Hyponatraemia 💧
Clinical guide for evaluating and managing hyponatraemia. Step-by-step algorithm, volume status assessment, SIADH, and correction rates for on-call doctors.
🚨 Life-Threatening Hyponatraemia
- ⚠️ ESCALATE IMMEDIATELY if life-threatening features present
- Life-threatening features: Severe neurological symptoms (seizures, coma, severe confusion), signs of cerebral oedema, very low Na (<120 mmol/L) with severe symptoms, or rapid onset with severe symptoms
- Management: Proceed to life-threatening hyponatraemia algorithm per IAEM guideline
- Note: This topic covers NON-LIFE-THREATENING hyponatraemia only
🧠 Definition & Classification
- Hyponatraemia = Serum Na⁺ < 135 mmol/L
- Most common electrolyte disorder in hospitalised patients
- Most common form: Hypotonic hyponatraemia (serum osmolality <275 mOsmol/kg)
- Use the interactive algorithm below for systematic evaluation (based on IAEM guideline Figure 1)
📋 Clinical History & Examination
- History: Focus on symptoms of hypothyroidism, glucocorticoid deficiency, CCF, renal/liver failure
- Note recent medication changes or extra fluid intake
- Symptoms (Na <130): Fatigue, lethargy, somnolence, nausea, vomiting, anorexia
- Signs of cerebral oedema: Seizures, confusion, lethargy
- Examination: Assess vital signs, examine for liver/renal/heart failure, hypothyroidism, Addison's disease
- Fluid status assessment: Pulse, BP, capillary refill, mucous membranes, skin turgor, pitting oedema, respiratory exam, JVP, urine output, weight trend
🩺 Volume Status Assessment
| Volume Status | Clinical Features |
|---|---|
| Hypovolaemic | Dry mucous membranes, decreased skin turgor, postural hypotension, tachycardia, low JVP, reduced urine output, poor capillary refill |
| Euvolaemic | Normal volume status, no signs of dehydration or overload. Most common cause: SIADH |
| Hypervolaemic | Peripheral oedema, raised JVP, ascites, pulmonary oedema. Associated with heart failure, cirrhosis, nephrotic syndrome, renal failure |
🧪 Core Investigations
- Serum sodium
- Serum osmolality (or calculate: 2 × Na + glucose + urea)
- Venous blood gas (VBG)
- Urinary sodium
- Urinary osmolality
- Urea and electrolytes
- Full blood count
- Liver function tests
- Thyroid function tests
- CXR (to assess for pulmonary oedema)
🔍 Diagnosis of Hypotonic Hyponatraemia (IAEM Table)
| Volume Status | Urinary Na | Likely Diagnoses |
|---|---|---|
| Hypervolaemic | <30 mmol/L | Liver failure, Cardiac failure |
| Hypervolaemic | >30 mmol/L | Renal failure, Diuretic treatment in cardiac failure |
| Euvolaemic | <30 mmol/L | Hypothyroidism, Iatrogenic (IV fluids), Water intoxication |
| Euvolaemic | >30 mmol/L | SIADH, Adrenocortical deficiency |
| Hypovolaemic | <30 mmol/L | Diarrhoea and vomiting, Reduced PO intake, Burns |
| Hypovolaemic | >30 mmol/L | Salt wasting (renal/cerebral), Diuretic medication, Addison's disease |
🔍 Other Forms of Hyponatraemia
| Type | Serum Osmolality | Causes |
|---|---|---|
| Isotonic | 275-295 mOsmol/kg | Renal failure, High serum lipids/protein/glucose |
| Hyperosmolar | >295 mOsmol/kg | Infusion of hypertonic solutions (e.g. mannitol) |
💊 Treatment by Volume Status (Non-Life-Threatening)
| Volume Status | Management |
|---|---|
| Hypervolaemic | Treat the cause, Fluid restrict if appropriate |
| Euvolaemic | Treat the cause, Stop potential contributing medications, Fluid restrict if appropriate |
| Hypovolaemic | Treat the cause, Administer IV 0.9% NaCl or Hartmann's at 0.5-1ml/kg/hr, Reassess Na every 8 hours, Target Na rise <10mmol/L in 24h (healthy) or <8mmol/L (malnourished) |
⚠️ Osmotic Demyelination Syndrome (ODS)
- Risk: If hyponatraemia present >48 hours, brain cell osmolality falls and brain becomes dehydrated
- Overcorrection can cause ODS: paresis, paralysis, seizures, coma
- Prevention: Target Na rise <10mmol/L in 24h (healthy) or <8mmol/L (malnourished)
- Never correct rapidly in chronic hyponatraemia
- Monitor closely during correction
📈 Monitoring During Correction
- Repeat clinical assessment and sodium monitoring every 8 hours (early in treatment)
- Frequency depends on patient stability and rate of sodium change
- Monitor urine output during treatment while fluid balance is being adjusted
- Watch for complications: Cerebral oedema (acute), ODS (chronic, overcorrection), fluid overload
- If Na rises too quickly: Stop correction, consider DDAVP or hypotonic fluids, consult endocrinology/nephrology
🔗 External Resources
Non-Life-Threatening Hyponatraemia Algorithm
Step-by-step assessment and diagnosis