Definition

Normal serum Na: 136-145 mmol/L
Severe Hyponatraemia: < 120 mmol/L

Serum Osmolarity = 2 x Na + Glucose + Urea

Presentation

• Headache

• Lethargy

• Nausea/Vomiting

• Confusion

• Seizure

• Coma

• Respiratory arrest

Pathophysiology

1. Too much water

↑ Water Intake

• Water intoxication (Psychogenic Polydipsia)

↓ Water Output

• Excess ADH secretion (SIADH) - ADH reabsorb water

2. Too little Sodium

↓ Intake

• Dietary cause is rare (daily demand is relatively easy to meet)

↑ Loss

• Insensible loss
  • Sweating
  • Vomiting
  • Diarrhoea
  • Burns
• Renal loss
  • ↓ Aldosterone (Addison's Disease)
  • Diuretics
    • Loop diuretics (e.g. Furosemide)
    • Thiazide diuretics (e.g. Indapamide)
    • Potassium-sparing diuretics (e.g. Spironolactone, Eplerenone)

Approach

1. Is it a true hyponatraemia?

Assess Serum osmolarity

2. Assess fluid status

Hypovolaemic (Dehydrated)

Due to too little Na+

• Renal Loss → Addison's, Diuretics
• Insensible loss due to burns etc.

Euvolaemic

Due to too much water

• ↑ intake
• ↑ reabsorption - SIADH

Hypervolaemic (Oedematous)

• congestive heart failure
• hypoalbuminaemia
  • nephrotic syndrome - CKD
  • liver failure

Prolonged reduction in cardiac output will cause ↑ADH

3. Urine sodium/osmolarity

Urine sodium should reflect serum osmolarity if kidney is working

High → renal cause
Low → other cause (kidney is working okay)

flowchart TD
Hypo["Hypovolaemic
(↓ Na) "]
Eu["Euvolaemic
(↑ H2O)"]
Hyper["Hypervolaemic"]

Hypo --> A1(High Urinary Na+) & B1(Low Urinary Na+)
A1 -->|Renal Loss| A2["- Addison
- Diuretics"]
B1 -->|Loss elsewhere| B2["Insensible Loss
e.g. burns, diarrhoea"]

Eu --> C1(↑ Urine Na+) & D1(↓ Urine Na+)
C1 -->|↑ Reabsorbtion| C2[SIADH]
D1 -->|↑ Intake| D2[Primary Polydipsia]

Hyper --> E[CCF] & F1[↓ Alubumin]
F1 --> F2["- Nephrotic Syndrome
- Liver Cirrhosis"]

Management

Hypovolaemic

1. 0.9% Saline

Euvolaemic

1. Fluid restrict 500-1000 mL/day
2. Consider ADH receptor antagonist (Vaptan)

Hypervolaemic

1. Fluid restrict 500-1000 mL/day
2. Consider Loop diuretics
3. Consider ADH receptor antagonist

Complication