Acromegaly In Cats
Acromegaly may be the cause of your diabetic cat having poorly-regulated blood sugar. Acromegaly is defined as an excess production of growth hormone.
Growth hormone (GH) is one of many steroid hormones produced by the body. GH is synthesized in the pituitary gland, inside the brain. Acromegaly occurs when a functional, non-cancerous (benign) tumor in the pituitary produces excessive amounts of growth hormone.
Acromegaly occurs in people, dogs and cats. It is a far less common condition in dogs than in cats. In people, it occurs in approximately two males to every female. In cats, the ratio is more like ten males to one female. However, older male cats are more likely to be diagnosed with diabetes than females, so the data may be skewed by that factor. There is no known breed predisposition among cats.
Acromegalic cats are usually presented for signs of unregulated feline diabetes. If the cat was previously diagnosed with diabetes, the owner may notice an uptick in water intake and appetite, which may also be accompanied by weight gain or loss (weight gain is more common than weight loss, and can be dramatic). Astute owners may observe physical changes associated with acromegaly, including:
• broadening of the face
• protrusion of the mandible (lower jaw)
• increased space between teeth as the lower jaw enlarges and thus moves the teeth further apart
• increased foot size
• dull hair coat
• snoring (even when awake) caused by growth of soft palate and tissues around the larynx
Your pet’s doctor’s examination findings may include:
• palpation of the abdomen may reveal enlarged abdominal organs
• heart changes may result in hypertrophic cardiomyopathy, a heart murmur, high blood pressure, irregular heart rhythm and congestive heart failure
• laboratory testing may reveal protein and glucose and/or ketones in the urine, low specific gravity, renal failure. Blood tests may help confirm kidney failure, and be accompanied by elevated liver enzymes, high cholesterol, high phosphorous levels, increased globulin protein (from the immune system) and an increased red blood cell count.
• lameness, which results from changes in and around joints, combined with diabetic neuropathy
• rarely, pituitary tumors may become large enough to result in mental dullness, lethargy, circling and blindness
For people, specific assays for growth hormone exist. There is insufficient market for monetary incentive to produce a test for cats in the United States. A test is available in Europe, but is not imported here. Further, there are problems with diagnosing acromegaly in cats by growth hormone levels alone:
• levels of the hormone may be low early in the disease process, even though the patient suffers from acromegaly
• growth hormone levels are not constant. They cycle up and down throughout the day, therefore, repetitive tests are required to identify a trend.
• because of a quirk of metabolism in diabetic cats, growth hormone levels may already be elevated, even if the cat is not acromegalic
• even though human medicine has an accurate and available test, growth hormone level is not used alone to diagnose the condition
Another laboratory test, which assays for insulin-like growth factor 1 (IGF-1), can be a strong aid to diagnosis in cats, people and dogs suspected of being acromegalic. Produced by the liver in proportion to the level of growth hormone, it varies little in the course of the day. However, it lacks validity in untreated diabetic cats because the liver requires sufficient levels of insulin to produce IGF-1. Also, treated, recovering diabetic cats (especially those who are beginning to produce their own insulin again) may have higher IGF-1 levels than non-diabetic cats.
In another study, untreated diabetic cats’ IGF-1 levels were measured, and most were low to normal. After beginning insulin, those patients’ IGF-1 levels increased. Therefore, it is recommended that acromegaly suspects have their IGF-1 levels retested a few weeks after beginning insulin therapy or increasing insulin dosage.
As you see, the diagnosis of acromegaly in cats relies strongly on the physical examination and laboratory testing with an eye toward abnormalities expected in the livers and kidneys of acromegalic cats.
Radiographs (X-rays) may show joint collapse, arthritis, soft tissue changes around the joints, bony changes in the skull, deterioration of the spine, elongation of the mandible and increased heart size.
Magnetic resonance imaging (MRI) may reveal a mass in the pituitary gland. However, pituitary tumors associated with acromegaly are not the only masses that can grow in pituitaries. Also, there is one case on record in which no tumor was visible on MRI, but one was confirmed on necropsy when the patient succumbed. It is possible, albeit difficult, to biopsy a pituitary in the living patient. Board-certified veterinary neurosurgeons do sometimes surgically remove pituitary glands, but that opens a host of new complications.
Feline Cushing’s disease, which can also be caused by a benign pituitary tumor, can present with similar symptoms.
Treatment of feline acromegaly remains problematic. Most owners and practitioners settle for increasing insulin dosages in an effort to obtain some measure of diabetes control, even though this is not an ideal approach.
Medications used with mixed success in people show even less success in cats.
Surgical removal of the primary tumor is challenging and associated with complications.
Radiation therapy can help, but side effects, cost and limited availability present barriers.
See you next week, Dr. Randolph.