Feline Acromegaly Study Guide

Overview and Clinical Importance

Feline acromegaly (also known as hypersomatotropism) is an endocrine disorder caused by chronic excessive secretion of growth hormone (GH) from a functional pituitary adenoma. This condition has emerged as a critically important differential diagnosis for difficult-to-regulate diabetic cats and represents a significant topic on the NAVLE examination.

Recent studies have revealed that acromegaly affects approximately 25-32% of diabetic cats, making it far more common than previously recognized. The disease is caused by a GH-secreting adenoma of the anterior pituitary (pars distalis), which leads to insulin resistance, diabetes mellitus, and characteristic soft tissue and skeletal changes.

Etiology and Pathophysiology

Cause of Feline Acromegaly

The predominant cause of feline acromegaly is a functional somatotropic adenoma (acidophil adenoma) in the pars distalis of the anterior pituitary gland. These tumors grow slowly and may be present for a prolonged period before clinical signs become apparent. Rarely, somatotropic hyperplasia may be the underlying cause. Unlike dogs, where acromegaly is typically caused by progestogen-induced mammary GH secretion during diestrus, feline acromegaly is almost exclusively pituitary in origin.

Growth Hormone Regulation

Growth hormone is normally secreted in a pulsatile fashion by somatotrophs in the anterior pituitary. Regulation involves two primary hypothalamic hormones: Growth Hormone-Releasing Hormone (GHRH) which stimulates GH release, and Somatostatin which inhibits GH release. Negative feedback is provided by both GH and IGF-1. In acromegaly, the adenoma secretes GH autonomously, bypassing normal feedback mechanisms.

Effects of Growth Hormone Excess

Chronic GH excess produces both catabolic and anabolic effects:

Signalment and Epidemiology

Clinical Signs and Physical Examination

Historical Findings

The clinical signs of acromegaly develop gradually over months to years. Many cats are diagnosed during investigation of poorly controlled diabetes mellitus. The classic triad of diabetes-related signs includes polyuria, polydipsia, and polyphagia.

Physical Examination Findings

Physical changes may be subtle or dramatic depending on disease duration. Classic acromegalic features take time to develop and are not present in all cats at diagnosis. Comparison of current photos with older photos can be helpful.

Cardiovascular Manifestations

Cardiac disease is a major cause of morbidity and mortality in acromegalic cats. The cardiac changes mirror those seen in humans and are termed acromegalic cardiomyopathy. GH and IGF-1 exert direct effects on the myocardium, causing hypertrophy and potentially leading to heart failure.

Diagnosis

No single diagnostic test definitively confirms feline acromegaly. Diagnosis relies on a combination of compatible clinical signs, elevated IGF-1 concentrations, and pituitary imaging.

When to Suspect Acromegaly

• Diabetic cat failing to achieve glycemic control despite insulin doses greater than 1.5-2.0 U/kg per injection
• Cat requiring greater than 6-8 units of insulin per injection
• Weight gain or maintenance despite poorly controlled diabetes
• Physical changes suggestive of acromegaly (broad face, prognathia, large paws)
• Unexplained organomegaly (hepatomegaly, renomegaly)
• Respiratory stridor or snoring of unknown cause

Laboratory Findings

IGF-1 Measurement

Serum IGF-1 (Insulin-like Growth Factor-1) is the primary screening test for feline acromegaly. IGF-1 is preferred over GH measurement because it is not secreted in a pulsatile fashion, has excellent species homology, and commercial assays are available.

Pituitary Imaging

Advanced imaging with CT or MRI is essential to confirm the presence of a pituitary mass, assess tumor size, and plan treatment. MRI is considered more sensitive than CT for detecting small adenomas.

Differential Diagnosis

The key differential diagnosis for acromegaly is pituitary-dependent hyperadrenocorticism (PDH). Both conditions can cause insulin-resistant diabetes and pituitary enlargement.

Treatment Options

Conservative Management Details

When definitive treatment is not available or declined, the goal is to manage insulin-resistant diabetes with high-dose insulin therapy. Key points include:

• Long-acting insulins (glargine, detemir) are preferred
• Doses may need to exceed 15-20 units per injection; some cats require TID-QID dosing
• Avoid exceeding 12-15 units per injection without close monitoring (hypoglycemia risk)
• Blood glucose typically maintained 200-400 mg/dL (goal: avoid ketosis, not perfect control)
• Monitor for cardiac disease progression; may need cardiac medications

Prognosis

Common Causes of Death: Congestive heart failure (13%), neurologic disease (13%), chronic kidney disease (11%), and complications of expanding pituitary mass. Early diagnosis and treatment significantly improve prognosis.