Hyponatraemia and syndrome of inappropriate ADH secretion

Question 1

What is hyponatraemia commonly defined as

Answer 1

a serum sodium concentration of less than 135mmol/L

Question 2

What are the majority of causes of hyponatraemia associated with

Answer 2

low plasma osmolality and increased antidiuretic hormone levels

Question 3

What are most cases of hyponatraemia caused by

Answer 3

an increase in extracellular water relative to extracellular sodium
Due to an impairment of renal water excretion capacity and water retention caused by increased plasma ADH levels

Question 4

What mediates the increased secretion of ADH in patients with volume depletion (hypovolaemia)

Answer 4

Carotid sinus baroreceptors which sense the reduced pressure

Question 5

What are the most common cause of hyponatraemia in adults

Answer 5

Thiazide diuretics

Question 6

In hypothyroidism, what might cause increased plasma ADH levels

Answer 6

reduced cardiac output and activation of the carotid sinus baroreceptors

Question 7

In adrenal insufficiency, what causes an increase in plasma ADH levels

Answer 7

reduced systemic blood pressure and cardiac output (due to a lack of cortisol)
hypovolaemia (due to aldosterone deficiency)
Removal of the inhibitory effect of cortisol on corticotrophin-releasing hormone and ADH

Question 8

What might cause SIADH (Syndrome of inappropriate ADH secretion)

Answer 8

CNS pathology
pulmonary pathology
malignancy (ADH secreted by the tumour)
drugs

Question 9

What detects an elevated level of fluid

Answer 9

The renal juxtaglomerular cells

Question 10

What do the juxtaglomerular cells do if they detect an elevated fluid level

Answer 10

Cause a reduction in renin and aldosterone levels causing an increased sodium excretion and thus preventing fluid overload

Question 11

What patients may have an excessive water intake (more than 10L per day)

Answer 11

psychiatric patients with polydipsia
following ecstasy
marathon runners

Question 12

When might the aqueous fraction of the plasma volume be reduced

Answer 12

Patients with hyperlipidaemia (uncontrolled diabetes) or hyperproteinaemia (multiple myeloma)

Question 13

If the aqueous fraction of the plasma volume is reduced, what happens to the sodium concentration

Answer 13

it is reduced

Question 14

How can you calculate osmolality

Answer 14

2x (Na+ + K+) + urea + glucose)

Question 15

What is sometimes seen in patients with hyperglycaemia

Why ?

Answer 15

Hyponatraemia with high plasma osmolality
The rise in plasma glucose pulls water out of the cells and results in a reduction in plasma sodium concentration by dilution

Question 16

What is sometimes seen in renal failure

Answer 16

water retention which leads to hyponatraemia with normal plasma osmolality as the decrease in osmolality due to low sodium is offset by the increased urea

Question 17

How do most patients present with a serum sodium concentration of more than 125mmol/L

Answer 17

asymptomatic

Question 18

What are some symptoms if the decrease in serum sodium is large or happened suddenly

Answer 18

Headache 
anorexia, nausea, vomiting 
lethargy 
muscle cramps 
depressed reflexes
confusion, disorientation 
seizures
coma, death

Question 19

What does hypotonic hyponatraemia cause

Answer 19

entry of water into the brain, resulting in cerebral oedema and intracranial hypertension

Question 20

What must the correction of Na concentration not exceed

Answer 20

10nmol/L in the first 24 hours

28mmol/L in the first 48 hours

Question 21

What can rapid correction of hyponatraemia cause

Answer 21

Shrinkage of the braine –> demyelination of the pontine and extrapontine neurones

Question 22

What is the osmotic demyelination also know as

Answer 22

cerebral pontine myelinolysis

Question 23

In what patients is the risk of osmotic demyelination even higher in

Answer 23

alcohoics
malnourished
liver failure
potassium depletion

Question 24

In hyponatraemia, what should also be measured and why

Answer 24

lipids and protein to rule out pseudohyponatraemia
Blood glucose
TSH, T4 and cortisol - hypothyroidism and adrenal insufficiency

Question 25

What patients will have a high spot urinary sodium

Answer 25

Hypovolaemic patients with a renal cause of fluid and sodium loss (thiazide diuretics)

Question 26

What patients will have a low spot urinary sodium

Answer 26

Those with an external cause of fluid and sodium loss (diarrhoea or vomiting

Question 27

What are the key test for investigating SIADH

Answer 27

paired plasma osmolality and urine osmolality

sodium concentration

Question 28

What would be the results if a patient was positive for SIADH

Answer 28

low plasma osmolality

high urine osmolality

Question 29

How should patients be investigated further if they have SIADH

Answer 29

brain imaging - CT contrast
MRI
CT chest

Question 30

What is the treatment of hyponatraemia

Answer 30

correction of the underlying cause - stopping the causative drug or administration of hydrocortisone and mineralocorticoids to patients with adrenal insufficiency and thyroxin to hypothyroid patients

Question 31

How do we treat a patient with hypovolaemia

Answer 31

rehydration with isotonic saline (0.9%)

This causes ADH release and allows excretion of the excess water

Question 32

How do we treat hypervolaemic patient

Answer 32

Fluid restriction and drug review

Question 33

What is the treatment for mild-moderate hyponatraemia in euvolaemic patients with SIADH

Answer 33

Fluid restriction or increased dietary intake of salt

If not tolerated, then demeclocycline

Question 34

How does demeclocycline work

Answer 34

it reduces the responsiveness of the collecting tubule cells to ADH and therefore increases water excretion

Question 35

How do we treat patients with severe hyponatraemia in euvolaemic patients with SIADH

Answer 35

Hypertonic saline with extreme caution

Loop diuretic - inhibits sodium chloride reabsorption in the thick ascending limb of the loop of Henle

Question 36

What can giving potassium do

Answer 36

Raise the plasma sodium concentration

Question 37

How do vaspressin recepto antagonists work

Answer 37

they cause a selective water diuresis without affecting sodium and potassium excretion

Question 38

Who are most likely to benefit from vasopressin receptor antagonists

Answer 38

Moderate chronic hyponatraemia if water restriction is insufficient