Hyponatraemia is a lower-than-normal concentration of sodium in the blood. Society for Endocrinology emergency guidance describes hyponatraemia as a serum sodium concentration below 135 mmol/L.
Sodium is an important electrolyte involved in fluid balance, nerve signalling, and muscle function. Hyponatraemia is often not a simple shortage of sodium; it commonly reflects too much water relative to sodium in the bloodstream.
Severity
Severity is usually described by the sodium concentration and the person's symptoms. Mild hyponatraemia may cause no obvious symptoms. More severe or rapidly developing hyponatraemia can cause neurological symptoms because water moves into brain cells.
Clinical thresholds vary slightly between guidelines, but sodium below 135 mmol/L is generally hyponatraemia. Very low sodium, a rapid fall, or symptoms such as confusion, seizures, or reduced consciousness are treated as urgent.
Symptoms
Symptoms may include:
- Nausea.
- Headache.
- Tiredness or lethargy.
- Muscle cramps or weakness.
- Unsteadiness.
- Confusion.
- Agitation.
- Seizures.
- Reduced consciousness or coma in severe cases.
Symptoms depend not only on the sodium number but also on how quickly it has fallen and the person's general health.
Causes
Hyponatraemia has many causes. Common groups include:
- Medicines, including some diuretics, antidepressants, and anti-epileptic medicines.
- Heart failure, liver disease, or kidney disease.
- Syndrome of inappropriate antidiuretic hormone secretion, known as SIADH.
- Adrenal insufficiency.
- Low thyroid function in some cases.
- Vomiting, diarrhoea, or fluid loss with replacement by water alone.
- Excessive water intake.
- Post-operative fluids.
- Endurance exercise with excess hypotonic fluid intake.
High blood glucose and some laboratory artefacts can make sodium appear low or alter interpretation, so assessment considers the clinical setting and blood chemistry as a whole.
Diagnosis
Diagnosis begins with a blood test. Further assessment may include repeat sodium measurement, kidney function, glucose, serum osmolality, urine sodium, urine osmolality, thyroid testing, cortisol testing, medication review, and assessment of fluid status.
The main diagnostic questions are whether the result is real, whether symptoms are present, whether the fall is acute or chronic, and whether the person appears dehydrated, overloaded with fluid, or clinically euvolaemic.
Treatment
Treatment depends on severity, symptoms, speed of onset, and cause. It may include:
- Treating the underlying condition.
- Stopping or changing a medicine that is contributing.
- Fluid restriction in some cases.
- Careful sodium replacement.
- Intravenous hypertonic saline for severe symptomatic cases under close monitoring.
Correction has to be controlled. Raising sodium too quickly can cause serious neurological injury, so severe cases are usually managed in hospital with repeated blood tests.
Emergency Features
Seizures, marked confusion, reduced consciousness, or severe symptoms with low sodium require urgent medical assessment. The Society for Endocrinology describes severe symptomatic hyponatraemia as potentially life-threatening and needing emergency assessment and treatment.
References
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