🩺 FCPS Paediatrics TOACS · SIADH (Syndrome of Inappropriate Antidiuretic Hormone)
📖 4‑year‑old with tachypnoea, chest pain, fever & severe hyponatraemia (Na 128) – Approach to euvolemic hyponatremia
📚 paeds.online – Paeds Online
🩺 OBSERVED/INTERACTIVE STATION · CPSP FORMAT · 8 MINUTES · FLUID & ELECTROLYTE EMERGENCY (SIADH)
🌾 Observed station – “4‑year‑old girl from rural area: tachypnoea, chest pain, fever and severe hyponatraemia”
👧🏽 Clinical scenario (displayed / read to candidate):
A 4‑year‑old girl who has just returned from a rural area presents with tachypnoea, chest pain and mild fever (38.2°C). Parents report reduced oral intake but no vomiting or diarrhoea. On examination: alert but mildly lethargic, mucous membranes moist, no oedema, normal skin turgor. Chest auscultation reveals crackles in the right lower zone.
Laboratory results:
• Na 128 mmol/L (low), K 4.3, Urea 6.8 mmol/L, Cr 80 µmol/L
• Hb 12.3 g/dL, Plt 400 x 10⁹/L, WCC 22 x 10⁹/L (N 70%) → leukocytosis, neutrophilia
• Blood gas: pH 7.33, HCO₃ 18, PCO₂ 4.5, BE 0.0 → mild metabolic acidosis (normal AG?)
• CRP 420 mg/L → marked inflammation
• Urine: Na 40 mmol/L (spot), RBCs 0, Glucose 0, Ketones +
🎯 Task (examiner observed): Recognize SIADH (Syndrome of Inappropriate Antidiuretic Hormone) secondary to community‑acquired pneumonia (likely bacterial). Discuss diagnostic criteria for SIADH, differentiate from cerebral salt wasting and hypovolaemic hyponatraemia, propose fluid management (fluid restriction vs hypertonic saline), and address the underlying infection.
A 4‑year‑old girl who has just returned from a rural area presents with tachypnoea, chest pain and mild fever (38.2°C). Parents report reduced oral intake but no vomiting or diarrhoea. On examination: alert but mildly lethargic, mucous membranes moist, no oedema, normal skin turgor. Chest auscultation reveals crackles in the right lower zone.
Laboratory results:
• Na 128 mmol/L (low), K 4.3, Urea 6.8 mmol/L, Cr 80 µmol/L
• Hb 12.3 g/dL, Plt 400 x 10⁹/L, WCC 22 x 10⁹/L (N 70%) → leukocytosis, neutrophilia
• Blood gas: pH 7.33, HCO₃ 18, PCO₂ 4.5, BE 0.0 → mild metabolic acidosis (normal AG?)
• CRP 420 mg/L → marked inflammation
• Urine: Na 40 mmol/L (spot), RBCs 0, Glucose 0, Ketones +
🎯 Task (examiner observed): Recognize SIADH (Syndrome of Inappropriate Antidiuretic Hormone) secondary to community‑acquired pneumonia (likely bacterial). Discuss diagnostic criteria for SIADH, differentiate from cerebral salt wasting and hypovolaemic hyponatraemia, propose fluid management (fluid restriction vs hypertonic saline), and address the underlying infection.
⚠️ Clinical red flags (examiner expects):
• Na 128 mmol/L with symptoms (lethargy) → risk of cerebral oedema and seizures.
• Urine Na 40 mmol/L ( >20-30) + euvolemia (no dehydration/oedema) → suggests SIADH (not dehydration which would have urine Na <20).
• Elevated CRP and chest findings → underlying pneumonia (mycoplasma? bacterial?) triggers ADH release.
• Na 128 mmol/L with symptoms (lethargy) → risk of cerebral oedema and seizures.
• Urine Na 40 mmol/L ( >20-30) + euvolemia (no dehydration/oedema) → suggests SIADH (not dehydration which would have urine Na <20).
• Elevated CRP and chest findings → underlying pneumonia (mycoplasma? bacterial?) triggers ADH release.
🔬 Key concept – SIADH (Nelson's Chapter 55 – Disorders of Water Balance):
SIADH is defined by euvolemic hyponatremia with inappropriately concentrated urine (urine osmolality >100 mOsm/kg), urine Na >20-40 mmol/L, normal renal, adrenal and thyroid function. Common causes: pneumonia (especially Mycoplasma pneumoniae, Legionella, viral), CNS disorders, drugs, post‑surgery.
SIADH is defined by euvolemic hyponatremia with inappropriately concentrated urine (urine osmolality >100 mOsm/kg), urine Na >20-40 mmol/L, normal renal, adrenal and thyroid function. Common causes: pneumonia (especially Mycoplasma pneumoniae, Legionella, viral), CNS disorders, drugs, post‑surgery.
