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Veterinary Forum July 2009 (Vol 26, No 7)

Insulin resistance in cats: Cushing's or acromegaly?

by Paul Basilio

    LAS VEGAS — An insulin-resistant cat can present a frustrating diagnostic quandary for any veterinarian, said Catharine Scott-Moncrieff, MS, VetMB, DACVIM, professor of small animal medicine at the Purdue University School of Veterinary Medicine.

    "Any severe concurrent illness can cause insulin resistance," she said here at the Western Veterinary Conference. "If a cat presents with insulin resistance, how do you create a diagnostic approach to that [problem]?"

    Scott-Moncrieff said that she first looks for insulin resistance caused by drug administration and other parameters that may indicate the underlying disease by performing a thorough examination. "A good physical examination is important," she explained. "One of the things that happens to diabetic cats is that they come in for a blood glucose curve, our technicians look at them, but [the veterinarian] never checks back on them. If you have a cat that's in trouble with its glycemic control, bring it in for a full appointment and repeat the physical exam to palpate the abdomen and listen to the heart. Take a minimum database for a blood panel and biochemical panel, as well as a T4 if the cat is elderly."

    A blood glucose curve should be conducted to confirm the severity of insulin resistance. Scott-Moncrieff said she prefers to obtain thoracic radiographs to look for underlying neoplasia or infection, as well as an abdominal ultrasound to look at the adrenal glands, kidneys, pancreas and bladder.

    Because infection is the most common cause of insulin resistance, she also recommends conducting as many cultures as possible. "Culture any fluids you can get your hands on."

    If a thorough workup has ruled out drug administration, infection, renal disease, pancreatic disease, pregnancy, diestrus, hypolipidemia, obesity, thyroid disease and neoplasia, Scott-Moncrieff said, you are left with two unusual diseases: Cushing's disease and acromegaly.

    "By the time you've finished the physical exam, history and workup, you can decide whether you should look for acromegaly or Cushing's. You're likely not going to be looking for both," she said, adding that both conditions can cause insulin resistance, but the similarities end there.

    Acromegaly

    Acromegaly involves excessive secretion of growth hormones caused by a pituitary adenoma. It results in profound insulin resistance from the postreceptor defect in insulin action.

    "You could treat these cats with 40 to 50 units of insulin, although I wouldn't recommend it," Scott-Moncrieff said. "Generally, these cats do not become hypoglycemic."

    With acromegaly, an elevation of insulin-like growth factor-1 (IGF-1) causes anabolic characteristics, such as overgrowth of soft tissue on the viscera. The clinical signs are similar to those of diabetes, except that cats with acromegaly tend not to lose weight, at least in the short term. Other signs include abdominal organomegaly, inferior prognathia, cataracts, broad facial features, cardiac murmurs and respiratory stridor because of soft tissue overgrowth.

    "You'd think you could recognize these cats, right?" Scott-Moncrieff asked. "The problem is that most cats with acromegaly aren't that obvious. It's a chronic, insidious disease. Many cats have profound insulin resistance long before developing other signs. The key finding is having a stable weight despite poor glycemic control."

    Acromegaly can be diagnosed by measuring the IGF-1 level or by looking for a pituitary mass on an MRI. Imaging studies can be used not only to diagnose but also to plan treatment, Scott-Moncrieff said. "Radiation is a good therapy for this disease. If the owner doesn't want radiation therapy, it's worthwhile to try insulin therapy. You're unlikely to make acromegalic cats hypoglycemic."

    Cushing's disease

    Cushing's disease is caused by a functional pituitary tumor or a functional adrenocortical tumor. Clinical signs include insulin-resistant diabetes mellitus, lethargy, abdominal enlargement, unkempt haircoat, symmetrical alopecia, recurrent infection and thin skin that is easily compressed.

    "A minimum database is not helpful because it is consistent with diabetes mellitus," Scott-Moncrieff said. "I wouldn't recommend starting with a dexamethasone suppression test. Remember it is a higher dose than that used in a dog — typically 0.1 mg/kg intravenously. Some cats will fail to suppress with the lower dose. Get an 8-hour cortisol level. You can do a 4-hour test, but no one really knows how to interpret that in a cat. An ACTH stimulation test is not that sensitive, but it can help."

    Scott-Moncrieff usually conducts abdominal ultrasonography and an endogenous ACTH test to diagnose Cushing's. "If you're worried about a pituitary tumor or want to evaluate the abdomen further, you can use CT or MRI, she said."

    Treatment for Cushing's is frustrating, she admitted. Hypophysectomy is rarely reported, and cats have difficulty recovering from bilateral adrenalectomy.

    "Trilostane looks like the best drug right now for treating Cushing's in cats," Scott-Moncrieff said.

    For more information:

    Scott-Moncrieff CJ. Insulin-resistant diabetes mellitus: is it Cushing's or acromegaly? Presented at: The Western Veterinary Conference. Las Vegas; Feb. 15-19, 2009.

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