QuestionSBA
QuestionChoice
This would cause low serum osmolality and a genuine hypovolaemic hyponatraemia.
10015668
c
Loop diuretic use
QuestionChoice
This is a pseudohyponatremia as the serum osmolality is normal. All the other options cause a genuine hyponatraemia. Other causes of a similar picture could be hyperglycaemia or the presence of paraproteins in the plasma (such as in myeloma). Hyperlipidaemia causes psuedohyponatremia as sodium is dissolved in plasma and in extreme hyperlipidemia the plasma is displaced by lipids, leading to an inaccurate measurement of sodium
10015666
a
Hyperlipidaemia
QuestionChoice
This would cause low serum osmolality and a genuine hypervolaemic hyponatraemia.
10015669
d
Cirrhosis of the liver
QuestionChoice
This would cause low serum osmolality and a genuine euvolaemic hyponatraemia.
10015667
b
SIADH
QuestionChoice
This would cause low serum osmolality and a genuine hyponatraemia.
10015670
e
Psychogenic polydipsia
QuestionComment
DAR SH PuNCH needs more salt = hypovolemic: diuretics (loop, thiazide), Addison's, renal failure - euvolemic: SIADH, hypothyroidism - hypervolemic: psychogenic, nephrotic syndrome, cirrhosis of liver, heart failure
36211
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what a shocking pneumonic
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ButtMuncher
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what an amazing name
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duodeno duodenos tommy
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duodeno duodenos tommy
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Very nice but a good question
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Versicolor Neoplasia
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maybe i shouldve spent more time on wards
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step back sis step back 
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Great Question
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Supraventricular Lion
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Dunno why you guys are praising this question, it's made of faecal matter.

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Simon Kaggwa Njala
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gOoD qUeStIoN
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C.difficult
Concept
Chapter
# Summary

Hyponatraemia is defined as a serum sodium concentration of less than 135 mmol/L. It may develop acutely or chronically, and is often asymptomatic especially in mild cases. There are many causes which may be classified by whether the patient is hypovolaemic, euvolaemic or hypervolaemic. Symptoms are more likely in severe hyponatraemia and where serum sodium has fallen acutely, including headache, nausea and vomiting and confusion. Key investigations include U&Es for sodium, potassium and renal function, cortisol and thyroid function to rule out other causes of hyponatraemia and urinary sodium and osmolality. Management differs depending on the cause of hyponatraemia - what may improve hyponatraemia in some cases may worsen it in others. In severe cases where more rapid correction of sodium is required, hypertonic saline may be administered with close monitoring of sodium levels.

# Definition

Normal serum sodium is between 135-145 mmol/L, and so levels below 135 mmol/L are classified as hyponatraemia. This may be mild (130-135 mmol/L), moderate (125-129 mmol/L) or severe (< 125 mmol/L). Acute cases develop within 48 hours, with most cases considered to be chronic (i.e. over 48 hours duration) if the onset is unclear. 

# Epidemiology

- Hyponatraemia is the commonest electrolyte disorder seen in clinical practice
- Approximately 15-20% of hospital inpatients are hyponatraemia
- Incidence increases with age and is particularly common in older people who are nursing home residents or in hospital
- Other risk factors include:
- Medications (e.g. diuretics, antidepressants)
- Comorbidities (e.g. chronic kidney disease, heart failure)

# Aetiology

## Hypovolaemic hyponatraemia

Both sodium and water are lost (but more sodium than water)

- Diarrhoea
- Vomiting
- Medications (e.g. thiazide diuretics)
- Adrenal insufficiency
- Burns
- Excessive sweating
- Cerebral salt-wasting (rare cause of hyponatraemia that occurs due to intracranial disease, e.g. subarachnoid haemorrhage or traumatic brain injury)
- Salt-wasting nephropathy (e.g. Bartter syndrome)
- Third space losses (e.g. sepsis, pancreatitis, bowel obstruction)

## Euvolaemic hyponatraemia

No loss of sodium but total body water increases causing a dilutional effect

- Syndrome of Inappropriate ADH release (SIADH)
- Beer potomania (alcohol excess combined with low solute intake)
- Primary polydipsia
- Hypothyroidism
- Secondary adrenal insufficiency
- Exercise-induced hyponatraemia 
- Hypotonic intravenous fluids

## Hypervolaemic hyponatraemia

Both total body water and sodium increase, with a greater increase in water resulting in oedema

- Heart failure
- Chronic liver disease
- Renal disease (nephrotic syndrome, chronic kidney disease or acute kidney injury)

# Signs and Symptoms

Many patients, especially those with mild and/or chronic hyponatraemia are asymptomatic and detected incidentally.

