Clinical Case Quiz — Electrolyte Disorder
History
Age / Sex:19-year-old woman
Complaint:Progressive generalised muscle weakness and lethargy
Medications:None (no regular medications)
Examination
BP:110/60 mmHg (normotensive)
BMI:19 kg/m²
General:No oedema, no cushingoid features
Investigations
Serum K⁺:2.7 mmol/L ↓ (hypokalemia)
Serum HCO₃⁻:34 mmol/L ↑ (metabolic alkalosis)
Serum Creatinine:72 µmol/L
eGFR:>90 ml/min (normal)
Serum Mg⁺⁺:0.60 mmol/L ↓ (hypomagnesaemia)
Serum Cl⁻:83 mmol/L ↓ (hypochloraemia)
Urinary Cl⁻:60 mmol/L ↑ (inappropriately high)
Urinary Na⁺:55 mmol/day
24h Urinary Ca²⁺:1.5 mmol (hypocalciuria)
Question
What is the most likely diagnosis in this patient?
✔ Correct Answer: C — Gitelman Syndrome
Answer Explanation: Gitelman syndrome is an autosomal recessive tubulopathy caused by loss-of-function mutations in the SLC12A3 gene encoding the thiazide-sensitive NaCl co-transporter (NCC) in the distal convoluted tubule (DCT). It presents in adolescents/young adults with hypokalemia, metabolic alkalosis, hypomagnesaemia, and — crucially — hypocalciuria (24h Ca²⁺ 1.5 mmol). Urinary Cl⁻ is elevated (60 mmol/L), confirming renal chloride wasting. Normal BP excludes hyperaldosteronism.
Why not others:
- A — Bartter syndrome: Similar electrolyte profile but causes hypercalciuria, not hypocalciuria. Presents in neonates/infants.
- B — Loop diuretic abuse: Mimics Bartter (hypercalciuria). No medications history; hypocalciuria argues against it.
- D — Primary hyperaldosteronism: Causes hypertension + hypokalemia. BP here is 110/60 mmHg — excludes this.
- E — Vomiting/Bulimia: Urinary Cl⁻ would be <20 mmol/L. Here it is 60 mmol/L — excludes extrarenal loss.
Clinical Pearls
1. Hypokalemia + metabolic alkalosis + hypomagnesaemia + hypocalciuria + normal BP = Gitelman syndrome until proven otherwise.
2. Key differentiator from Bartter: Gitelman = low urinary Ca²⁺; Bartter = high urinary Ca²⁺.
3. Urinary Cl⁻ ≥20 mmol/L confirms renal chloride wasting — excludes vomiting/laxative abuse.
4. Gitelman physiologically mimics chronic thiazide diuretic use (NCC is the thiazide target).
5. Hypomagnesaemia must be corrected first — hypokalemia will not resolve until Mg²⁺ is repleted.
6. Patients are normotensive or hypotensive due to chronic mild volume depletion.
7. Symptoms: muscle weakness, fatigue, cramps, tetany. Severe cases → QT prolongation, torsades de pointes.
8. Genetics: SLC12A3 mutation, chromosome 16q13, autosomal recessive.
9. Treatment: oral KCl + Mg²⁺ lifelong; amiloride, spironolactone, or indomethacin in refractory cases.
10. Most common inherited renal tubular disorder; prevalence ~1:40,000.
Frequently Asked Questions (FAQ)
1. How do I distinguish Gitelman from Bartter at the bedside?
Check 24h urinary calcium. Gitelman = hypocalciuria; Bartter = hypercalciuria. Bartter also presents in infancy; Gitelman in adolescence or adulthood.
2. Why is urinary Cl⁻ high despite low serum Cl⁻?
The NCC defect impairs DCT reabsorption of NaCl — chloride that should be reclaimed is lost in urine, hence inappropriately elevated urinary Cl⁻.
3. Why does Gitelman cause hypomagnesaemia?
NCC dysfunction impairs TRPM6 (Mg²⁺ channel) activity in the DCT, reducing magnesium reabsorption. Almost universally present.
4. Why is BP normal despite renal salt wasting?
Volume depletion activates RAAS, which partially compensates. Net result is mild volume depletion with normal or low BP.
5. What is the mechanism of metabolic alkalosis?
Volume contraction → aldosterone → increased H⁺ excretion → net HCO₃⁻ generation. Hypokalaemia also stimulates renal ammoniagenesis, worsening alkalosis.
6. Can Gitelman syndrome cause cardiac arrhythmias?
Yes. Severe hypokalemia and hypomagnesaemia prolong the QT interval and can cause torsades de pointes.
7. How is Gitelman syndrome treated?
Oral KCl and Mg²⁺ supplementation lifelong. Magnesium must be replaced first. Amiloride, spironolactone, or indomethacin used adjunctively.
8. Is genetic testing necessary?
Not always. Clinical and biochemical diagnosis is usually sufficient. SLC12A3 testing is used in atypical or familial cases.
9. What is the long-term prognosis?
Generally good. Renal function remains normal in most patients. Main risks are quality of life and arrhythmia with severe electrolyte derangement.
10. MRCP/FCPS exam tip: single test to distinguish Gitelman from vomiting?
Spot urinary chloride. Gitelman: ≥20 mmol/L (renal loss). Vomiting: <20 mmol/L (extrarenal — kidney conserves Cl⁻). High-yield exam point.
Gitelman syndrome
hypokalemia metabolic alkalosis
hypocalciuria differential
SLC12A3 mutation
Bartter vs Gitelman
renal tubular disorder MRCP
hypomagnesaemia hypokalemia
urinary chloride alkalosis
FCPS Part II nephrology
distal convoluted tubule defect
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