Bartter syndrome

Overview and Pathophysiology

Bartter syndrome encompasses a group of rare, primarily autosomal recessive, salt-losing tubulopathies.
The fundamental defect lies in the impairment of sodium chloride (NaCl) reabsorption within the thick ascending limb (TAL) of the loop of Henle.
Defective NaCl reabsorption directly causes severe renal salt wasting and subsequent extracellular fluid volume contraction.
This volume depletion acts as a potent stimulus for the renin-angiotensin-aldosterone system (RAAS), resulting in marked secondary hyperaldosteronism.
Elevated aldosterone levels drive compensatory sodium reabsorption in the distal tubule in exchange for the secretion of potassium ( $K^{+}$ ) and hydrogen ( $H^{+}$ ) ions, generating the hallmark hypokalemic, hypochloremic metabolic alkalosis.
The primary transport defect abolishes the lumen-positive transepithelial voltage in the TAL, which is necessary for the paracellular reabsorption of calcium; this leads to marked hypercalciuria and predisposes the patient to nephrocalcinosis.
Prostaglandin E2 (PGE2) synthesis is significantly upregulated, which further inhibits NaCl transport in the TAL, contributing to the massive polyuria and exacerbating the clinical severity.

Bartter syndrome exhibits considerable genetic heterogeneity and is classified according to the specific mutated transport protein.

Type	Gene	Affected Protein / Channel	Inheritance	Key Differentiating Features
Type I (Antenatal)	SLC12A1	NKCC2 ( $N a^{+} - K^{+} - 2 C l^{-}$ cotransporter)	Autosomal Recessive	Antenatal onset, severe polyhydramnios, prematurity, nephrocalcinosis.
Type II (Antenatal)	KCNJ1	ROMK (Apical $K^{+}$ channel)	Autosomal Recessive	Similar to Type I, but may feature transient neonatal hyperkalemia.
Type III (Classic)	CLCNKB	ClC-Kb (Basolateral $C l^{-}$ channel)	Autosomal Recessive	Variable onset (usually infancy/childhood), milder phenotype, nephrocalcinosis is rare.
Type IV (with Deafness)	BSND (or CLCNKA/B)	Barttin ( $β$ -subunit for ClC channels)	Autosomal Recessive	Associated with congenital sensorineural deafness; notably lacks nephrocalcinosis.
Type V (Transient)	MAGED2	MAGE-D2 protein	X-linked Recessive	Extreme prematurity and polyhydramnios, but spontaneously resolves by 2 to 18 months of age.

Typically presents in utero with severe maternal polyhydramnios, driven by massive fetal polyuria, frequently leading to premature delivery.
Neonates exhibit life-threatening episodes of hypovolemic dehydration, massive polyuria, and severe salt wasting.
Physical examination may reveal a characteristic triangular facies with a prominent forehead, large eyes, protruding ears, and a drooping mouth.
Growth retardation and failure to thrive are invariably observed early in the clinical course.
Profound hypercalciuria typically leads to the development of medullary nephrocalcinosis within the first few months of life, with the notable exception of Type IV.

Usually presents later in infancy or early childhood with polyuria, polydipsia, failure to thrive, and recurrent episodes of dehydration.
Patients frequently experience muscle weakness, cramps, and fatigue secondary to chronic, profound hypokalemia.

Initial laboratory evaluation reveals profound hypokalemia, hypochloremia, and metabolic alkalosis.
Urinary electrolytes demonstrate inappropriately elevated fractional excretion of sodium, potassium, and chloride despite systemic volume depletion.
Urinary calcium excretion is classically elevated, a key feature differentiating it from Gitelman syndrome, where hypocalciuria is typical.
Serum levels of renin and aldosterone are markedly elevated, alongside high urinary excretion of PGE2.
Renal ultrasonography is crucial to identify nephrocalcinosis, which is characteristic of the antenatal forms.
A definitive diagnosis and accurate subtyping are established via targeted molecular genetic testing.
Conditions such as cystic fibrosis, chronic vomiting, and diuretic abuse must be meticulously excluded, as they can perfectly mimic the biochemical profile (pseudo-Bartter syndrome).

Acute management focuses on vigorous intravenous fluid resuscitation and the correction of profound electrolyte deficits, particularly critical in premature neonates.
Long-term maintenance therapy relies on the generous, lifelong supplementation of sodium chloride and potassium chloride, often requiring doses of 1-3 mEq/kg/day or significantly higher.
Prostaglandin synthesis inhibitors, primarily indomethacin (1-2.5 mg/kg/day) or selective COX-2 inhibitors, form the cornerstone of pharmacological therapy; they effectively reduce polyuria, normalize renin levels, and improve linear growth.
Extreme caution is required when initiating NSAIDs in premature infants due to the heightened risks of necrotizing enterocolitis, gastrointestinal bleeding, and acute kidney injury.
Potassium-sparing diuretics, such as spironolactone or amiloride, are frequently added to mitigate persistent distal potassium losses and reduce the required dose of oral potassium supplements.
For patients with persistent hypomagnesemia, oral magnesium supplementation is required, though normalization of serum levels can be clinically challenging.
Multidisciplinary care and serial monitoring are essential to assess for NSAID-induced nephrotoxicity, track the progression of chronic kidney disease, and optimize nutritional status.