SIADH Flashcards
SIADH stands for
syndrome of inappropriate anti-diuretic hormone secretion
definition
hyponatraemia and hypo-osmolality caused by the inappropriate secretion of anti-diuretic hormone (despite normal or increased plasma volume)
anti-diuretic hormone acts on the
collecting ducts which resorb water back into the blood
the cells of the collecting ducts contains
proteins called aqaupaorins which allows water to pass through
the more ADH being secreted
the more aquaporins become available to facilitate water resorption
if ADH is continuously released even when plasma osmolality is normal
it causes increased water resorption which dilutes the sodium present in the blood
the extra fluid also
increases the blood volume which inhibits the RAAS system resulting in no production of aldosterone
less aldosterone means
sodium in the blood gets excreted in the urine, water follow sodium which normalises the plasma volume but causes reduced sodium plasma osmolality causes hyponatraemia
causes of SIADH
- brain damage
- ectopic ADH production
- infection
- drugs
brain damage causing SIADH
strokes, meningitis, subarachnoid haemorrhage
drugs causing SIADH
carbamepazine (anti-epileptic), SSRI’s (citalopram), chemotherapeutic agents (vincristine, vinblastine, cyclophosphamide)
Ectopic ADH production most common caused by
small cell lung cancer
infection causing SIADH
pneumonia, lung abscess, brain abscess
initial symptoms of SIADH
- nausea
- vomiting
- muscle cramps
- headaches
- tremors
later symptoms of SIADH as sodium plasma concentrations continue to fall
neurons in the brain swell causing cerebral oedema, hallucination, confusion, seizures, coma and death
biochemistry which should shout out at you this is SIDADH
hyponatraemia in a person who has symptoms
other biochemistry
- reduced serum osmolality
- elevated urine osmolality
- high sodium urine concentration
after you diagnose SIADH what should you do
find the cause
how do you find the cause
- thorough medication history for drugs that may cause SIADH
- chest CT is small cell lung cancer or lung infection suspected to be the cause
- head CT if brain damage/ brain abscess suspected to be the cause
ectopic ADH production from small cell lung cancer is a type of what
paraneoplastic syndrome
management if acute presentation of SIADH
- IV hypertonic saline (3% NaCl)
what should you be very careful about when replacing someones sodium
you must check there sodium levels every 2 hours, sodium levels must not be increased by more than 1-2mEq/l/hour as if you replace sodium to quickly you can cause central pontine myelinolysis
what else is required other than sodium replacement
treat the underlying cause
central pontine myelinolysis
severe devastating damage to the myelin sheath of the pons causing:
- acute paralysis
- dysphagia
- dysarthria (difficulty speaking)
