Endocrinology - SIADH Flashcards
What does ADH do?
It stimulates water reabsorption from the collecting ducts in the kidney
What can cause SIADH?
This may be the result of the posterior pituitary secreting too much ADH
Or ectopic ADH secretion e.g. small cell lung cancer.
What occurs in SIADH?
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
Symptoms of SIADH
Non-specific
Headache Fatigue Muscle aches and cramps Confusion Severe hyponatraemia can cause seizures and reduced consciousness
Causes of SIADH
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
Initial diagnosis
- 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
Hyponatraemia and history of smoking/weight loss think…
Small cell lung cancer secreting ADH (causing SIADH)
Management of SIADH
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)
First step in assessing hyponatraemia
Assess the fluid status
Hypovolaemic - e.g. diuretics, poor fluid intake, increased loss
Euvolaemic - SIADH
Hypervolaemic - e.g. heart, liver and renal failure
Management of symptomatic hyponatraemia
Correct with 3% hypertonic saline under senior guidance
Do not correct sodium too quickly (not more that 10mmol per 24h)
What is the risk of correcting sodium too quickly?
Osmotic demyelination syndrome
Due to shifts in the osmolality in the neurone, the neurone undergoes demyelination
How does osmotic demyelination syndrome present? What is the treatment?
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.