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Veterinary Forum March 2009 (Vol 26, No 3)

Expert describes best 'test' for diagnosing Cushing's syndrome

by Marie Rosenthal

    The best method to diagnose Cushing's syndrome is a history and physical examination, Edward C. Feldman, DVM, DACVIM, said here at the Western Veterinary Conference.

    "We want a test that can separate dogs that have Cushing's from dogs that don't have Cushing's. Which 'test' is the best? Which has the best sensitivity and best specificity? There is no doubt in my mind: It is history and physical examination," said Feldman, who is professor of small animal internal medicine at the University of California, Davis, College of Veterinary Medicine. The conference session was sponsored by Hill's Pet Nutrition.

    If the dog presents with polydipsia, polyuria, polyphagia, alopecia, muscle weakness and excessive panting, the dog has Cushing's, according to Feldman. Less common signs include skin infections, hyperpigmentation, testicular atrophy, hepatomegaly, calcinosis cutis and clitoral hypertrophy, along with bruising easily. Feldman said the number and severity of the signs vary.

    Often owners don't recognize the signs of disease because they think the signs are normal in an aging pet. "The thing to remember about dogs with Cushing's is that many of the changes — pot belly, muscle weakness, panting, even hair loss — are associated with aging.

    "The fact that dogs with Cushing's maintain an excellent appetite is a major stimulus not to bring the dog to you. In the United States and Canada, polyphagia is a sign of health," Feldman joked.

    What usually brings these dogs to the clinic is that a formerly housebroken animal is urinating in the house. "The single most life-threatening clinical sign in a dog with Cushing's is polyuria. Polyuria in a housebroken animal is a life-threatening problem. Owners only put up with polyuria for so long and then they say, 'You know, I have so much else going on, I can't deal with this.'"

    Before conducting any endocrine tests, Feldman recommended a complete blood count, urinalysis with culture and serum chemistry profile, as well as abdominal ultrasonography.

    "I would not recommend treating Cushing's based on a laboratory test of any kind. You only treat clinical signs. There are different conditions in veterinary medicine for which we treat a test result. If you have a hypercalcemic dog, you had better treat that dog. But a Cushing's dog that doesn't have clinical signs is an oxymoron because Cushing's is a clinical syndrome," Feldman said.

    "We know there are no perfect tests out there," he said, adding that he thinks Cushing's is overdiagnosed in dogs because some veterinarians diagnose the condition based on laboratory values, instead of history and physical examination findings.

    If the veterinarian does elect endocrine testing, Feldman recommended the low-dose dexamethasone suppression (LDDS) test because it is relatively sensitive and specific, although it is not perfect. LDDS is conducted by obtaining plasma samples for cortisol testing before and 4 and 8 hours after administering 0.01 mg/kg IV dexamethasone. At 8 hours, concentrations greater than 1.4 µg/dl are consistent with Cushing's syndrome, and about 90% of cushingoid dogs will have a high concentration of cortisol after 8 hours.

    For more information:

    Feldman E. Diagnosis and treatment of canine Cushing's: parts I and II. Presented at: The Western Veterinary Conference. Las Vegas, Nev.; Feb. 15-19, 2009.

    NEXT: FORUM FIVE — Working with rehab

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