SIADH Flashcards
What is vasopressin?
ADH
What is the effect of vasopressin on the kidneys?
ADH stimulates water reabsorption from the collecting ducts in the kidney, by activating aquaporins on the basolateral membrane.
Briefly define SIADH
Syndrome where there is inappropriately large amount of ADH
What happens to sodium in SIADH?
Hyponatraemia.
There is excessive water reabsorption, reflected by the low sodium concentration.
However, there is not enough fluid to cause a fluid overload, thus it is euvolaemic hyponatraemia.
There is high urine sodium.
What are the symptoms of SIADH?
Headache Fatigue Muscle aches and cramps Confusion Severe hyponatraemia can cause seizures and reduced consciousness
Name some causes of SIADH
Post-operative from major surgery
Infection, particularly atypical pneumonia and lung abscesses
Head injury
Medications (thiazide diuretics, carbamazepine, vincristine, cyclophosphamide, antipsychotics, SSRIs, NSAIDSs,)
Malignancy, particularly small cell lung cancer
Meningitis
How is SIADH diagnosed?
It is a diagnosis of exclusion:
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
What investigations can be used in SIADH?
U+Es will show a hyponatraemia. Urine sodium and osmolality will be high.
What will clinical examination show in SIADH?
Euvolaemia
How is SIADH managed?
Treat cause.
Correct the sodium slowly,
Fluid restriction,
Vaptans (tolvaptan)- ADH receptor blockers,
Demeclocycline- tetracycline antibiotic inhibits ADH
What is central pontine myelinolysis?
Osmotic demyelination syndrome.
Complication of long term severe hyponatraemia correction being treated too quickly (> 10 mmol/l increase over 24 hours).
How does the brain adapt to severe long term hyponatraemia?
As blood sodium level falls water will move by osmosis across the blood-brain barrier into the cells of the brain.
This causes the brain to swell. The brain adapts to this by reducing the solutes in the brain cells.
This adaptation takes a few days. Therefore, if the hyponatraemia has been present and severe for a long time the brain cells will also have a low osmolality. This is not a problem until the blood sodium levels rapidly rise. When this happens water will rapidly shift out of the brain cells and into the blood.
What is the first phase of symptoms of CPM?
This is due to the electrolyte imbalance and the patient presents as encephalopathic and confused. They may have a headache or nausea and vomiting. These symptoms often resolve prior to the onset of the second phase.
What is the second phase of symptoms of CPM?
This is due to the demyelination of the neurones, particularly in the pons. This occurs a few days after the rapid correction of sodium. This may present as spastic quadriparesis, pseudobulbar palsy and cognitive and behavioural changes. There is a significant risk of death.
How is CPM treated?
Treatment is only supportive. A proportion of patients make a clinical improvement but most are left with some neurological deficit.
