Acromegaly is due to excess secretion of growth hormone (GH) in adult animals. In cats, it is caused by GH-secreting tumors of the anterior pituitary. Signs related to diabetes mellitus are typically the first clinical signs of acromegaly to be noticed. Some cats with acromegaly show excess growth of the extremities, skull, and muscles. Cardiomegaly and azotemia develop late in the disease. Diagnosis is based on measurement of insulin-like growth factor 1 or on CT scan of the pituitary. Treatments include medication, radiation, or hypophysectomy, along with high doses of insulin to combat insulin resistance.
Acromegaly, or hypersomatotropism, results from chronic, excessive secretion of growth hormone in adult animals. Acromegaly in cats is due to a growth hormone–secreting tumor of the anterior pituitary. These tumors grow slowly in cats and can be present for a long time before clinical signs appear.
Epidemiology of Acromegaly in Cats
Acromegaly affects predominantly older cats (8–14 years old), and it appears to be more common in males.
Clinical Findings of Acromegaly in Cats
Clinical signs of uncontrolled diabetes mellitus (polyuria/polydipsia and polyphagia) are often the reason for initial evaluation of cats with acromegaly.
Net weight gain of lean body mass in cats with uncontrolled diabetes mellitus is a key sign of acromegaly. Organomegaly, including renomegaly, hepatomegaly, and enlargement of endocrine organs, is also observed.
Some cats show enlargement of extremities, overall body size, jaw, tongue, and forehead that is characteristic of acromegaly in humans (see acromegaly image). Some of the most striking manifestations of acromegaly in cats occur in the musculoskeletal system, including an increase in muscle mass and growth of the acral segments of the body, including the paws, chin, and skull.
Courtesy of Dr. Deborah Greco.
Stridor can occur as a result of increased soft tissue enlargement of the epiglottis and surrounding tissues. Cardiovascular abnormalities, such as cardiomegaly (radiographic and echocardiographic), systolic murmurs, and congestive heart failure, develop late in the disease course. Azotemia also develops late in the disease course in approximately 50% of cats with acromegaly.
Neurological signs of acromegaly in humans, such as peripheral neuropathies (paresthesias, carpal tunnel syndrome, sensory and motor defects) and parasellar manifestations (headache and visual field defects), are not generally detected in acromegalic cats.
Impaired glucose tolerance and insulin resistance resulting in diabetes mellitus are evident in all cats with acromegaly. Measurement of endogenous insulin reveals dramatically increased serum insulin concentrations. Despite severe insulin resistance and hyperglycemia, ketosis is rare. Hypercholesterolemia and mild increases in liver enzyme concentrations are attributed to diabetes.
Hyperphosphatemia without azotemia is also a common clinicopathological finding. Urinalysis is unremarkable except for persistent proteinuria. Acromegaly is the most common underlying cause (25% of cases) of non-insulin-dependent diabetes mellitus in cats.Acromegaly should be suspected in any cat that does not respond within 4 months to standard treatment (high-protein, low-carbohydrate diet and glargine insulin) of type II diabetes mellitus.
Lesions
Gross necropsy findings in acromegalic cats can include the following:
a large, expansile pituitary mass
hypertrophic cardiomyopathy with marked left ventricular and septal hypertrophy (early) or dilated cardiomyopathy (late)
hepatomegaly
renomegaly
degenerative joint disease
lumbar vertebral spondylosis
moderate enlargement of the parathyroid glands
adrenocortical hyperplasia
diffuse pancreatic enlargement with multifocal nodular hyperplasia
Histological examination of the endocrine glands reveals the following:
acidophil adenoma of the pituitary
adenomatous hyperplasia of the thyroid gland
nodular hyperplasia of the adrenal cortices, parathyroid glands, and pancreas
Diagnosis of Acromegaly in Cats
CT of the pituitary region
Definitive diagnosis of feline acromegaly requires measurement of increased plasma growth hormone or insulin-like growth factor 1 (IGF-1) concentrations in suspected cases. However, feline growth hormone assays are no longer available. As in humans with acromegaly, acromegalic cats have dramatically increased serum IGF-1 concentrations.
Currently, the most definitive diagnostic test is CT of the pituitary. Results of CT, coupled with exclusion of other disorders that lead to insulin resistance (eg, hyperthyroidism, hyperadrenocorticism) and characteristic clinical signs and laboratory abnormalities, support a diagnosis of acromegaly.
Treatment and Prognosis of Acromegaly in Cats
Dopamine agonists or somatostatin analogues
Radiotherapy
Hypophysectomy
High-dose insulin therapy to combat insulin resistance
Medical treatment of acromegaly in humans includes administration of dopamine agonists (eg, bromocriptine) and somatostatin analogues. Treatment with the novel somatostatin analogue pasireotide has been shown to be successful in cats with acromegaly; however, dosing of this drug requires further study, and treatment is relatively expensive.
Radiotherapy probably offers the greatest chance of success in treating feline acromegaly; it is associated with low rates of morbidity and mortality. The disadvantages of radiotherapy include the slow rate of tumor shrinkage (> 3 years) and the occurrence of hypopituitarism, cranial and optic nerve damage, and radiation injury to the hypothalamus.
Although its availability is limited, hypophysectomy is a viable treatment for cats with acromegaly. As with Cushing disease, hypophysectomy to treat acromegaly can be associated with a rapid drop in hormone concentrations, resulting in remission of clinical signs; however, adverse effects, such as transient diabetes insipidus, are commonly observed.
The short-term prognosis in cats with untreated acromegaly is fair to good. Insulin resistance is generally controlled satisfactorily with large doses of insulin divided and administered several times a day. Mild signs of cardiac disease can be managed with diuretics and vasodilators.
The longterm prognosis for feline acromegaly is relatively poor, however, and most affected cats die of congestive heart failure, chronic renal failure, or signs associated with an expanding pituitary mass. The longterm prognosis can improve with early diagnosis and treatment.
Key Points
Feline hypersomatotropism, or acromegaly, is an uncommon endocrine disorder of excess growth hormone secretion resulting from a pituitary adenoma or hyperplasia.
Clinical signs of acromegaly, such as those associated with poorly regulated diabetes mellitus, can be subtle, or they can be dramatic (eg, overgrowth of the appendicular skeleton resulting in enlargement of the skull and jaw and generalized weight gain without obesity).
Treatment of acromegaly can include medical treatment (pasireotide), radiotherapy, or hypophysectomy. Usually, cats are left untreated except for attempts to control insulin resistance with increasing doses of insulin.
For More Information
Bruyette, D. Feline acromegaly. World Small Animal Veterinary Association World Congress Proceedings, 2015.
Hurty CA, Flatland B. Feline acromegaly: a review of the syndrome. J Am Anim Hosp Assoc. 2005;41(5):292-297.
Greco DS. Feline acromegaly. Top Companion Anim Med. 2012;27(1):31-35.
Also see pet owner content regarding feline acromegaly.
