Question

A 10-year-old boy presents with recurrent episodes of muscle cramps and fatigue. Investigations reveal hypokalemia, hypomagnesemia, metabolic alkalosis, and low urinary calcium excretion. Blood pressure is normal. What is the most likely diagnosis?

• A. Bartter syndrome
• B. Gitelman syndrome
• C. Liddle syndrome
• D. Renal tubular acidosis (RTA)

Hint:

Bartter vs Gitelman vs Liddle:
$\bullet$ Bartter: Thick ascending limb (Loop diuretic-like) $\rightarrow$ Normotensive, Hypercalciuria, Nephrocalcinosis.
$\bullet$ Gitelman: Distal tubule (Thiazide-like) $\rightarrow$ Normotensive, Hypocalciuria + Hypomagnesemia.
$\bullet$ Liddle: ENaC gain of function $\rightarrow$ Hypertension + Hypokalemic alkalosis.

Answer 1

The Correct Option is B

Concept:
Hereditary tubulopathies affecting renal electrolyte handling typically present with distinct urinary electrolyte excretion patterns, acid-base imbalances, and blood pressure profiles.
Gitelman syndrome is an autosomal recessive salt-wasting tubulopathy of the distal convoluted tubule (DCT) that mimics the chronic administration of thiazide diuretics.
Explanation:
• Gitelman syndrome is caused by loss-of-function mutations in the SLC12A3 gene, which encodes the thiazide-sensitive Sodium-Chloride Cotransporter (NCCT) in the distal convoluted tubule.

• Impaired NaCl reabsorption in the DCT increases distal sodium and fluid delivery to the collecting duct, stimulating aldosterone-mediated sodium reabsorption in exchange for potassium and hydrogen ion excretion, leading to hypokalemia and metabolic alkalosis.

• The two distinguishing laboratory hallmarks that differentiate Gitelman syndrome from Bartter syndrome are:
1. Severe Hypomagnesemia with renal magnesium wasting
2. Hypocalciuria (low urinary calcium excretion, urine calcium-to-creatinine ratio $<0.2$) due to enhanced passive calcium reabsorption in the DCT.

• Bartter syndrome (loop-diuretic phenotype affecting NKCC2/ROMK in thick ascending limb) presents earlier in infancy with polyhydramnios, severe failure to thrive, and hypercalciuria with nephrocalcinosis.

• Liddle syndrome is characterized by constitutive activation of ENaC, causing hypokalemic metabolic alkalosis accompanied by severe hypertension and low renin/aldosterone levels.
Final Answer:
The combination of normal blood pressure, hypokalemia, metabolic alkalosis, hypomagnesemia, and characteristic hypocalciuria is diagnostic of Gitelman syndrome.