately after separation
and transported frozen to the laboratory for analysis. If equivocal results
are obtained with the glucose and insulin measurements, consider re-testing
on a second occasion or using the insulin:glucose ratio. Please call the
laboratory for further advice in this situation.
Treatment
Hypoglycaemic seizures should be treated with 1ml/kg 50% dextrose intravenously
over 5-10 minutes. This can be repeated as required. Dogs should be maintained
on a glucose infusion until eating again. At home, owners can use glucose
gels which are smeared on the gums. Surgical removal of the tumour is potentially
curative. However, metastatic spread is common and many patients are poor
anaesthetic candidates. With no evidence of metastases, 50% of dogs that
have surgery survive 18 months compared to 6 months if distant spread is
present at diagnosis. Mean survival time for medically managed patients
is 12 months from the onset of clinical signs. Medical management involves
drug therapy and dietary modification. The goals of medical therapy are
to control the clinical signs, not to re-establish euglycaemia. Affected
dogs should be given small meals frequently. Exercise should be restricted
and heavy exertion avoided. Prednisolone (0.5-1mg/kg twice daily)
(alter dose to effect) antagonises insulin activity and promotes
gluconeogenesis and is useful in managing affected dogs. The lowest effective
dose should be used since therapy is lifelong and the side effects can be
problematic. Diazoxide (5-30mg/kg twice daily) ("Eudemine",
unlicensed) acts by inhibiting insulin secretion. It also promotes gluconeogenesis
and can also be a useful addition (usually alongside prednisolone).
However gastrointestinal side effects or occasionally blood dyscrasias are
a potential complication of this drug. Both drugs may have progressively
less effect with time. Octreotide may have some beneficial effects but this
has not been consistently demonstrated and it requires frequent subcutaneous
administration. Somatostatin and Alloxan have been used as "last
resorts" and may have some beneficial effect. However, they are
not routinely used due to variable effectiveness and their potential for
complications. |