Key Facts

Hyponatraemia is the most frequently encountered electrolyte abnormality in clinical practice. It ranges from mild and asymptomatic to acutely life-threatening. A systematic approach to diagnosis based on osmolality and volume status is essential.

Epidemiology

• Present in 15-30% of hospitalised patients (mild); 5% have Na <130 mmol/L
• More common in elderly, patients on diuretics, and those with chronic diseases
• Associated with increased hospital length of stay and mortality

Aetiology

Hypovolaemic (low total body water and sodium):

• Vomiting, diarrhoea
• Thiazide diuretics (most common drug cause)
• Addison disease (adrenal insufficiency)
• Cerebral salt wasting
• Burns

Euvolaemic (excess water relative to sodium):

• SIADH (most common cause of euvolaemic hyponatraemia)
• Hypothyroidism (severe)
• Psychogenic polydipsia
• Drugs: SSRIs, carbamazepine, cyclophosphamide

Hypervolaemic (excess total body water > sodium):

• Heart failure
• Cirrhosis
• Nephrotic syndrome
• CKD

Pathophysiology

• Hyponatraemia reflects relative excess of water to sodium
• True hyponatraemia is associated with low serum osmolality (<275 mOsm/kg)
• Pseudohyponatraemia: normal osmolality with high lipids/proteins
• Hypertonic hyponatraemia: high osmolality due to glucose (each 5.6 mmol/L glucose rise → ~2.4 mmol/L fall in sodium)
• In SIADH: ADH secretion is inappropriate → water retention, dilutional hyponatraemia, concentrated urine

Mild Hyponatraemia (130-134 mmol/L)

• Often asymptomatic
• Subtle cognitive impairment, increased fall risk in elderly
• May have gait disturbance

Moderate Hyponatraemia (125-129 mmol/L)

• Nausea, headache
• Confusion, disorientation
• Muscle cramps, weakness

Severe Hyponatraemia (<125 mmol/L)

• Seizures
• Reduced consciousness, coma
• Respiratory arrest (brainstem herniation)

Red Flags

• Acute symptomatic hyponatraemia (<48h with seizures/reduced consciousness) → medical emergency
• Post-operative hyponatraemia → common and potentially dangerous
• Features of adrenal crisis (hypotension, hyperkalaemia) → urgent Synacthen test
• Rapid correction → risk of osmotic demyelination syndrome (ODS)

Bedside

• Volume status assessment: JVP, skin turgor, mucous membranes, oedema, postural BP
• Fluid balance chart review
• Drug history: thiazides, SSRIs, carbamazepine

Bloods

• Serum sodium: confirm hyponatraemia and severity
• Serum osmolality: low (<275 mOsm/kg) confirms true hyponatraemia
• Urine osmolality: >100 mOsm/kg suggests impaired water excretion
• Urine sodium: >30 mmol/L suggests renal sodium loss (SIADH, diuretics, Addison); <30 mmol/L suggests extrarenal loss (vomiting, diarrhoea)
• TFTs: exclude hypothyroidism
• 9am cortisol/Short Synacthen test: exclude adrenal insufficiency
• Glucose: exclude hyperglycaemic hyponatraemia
• Lipid profile, protein: exclude pseudohyponatraemia

Special Tests

• Paired serum and urine osmolality with urine sodium: essential for aetiological diagnosis
• BNP/NT-proBNP: if heart failure suspected
• CT head/chest: if SIADH suspected (identify underlying cause – lung cancer, CNS pathology)

SIADH Diagnostic Criteria

1. Serum osmolality <275 mOsm/kg
2. Urine osmolality >100 mOsm/kg (inappropriately concentrated)
3. Urine sodium >30 mmol/L (usually >40)
4. Euvolaemic
5. Normal thyroid, adrenal, renal function
6. No diuretics

Acute Symptomatic Hyponatraemia (seizures, reduced consciousness)

• Hypertonic saline (3% NaCl): 100-150mL IV over 20 minutes, can repeat x2
• Aim to raise Na by 4-6 mmol/L in first 6 hours (sufficient to reduce cerebral oedema)
• Monitor Na every 2-4 hours
• Do not exceed 8-10 mmol/L correction in 24 hours

Chronic/Asymptomatic Hyponatraemia

Treat underlying cause:

• Hypovolaemic: IV 0.9% NaCl (restore volume)
• Euvolaemic (SIADH):
  • Fluid restriction 750-1000 mL/day (first-line)
  • Demeclocycline 600-1200mg daily (ADH antagonist – rarely used now)
  • Tolvaptan 15mg OD (vasopressin V2 receptor antagonist – specialist initiation only, monitor Na closely due to risk of overcorrection)
  • Treat underlying cause of SIADH
• Hypervolaemic: fluid restriction + treat underlying cause (diuretics for HF, spironolactone for cirrhosis)
• Drug-induced: withdraw offending drug (thiazides, SSRIs)
• Adrenal insufficiency: hydrocortisone replacement

Preventing Osmotic Demyelination Syndrome (ODS)

• Correct Na by ≤8-10 mmol/L in first 24 hours and ≤18 mmol/L in first 48 hours
• High-risk patients (Na <105, alcoholism, malnutrition, hypokalaemia): limit to 6 mmol/L in 24 hours
• If over-corrected: desmopressin 2 mcg IV + 5% dextrose to lower Na back down

Referral Criteria

• Severe or symptomatic hyponatraemia → acute medicine/ITU
• Persistent SIADH requiring tolvaptan → endocrinology
• Suspected underlying malignancy → oncology

• Mild hyponatraemia: associated with increased fall risk and fractures in elderly, longer hospital stays
• Severe acute hyponatraemia: mortality up to 50% if untreated
• ODS: can cause permanent neurological damage (locked-in syndrome, quadriparesis) – develops 2-7 days after overly rapid correction
• SIADH prognosis depends on underlying cause (drug-induced = excellent; malignancy = poor)
• Chronic mild hyponatraemia associated with osteoporosis and cognitive impairment in elderly

Diagnostic Algorithm

Common Causes of SIADH