FCPS Paediatrics TOACS Mock · SIADH (Data Interpretation & Management)

📋 OBSERVED STATION · CPSP FORMAT

🌾 4‑year‑old girl – tachypnoea, chest pain, fever & severe hyponatraemia (Na 128)

History: Returned from rural area, reduced oral intake, no vomiting/diarrhoea. On examination: alert but mildly lethargic, mucous membranes moist, no oedema, chest crackles right lower zone. Fever 38.2°C.

📊 Key Investigations:
• Na 128 mmol/L | K 4.3 | Urea 6.8 | Cr 80 µmol/L
• CRP 420 mg/L (↑), WCC 22 x 10⁹/L (neutrophilia)
• Blood gas: pH 7.33, HCO₃ 18, PCO₂ 4.5 → mild metabolic acidosis
• Urine: Na 40 mmol/L, Ketones +, no glucose/RBCs
• Chest X‑ray: right lower lobe infiltrate (pneumonia)

⚠️ Clinical context: Euvolemic hyponatremia (no dehydration, no edema) + urine Na >30 + underlying pneumonia .

💡 Examiner instruction (TOACS): You are expected to interpret the data, establish diagnostic criteria, differentiate from other hyponatremia causes, propose immediate management, counsel parents, and answer follow‑up questions.

❓ Q1. What are the formal diagnostic criteria for SIADH (Schwartz-Bartter criteria)? List the essential components based on this case.

✅ Model answer:
• Serum Na <135 mmol/L with serum osmolality <275 mOsm/kg (Na=128 → hypotonic).
• Urine osmolality >100 mOsm/kg during hypotonicity (inappropriately concentrated).
• Clinical euvolemia: no dehydration, no oedema (present in this child).
• Urine Na >20-40 mmol/L with normal salt intake (here urine Na 40).
• Normal thyroid, adrenal, renal function (exclude hypocortisolism, hypothyroidism).
• No recent diuretics.

❓ Q2. How do you differentiate SIADH from hypovolaemic hyponatremia (dehydration) and cerebral salt wasting (CSW)? Focus on volume status and urine electrolytes.

✅ Differentiation:
• Dehydration (hypovolaemic): dry mucous membranes, poor skin turgor, tachycardia, urine Na <10-20 mmol/L, BUN/Cr ratio ↑. Fluid resuscitation corrects Na.
• Cerebral salt wasting (CSW): hypovolaemia (low CVP, orthostasis), high urine output, high urine Na but volume depleted. Typically CNS pathology.
• SIADH: euvolemia, urine Na >30, concentrated urine. This child has euvolemic exam and urine Na 40 → SIADH.

❓ Q3. This child has Na 128 mmol/L with mild lethargy. What is your immediate fluid management? Include specific measures.

✅ Immediate management (mild-moderate symptoms):
• Fluid restriction: restrict to 50-70% of maintenance (e.g., 2/3 maintenance). Avoid hypotonic fluids (D5W, 0.45% NS).
• Treat underlying pneumonia – start appropriate antibiotics (cover Mycoplasma/bacterial).
• Monitor serum Na every 4-6h, daily weight, urine output.
• If severe symptoms (seizures/coma): 3% hypertonic saline 2-3 mL/kg over 15-20 min to raise Na by 4-6 mmol/L, then switch to fluid restriction. Correction <8-10 mmol/L in 24h.

❓ Q4. Enumerate common causes of SIADH in paediatric age group. Which one is most likely in this patient?

✅ Common causes:
• Pulmonary: pneumonia (Mycoplasma, viral, bacterial), asthma, TB, mechanical ventilation.
• CNS: meningitis, encephalitis, head trauma, brain tumour.
• Drugs: carbamazepine, SSRIs, vincristine, cyclophosphamide.
• Post-operative pain, nausea; idiopathic.
➜ In this child, community-acquired pneumonia (right lower lobe infiltrate, high CRP) is the most likely trigger.

❓ Q5. What is the safe rate of sodium correction to avoid osmotic demyelination? If this child had a seizure, how would you use hypertonic saline?

✅ Safe correction limits:
• Increase serum Na by 8-10 mmol/L in first 24 hours and <18 mmol/L in 48 hours.
• For seizure/coma: 3% hypertonic saline 2-3 mL/kg IV over 15-20 min; can repeat once if needed to raise Na by 4-6 mmol/L, then stop and initiate fluid restriction. Central line preferred.
• Rapid correction (>12 mmol/L/day) risks osmotic demyelination syndrome (quadriparesis, pseudobulbar palsy).

❓ Q6. This child returned from a rural area. What additional investigations would you order to confirm aetiology and exclude mimics?

✅ Recommended workup:
• Serum osmolality & urine osmolality (confirm hypotonicity + concentrated urine).
• Thyroid function (TSH, fT4), morning cortisol (exclude adrenal insufficiency).
• Mycoplasma pneumoniae serology (IgM) + cold agglutinins.
• Blood cultures, CRP trend; consider HIV/TB if risk factors.
• Renal function, serum uric acid (low in SIADH).
• Chest X‑ray (already done) ± CT chest if refractory.

❓ Q7. (Counseling task) Parents ask: “Why does she need fluid restriction when she already drinks very little?” Give an explanation in simple language.

✅ Explanation for parents:
“In SIADH, the body makes too much anti‑diuretic hormone, which tells the kidneys to hold onto water. Even though your daughter is not drinking much, the water inside her body is being retained, diluting the blood salt level. This can cause brain swelling. By restricting fluids, we allow the kidneys to remove excess water and slowly bring the salt back to normal. Once her pneumonia improves, the hormone level will normalise and she can drink freely again.”

❓ Q8. If fluid restriction fails to correct hyponatremia, what pharmacological options are available (mention paediatric considerations)?

✅ Second-line / refractory options:
• Demeclocycline (induces nephrogenic DI) – rarely used in children, risk of nephrotoxicity.
• Vaptans (tolvaptan, conivaptan) – ADH receptor antagonists; NOT routinely used in children due to risk of rapid correction and hepatotoxicity. Reserved for severe refractory cases under specialist care.
• Oral urea (osmotic water diuresis) – unpalatable, but can be used in chronic SIADH.
• In most children, treating underlying infection and fluid restriction is sufficient.

📋 TOACS Station Feedback (SIADH)

⭐ Examiner takeaway points: SIADH = euvolemic hyponatremia + urine Na >30. First-line: fluid restriction. Avoid hypotonic IV fluids. Hypertonic saline only for severe symptoms. Treat pneumonia. Always exclude adrenal/thyroid dysfunction.

🩺 FCPS Paediatrics TOACS · Mock Station