Hyponatraemia

Serum sodium below <130mmol/L occurs in up to 30% of inpatients. But, sudden and dramatic falls in sodium to <120mmol/L can be life-threatening and requires evaluation of fluid status to determine emergency management.

Mild hyponatraemia

Mild chronic and asymptomatic hyponatraemia >130mmol/l does not usually require any specific treatment. 

Hypovolaemic hyponatraemia

Patients with obvious hypovolaemia can be safely resuscitated with IV normal saline. Reassess serum sodium at least 12 hourly for the first 24 hours, then daily until improving.

Moderate or severe hyponatraemia

Stop interfering drugs

Diuretics are the most important drugs to stop due to their natriuretic effects.

Proton pump inhibitors, non steroidal anti-inflammatory drugs and ACE inhibitors should also generally be stopped.

Neurology or psychiatric advice should be sought regarding the safety and appropriateness of stopping other agents e.g. carbemazepine and anti-depressants.

Treat the underlying cause

Patients with hypothyroidism or with glucocorticoid deficiency need to receive treatment for the underlying cause of their hyponatraemia. They do not usually require any specific treatment of the associated sodium abnormality.

Patients with hypervolaemic hyponatraemia - e.g. due to heart failure, nephrotic syndrome or cirrhosis - may require fluid restriction but also require treatment of the underlying cause, such as diuretics.

Determine the patient's fluid status

Fluid status, in particular whether the patient is hypovolaemic or not, is crucial to determine the right course of management.

Eu- or hyper-volaemic hyponatraemia

Fluid restriction

The main stay of treatmtent is fluid restriction. Virtually all patients will need to be fluid restricted to <1 litre/24 hours, depending on the rapidity of onset of symptoms, usual fluid intake and size of the patient.

For example, a patient of low body weight e.g. <50kg and who habitually only drinks small volumes is likely to require a tighter restriction e.g. to 800ml/24hours from day one.

Sodium levels above 125mmol/l

These are usually well tolerated. Daily monitoring to confirm response to fluid restriction is generally satisfactory. 

Sodium levels 120-125mmol/l

With hyponatraemia 120-125mmol/l repeat serum sodium 12 hourly for 24hours, then daily if it is improving. 

Severe hyponatraemia <120mmol/l

With hyponatraemia <120mmol/l, repeat serum sodium testing at 4-6 hours to ensure that it has not deteriorated further, and to ensure that correction is not too rapid.

Serum sodium should not rise by >0.5mmol/l per hour and never by more than 10-12mmol/l per 24 hours.

What if the serum sodium falls further?

In this case, the patient and their fluid charts should be reassessed. The most likely explanations are that the fluid restriction has not been fully enforced if the patient was truly euvolaemic, or that the patient was in fact hypovolaemic.

If in doubt, the patient can be given a small volume of saline (500mls 0.9% or normal saline over 4 hours). An improvement in serum sodium suggests that the patient was in fact hypovolaemic and so this treatment should be continued with close monitoring.

What if the serum sodium is not responding, or responding too slowly?   

The patient and their fluid charts should be reassessed.

The most likely explanations are that the fluid restriction has not been fully enforced if the patient was truly euvolaemic, or that the fluid restriction needs to be tighter. If the patient remains stable and still appears euvolaemic, reduce the fluid allowance further e.g. to 700-800mls.

If the patient is taking several days to improve despite appropriate fluid restriction, consider demeclocycline or one of the new vaspopressor receptor antagonists the vaptan class of drugs under specialist advice and with close monitoring.

What if too rapid correction has occurred?

Sodium increases of more that 0.5mmol/l per hour may be associated with pontine and extrapontine demyelination which can cause irreversible CNS damage.

If the sodium rise has exceeded this, the patient must be reassessed urgently.

If there is clinical concern e.g. the patient's neurological status has altered, a small infusion of 0.8% saline can be infused to arrest the rise in serum sodium, although this will require very close monitoring of the serum sodium in an HDU setting.

Emergency treatment of the patient who is fitting or the unstable patient with deteriorating GCS

This life threatening situation is the only situation in which the eu-volaemic patient should be considered for hypertonic saline.

This must be done with HDU support and near continuous (at least hourly) monitoring. An IV bolus of up to 100 mls of hypertonic saline (1.8%) through an infusion pump can be given over 30 mins, with frequent sodium monitoring. Larger volumes or higher concentrations of saline carry an increased risk of demyelination. 

The aim of this treatment is to increase the serum sodium by 1-2 mmol/hr in the first 2 hours only, and still to no more than 10-12mmol/L in the first 24 hours. IV therapy should not then be continued, but fluid restriction and standard management as above reinstituted once the patient is stabilised and has stopped fitting.

Serum sodium should be then monitored every 2-4 hours with the aim to increase serum sodium by no more than 10-12mmol/l per 24 hours.