Hyponatraemia

Related Subjects: | Sodium Physiology | Hyponatraemia | TURP Hyponatraemia syndrome | Hypernatraemia | Diabetes Insipidus

⚠️ Hyponatraemia: A sudden drop in serum sodium (Na⁺) can cause cerebral oedema → cellular swelling, seizures, and coma. 🧠 Severe cases risk brain herniation. ⏳ Chronic/gradual decline is better tolerated. 📈 Correction must be <12 mmol/L per day (ideally 6–8 mmol/L/24h) to avoid Central Pontine Myelinolysis (CPM).

🧬 Physiology

• Low Na⁺ → suppresses ADH release (normally increases water excretion).
• SIADH → inappropriate ADH secretion → water retention, ↑ urine osmolality.
• Hypovolaemia → ADH & thirst ↑, aldosterone promotes Na reabsorption.
• Result = water shifts intracellularly → cerebral oedema.

📊 Clinical Evaluation

• Check drug history, fluid balance, exam hydration status.
• Hypovolaemic: Dehydrated, dry mucous membranes, hypotension. (Causes: GI losses, Addison’s, burns, diuretics).
• Euvolaemic: Normal exam, SIADH common. (Causes: SSRIs, NSAIDs, post-op, pain, stress, psychogenic polydipsia).
• Hypervolaemic: Oedema, ascites, HF, CKD, cirrhosis, iatrogenic fluids, post-TURP syndrome.

🧾 Causes of Hyponatraemia

• Hypovolaemic: GI losses, diuretics, Addison’s, salt-wasting nephropathy.
• Euvolaemic: SIADH, drugs, pain, stress, post-TURP, psychogenic polydipsia.
• Hypervolaemic: HF, CKD, cirrhosis, iatrogenic fluids.
• Pseudohyponatraemia: Hyperlipidaemia or hyperproteinaemia → lab artefact.

📉 Severity & Symptoms

• 🔺 Rapid fall or Na <120 mmol/L → symptomatic risk ↑.
• Na 110–115 → headache, confusion, irritability.
• Na <110 → seizures, coma, ataxia (medical emergency).

🔬 Investigations

• 🧪 U&E: Mild (135–130), Moderate (125–129), Severe (<125).
• 🧪 Serum Osmolality: <275 in true hypoNa.
• 🧪 Urine Osmolality: >100 in SIADH.
• 🧪 Urine Na: <20 in hypovolaemia, >30 in SIADH.
• 🔎 TFTs & Cortisol (exclude hypothyroid/adrenal insufficiency).
• 📷 CXR (lung cancer, pneumonia → SIADH).
• 🧠 CT Head (exclude bleed, tumour, infection).

⚠️ Central Pontine Myelinolysis (CPM)

🧠 Risk of overly rapid correction of chronic hypoNa → demyelination (esp. pons). ➡️ Irreversible deficits: dysarthria, quadriplegia, locked-in syndrome. ⛔ Keep correction ≤12 mmol/L/day.

📈 Algorithm

🩺 Management Principles

• ✅ ABC first. Admit severe cases to HDU/ITU.
• 🔄 Frequent Na checks (2–4h), strict fluid balance.
• 💉 Hypovolaemia: IV normal saline → reassess.
• 🚰 SIADH/Euvolaemia: Fluid restrict (800–1000 ml/day). In seizures/coma → 3% saline bolus (~5 mmol/L correction).
• 💊 Persistent SIADH: Tolvaptan or Demeclocycline.
• 🧾 Addison’s: Short Synacthen Test → IV hydrocortisone.
• 🌊 Hypervolaemic: Restrict salt & water, loop diuretics if needed.

📚 References

• Diagnosis & Management of Inpatient Hyponatraemia and SIADH
• Guidelines for Hyponatraemia Management (RLBUHT)

🩺 Case 1 — Hypovolaemic Hyponatraemia

A 72-year-old woman presents with vomiting and diarrhoea for 4 days. She is confused, BP 90/60 mmHg, pulse 110 bpm, mucous membranes dry. Labs: Na⁺ 124 mmol/L, urea and creatinine raised. Management: 💧 IV 0.9% saline cautiously to restore volume, monitor sodium rise. Identify and treat cause (e.g. gastroenteritis). Avoid: ❌ Rapid correction (>8–10 mmol/L per 24 h) as this risks osmotic demyelination syndrome (ODS).

🩺 Case 2 — Euvolaemic Hyponatraemia (SIADH)

A 68-year-old man with a history of small-cell lung cancer presents with confusion and seizures. He is euvolaemic on exam. Labs: Na⁺ 116 mmol/L, low plasma osmolality, inappropriately concentrated urine. Management: 🚰 Fluid restriction, consider demeclocycline or vasopressin receptor antagonists in refractory cases; treat underlying cause (e.g. lung cancer). Hypertonic saline in severe symptomatic cases under specialist care. Avoid: ❌ Isotonic saline — may worsen hyponatraemia in SIADH. Avoid rapid correction.

🩺 Case 3 — Hypervolaemic Hyponatraemia

An 80-year-old man with decompensated heart failure presents with breathlessness, ankle swelling, and Na⁺ 128 mmol/L. Exam shows raised JVP, pitting oedema, bibasal crackles. Management: 💊 Fluid and salt restriction, loop diuretics, optimise heart failure therapy (ACEI/ARNI, beta-blocker, MRA). Avoid: ❌ Rapid overcorrection; avoid excessive IV fluids which worsen overload.