In addition to correcting the serum sodium, the management of hyponatraemia must always include treatment of the underlying cause. This could be withdrawing the probable causative drug, treating postoperative pain, treating hormonal abnormalities and treating identifiable causes of the syndrome of inappropriate antidiuretic hormone secretion.
Severe or symptomatic hyponatraemia should be treated as a medical emergency. Failure to act makes progression to altered consciousness, seizures, and permanent brain damage or death probable. These patients must be managed in a hospital with onsite 24-hour pathology and appropriate medical and specialist staffing.
Euvolaemic hyponatraemia
While fluid restriction is the initial treatment of choice in asymptomatic patients, more active and urgent treatment is required in the symptomatic patient with severe euvolaemic hyponatraemia. It is the commonest type of severe hyponatraemia and prompt intervention to raise the serum sodium is indicated. Isotonic saline is contraindicated as it can be associated with a further fall in serum sodium. Hypertonic saline should only be used in symptomatic patients with very low serum sodium concentrations. The infusion aims to increase the serum sodium to a 'safe' level, usually considered to be greater than 120–125 mmol/L depending on the initial concentration. The rate of increase in serum sodium should be limited to prevent complications. There are a variety of formulae for predicting the increase in serum sodium expected from alterations to the rate and volume of hypertonic saline infusion. However, these formulae often result in over-correction, probably due to alterations in the clinical state not predicted at the time the calculation was performed, such as the commencement of a diuresis following the start of the saline infusion, placing the patient at risk of adverse effects.5
A recent review has suggested that immediate treatment should be the infusion of 100 mL of 3% sodium chloride over one hour.5If symptomatic hyponatraemia with fitting persists, a further 200 mL over the next two hours can be given. The aim of treatment is to raise the serum sodium into a 'safe' range usually recognised as greater than 120 mmol/L, and to abolish the patient's symptoms. Once these goals have been achieved further hypertonic saline should not be given, although ongoing fluid restriction will usually be required. It may take 48–72 hours for the patient's symptoms to improve.
The maximum rate of increase in serum sodium should not exceed 10 mmol/L over 24 hours and 18 mmol/L over 48 hours to minimise the risk of osmotic demyelination.5,6In patients with liver disease a slower rate of correction is indicated in view of their greater risk of osmotic demyelination. Hypertonic saline can only be administered safely in a hospital with intensive care facilities associated with 24-hour onsite pathology, as the patients must be closely monitored and have their electrolytes checked every two hours. However, the initial infusion of hypertonic saline may need to be given before transfer to that higher level care in consultation with the accepting team.
Hypovolaemic hyponatraemia
The history and clinical examination will be supported by biochemistry consistent with hypovolaemia. The urine sodium will be below 20 mmol/L except in cases of renal salt wasting and renal failure. Treatment consists of volume expansion with isotonic saline. This is the only situation in which the use of isotonic saline is appropriate treatment for hyponatraemia.
Hypervolaemic hyponatraemia
Cardiac failure, hepatic cirrhosis or renal disease should be easily recognisable by history, clinical examination and the results of renal and liver function tests. The management is to treat the underlying disease process and will usually include fluid restriction and diuretic therapy.