Feline Hyperaldosteronism Study Guide

Overview and Clinical Importance

Primary hyperaldosteronism (PHA), also known as Conn's syndrome, is the most common adrenocortical disorder in cats. This condition results from autonomous hypersecretion of aldosterone, independent of the renin-angiotensin-aldosterone system (RAAS). Despite being relatively common, hyperaldosteronism is significantly underdiagnosed in feline practice because its clinical signs overlap with chronic kidney disease (CKD).

The classic triad of feline hyperaldosteronism includes hypokalemia, systemic arterial hypertension, and elevated plasma aldosterone concentration. Understanding this condition is essential for NAVLE success, as questions frequently test the ability to differentiate primary hyperaldosteronism from secondary causes associated with CKD.

Anatomy and Physiology of Aldosterone

Adrenal Gland Anatomy

The feline adrenal glands are paired retroperitoneal organs located craniomedial to the kidneys. Unlike dogs, feline adrenal glands have a similar bilobed or bean-shaped appearance on both sides. The adrenal cortex consists of three zones:

The Renin-Angiotensin-Aldosterone System (RAAS)

Under normal physiological conditions, aldosterone secretion is regulated by the RAAS. When renal perfusion decreases, the juxtaglomerular cells release renin, which converts angiotensinogen to angiotensin I. Angiotensin-converting enzyme (ACE) then converts angiotensin I to angiotensin II, which stimulates the zona glomerulosa to secrete aldosterone.

Aldosterone actions at the collecting duct: Aldosterone binds to mineralocorticoid receptors on principal cells, increasing expression of epithelial sodium channels (ENaC). This promotes sodium reabsorption from urine, with water following passively. Potassium is excreted in exchange for sodium, leading to the characteristic hypokalemia when aldosterone is elevated.

Etiology and Pathophysiology

Primary vs Secondary Hyperaldosteronism

Causes of Primary Hyperaldosteronism in Cats

Unilateral adrenocortical tumor (most common): Adenomas and adenocarcinomas occur with approximately equal frequency. These tumors arise from the zona glomerulosa and autonomously secrete aldosterone. Carcinomas may invade the caudal vena cava (approximately 10% of cases) or metastasize.

Bilateral adrenocortical hyperplasia (idiopathic hyperaldosteronism): This involves bilateral hyperplasia of the zona glomerulosa without tumor formation. While less commonly diagnosed than tumors, this form may be underdiagnosed because affected cats are typically treated medically without histopathological confirmation.

Clinical Presentation

Signalment

Affected cats are typically middle-aged to geriatric (median age 13 years, range 5-20 years). No breed or sex predilection has been documented. Because cats are often geriatric, owners frequently attribute clinical signs to normal aging changes.

Clinical Signs

Clinical manifestations relate to the two major consequences of aldosterone excess: hypokalemia and systemic hypertension.

Why Cats are Predisposed to Cervical Ventroflexion

Unlike dogs and large domestic animals, cats lack a nuchal ligament - an elastic ligament that helps passively support the weight of the head in other species. When hypokalemia causes generalized muscle weakness, cats cannot maintain normal head posture, resulting in the characteristic cervical ventroflexion. This sign is relatively specific to cats and should immediately trigger consideration of hypokalemic myopathy.

Diagnostic Approach

Minimum Database Findings

Endocrine Testing

Plasma Aldosterone Concentration (PAC)

Reference range: 10-243 pg/mL (varies by laboratory and assay method). Cats with aldosterone-secreting adrenal tumors typically have PAC greater than 1000 pmol/L (often greater than 5000 pmol/L). Cats with bilateral hyperplasia may have only mildly elevated or high-normal values.

Critical interpretation point: Aldosterone concentration must be interpreted in light of potassium status. Since hypokalemia normally suppresses aldosterone secretion, even a "normal" aldosterone level in a hypokalemic cat is inappropriately high and suggests primary hyperaldosteronism.

Aldosterone:Renin Ratio (ARR)

The ARR is considered the gold standard screening test for primary hyperaldosteronism. A high PAC with low or suppressed plasma renin activity (PRA) indicates autonomous aldosterone production independent of RAAS stimulation. In cats with adrenal tumors, the ratio is typically very high because renin is completely suppressed. The ratio may be less dramatic in bilateral hyperplasia where some renin activity persists.

Diagnostic Imaging

Abdominal Ultrasound

Ultrasound is the most commonly used imaging modality. Normal feline adrenal glands are bilobed, hypoechoic, and surrounded by hyperechoic fat. Normal maximum dorsoventral thickness is less than 4.0-5.0 mm (depending on body weight). Findings in hyperaldosteronism include:

• Unilateral adrenal mass (most common finding)
• Masses can range from small nodules to greater than 5 cm diameter
• Contralateral adrenal gland may be small/atrophic
• Bilateral enlargement in idiopathic hyperaldosteronism
• Mineralization may be present (incidental in older cats)
• Cannot reliably differentiate adenoma from carcinoma on imaging

CT/MRI

Cross-sectional imaging provides superior detail for surgical planning. CT with contrast enhancement is particularly useful for evaluating vascular invasion into the caudal vena cava, detecting metastases to lymph nodes or liver, and assessing tumor margins. CT is recommended before adrenalectomy, especially for right-sided or large masses.

Differential Diagnosis

Differentiating PHA from CKD

Both conditions can cause hypokalemia and hypertension, making differentiation challenging. Key distinguishing features:

Other Causes of Cervical Ventroflexion

• Thiamine deficiency: Often spastic rather than flaccid; associated with inappropriate diet
• Hyperthyroidism: Can cause hypokalemia; check T4
• Periodic hypokalemic polymyopathy (Burmese cats): Hereditary; episodic; young cats
• Myasthenia gravis: Rare in cats; responds to edrophonium
• Organophosphate toxicity: History of exposure; cholinergic signs

Treatment

Surgical Treatment

Unilateral adrenalectomy is the treatment of choice for confirmed unilateral primary hyperaldosteronism without evidence of metastasis. Surgery can be curative for both adenomas and adenocarcinomas, with resolution of hypertension and hypokalemia and no need for long-term medication in most cases.

Preoperative Stabilization

Cats should be medically stabilized for several weeks before surgery:

• Correct hypokalemia with IV potassium chloride (do not exceed 0.5 mEq/kg/hour)
• Initiate spironolactone (1-2 mg/kg PO BID)
• Control hypertension with amlodipine (0.625-1.25 mg/cat PO once daily)
• Target serum potassium greater than 3.5 mEq/L and BP less than 160 mmHg

Surgical Considerations

Adrenalectomy can be performed via ventral midline celiotomy, paracostal approach, or laparoscopically. Right-sided tumors are more challenging due to proximity to the caudal vena cava. Survival rates have improved significantly (60-80% historically to greater than 97% in recent studies). Potential complications include hemorrhage, thromboembolism, acute renal failure, and transient hypoaldosteronism.

Medical Treatment

Medical management is indicated for bilateral disease, metastatic disease, or when surgery is declined. The goal is to control clinical signs, not cure the underlying disease.

Treatment goal: Target serum potassium of 3.0-4.0 mEq/L and systolic BP less than 160 mmHg. Achieving normal potassium levels is difficult; the goal is to prevent clinical signs, not normalize values.

Prognosis