Hypernatraemia
Background Information
Definition
Hypernatraemia is defined as serum sodium >145 mmol/L
Severe hypernatraemia is generally defined as >160 mmol/L
Clinical Features
Key features: [Ref]
| Mild to moderate hypernatraemia |
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| Severe hypernatraemia (>160 mmol/L) |
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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.
Chronic hypernatraemia is often asymptomatic, esp. in elderly patients. [Ref]
Hypernatraemia in infants can cause hyperpnoea, muscle weakness, restlessness and a characteristic high-pitched cry. [Ref]
Diagnosis
Diagnostic Approach
Key tests to order
- Paired serum and urine osmolality
- Urine sodium
Other tests:
- Serum glucose
- U&E
- Serum sodium
Hypernatraemia always gives a hypertonic state (unlike hyponatraemia, which can be hypotonic / isotonic / hypertonic):
- ↑ Serum osmolality
- ↑ Haematocrit
Identifying the Underlying Cause of Hypernatraemia
Urine osmolality is the primary discriminator:
- Low urine osmolality: intra-renal loss (kidney is not concentrating urine)
- High urine osmolality: extra-renal loss (kidney is concentrating urine maximally)
| Intra-renal loss causes |
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| Extra-renal loss causes | Essential causes of water loss (not enough input or too much output):
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The causes of hypernatraemia can be further categorised based on fluid status and urine sodium, but are not included here as they are less clinically relevant and exam-important (as opposed to hyponatraemia).
Management
Treat the underlying cause once identified.
Mild Hypernatraemia
Replace missing water with: [Ref]
- Oral water (NOT electrolyte drinks), or
- IV glucose 5%
IV glucose (e.g., glucose 5%) is preferred over normal saline for correcting hypernatraemia. Because once the glucose is metabolised, it provides free water.
In contrast, normal saline is isotonic to slightly hypertonic and therefore does not lower serum sodium.
Severe Hypernatraemia (>170 mmol/L)
Give IV glucose 5% [Ref]
- Only give IV sodium chloride 0.9% if the patient is volume-depleted and hypotensive
IV glucose (e.g., glucose 5%) is preferred over normal saline for correcting hypernatraemia. Because once the glucose is metabolised, it provides free water.
In contrast, normal saline is isotonic to slightly hypertonic and therefore does not lower serum sodium.
It is important NOT to correct hypernatraemia too quickly, due to the risk of cerebral oedema (rapid decrease of serum osmolality will cause water shifting into the brain cells rapidly, causing swelling)
To prevent cerebral oedema from occurring:
- Maximum correction rate is 10 mmol/L per day
On the other hand, if hyponatraemia is corrected too rapidly, there is a risk of developing osmotic demyelination syndrome (rapid increase in serum osmolality will cause water shifting out of brain cells rapidly).