SIADH Flashcards
define SIADH
Excess ADH secretion from the posterior pituitary gland. SIADH = Syndrome of Inappropriate ADH
describe the pathophysiology behind SIADH
- Excessive release of ADH from the posterior pituitary (supraoptic nucleus).
- Acts on V2 receptors in the CD and late DCT.
- Acts via GPCR mechanism which increases AQP2 insertion in the apical membrane of CD and DCT
- More water retained from kidney tubules into blood
what can ensue when more water is retained from kidney tubes into blood IN SIADH?
acute and chronic hyponatraemia
what is the serum osmolality needed for a diagnosis of SIADH?
Serum osmolality <275mmol/Kg
what is the serum Na+ needed for a diagnosis of SIADH?
Serum Na+ <135mmol/L
what are the key investigations needed for SIADH?
Serum osmolality and Na+
Urine osmolality and Na+
(Random blood sugar, LFT, TFT, cortisol, lipid profile – to rule out other causes)
what are the 4 causes of SIADH?
- CNS disturbances
- Tumour
- Drugs
-Respiratory
what are some examples of CNS disturbances that causes SIADHs?
e.g. stroke, infection, trauma, haemorrhage
what is an example of a tumour that causes SIADHs?
Small Cell Lung Cancer (SCLC) common cause of ectopic ADH
what are some examples of drugs that can cause SIADHs?
e.g. carbamazepine, cyclophosphamide, SSRIs etc
what is an example of a respiratory cause for SIADHs?
Particularly pneumonia
what are the signs/ symptoms for SIADHs?
- low serum osmolality
- NO oedema
- typically NORMAL circulating volume
- Hyponatraemia symptoms
describe further the typically NORMAL circulating volume seen as a sign of SIADHs
-Euvolemic hyponatraemia
-(can present w hypervolaemia instead however)
what are moderate hyponatraemia symptoms that can be found in SIADHs ?
nausea, vomiting, headache, mental slowing, instability, irritability
what are severe hyponatraemia symptoms that can be found in SIADHs ?
convulsions, coma, respiratory arrest
how can there be euvolemic hyponatraemia be present in SIADHs?
- although water is retained, the osmoregulatory mechanisms are still occurring, such as -ANP, RAAS systems
- therefore excess fluid can still be excreted
- there’s still hyponatraemia due to salts being lost via fluid loss
what is the treatment/ management for SIADHs?
- treat the underlying cause and correct Na+ levels
if its moderate/mild Na+ levels- what treatment/management do you do IN SIADH?
- restrict water intake
Persistent– salt tablets, IV saline
if its severe Na+ levels- what treatment/management do you do in SIADH ?
-Hypertonic saline (3%)
- If Persistent – Tolvaptan (competitive V2 receptor antagonist)
what should you be cautious about when treating chronic hyponatraemia in SIADH?
The body (especially the brain) has adapted to hyponatraemia so sudden administration of hypertonic solution will rapidly draw water out of cells - can lead to Osmotic DemyelinationSyndrome (ODS)
