Hyponatraemia is defined as a serum sodium concentration below 135 mmol/l. Hyponatremia is the most common type of electrolyte imbalance and occurs in 20% of those admitted to hospital.
The degree of hyponatraemia is classified as Mild: 130–135mmol/L, Moderate: 125– 129mmol/L and Severe: < 125mmol/L.
There are multiple different causes of hyponatraemia, including:
Hypovolemic hyponatraemia- this occurs when both the total body water and sodium content are decreased but the relative decrease in total body sodium is greater than the decrease in total body water.
Hypervolemic hyponatraemia- this occurs when both the total body water and sodium content increase but the relative increase in total body water is greater than the increase in total body sodium, resulting in oedema.
Euvolemic hyponatraemia- this occurs when the total body water increases but the total body sodium remains unchanged, thereby producing a dilutional effect.
If the hyponatraemia is mild and has developed slowly then this is commonly asymptomatic and it is often an incidental finding based on blood tests. If however there are rapid changes in serum sodium levels or the hyponatraemia is severe this can cause symptoms of drowsiness, vomiting, headache, and seizures.
In order to treat hyponatraemia, it is important to understand the underlying cause as a treatment for low blood sodium varies depending on the cause. It can include: • Adjusting the dosage of diuretics; • Restricting fluid intake; • Treating underlying conditions; • Infusing an intravenous sodium solution.
It is imperative that if hyponatraemia is suspected or diagnosed that clinicians provide appropriate care by arranging appropriate investigations, arranging a referral to an appropriate specialist such as an endocrinologist and/or admitting the patient to hospital for treatment.