Primary aldosteronism

Primary aldosteronism is a condition caused by an excess of the adrenal hormone aldosterone independent of the renin-angiotensin-aldosterone axis.

The hallmarks of disease are hypertension and hypokalaemia – though hypokalaemia is often absent. Primary aldosteronism is increasingly being recognised as a common and under-diagnosed cause of secondary hypertension. Today prevalence is thought to be between 5-15% of patients with hypertension.

Disease is caused by one of a number of subtypes:

• Bilateral idiopathic hyperaldosternism (60-70%)
• Aldosterone-producing adenoma (30-40%)
• Unilateral hyperplasia (approx 3%)
• Other (familial hyperaldosteronism, adrenal carcinoma)

Treatment depends on the underlying cause and whether it is unilateral or bilateral. It may involve surgery (adrenalectomy) or aldosterone antagonists (e.g. spironolactone).

The adrenal gland is divided into two functionally distinct components, an outer cortex and an inner medulla.

Also termed the ‘suprarenal’ gland, the adrenal gland sits atop each kidney. The shape can distinguish laterality with the right gland a pyramidal shape whilst the left is more semilunar.

Adrenal cortex

The adrenal cortex is the outermost aspect of the adrenal gland. It is composed of three layers:

• Zona Glomerulosa: mineralocorticoids – aldosterone is the key mineralocorticoid, release primarily controlled by the renin-angiotensin system (described below).
• Zona Fasciculata: glucocorticoids – release controlled by hypothalamic-pituitary-adrenal axis (described below).
• Zona Reticularis: androgens – produces dehydroepiandrosterone (DHEA).

Adrenal medulla

The innermost aspect of the adrenal gland, composed of a single layer. Responsible for the production of three hormones:

• Adrenaline
• Noradrenaline
• Dopamine

Under normal conditions aldosterone release is primarily controlled by the renin-angiotensin system.

1. Renin

Renin, a proteolytic enzyme, is released by granular cells of the juxtaglomerular apparatus in response to:

• Renal artery hypotension
• Sympathetic stimulation
• Reduced sodium levels in the distal tubal

In the blood renin cleaves angiotensinogen into angiotensin I.

2. Angiotensin

Angiotensin-converting enzyme (ACE), found primarily in the vascular endothelium of the lungs, cleaves angiotensin I to give angiotensin II.

Angiotensin II has multiple functions:

• Stimulates adrenal cortex to release aldosterone
• Causes vasoconstriction
• Increases sodium reabsorption
• Stimulates the release of anti-diuretic hormone (ADH)
• Increases sympathetic permissiveness

3. Aldosterone

Aldosterone is a mineralocorticoid released from the zona glomerulosa of the adrenal cortex. It is released in response to:

• Angiotensin II (primary stimulus)
• ACTH
• Potassium levels

Its primary action is to increase the number of epithelial sodium channels in the distal tubule. This results in sodium and water reabsorption and potassium excretion.

Hypersecretion of aldosterone leads to sodium and water retention and potassium wasting.

Hypertension & hypokalaemia

As described above aldosterone acts on the kidneys, primarily the collecting duct where it increases the number of sodium channels in principal cells. This results in sodium reabsorption followed by water, potassium is excreted as it moves down the electrical gradient.

In the presence of aldosterone excess the human body begins to retain sodium and water. The increase in volume and changes to systemic vascular resistance lead to hypertension. Hypokalaemia may result in 15-30% of cases from the increase in potassium excretion.

Aldosterone escape

The volume expansion prompted by increased water reabsorption results in an ‘escape’ phenomenon. This is a diuresis that occurs in response to the water and sodium retention caused by raised aldosterone. The exact mechanisms are unclear but appear to involve:

• Pressure natriuresis
• Atrial natriuretic peptide release
• Changes to reabsorption in the distal tubule

In the vast majority of cases primary aldosteronism is caused by adrenal adenomas or adrenal hyperplasia.

Bilateral idiopathic hyperplasia

Also called idiopathic adrenal hyperplasia, this the most common cause of primary aldosteronism, implicated in 60-70% of cases. It tends to cause a primary aldosteronism that is less severe than that caused by aldosterone-producing adenomas.

Aldosterone-producing adenoma

Primary aldosteronism was first described by Conn in the 1950’s in patients with adrenal adenomas (hence the alternate name Conn’s syndrome). The autonomous adenoma produces aldosterone independent of normal mechanisms.

Once thought to be the most common cause of primary aldosteronism, it is implicated in around 30-40% of cases.

Unilateral hyperplasia

This is caused by hyperplasia only affecting one gland. Far less common than the bilateral form and may be treated more like an aldosterone-producing adenoma.

Other (rare)

Familial hyperaldosteronism:

• Type 1: Glucocorticoid-remediable aldosteronism
• Type 2: Poorly understood, likely most common familial cause, appears to be due to an inherited adrenal adenoma or idiopathic adrenal hyperplasia.
• Type 3: Mutations in KCNJ5 gene

Adrenal carcinoma is a rare cause.

Frequently asymptomatic, clinical features of primary aldosteronism are those of hypokalaemia (30%) and hypertension.

Hypertension

Hypertension is generally asymptomatic but present in the vast majority of patients with primary aldosteronism. Long-standing hypertension may result in end-organ damage:

• Chronic kidney disease
• Cerebrovascular disease
• Heart failure
• Retinopathy

Hypokalaemia

Hypokalaemia affects around 10-30% of patients with primary aldosteronism. It is normally asymptomatic but may present with non-specific signs and symptoms including:

• Muscle weakness
• Paraesthesia
• Mood disturbance
• Polyuria/nocturia

Hypertension, hypokalaemia and metabolic alkalosis are the classical findings.

Suggestive findings

Hypertension is essentially universal in primary aldosteronism. All other features are variably present:

• Metabolic alkalosis: caused by increased urinary hydrogen excretion, multifactorial, due to hypokalaemia and direct effects of aldosterone on intercalated cells.
• Hypokalaemia: low serum levels are present in 15-30% of patients. Urinary potassium may demonstrate inappropriate potassium loss in the setting of hypokalaemia.

NOTE: Primary aldosteronism has been described with hypokalaemia but an absence of hypertension – this is normally in young women.

Triggers for testing

Patients should be tested where a secondary cause for hypertension is suspected. This includes:

• Hypertension and hypokalaemia (spontaneous or diuretic-induced)
• Severe hypertension (systolic > 150, diastolic > 100)
• Hypertension resistant to treatment
• Hypertension and:
  • Adrenal incidentaloma
  • Sleep apnea
  • Family history of early onset hypertension
  • Family history of early onset CVA
  • Primary aldosteronism affecting all 1st degree relatives with hypertension

Diagnosis of primary aldosteronism requires a multi-tier testing approach.

1. Screening

Management of primary aldosteronism involves both surgical and medical modalities.

Unilateral disease

This includes patients with aldosterone-producing adenomas and unilateral hyperplasia. In those whom are fit and willing, unilateral adrenalectomy offers cure. This may be either laparoscopic or open depending on patient factors and local expertise.

Medical management with mineralocorticoid receptor antagonists (e.g. spironolactone) can be used as a bridge to surgery or as definitive management in those not for surgical intervention.

Bilateral disease

Medical management is typically used:

• Mineralocorticoid receptor antagonists: Spironolactone, Eplerenone (newer, unlike spironolactone does not cause gynaecomastia).
• ENaC inhibitor: Amiloride, a potassium-sparing diuretic, may be used if aldosterone antagonists are not tolerated.

Other antihypertensives may be added.