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Hyponatraemia

Checked: 23-08-2021 by Vicky Ryan Next Review: 23-08-2022

Overview

These guidelines have been provided by the Endocrinology Team at NBT so pathways might be slightly different at UHBW

Definition

A serum sodium ≤135mmol/l and is the most common disorder of fluid and electrolyte imbalance in clinical practice
Can be classified by biochemical severity:

Mild – serum sodium 130-135 mmol/l
Moderate - serum sodium 125-129 mmol/l
Severe – serum sodium <125 mmol/l

OR by clinical symptoms:

Moderate – headache, confusion, nausea without vomiting
Severe – vomiting, cardiorespiratory distress, low GCS, seizures

Important points

Always review previous results and duration of hyponatraemia:

Acute hyponatraemia is defined as <48 hours duration
Chronic hyponatraemia is >48 hours duration
If it cannot be classified, it is assumed to be chronic unless clinical features suggest otherwise

If chronic, it might represent "reset osmostat" which is a common phenomenon in the elderly. These patients regulate their serum osmolality around a reduced set point but are also able to dilute their urine in response to a water load to keep the serum osmolality around this set point. Assuming other causes have been excluded e.g. hypothyroidism, hypoadrenalism etc. decide on an acceptable adjusted range for that individual’s sodium target based on past results and symptoms

Whilst recommended as part of the overall patient assessment, clinical judgment of fluid status alone is of low sensitivity and should therefore be used in conjunction with the investigations outlined below

The underlying cause can often be multi- factorial

Who to refer

If the cause is likely hypervolaemic hyponatraemia e.g. cirrhosis, heart failure, nephrotic syndrome, consider referral to the relevant specialty
Mild (sodium 130-135mmol/l) asymptomatic hyponatraemia can usually be managed in primary care. Advise:

Repeat sodium to ensure not falling rapidly
Stop/switch any potential causative medications as below
Treat any underling concurrent illnesses that could be contributory e.g. diarrhoea, lower respiratory tract infection, fluid overload
After doing the above, repeat the sodium in 2 weeks and if it remains low despite the above measures, send a routine eRS referral

For the following situations, send a routine ERS referral for further management advice or to establish whether secondary care review is needed

If the person has moderate (sodium 125-129mmol/l) asymptomatic hyponatraemia or the cause of the hyponatraemia is unclear
There is suspicion of adrenal insufficiency as a cause
When the biochemistry suggests SIADH with either moderate hyponatraemia or persistent mild hyponatraemia (>2 weeks)

Red Flags

If acute or severe hyponatraemia (biochemically or clinically) – requires urgent medical treatment – refer to the Medical take

Before referral

Further investigation

Review all medications (particularly those that are new). The commonest drugs to cause hyponatraemia (usually via a mechanism of SIADH) include – thiazides diuretics, SSRIs, anti-convulsants e.g. carbamazepine and PPIs. Where possible, stop the relevant medication(s)/switch to an alternative and repeat the sodium 2 weeks later
Exclude hyperglycaemia and other causes of non-hypotonic hyponatraemia e.g. serum glucose, triglycerides and total protein
Other investigations:

LFTs
TSH – within the last 6 months
9am cortisol – if ≥350 nmol/L (assuming not on steroids), adrenal insufficiency is unlikely. If any clinical suspicion of adrenal insufficiency, see separate adrenal insufficiency guidance
Paired serum and urine osmolality and urine sodium
Consider CT head and CXR (to assess for malignancy) if the biochemistry is suggestive of SIADH

https://eje.bioscientifica.com/view/journals/eje/170/3/G1.xml

Services

Endocrinology NBT

Endocrinology UHBW

Resources

References

Efforts are made to ensure the accuracy and agreement of these guidelines, including any content uploaded, referred to or linked to from the system. However, BNSSG ICB cannot guarantee this. This guidance does not override the individual responsibility of healthcare professionals to make decisions appropriate to the circumstances of the individual patient, in consultation with the patient and/or guardian or carer, in accordance with the mental capacity act, and informed by the summary of product characteristics of any drugs they are considering. Practitioners are required to perform their duties in accordance with the law and their regulators and nothing in this guidance should be interpreted in a way that would be inconsistent with compliance with those duties.

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