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==Pathophysiology==
==Pathophysiology==
The [[Loop of Henle|thick ascending limb of the loop of Henle]] is not permeable to water and reabsorbs a large proportion of the filtered [[sodium chloride]] as shown in the figure, which leads to interstitial hypertonicity that powers the [[countercurrent exchange]] and urinary concentration mechanisms. In case of impairment of this function, a major loss of [[water]] and [[sodium]] occurs, as seen with [[Loop diuretic|loop diuretics]].[[Bartter syndrome]] is a [[Renal tubular disorder|renal tubular salt-wasting disorder]] in which the kidneys cannot reabsorb [[sodium]] and [[chloride]] in the [[Loop of Henle|thick ascending limb of the loop of Henle]]. Impairment of [[sodium]] and [[chloride]] reabsorption is the primary defect in the [[Bartter syndrome]] that initiates the cascade. This leads to increased delivery of salt to the [[Distal convoluted tubule|distal tubules]] and excessive salt and water loss from the body. The resultant volume depletion causes activation of the [[Renin-angiotensin system|renin-angiotensin-aldosterone system (RAAS)]] and subsequent [[Hyperaldosteronism|secondary hyperaldosteronism]]. Long-term stimulation causes hyperplasia of the [[juxtaglomerular apparatus]] and elevates [[Renin|renin levels]]. Excessive distal delivery of [[sodium]] follows by [[sodium]] (Na) reabsorption in the [[distal convoluted tubule]]. [[Na+-exporting ATPase|Na]] reabsorption exchange with the secretion of positively charged [[potassium]] or [[hydrogen]] ion and leads to increased loss of [[potassium]] (K+) in urine and increased [[Hydrogen|hydrogen (H+)]] secretion. Decreased [[Chloride|chloride (Cl-)]] reabsorption decreases the exchange with [[bicarbonate]] (HCO3-). Thus, increased [[bicarbonate]] retention and [[hypokalemia]] result in [[metabolic alkalosis]]. [[Calcium]] and [[magnesium]] reabsorb in the [[Loop of Henle|ascending limb of the loop of Henle]] as a result of a [[Electrochemical gradient|positive electrochemical gradient]] in the lumen created by the [[Potassium ion channels|back leak of K+ ion]] in the lumen, drives passive paracellular sodium, calcium, and magnesium reabsorption as shown in the figure. The defective [[sodium chloride]] transport in the [[Loop of Henle|ascending limb of the loop of Henle]] associated with [[Bartter syndrome]] leads to the impaired [[electrochemical gradient]] leading to increased urinary loss of [[calcium]] and [[magnesium]]. This leads to the development of [[nephrocalcinosis]] in [[Bartter syndrome]].


==Causes==
==Causes==

Revision as of 11:11, 7 August 2020

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Main article:Bartter syndrome

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Tayyaba Ali, M.D.[2]

Overview

Historical Perspective

Bartter syndrome was first discovered by Bartter et al.and introduced in a seminal paper in the December issue of the American Journal of Medicine in 1962. Authors in the paper reported two pediatric patients with growth and developmental delay associated with hypokalemic alkalosis and normal blood pressure despite high aldosterone production. The syndrome named after Bartter. This disease was observed in children as well as in adults, females as well as males.Authors described in the paper that this disease is characterized by hypokalemia, metabolic alkalosis, hyperreninemia, secondary hyperaldosteronism, and normal blood pressure.

Classification

Bartter Syndrome can be classified into five different types based on genotype. Bartter syndrome can result from homozygous or mixed heterozygous mutations in any of the genes. Thus, affecting the function of genes responsible for synthesis or membrane insertion of the transporters in the ascending limb of the loop of Henle.

Bartter syndrome types 1, 2, and 4 present at a younger age. They present with symptoms, often quite severe in the neonatal period. Bartter syndrome type 3 also called classic Bartter syndrome present later in life and maybe sporadically asymptomatic or mildly symptomatic.

Pathophysiology

The thick ascending limb of the loop of Henle is not permeable to water and reabsorbs a large proportion of the filtered sodium chloride as shown in the figure, which leads to interstitial hypertonicity that powers the countercurrent exchange and urinary concentration mechanisms. In case of impairment of this function, a major loss of water and sodium occurs, as seen with loop diuretics.Bartter syndrome is a renal tubular salt-wasting disorder in which the kidneys cannot reabsorb sodium and chloride in the thick ascending limb of the loop of Henle. Impairment of sodium and chloride reabsorption is the primary defect in the Bartter syndrome that initiates the cascade. This leads to increased delivery of salt to the distal tubules and excessive salt and water loss from the body. The resultant volume depletion causes activation of the renin-angiotensin-aldosterone system (RAAS) and subsequent secondary hyperaldosteronism. Long-term stimulation causes hyperplasia of the juxtaglomerular apparatus and elevates renin levels. Excessive distal delivery of sodium follows by sodium (Na) reabsorption in the distal convoluted tubule. Na reabsorption exchange with the secretion of positively charged potassium or hydrogen ion and leads to increased loss of potassium (K+) in urine and increased hydrogen (H+) secretion. Decreased chloride (Cl-) reabsorption decreases the exchange with bicarbonate (HCO3-). Thus, increased bicarbonate retention and hypokalemia result in metabolic alkalosis. Calcium and magnesium reabsorb in the ascending limb of the loop of Henle as a result of a positive electrochemical gradient in the lumen created by the back leak of K+ ion in the lumen, drives passive paracellular sodium, calcium, and magnesium reabsorption as shown in the figure. The defective sodium chloride transport in the ascending limb of the loop of Henle associated with Bartter syndrome leads to the impaired electrochemical gradient leading to increased urinary loss of calcium and magnesium. This leads to the development of nephrocalcinosis in Bartter syndrome.

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