SIADH Flashcards
What does SIADH stand for?
Syndrome of Inappropriate ADH
SIADH refers to the decreased/increased release of ADH from the anterior/posterior pituitary?
increased release
posterior pituitary
SIADH leads to increased water reabsorption from the urine, diluting the blood and leading to h_____
hyponatraemia
Where is ADH made?
Hypothalamus
Where is ADH stored and secreted from?
Posterior pituitary gland
ADH stimulate water reabsorption from where in the kidneys?
Collecting ducts
Inserts more aquaporin II channels to reabsorb water
What causes SIADH?
Think SIADH
Small cell lung cancer
Infection (eg. TB, pneumonia, meningitis, HIV)
Abscesses (other tumours)
Drugs (SSRIs - selective serotonin reuptake inhibitors, antidepressant)
Head trauma / Post-operative (stress response increase ADH release)
What are 2 potential sources of excessive ADH?
Increased secretion by posterior pituitary
Ectopic ADH
What is the most common cause of ectopic ADH?
small cell lung cancer
True of false: SIADH causes so much extra water that there is fluid overload?
False
Enough to reduce sodium concentration (hyponatraemia), not much to change volume of blood by much.
SIADH results in e______ hyponatraemia
euvolemic
(normal volume of blood)
What happens to the urine in SIADH?
It becomes more concentrated
High urine osmolality and high urine sodium
How may a patient with SIADH present?
May be asymptomatic
Vomiting
Headache
Muscle cramps
Fatigue
Confusion / decreased GCS (Glasgow coma scale)
Brain stem herniation
What can severe hyponatraemia cause?
Seizures and reduced consciousness
Why can brainstem herniation be caused by SIADH?
Low sodium means increased water enters skull to compensate. This increases the intra-cranial pressure and can cause hyponatraemic encephalopathy with risk of the brainstem herniating through foramen magnum.
How do you diagnose SIADH?
Euvolaemia
Hyponatraemia
Low serum osmolality
High urine sodium
High urine osmolality
What other causes of hyponatraemia need to be ruled out?
adrenal insufficiency (less aldosterone)- short synacthen test
Diuretic meds
D&V, burns, fistula, excessive sweating
No excessive water intake
No CKD or AKI (more water to sodium)
No heart failure (fluid retention) or liver disease
Na+ depletion - give 0.9% saline and should normalise, won’t in SIADH.
What other condition also causes euvolaemic hyponatraemia?
Primary polydipsia
Excessive water consumption with no cause diluting the blood and urine.
How can you differentiate primary polydipsia and SIADH?
primary polydipsia has low urine sodium levels and urine osmolality
SIAD has high urine sodium and urine osmolality
How do you manage SIADH?
Treat the underlying cause (eg stop causative SSRI or treat infection)
Fluid restriction
Hypertonic saline (to concentrate the blood)
For chronic cases, use a vasopressin receptor antagonist eg tolvaptan
Why is it important to correct the sodium slowly?
To prevent osmotic demyelination (don’t really need to know yet)
Na+ concentration shouldn’t change more than 10mmol/l in 24 hours
