Hyponatraemia
Background Information
Definition
Hyponatraemia is defined as serum sodium <135 mmol/L
The general cut-off of severe hyponatraemia is <120 mmol/L
Clinical Features
Clinical features of hyponatraemia are highly non-specific.
Clinical features in moderately severe hyponatraemia:
- Nausea (without vomiting)
- Confusion
- Headache
Clinical features in severe hyponatraemia (from cerebral oedema):
- Vomiting
- Abnormal and deep somnolence
- Cardiorespiratory depress
- Seizures
- Coma
- Hyporeflexia
Also, varying fluid status and features of the underlying cause.
Sodium disturbances (hypernatraemia or hyponatraemia) do NOT typically cause ECG changes or arrhythmias.
Unlike abnormalities in potassium, calcium, or magnesium, which directly affect cardiac electrophysiology.
Complications
Key complication of acute hyponatraemia: cerebral oedema
Complications of chronic hyponatraemia:
- Cognitive deficits
- Gait disturbances
- Falls
- Fragility fractures
Diagnosis
Diagnostic Approach
Key tests to order
- Non-specific tests (to categorise the hyponatraemia)
- Paired serum and urine osmolality
- Paired serum and urine electrolytes
- Serum glucose level
- Fluid status assessment
- Specific tests
- Renal function (urea, creatinine, eGFR)
- TFT (to exclude hypothyroidism)
- Serum cortisol (to exclude adrenal insufficiency)
Identifying the Underlying Cause of Hyponatraemia
The causes of hyponatraemia are first classified based on serum osmolality.
Hypotonic Hyponatraemia (True Hyponatraemia)
Real, physiological reduction in plasma sodium concentration, leading to a decrease in serum osmolality
Causes are sub-classified by fluid status and renal sodium:
| Renal causes (urine sodium >20 mmol/L) | Extra-renal cause (urine sodium ≤20 mmol/L) | |
|---|---|---|
| Hypovolaemic |
|
|
| Euvolaemic |
|
|
| Hypervolaemic |
|
|
Fluid status:
- Hypovolaemic: ↓ skin turgor, dry mucous membrane, tachycardia, hypotension
- Hypervolaemic: peripheral oedema, ↑ JVP, pulmonary oedema (bi-basal crackles)
Isotonic Hyponatraemia
Causes:
- Pseudohyponatremia (laboratory artefact, caused by hyperlipidaemia, multiple myeloma)
- TURB syndrome
Hypertonic Hyponatremia
Causes:
- Hyperglycaemia
- Severe uraemia
- Mannitol use
Management
Severe, Symptomatic Hyponatraemia (<120 mmol/L)
Treat immediately with hypertonic saline (options include 3%, 2.7%, 1.8% saline)
- Patient should be in HDU / ICU, under the guidance of a senior
- The aim is to improve symptoms, NOT correct sodium back to normal
Other Patients
If hypertonic saline is not necessary (see above), hyponatraemia is treated depending on fluid status and likely underlying cause
| Fluid status | Management |
|---|---|
| Hypovolaemic | Treat with 0.9% saline |
| Euvolaemic | Depends on the underlying cause:
|
| Hypervolaemic | Fluid restriction +/- diuretics needed
Exact management depends on the underlying cause (usually due to AKI / heart failure / cirrhosis / nephrotic syndrome) |
It is important NOT to correct hyponatraemia too quickly, due to the risk of osmotic demyelination syndrome (rapid increase of serum osmolality will cause water shifting out of the brain cells rapidly, leading to dehydration and injury of oligodendrocytes)
To prevent osmotic demyelination syndrome from occurring:
- Maximum correction rate is 10 mmol/L in the first 24 hours
- Then, 8 mmol/L in the subsequent 24 hours
On the other hand, if hypernatraemia is corrected too rapidly, there is a risk of developing cerebral oedema (rapid reduction of serum osmolality will cause water shifting into brain cells rapidly).