Wednesday, May 15, 2013

Insulinoma in Dogs, Cat, and Ferrets: Confirming the Diagnosis

Insulinoma, or functional beta-cell tumor, originates from the islet cells of the endocrine portion of the pancreas (1,2). Insulinoma has been described most commonly in dogs and ferrets, and less commonly in cats. Although insulinoma cells produce a variety of polypeptides, most animals with insulinoma are examined because of clinical signs related to hyperinsulinism and resultant hypoglycemia.

Insulinoma has been reported in dogs ranging from 3-15 years old but is most common in dogs older than 8 years old (3-8). Insulinoma is very common in domestic ferrets, with an age range from 2-7 years (9-11). No sex predilection has been reported in dogs, but male ferrets seem to be affected more commonly than females. Insulinoma appears to be a rare condition in cats, with only five cats having been reported; these cats ranged in age from 12-17 years (12-15).

Clinical Signs
Clinical signs in animals with insulinoma are caused by hyperinsulinism, which leads to hypoglycemia. In response to a low blood glucose concentration, catecholamines, glucagon, cortisol, ACTH, and growth hormone are released.

When there is a drop in blood glucose in clinically normal animals, these hormones (i.e., catecholamines, glucagon, cortisol, and growth hormone), in conjunction with a decrease in circulating insulin, help prevent progressive and potentially dangerously low blood glucose concentration. In animals with insulinoma, insulin is secreted even in the face of hypoglycemia and the increase in the counterregulatory hormones listed above. In these patients, the blood glucose is not stabilized, but continues to fall.

Dogs with insulinoma may be examined because of clinical signs related to neuroglycopenic symptoms produced by glucose deprivation of the central nervous system (e.g., hypoglycemia). Less commonly, some animals show adrenergic symptoms caused by catecholamines such as epinephrine (e.g., nervousness, tachycardia) (1,2,16).

The most common complaint for dogs with insulinoma is seizures (1-8). Other signs include collapse, lethargy, weakness, ataxia, mental dullness, muscle fasciculation, trembling, and nervousness (Table 1). Similar signs have been reported in cats with insulinoma (1,12-15). Peripheral neuropathy in association with insulinoma and hypoglycemia has been rarely reported in dogs with insulinoma (17,18).
Table 1: Clinical signs associated with insulinoma in dogs, cats, or ferrets (from reference 2).

Ferrets with insulinoma also commonly show signs of weakness and lethargy (9-11). As in dogs, these symptoms may be episodic. However, seizures are relatively uncommon in this species. Ptyalism is a clinical sign associated with insulinoma in ferrets that has not been described in dogs. The cause of this sign is not known, but ptyalism in ferrets may indicate nausea.

Confirming the Diagnosis— Fulfilling Whipple's Triad
A complete history may lead the clinician to suspect that a patient’s presenting clinical signs are related to hypoglycemia, and thus consider insulinoma as a differential diagnosis. A plasma glucose concentration of 40 mg/dl (2 mmol/L) or less supports the conclusion that the signs are caused by hypoglycemia (16).

If administration of glucose relieves the clinical signs of weakness, disorientation, seizures or trembling, we can conclude that these symptoms are caused by hypoglycemia. This full fills Whipple's triad (i.e, signs of hypoglycemia, biochemical confirmation of low blood glucose at time of clinical signs, and relief of clinical signs after glucose administration) (19,20). This positive response may be seen in animals with hypoglycemia for any reason, however, and is not diagnostic of insulinoma.

Other Causes of Hypoglycemia
In addition to insulinoma, there are many other possible causes of hypoglycemia in animals (1,2,16,21) (Table 2).

Table 2: Causes of hypoglycemia in the mature animal

Many of these differential diagnoses can be ruled out quickly during the initial history and physical examination. After consideration of these diseases is eliminated, insulinoma should be seriously considered in a mature patient with clinical signs of hypoglycemia.

Confirming the Diagnosis—Documenting Hyperinsulinemia
Hyperinsulinism is best diagnosed by the interpretation of serum insulin and glucose concentrations obtained from the patient at the same time. If the clinician suspects hyperinsulinism at the time of initial examination of an animal showing signs of hypoglycemia, serum samples for glucose and insulin measurements are best obtained at that time.

If attempts are made to document hyperinsulinism at a later date, blood samples should be obtained after fasting when the glucose is less than 50 mg/dl (<3.0 mmol/L). It is essential that patients suspected of having hyperinsulinism fast under supervision to allow intervention should signs of hypoglycemia occur.

A high insulin concentration in any animal with concurrent hypoglycemia is consistent with hyperinsulinism (1,2,16,22). If a hypoglycemic patient has an insulin concentration that is within the reference range, the animal again should fast, and the test should be repeated when two consecutive serum glucose readings of 50 mg/dl or less are obtained. If the patient is consistently hypoglycemic, an insulin level within the normal range is considered inappropriate and the patient likely has hyperinsulinism.

Identifying the Pancreatic Nodule
Whenever possible, abdominal ultrasound should be performed in dogs and cats with suspected insulinoma. It can be difficult to detect small pancreatic nodules via ultrasound, but it may be helpful in identifying abdominal metastases (2,22,23). In all species, abdominal ultrasonography may help rule out other neoplasms as a cause of hypoglycemia.

Computed tomography (CT) can also be used to accurately identify pancreatic nodules, and this procedure may be helpful in surgical planning (23).

Bottom Line

Although an accurate diagnosis of insulinoma can generally be made by clinical pathologic testing, histologic examination is required for a definitive diagnosis. Exploratory celiotomy is recommended in all patients with insulinoma if the owner wishes to pursue treatment, but long-term medical management can be helpful in many of these animals.

In my next post, I'll be discussing emergency management of hypoglycemia associated with insulinoma.  This is a critical issue—  if we can't control the immediate clinical signs of hypoglycemia, we will never be able to proceed to definitive or long-term treatment of this serious disorder.

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