Resistant hypertension is a condition in which blood pressure continues to be greater than 140/90 mm Hg despite compliance with a regimen of 3 antihypertensive agents including a diuretic. Liddle syndrome is one of the rare causes of resistant hypertension and it usually presents since early childhood. Grant Liddle et al. first explained Liddle syndrome in 1963. It is also known as pseudohyperaldosteronism. It is a rare cause of secondary hypertension due to a genetic mutation, affecting the function of the collecting tubule sodium channel.
Liddle syndrome is a congenital disorder due to a single gene mutation. Patients inherit the disorder in an autosomal dominant with early penetrance. Diverse populations can be affected by the syndrome. Genetic studies showed that this syndrome occurs from a gain of function mutations in the epithelial sodium in the distal nephron. In 1995, Hansson et al. discovered the germline mutation in the SCNN1G gene as a cause of the Liddle syndrome. Later, researchers showed that the epithelial sodium channels (ENaC) comprise of 3 homologous subunits alpha, beta, and gamma coded by SCNN1A, SCNN1B, and SCNN1G genes. Mutation in beta or gamma subunits of the ENaC leads to an amplified activity of this channel, independent of aldosterone activity. While these are the well-described mutations associated with Liddle Syndrome, a systemic review by Tetti et al in 2018 discovered 31 different causative mutations from over 72 different families.
Researchers have not extensively studied the incidence of Liddle syndrome in the hypertensive population. Lin-Ping Wang et al. and Liu et. al studied the prevalence of Liddle syndrome. They found that among younger Chinese patients with unexplained resistant hypertension, the prevalence of Liddle Syndrome was 1.52% and 1.72% respectively. In the first study by Lin-Ping Wang et al, the only participants who were actually tested for the genetic mutation had to be hypokalemic, however, Liddle syndrome can be seen in patients with normal potassium levels. Therefore, if different researches studied the prevalence of Liddle syndrome in patients with normal potassium and resistant hypertension, it would likely be higher. There is no gender or race predisposition.
Researchers of one study in US veterans concluded that there was approximately a 6% prevalence of symptomatic Liddle syndrome.
ENaC is present in the distal colon, ducts of exocrine glands, lungs, and apical surface of the epithelial surface of distal nephron. Patients with Liddle syndrome have an abnormality in the ENaC in distal nephrons due to mutations in 1 of the 3 subunits. Due to this mutation, the degradation of the sodium channels has been impaired; therefore, the quantity of these channels on the apical surface of distal nephron increases inappropriately. The sodium feedback inhibition system is also impaired in patients with Liddle syndrome. Typically, increased intracellular sodium in distal nephron cells inhibits apical epithelial sodium channels, but patients with Liddle syndrome become insensitive to sodium concentration. Increased sodium channels cause increased sodium reabsorption which results in chronic volume retention with a hypertensive state and suppressed renin and aldosterone levels. In this population, renal biopsy showed atrophy of juxtaglomerular cells due to chronically suppressed renin and aldosterone levels.
Additionally, the increased influx of sodium through the ENaC channel causes a number of different effects on other channels within the membrane. First, it causes increased activity of the Na+/K+ ATPase at the basolateral membrane resulting in increased potassium influx into the cell from the basolateral side. Second, the depolarization of the apical membrane of the cell secondary to sodium entry causes potassium secretion through apical potassium channels and thus potassium is excreted in the urine resulting in hypokalemia.
Patients with Liddle syndrome can be symptomatic or asymptomatic. It usually presents with early-onset resistant hypertension between the ages of 11 to 31 years old due to sodium reabsorption at the level of the distal nephron. It may take years or even decades for healthcare providers to diagnose it. Providers can often misdiagnose the symptoms. Hypertension due to Liddle syndrome is sensitive to a salt-restricted diet and it can present with a headache, dizziness, retinopathy, chronic kidney disease, left ventricular hypertrophy, and sudden death. Due to resistant hypertension, hypokalemia, and ventricular hypertrophy, a patient can develop lethal arrhythmias which can potentially lead to sudden death.
Resistant hypertension can cause muscle weakness, polyuria, and polydipsia due to hypokalemia. Hypokalemia and metabolic alkalosis happen due to excessive loss of potassium in the urine at the expense of sodium reabsorption.
