Syndrome of inappropriate ADH (SIADH)

Question 1

1) What does excessive ADH secretion result in?
2) How does this impact sodium conc?
3) Why is this called an euvolemic hyponatremia?
4) What effect does SIADH have on urine conc?

Answer 1

1) Excessive water reabsorption in the collecting ducts
2) Hyponatraemia
3) The excessive water reabsorption is not usually significant enough to cause a fluid overload
4) High conc urine (high osmolality)

Question 2

The symptoms of SIADH are non-specific - name 3 of these

Answer 2

• Headache
• Fatigue
• Muscle aches and cramps
• Confusion
• Seizures and reduced consciousness (severe hypovolaemia)

Question 3

Name 3 causes of SIADH

Answer 3

• Post-operative from major surgery
• Infection, particularly atypical pneumonia and lung abscesses
• Head injury
• Medications (thiazide diuretics, carbamazepine, cyclophosphamide, antipsychotics, SSRIs, NSAIDs,)
• Malignancy, particularly small cell lung cancer
• Meningitis

Question 4

SIADH is largely a diagnosis of exclusion. Name 2 conditions that result in hyponatremia that need to be excluded

Answer 4

• Negative short synacthen test
• No history of diuretic use
• No diarrhoea, vomiting, burns, fistula or excessive sweating
• No excessive water intake
• No CKD or AKI

Question 5

As well as treating the cause (if possible) and fluid restriction, ADH antagonists can be used. Name one of these

Answer 5

Tolvaptan

Question 6

Central pontine myelinolysis (1)
1) CPM is usually a complication of what?
2) What is the pathophysiology that results in the 2 phases of symptoms?

Answer 6

1) Chronic severe hyponatremia being treated too quickly
2) As blood sodium level falls water will move by osmosis across the BBB into the cells of the brain, causing it to swell. The brain adapts to this by reducing the solutes in the brain cells so that water is balanced across the BBB. If hyponatraemia has been present for a long time, the osmolality of brain cells will be low. This is a problem if blood sodium levels rapidly rise, as water will rapidly shift out of brain cells into the blood - resulting in 2 phases of symptoms

Question 7

Central pontine myelinolysis (2)
1) What is the 1st phase of symptoms due to?
2) Name a way a patient may present in this phase
3) What is the 2nd phase of symptoms caused by?
4) How long after the rapid correction of sodium would this occur?
5) Name 2 ways it may present

Answer 7

1) Electrolyte imbalances
2) Encephalopathic and confused, may have a headache and be vomiting
3) Demyelination of the neurons, particularly in the pons
4) 2 days
5) Spastic quadriparesis, pseudobulbar palsy and cognitive and behavioural changes