Grading hyponatraemia: severity and acute versus chronic
Hyponatraemia (a low serum sodium concentration, below 135 mmol/L) is described on two separate axes: how low the level is and how fast it fell.
Remember
Grade the level and the timing separately: severe means a sodium below 125 mmol/L, acute means a documented fall within 48 hours, and chronic means longer than that or an unknown duration.
Hyponatraemia means a serum sodium (the concentration of sodium in the blood) below 135 mmol/L. It is then described on two axes, and when the timing is known, both should be named.
The first axis is the level. Conventionally, the level is graded as mild at 130 to 134 mmol/L, moderate at 125 to 129 mmol/L and severe below 125 mmol/L. Some guidelines call the lowest band profound rather than severe. The thresholds step down by five from 135, which makes them easy to recall.
The second axis is the timing. Acute means the low sodium is known to have developed within 48 hours. Chronic means the low sodium has been present for longer than 48 hours, or the duration is unknown. Unknown timing is treated as chronic, and a patient should be treated as chronic if there is any doubt. One laboratory value gives the level but not the timing, so on its own it cannot show whether the fall was acute or chronic.
The clock matters because of the brain. After a rapid fall, brain cells have no time to lose osmolytes (small dissolved particles), so water moves into them and the brain swells, which is called cerebral oedema. That swelling can cause nausea, headache, confusion and seizures. A slow fall gives the cells time to lose osmolytes, so chronic cases are often much quieter.
For the same reason, the grade does not reliably predict symptoms. A very low sodium may cause no symptoms, while a moderate fall that happened quickly may cause many.
Key facts
- Definition of hyponatraemia
- a serum sodium below 135 mmol/L.
- Mild band
- 130 to 134 mmol/L.
- Moderate band
- 125 to 129 mmol/L.
- Severe band
- below 125 mmol/L, called profound in some guidelines.
- Acute
- documented to have developed within 48 hours.
- Chronic
- present for longer than 48 hours, or the duration is unknown.
- Grade and symptoms
- the level does not reliably predict symptoms, because the speed of the fall matters.
Exam traps
- A sodium of 125 to 129 mmol/L belongs to the moderate band and not the severe one, because severe starts below 125 mmol/L, not at it.
- A fall known to have happened within 48 hours defines acute hyponatraemia, which describes how fast the sodium fell rather than how low it is, so a sodium just under 135 mmol/L that fell quickly is acute but still mild.
- Acute is a statement about timing, so it is not a severity grade, and one laboratory value cannot show how quickly the sodium fell.
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