Algorithm for hyponatremia: assess volume status → euvolemic + urine Na >30 → SIADH is most likely.
💡 Examiner probe: “Why is urine sodium elevated in SIADH but low in dehydration?” → In SIADH, ADH‑mediated water retention expands volume slightly → renal perfusion increased → natriuresis (urine Na >30). In dehydration, hypovolaemia activates RAAS → avid Na retention → urine Na <10-20.
🩸 Diagnostic workup – confirming SIADH & excluding mimics
1 Confirm euvolemia
Clinical: normal skin turgor, moist mucous membranes, no oedema, no postural hypotension. Normal serum urea/creatinine ratio (Urea 6.8, Cr 80).
Clinical: normal skin turgor, moist mucous membranes, no oedema, no postural hypotension. Normal serum urea/creatinine ratio (Urea 6.8, Cr 80).
2 Laboratory criteria for SIADH
• Serum Na <135 mmol/L
• Serum osmolality <275 mOsm/kg
• Urine osmolality >100 mOsm/kg (inappropriately concentrated)
• Urine Na >20-40 mmol/L (normal salt intake)
• Normal thyroid & adrenal function (exclude hypocortisolism, hypothyroidism)
• Serum Na <135 mmol/L
• Serum osmolality <275 mOsm/kg
• Urine osmolality >100 mOsm/kg (inappropriately concentrated)
• Urine Na >20-40 mmol/L (normal salt intake)
• Normal thyroid & adrenal function (exclude hypocortisolism, hypothyroidism)
3 Exclude other causes
• Thyroid function tests (TSH, fT4)
• Cortisol (8 am) or ACTH stimulation test (exclude adrenal insufficiency)
• Renal function, serum uric acid (low in SIADH, normal in CSW)
• Thyroid function tests (TSH, fT4)
• Cortisol (8 am) or ACTH stimulation test (exclude adrenal insufficiency)
• Renal function, serum uric acid (low in SIADH, normal in CSW)
4 Identify underlying trigger
Chest X‑ray (pneumonia). Mycoplasma serology, blood cultures. Consider lumbar puncture if CNS infection suspected.
Chest X‑ray (pneumonia). Mycoplasma serology, blood cultures. Consider lumbar puncture if CNS infection suspected.
📊 Differential diagnosis of euvolemic hyponatremia:
• SIADH – urine Na >30, urine osmolality >100.
• Cerebral salt wasting (CSW) – hypovolaemia (low CVP, tachycardia), high urine Na, high urine output. Seen in CNS disorders.
• Hypothyroidism / adrenal insufficiency – cortisol low, TSH high.
• Psychogenic polydipsia – urine osmolality very low (<100).
• Diuretic use – history.
• SIADH – urine Na >30, urine osmolality >100.
• Cerebral salt wasting (CSW) – hypovolaemia (low CVP, tachycardia), high urine Na, high urine output. Seen in CNS disorders.
• Hypothyroidism / adrenal insufficiency – cortisol low, TSH high.
• Psychogenic polydipsia – urine osmolality very low (<100).
• Diuretic use – history.
🧠 Key point for TOACS: In a child with pneumonia + hyponatraemia, SIADH is the most common cause. Avoid over‑aggressive fluid correction unless seizures/coma.
🗨️ Examiner Q&A · SIADH pathophysiology, treatment & parental counseling
❓ Q1 (Examiner): “What are the formal diagnostic criteria for SIADH (Schwartz-Bartter criteria)?”
✅ Essential criteria:
1. Serum Na <135 mmol/L with serum osmolality <275 mOsm/kg.
2. Urine osmolality >100 mOsm/kg during hypotonicity.
3. Clinical euvolemia (no dehydration, no oedema).
4. Urine Na >20-40 mmol/L with normal salt intake.
5. Normal thyroid, adrenal, renal function.
6. No recent diuretic use.
1. Serum Na <135 mmol/L with serum osmolality <275 mOsm/kg.
2. Urine osmolality >100 mOsm/kg during hypotonicity.
3. Clinical euvolemia (no dehydration, no oedema).
4. Urine Na >20-40 mmol/L with normal salt intake.
5. Normal thyroid, adrenal, renal function.
6. No recent diuretic use.
❓ Q2: “How do you differentiate SIADH from cerebral salt wasting (CSW)?”
✅ Key differences:
• Volume status: SIADH = euvolemic; CSW = hypovolaemic (tachycardia, hypotension, poor skin turgor).
• Fluid balance: SIADH – fluid restriction improves Na; CSW – fluid resuscitation improves Na.
• Uric acid: low in both but CSW may have lower fractional excretion.
• Treatment: SIADH → fluid restriction; CSW → saline replacement + fludrocortisone.
