Syndrome of Inappropriate ADH

Question 1

def

Answer 1

secretion of ADH despite absence of normal stimuli for secretion (i.e. increased serum osmolality or decreased blood volume)

Question 2

what are the stimuli for ADH secretion

Answer 2

increased serum osmolality

decreased blood volume

Question 3

aetiology

Answer 3

brain
-haemorrhage/thrombosis
-meningitis
-trauma
-abscess
-gullain-barre syndrome
lung
-pneumonia
-TB
-abscess
tumours
-small cell lung cancer
-brain tumour
-mesothelioma
-by ectopic ADH secretion
drugs
-vincristine
-opiates
-carbamazepine
metabolic
-porphyria
-alcohol withdrawal

Question 4

what is porphyria

Answer 4

nerve or skin problems

Question 5

epi

Answer 5

almost half of all severe hyponatraemia is due to SIADH

Question 6

history

Answer 6

mild hyponatraemia may be asymptomatic (125-135)
severe hyponatraemia (<125)
-headache
-nausea/vomiting
-muscle cramp/weakness
-drowsiness &amp; confusion
-convulsions
-coma

Question 7

examination

Answer 7

mild hyponatraemia - none
severe hyponatraemia
-reduced reflexes
-positive babinski
signs of underlying cause

Question 8

why is hyponatraemia present in SIADH

Answer 8

due to dilation from increased body water

not due to decreases in na

Question 9

investigations

Answer 9

low Na, creatinine, glucose, serum protein & lipids
free T4 & TSH to exclude hypothyroidism
short ACTH stimulation test to exclude adrenal insufficiency

SIADH diagnosis
-low plasma osmolality
-low plasma Na
-high urine osmolality (>100mosml/kg)
-high urine Na (>20mmol/L)
PLUS
-absence of hypovolaemia/hypotension, oedema, renal failure

imaging for cause

Question 10

why are serum protein & lipids measured in SIADH

Answer 10

to exclude pseudohyponatraemia

which has high serum protein & lipids

Question 11

management

Answer 11

treat underlying cause
1 water restriction
2 if water restriction ineffective give democlocyline
3 vasopressin (V2) receptor antagonists (tolvaptan) in chronic moderate hyponatraemia if fluid restriction is insufficient
4 in severe cases (seizures and reduced consciousness) give slow IV hypertonic 3% saline (and furosemide)
change in Na conc cannot exceed 10mmol/l in 24h or 18mmol/l in 48h

Question 12

what is democlocyline

Answer 12

reduces responsiveness of collecting tubules to ADH

Question 13

what could rapid correction of hyponatraemia cause

Answer 13

central pontine myelinolysis

• respiratory arrest
• fits

Question 14

complications

Answer 14

convulsions
coma
death

Question 15

prognosis

Answer 15

dependent on cause
high morbidity and mortality if Na <110mmol/L
50% mortality with central pontine myelinolysis