SIADH

Question 1

define SIADH

Answer 1

Excess ADH secretion from the posterior pituitary gland. SIADH = Syndrome of Inappropriate ADH

Question 2

describe the pathophysiology behind SIADH

Answer 2

1. Excessive release of ADH from the posterior pituitary (supraoptic nucleus).
2. Acts on V2 receptors in the CD and late DCT.
3. Acts via GPCR mechanism which increases AQP2 insertion in the apical membrane of CD and DCT
4. More water retained from kidney tubules into blood

Question 3

what can ensue when more water is retained from kidney tubes into blood IN SIADH?

Answer 3

acute and chronic hyponatraemia

Question 4

what is the serum osmolality needed for a diagnosis of SIADH?

Answer 4

Serum osmolality <275mmol/Kg

Question 5

what is the serum Na+ needed for a diagnosis of SIADH?

Answer 5

Serum Na+ <135mmol/L

Question 6

what are the key investigations needed for SIADH?

Answer 6

Serum osmolality and Na+
Urine osmolality and Na+

(Random blood sugar, LFT, TFT, cortisol, lipid profile – to rule out other causes)

Question 7

what are the 4 causes of SIADH?

Answer 7

• CNS disturbances
• Tumour
• Drugs
  -Respiratory

Question 8

what are some examples of CNS disturbances that causes SIADHs?

Answer 8

e.g. stroke, infection, trauma, haemorrhage 

Question 9

what is an example of a tumour that causes SIADHs?

Answer 9

Small Cell Lung Cancer (SCLC) common cause of ectopic ADH

Question 10

what are some examples of drugs that can cause SIADHs?

Answer 10

e.g. carbamazepine, cyclophosphamide, SSRIs etc

Question 11

what is an example of a respiratory cause for SIADHs?

Answer 11

Particularly pneumonia

Question 12

what are the signs/ symptoms for SIADHs?

Answer 12

• low serum osmolality
• NO oedema
• typically NORMAL circulating volume
• Hyponatraemia symptoms

Question 13

describe further the typically NORMAL circulating volume seen as a sign of SIADHs

Answer 13

-Euvolemic hyponatraemia
-(can present w hypervolaemia instead however)

Question 14

what are moderate hyponatraemia symptoms that can be found in SIADHs ?

Answer 14

nausea, vomiting, headache, mental slowing, instability, irritability

Question 15

what are severe hyponatraemia symptoms that can be found in SIADHs ?

Answer 15

convulsions, coma, respiratory arrest

Question 16

how can there be euvolemic hyponatraemia be present in SIADHs?

Answer 16

• 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

Question 17

what is the treatment/ management for SIADHs?

Answer 17

• treat the underlying cause and correct Na+ levels

Question 18

if its moderate/mild Na+ levels- what treatment/management do you do IN SIADH?

Answer 18

• restrict water intake
  Persistent– salt tablets, IV saline

Question 19

if its severe Na+ levels- what treatment/management do you do in SIADH ?

Answer 19

-Hypertonic saline (3%)
- If Persistent – Tolvaptan (competitive V2 receptor antagonist)

Question 20

what should you be cautious about when treating chronic hyponatraemia in SIADH?

Answer 20

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)