• Volume status: SIADH = euvolemic; CSW = hypovolaemic (tachycardia, hypotension, poor skin turgor).
• Fluid balance: SIADH – fluid restriction improves Na; CSW – fluid resuscitation improves Na.
• Uric acid: low in both but CSW may have lower fractional excretion.
• Treatment: SIADH → fluid restriction; CSW → saline replacement + fludrocortisone.
❓ Q3: “This child has Na 128 mmol/L and mild lethargy. What is the initial fluid management?”
✅ • Asymptomatic/mild symptoms: Fluid restriction – restrict to 2/3 of maintenance (e.g., 50-60% of maintenance) + treat underlying pneumonia.
• Monitor serum Na every 4-6 hours.
• If severe symptoms (seizures, coma): 3% hypertonic saline (2-3 mL/kg over 15-20 min) to raise Na by 4-6 mmol/L initially, then slower correction.
• Avoid rapid correction >8-10 mmol/L in 24h to prevent osmotic demyelination.
• Monitor serum Na every 4-6 hours.
• If severe symptoms (seizures, coma): 3% hypertonic saline (2-3 mL/kg over 15-20 min) to raise Na by 4-6 mmol/L initially, then slower correction.
• Avoid rapid correction >8-10 mmol/L in 24h to prevent osmotic demyelination.
❓ Q4: “What are the common causes of SIADH in children?”
✅ Causes (mnemonic SIADH):
• Pulmonary – pneumonia (Mycoplasma, viral, bacterial), abscess, asthma, TB, mechanical ventilation.
• CNS – meningitis, encephalitis, head trauma, brain tumour, stroke.
• Drugs – carbamazepine, SSRIs, vincristine, cyclophosphamide, general anaesthetics.
• Post‑operative (pain, nausea).
• Idiopathic.
This patient likely has Mycoplasma or bacterial pneumonia.
• Pulmonary – pneumonia (Mycoplasma, viral, bacterial), abscess, asthma, TB, mechanical ventilation.
• CNS – meningitis, encephalitis, head trauma, brain tumour, stroke.
• Drugs – carbamazepine, SSRIs, vincristine, cyclophosphamide, general anaesthetics.
• Post‑operative (pain, nausea).
• Idiopathic.
This patient likely has Mycoplasma or bacterial pneumonia.
❓ Q5 (Counseling): “Parents ask why their daughter needs fluid restriction when she already drinks little. How do you explain?”
✅ “In SIADH, the body makes too much anti‑diuretic hormone, which tells the kidneys to hold onto water even though salt level is low. So any water she drinks will dilute her blood sodium further, which can cause brain swelling. By restricting fluids, we let the kidneys excrete the excess water and slowly bring sodium back to normal. Once the pneumonia improves, ADH will normalise and she can drink normally again.”
❓ Q6: “What medication can be used in refractory SIADH?”
✅ • Demeclocycline (induces nephrogenic diabetes insipidus) – rarely used in children.
• Vaptans (conivaptan, tolvaptan) – ADH receptor antagonists; not routinely used in children due to risk of rapid correction and hepatotoxicity. Reserved for severe, refractory euvolemic hyponatremia under specialist care.
• Urea (oral) – promotes osmotic water diuresis, but unpalatable.
• Vaptans (conivaptan, tolvaptan) – ADH receptor antagonists; not routinely used in children due to risk of rapid correction and hepatotoxicity. Reserved for severe, refractory euvolemic hyponatremia under specialist care.
• Urea (oral) – promotes osmotic water diuresis, but unpalatable.
❓ Q7: “What is the rate of sodium correction to avoid osmotic demyelination?”
✅ • Safe correction: Increase serum Na by 8-10 mmol/L in first 24 hours and <18 mmol/L in 48 hours.
• In severe symptomatic hyponatremia (seizures), raise by 4-6 mmol/L acutely with 3% saline, then stop and switch to fluid restriction.
• Overly rapid correction (>12 mmol/L/day) risks osmotic demyelination syndrome (quadriparesis, pseudobulbar palsy, behavioural changes).
• In severe symptomatic hyponatremia (seizures), raise by 4-6 mmol/L acutely with 3% saline, then stop and switch to fluid restriction.
• Overly rapid correction (>12 mmol/L/day) risks osmotic demyelination syndrome (quadriparesis, pseudobulbar palsy, behavioural changes).
❓ Q8: “What additional investigations would you order for this child (given rural return)?”
✅ • Chest X‑ray (to confirm pneumonia, rule out effusion).
• Mycoplasma pneumoniae serology (IgM) + cold agglutinins – common cause of SIADH in older children.
• Blood cultures, CRP monitoring.
• If no response to treatment, consider CT chest, HIV, TB workup.
• Serum osmolality & urine osmolality (confirm diagnostic criteria).