The incidence of hypertension and hypokalemia in patients with Liddle syndrome is about 92.4% and 71.8% respectively. A systemic review also found that 58.2% of patients with Liddle syndrome also present with hypoaldosteronism. Healthcare providers can order genetic testing to diagnose patients with Liddle syndrome who do not have hypertension or hypokalemia. In these individuals, genetic testing is usually done because of significant family history.
A patient with Liddle syndrome often presents with secondary or resistant hypertension. Laboratory investigation may reveal hypokalemia and metabolic alkalosis. Hyperaldosteronism can also present with the same features and biochemical abnormalities. Renin and aldosterone levels should be checked to differentiate between true hyperaldosteronism and pseudo-hyperaldosteronism (Liddle syndrome). In patients with Liddle syndrome, renin and aldosterone levels are low in contrast to patients with hyperaldosteronism in whom aldosterone levels are high. Due to low levels of aldosterone, spironolactone does not work for patients with Liddle syndrome.
Once the patients are diagnosed with low-renin and low-aldosterone levels, they should be prescribed aldosterone for two months and stay under close observation by a healthcare provider. Some conditions like glucocorticoid resistance syndrome, apparent mineralocorticoid excess syndrome, and congenital adrenal hyperplasia all respond well to this drug. If the patient does not respond to aldosterone, then the clinician should suspect Liddle syndrome.
Ultimately the final diagnosis is made by genetic analysis of the gene that regulates the ENaC.
As discussed above, low levels of aldosterone render spironolactone ineffective in Liddle syndrome patients. The drug of choice is amiloride and it works well because it directly inhibits ENaC. Amiloride is prescribed daily at a dose that ranges from 5 to 20 mg. Triamterene, another potassium-sparing diuretic similar to amiloride, can also be used to manage this syndrome. A sodium-restricted diet showed a cumulative effect with these drugs. However, excessive sodium accumulation on the receptor makes it unavailable for the medication. If renal function is normal, then the incidence of hyperkalemia is very rare. Avoidance of excessive potassium in the diet is suggested along with the use of potassium-sparing diuretics. Amiloride is safe in pregnancy.
Low renin hypertension can be classified as follows:
Liddle syndrome is classified under low renin with low aldosterone. Others causes of hypertension which are classified under low renin with low aldosterone are as follows:
Mineralocorticoid excess is an autosomal recessive syndrome due to 11beta-hydroxysteroid dehydrogenase type 2 enzyme deficiency. This enzyme converts cortisol (active) into cortisone (inactive) form and this inactive form is unable to bind to the mineralocorticoid receptor.
Gordon syndrome is an autosomal dominant condition due to a gene mutation responsible for ion transport in the kidney which results in increased reabsorption of sodium and decreased excretion of potassium.
Potassium-sparing diuretics including triamterene and amiloride can cause hyperkalemia, if the glomerular filtration rate is not within normal range. Other major side effects of amiloride are aplastic anemia and neutropenia.
Triamterene can cause ventricular arrhythmias.
Patients with Liddle syndrome respond well to medical therapy such as potassium-sparing diuretics. There aren't any studies available on the long-term mortality of Liddle syndrome. Clinicians often undertreat and misdiagnose these patients. Further studies are needed to establish mortality in the population suffering from secondary hypertension due to this syndrome.
Complications of Liddle syndrome include hypokalemia, metabolic alkalosis, and resistant hypertension. The patients with resistant hypertension can develop end-organ damage that includes myocardial infarction, transient ischemic attack or cerebrovascular accident, pulmonary edema and ventricular hypertrophy.
Consultations that could manage these patients better should include Nephrology, Pediatrics, and Endocrinology, or Cardiology that specializes in hypertension treatment.
Late diagnosis of the Liddle syndrome carries important adverse clinical outcomes; therefore, early diagnosis is essential. Coordination between pediatrician and pediatric nephrologist is very important. It is highly recommended that genetic counseling is offered to the other family members.
Clinical genetic testing is available through the Genetic Testing Registry (GTR) and the geneticists will sequence exon 13 of SCNN1B and SCNN1G.
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