21st May 2025

Hyponatraemia

Hyponatraemia is thought to occur in 15-28% of patients admitted to hospital, and approximately 25% of elderly patients with hyponatremia may have SIADH (Syndrome of inappropriate secretion of antidiuretic hormone) of which the most common causes are respiratory disease and malignancy. 

NICE guidelines state:

“Hyponatraemia is defined as a serum sodium concentration of less than 135 mmol/L.”

Normal range: serum sodium concentration of 135-145 mmol/L

Mild: 130-135 mmol/L (serum concentration of sodium)

Moderate: 125-129 mmol/L (serum concentration of sodium)

Severe: Less than 125 mmol/L (serum concentration of sodium)”

Causes of Hyponatraemia

Table 1: Causes of Hyponatraemia

Pulmonary embolism is also a common cause of hyponatraemia within hospital in-patients. 

Drug-related causes of hyponatraemia 

Table 2: Drug-related causes of hyponatraemia

Classification of Hyponatraemia

Acute vs Chronic 

It is important to define whether a hyponatraemia is acute i.e. a duration of less than 48 hours or chronic i.e. a duration greater than 48 hours. See below sections for further information on the management. 

Fluid status 

Hyponatraemia is classified according to 3 sub-groups:

1. Hypovolaemia – volume depletion i.e. dehydration
2. Euvolaemia – a normal volume status 
3. Hypervolaemia – volume overload  

Therefore when defining a hyponatraemia we should consider:

1. Is the hyponatraemia mild, moderate or severe?
2. Is this acute or chronic? i.e. less than or greater than 48 hours in duration
3. What is the patient’s fluid status? i.e. hypovolaemic/ euvolaemic/ hypervolaemic
4. Is the patient symptomatic?

Being able to answer these 4 questions will allow us to determine an appropriate management plan for each patient.

Presentation

It is important to note that patients with long-term, chronic hyponatraemia are often asymptomatic. However, the symptoms to look out for, especially in patients with acute changes to serum sodium levels include: 

• nausea
• vomiting
• confusion
• muscle cramping
• cognitive impairment
• gait disturbances
• depression
• tremors 
• hallucinations
• respiratory depression
• coma

Management

Management will vary based on the duration of hyponatraemia, patient’s volume status and whether or not the patient is symptomatic.

Acute onset (<48 hours) + Asymptomatic/ mild symptoms

1. Stop potential offending medications

2. Fluid intake should depend on the patient’s fluid status:

• Hypovolaemic: replace losses with 0.9% saline to restore the extracellular volume

• Euvolaemic: ensure all fluids given are essential and that any non-essential fluids are stopped

• Hypervolaemic: Fluid restrict these patients

3. Investigate for a cause and treat accordingly

Acute onset (<48 hours) + Moderate/ severe symptoms

1. Administer hypertonic saline

GAIN guidelines state that 200mls of 2.7% saline should be administered over 30 minutes. If this is required, sodium levels should be checked two hourly with an aim to increase levels by 1-2 mmol/L every hour until the patient’s symptoms resolve.

2. Fluids restrict these patients to 1-1.5 L daily and investigate for a cause including infection, stress, medication or other hypotonic fluids.

3. Monitor sodium daily.

It is often useful to discuss these patients with a consultant biochemist who will often be able to provide additional advice and support.

Chronic onset (>48 hours/unknown onset) without severe symptoms

General management

• Review medications and IV fluid prescriptions – stop any non-essential ones that may be contributing
• Investigations (bloods, paired serum and urine sodium and osmolalities)
  • Consider further investigation for underlying cause if SIADH diagnosed (see below section) 
• If mild hyponatraemia, treat the underlying cause
• If moderate/severe hyponatraemia, check serum levels daily until stable, and increase serum sodium by no more than 10mmol/L in the first 24 hours, and no more than 8mmol/L/24 hours thereafter
• If patient is fluid overloaded or has SIADH, treat with fluid restriction and consider diuresis
• If hypovolaemic, treat with IV crystalloid such as 0.9% sodium chloride

Chronic onset with severe symptoms

Treat as for acute onset with severe symptoms. 

If patients are haemodynamically unstable, resuscitation should be prioritised over concerns about rapid correction of serum sodium levels, and adequate resuscitation fluid should be given.