• Cortisol & TSH (exclude adrenal/thyroid insufficiency).
• Mycoplasma pneumoniae serology (IgM) + cold agglutinins – common cause of SIADH in older children.
• Blood cultures, CRP monitoring.
• If no response to treatment, consider CT chest, HIV, TB workup.
• Serum osmolality & urine osmolality (confirm diagnostic criteria).
• Cortisol & TSH (exclude adrenal/thyroid insufficiency).
📘 Management algorithm & summary (Nelson's Chapter 55)
🩺 Treat underlying cause
Antibiotics for pneumonia (macrolide/empiric). Supportive care. Resolution of infection → ADH normalises.
Antibiotics for pneumonia (macrolide/empiric). Supportive care. Resolution of infection → ADH normalises.
💧 Fluid restriction (first‑line)
Restrict to 50-70% of maintenance. Monitor daily weight, I/O, Na q4-6h initially. If Na rises too fast → liberalise fluids.
Restrict to 50-70% of maintenance. Monitor daily weight, I/O, Na q4-6h initially. If Na rises too fast → liberalise fluids.
🧂 Hypertonic saline (3%)
Indications: seizures, coma, severe lethargy, Na <120. Infusion 2-3 mL/kg over 15-20 min, reassess. Central line preferred.
Indications: seizures, coma, severe lethargy, Na <120. Infusion 2-3 mL/kg over 15-20 min, reassess. Central line preferred.
⚠️ Avoid
Hypotonic fluids (0.45% NS, D5W, free water). Routine saline infusion (may worsen hyponatraemia if ADH high).
Hypotonic fluids (0.45% NS, D5W, free water). Routine saline infusion (may worsen hyponatraemia if ADH high).
📈 Monitoring
Neurological status, serum Na every 4-6h until stable >130, urine output, daily weight, urine electrolytes.
Neurological status, serum Na every 4-6h until stable >130, urine output, daily weight, urine electrolytes.
🔁 When to stop restriction
Once serum Na normalises (>135) and underlying pneumonia improves. If chronic, manage as outpatient with mild fluid restriction.
Once serum Na normalises (>135) and underlying pneumonia improves. If chronic, manage as outpatient with mild fluid restriction.
📖 Nelson's Textbook of Pediatrics (Chapter 55 – Disorders of Water Balance):
“SIADH is the most common cause of euvolemic hyponatremia in hospitalized children. Management focuses on treating the underlying condition and fluid restriction. Hypertonic saline is reserved for severe symptomatic hyponatremia with neurological compromise. Overly rapid correction must be avoided to prevent central pontine myelinolysis.”
“SIADH is the most common cause of euvolemic hyponatremia in hospitalized children. Management focuses on treating the underlying condition and fluid restriction. Hypertonic saline is reserved for severe symptomatic hyponatremia with neurological compromise. Overly rapid correction must be avoided to prevent central pontine myelinolysis.”
⭐ TOACS TAKE‑HOME POINTS (candidate must articulate):
1. SIADH = euvolemic hyponatremia + urine Na >30 + concentrated urine (Uosm >100).
2. In a child with pneumonia and hyponatremia, think SIADH – do not mistake for dehydration.
3. First‑line treatment: fluid restriction (50-70% maintenance). Avoid hypotonic IV fluids.
4. Hypertonic saline (3%) only if seizures, coma, or very severe symptoms – correct slowly (max 8-10 mmol/L/24h).
5. Treat the underlying infection – SIADH resolves as pneumonia improves.
6. Exclude adrenal insufficiency and hypothyroidism before diagnosing SIADH.
7. Educate parents: fluid restriction is temporary; overhydration worsens brain swelling.
1. SIADH = euvolemic hyponatremia + urine Na >30 + concentrated urine (Uosm >100).
2. In a child with pneumonia and hyponatremia, think SIADH – do not mistake for dehydration.
3. First‑line treatment: fluid restriction (50-70% maintenance). Avoid hypotonic IV fluids.
4. Hypertonic saline (3%) only if seizures, coma, or very severe symptoms – correct slowly (max 8-10 mmol/L/24h).
5. Treat the underlying infection – SIADH resolves as pneumonia improves.
6. Exclude adrenal insufficiency and hypothyroidism before diagnosing SIADH.
7. Educate parents: fluid restriction is temporary; overhydration worsens brain swelling.
Pathophysiology: excess ADH → water retention → dilutional hyponatremia → cerebral oedema if severe.
🔗 Online resources for FCPS candidates:
• Paeds Online – TOACS station bank & SIADH summary
• UpToDate: SIADH in children
• Nelson’s Chapter 55 – Fluid and Electrolyte Disorders
• Paeds Online – TOACS station bank & SIADH summary
• UpToDate: SIADH in children
• Nelson’s Chapter 55 – Fluid and Electrolyte Disorders