Symptoms and signs include:

- Lethargy
- Anorexia
- Headache
- Nausea and vomiting
- Muscle cramps
- Drowsiness
- Confusion
- Seizures

Patients may also have features of either dehydration (in hypovolaemic hyponatraemia) or fluid overload (in hypervolaemic hypernatraemia - see separate chapters for details.

# Differential Diagnosis

- **Pseudohyponatraemia** refers to a falsely low serum sodium reading due to an increase in serum lipid or protein content
- Serum osmolality will usually be normal or high (unlike in true hyponatraemia where it is low)
- Causes include:
- Hypertriglyceridemia
- Hypercholesterolemia
- Hyperglycaemia
- Paraproteinaemia (e.g. in multiple myeloma)

# Investigations

**Bedside:**

- **Venous blood gas** to confirm hyponatraemia - if there is a significant discrepancy between the sodium on the gas and the U&Es, suspect pseudohyponatraemia
- **Urine osmolality** is used to assess ADH activity - it will be high (i.e. urine is concentrated) if ADH is acting, e.g. in SIADH, and low (i.e. urine is dilute) if ADH is not acting, e.g. in polydipsia
- **Urine sodium** can be used to help determine the cause of hyponatraemia; in patients who are hypovolaemic:
- If urine sodium is appropriately low, this indicates extrarenal salt loss - consider vomiting/diarrhoea/third spacing
- If urine sodium is high (> 30 mmol/L), this indicates renal salt loss - consider diuretics, adrenal insufficiency or cerebral salt-wasting
- NB urine osmolality and sodium cannot be interpreted in patients taking diuretics and so these may need to be stopped prior to investigations
- **Urine dip** looking for blood and protein if renal disease is suspected

**Blood tests:**

- **U&Es** to confirm hyponatraemia and to check potassium and renal function
- **Serum osmolality** should be low in true hyponatraemia - if this is normal or raised suspect pseudohyponatraemia (hypertonic fluids such as mannitol may cause a true hyponatraemia with a raised serum osmolality)
- **Thyroid function tests** to exclude hypothyroidism as a cause of hyponatraemia
- **9am serum cortisol** to exclude adrenal insufficiency as a cause of hyponatraemia
- **NT-proBNP** if heart failure is suspected
- **Liver function tests** if cirrhosis is suspected (e.g. in patients with ascites)

# Management

**Conservative:**

- Identify and treat the underlying cause (e.g. stopping causative medications where possible)
- Patients with hyponatraemia which is severe, acute or symptomatic should usually be admitted to hospital for treatment
- Consider referral to the appropriate speciality for advice on management of the underlying cause of hyponatraemia (e.g. cardiology for heart failure, endocrinology for adrenal insufficiency)
- Closely monitor sodium levels and ensure these are corrected no faster than 8-10 mmol/L per 24 hours 
- Fluid restriction is the first-line management for SIADH and may also be used in some cases of hypervolaemic hyponatraemia

**Medical:**

- Severe hyponatraemia should be treated with hypertonic saline, usually in the intensive care setting
- Hypovolaemic hyponatraemia is treated with IV normal saline
- In some cases of SIADH tolvaptan may be considered - this is a vasopressin V2-receptor antagonist
- Correction of endocrine abnormalities may be required e.g. levothyroxine for hypothyroidism; corticosteroids for adrenal insufficiency

# Complications

- **Cerebral oedema** may complicate severe and/or acute hyponatraemia and may be life-threatening - patients develop raised intracranial pressure with symptoms of vomiting, headache, seizures and coma
- **Central pontine myelinolysis** is a complication of rapid correction of hyponatraemia - patients may present with confusion, ataxia, spastic quadriparesis and pseudobulbar palsy with dysphagia and dysarthria
- Chronic hyponatraemia, especially in the elderly, increases the risk of **falls** as well as **cognitive impairment**

# NICE Guidelines

[NICE CKS - Hyponatraemia](https://cks.nice.org.uk/topics/hyponatraemia/)

# References

[Life in the Fast Lane - Hyponatraemia](https://litfl.com/hyponatraemia/)

[Whittington Hospital Hyponatraemia Guideline](https://www.whittington.nhs.uk/document.ashx?id=6089)
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Hyperlipidemia
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In cases of severe hyperlipidaemia, the displacement of plasma by lipids may result in an artificially low sodium measurement, leading to a diagnosis of hyponatraemia.
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A 58-year-old male has some routine blood tests as part of a wellness check. They are found to be hyponatraemic. As a part of further investigation, their GP checks their serum osmolality.

Serum osmolality 285 mosm/kg (275 – 295 mosm/kg)

What is the most likely cause of these blood results?
a