Endocrinology - SIADH

Question 1

What does ADH do?

Answer 1

It stimulates water reabsorption from the collecting ducts in the kidney

Question 2

What can cause SIADH?

Answer 2

This may be the result of the posterior pituitary secreting too much ADH

Or ectopic ADH secretion e.g. small cell lung cancer.

Question 3

What occurs in SIADH?

Answer 3

Excessive ADH results in excessive water reabsorption

The water dilutes sodium causing hyponatraemia

The excessive water reabsorption is not usually significant enough to cause a fluid overload, therefore you end up with a “euvolaemic hyponatraemia”.

Urine becomes more concentrated as less water excreted by the kidneys, so:
- High urine osmolality and high urine sodium

Question 4

Symptoms of SIADH

Answer 4

Non-specific

Headache
Fatigue
Muscle aches and cramps
Confusion
Severe hyponatraemia can cause seizures and reduced consciousness

Question 5

Causes of SIADH

Answer 5

Post-operative from major surgery

Infection, particularly atypical pneumonia and lung abscesses

Head injury

Medications;

• Carbamazepine
• PPIs
• Antipsychotics
• SSRIs
• Cyclophosphamide
• NSAIDs

Malignancy, particularly small cell lung cancer
Meningitis

Question 6

Initial diagnosis

Answer 6

• Clinical examination will show euvolaemia
• U&Es - hyponatraemia
• Urine sodium and osmolality high

SIADH is a diagnosis of exclusion

Other causes of hyponatraemia need to be excluded:

• Negative short synacthen test to exclude adrenal insufficiency
• No history of diuretic use
• No diarrhoea, vomiting, burns, fistula or excessive sweating
• No excessive water intake
• No chronic kidney disease or acute kidney injury

Question 7

Hyponatraemia and history of smoking/weight loss think…

Answer 7

Small cell lung cancer secreting ADH (causing SIADH)

Question 8

Management of SIADH

Answer 8

Investigate for and treat the underlying cause

Stop causative medications

Correct sodium slowly

Fluid restriction for SIADH (500ml-1L fluid intake)

Tolvaptan can be used in refractory cases under guidance from specialist (vaptans are ADH receptor blockers)

Question 9

First step in assessing hyponatraemia

Answer 9

Assess the fluid status

Hypovolaemic - e.g. diuretics, poor fluid intake, increased loss

Euvolaemic - SIADH

Hypervolaemic - e.g. heart, liver and renal failure

Question 10

Management of symptomatic hyponatraemia

Answer 10

Correct with 3% hypertonic saline under senior guidance

Do not correct sodium too quickly (not more that 10mmol per 24h)

Question 11

What is the risk of correcting sodium too quickly?

Answer 11

Osmotic demyelination syndrome

Due to shifts in the osmolality in the neurone, the neurone undergoes demyelination

Question 12

How does osmotic demyelination syndrome present? What is the treatment?

Answer 12

This may present as spastic quadriparesis, pseudobulbar palsy and cognitive and behavioural changes.

There is a significant risk of death.

Treatment is only supportive if this occurs and most are left with some neurological deficit so prevention is key.