Feline acromegaly is a disease characterized by excessive growth hormone secretion, leading to a wide array of clinical signs
caused by the hormone's effects on multiple organ systems. These effects can be divided into two major classes: catabolic
and anabolic. The catabolic actions of growth hormone include insulin antagonism and lipolysis, with the net effect of promoting
hyperglycemia. The slow anabolic (or hypertrophic) effects of growth hormone are mediated by insulin-like growth factors.
Growth hormone stimulates the production of insulin-like growth factors in several tissues throughout the body. Insulin-like
growth factor-1 (IGF-1), which is produced in the liver, is thought to be the key factor that facilitates the anabolic effects
of growth hormone that are responsible for the characteristic appearance of people, dogs, and cats with acromegaly.
Similar to its etiology in people, acromegaly in cats is the result of a functional adenoma of the pituitary gland that releases
excessive growth hormone despite negative feedback.1
ANATOMY AND PHYSIOLOGY
Growth hormone is produced in an anterior lobe of the pituitary gland, specifically by cells called somatotrophs. The regulation of growth hormone is complex, and many factors—both environmental and endogenous—are responsible for its
control. The two most important regulators of growth hormone production and release are growth hormone-releasing hormone (GHRH)
and somatostatin, which are produced in the hypothalamus. While growth hormone release is stimulated by GHRH, it is inhibited
by somatostatin as well as by negative feedback from itself and IGF-1.1
Feline acromegaly is an uncommon disease, although it is thought to be underdiagnosed. It most commonly affects middle-aged
and older, male castrated cats. In one study, 13 of 14 cats with acromegaly were males, with an average age of 10.2 years.2 This association may be biased, however, as most cats in which acromegaly is diagnosed are presented for complications associated
with diabetes mellitus, which is also common in older, male castrated cats. Based on available data, no known breed association
for feline acromegaly exists.
Cats with acromegaly are commonly presented for insulin-resistant diabetes mellitus (insulin doses dependent on insulin type)
with concurrent weight gain rather than weight loss.2 Other clinical signs vary because of the wide range of effects the disease has on the body.
1. This domestic shorthaired cat with presumptive acromegaly is exhibiting a broadened face, a physical change commonly associated
with feline acromegaly. The cat was presented for unregulated diabetes.
Physical changes associated with feline acromegaly include increased body weight, a broadened face, enlarged feet, protrusion
of the mandible (prognathia inferior), increased interdental spacing, organomegaly, and a poor coat (Figures 1-3).
Respiratory disease may result from excessive growth of the soft palate and laryngeal tissues, leading to stertorous breathing
and even upper airway obstruction. Cardiovascular signs include the presence of a heart murmur, hypertension, arrhythmia,
and congestive heart failure associated with hypertrophic cardiomyopathy.3
2. The same cat as in Figure 1 exhibiting another physical change associated with feline acromegaly—protrusion of the mandible.
Neurologic disease associated with feline acromegaly is uncommon but can occur with large pituitary adenomas. Neurologic signs
that have been observed with acromegaly include dullness, lethargy, abnormal behavior, circling, and blindness.
Glomerulopathy and secondary renal failure have also been associated with feline acromegaly. Histologic evaluation of the
kidneys of cats with acromegaly has revealed thickening of the glomerular basement membrane and Bowman's capsule, periglomerular
fibrosis, and degeneration of the renal tubules.2
3. This close-up of the cat's teeth (the same cat as in Figures 1 & 2) highlights increased interdental spacing, another physical
change associated with feline acromegaly.
Because of an associated degenerative arthropathy and peripheral (diabetic) neuropathy, lameness has also been noted in cats