Hyponatraemia and syndrome of inappropriate ADH secretion Flashcards Preview

Lecture Notes - Endocrinology > Hyponatraemia and syndrome of inappropriate ADH secretion > Flashcards

Flashcards in Hyponatraemia and syndrome of inappropriate ADH secretion Deck (38)
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1
Q

What is hyponatraemia commonly defined as

A

a serum sodium concentration of less than 135mmol/L

2
Q

What are the majority of causes of hyponatraemia associated with

A

low plasma osmolality and increased antidiuretic hormone levels

3
Q

What are most cases of hyponatraemia caused by

A

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

4
Q

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

A

Carotid sinus baroreceptors which sense the reduced pressure

5
Q

What are the most common cause of hyponatraemia in adults

A

Thiazide diuretics

6
Q

In hypothyroidism, what might cause increased plasma ADH levels

A

reduced cardiac output and activation of the carotid sinus baroreceptors

7
Q

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

A

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

8
Q

What might cause SIADH (Syndrome of inappropriate ADH secretion)

A

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

9
Q

What detects an elevated level of fluid

A

The renal juxtaglomerular cells

10
Q

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

A

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

11
Q

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

A

psychiatric patients with polydipsia
following ecstasy
marathon runners

12
Q

When might the aqueous fraction of the plasma volume be reduced

A

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

13
Q

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

A

it is reduced

14
Q

How can you calculate osmolality

A

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

15
Q

What is sometimes seen in patients with hyperglycaemia

Why ?

A

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

16
Q

What is sometimes seen in renal failure

A

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

17
Q

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

A

asymptomatic

18
Q

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

A
Headache 
anorexia, nausea, vomiting 
lethargy 
muscle cramps 
depressed reflexes
confusion, disorientation 
seizures
coma, death
19
Q

What does hypotonic hyponatraemia cause

A

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

20
Q

What must the correction of Na concentration not exceed

A

10nmol/L in the first 24 hours

28mmol/L in the first 48 hours

21
Q

What can rapid correction of hyponatraemia cause

A

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

22
Q

What is the osmotic demyelination also know as

A

cerebral pontine myelinolysis

23
Q

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

A

alcohoics
malnourished
liver failure
potassium depletion

24
Q

In hyponatraemia, what should also be measured and why

A

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

25
Q

What patients will have a high spot urinary sodium

A

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

26
Q

What patients will have a low spot urinary sodium

A

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

27
Q

What are the key test for investigating SIADH

A

paired plasma osmolality and urine osmolality

sodium concentration

28
Q

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

A

low plasma osmolality

high urine osmolality

29
Q

How should patients be investigated further if they have SIADH

A

brain imaging - CT contrast
MRI
CT chest

30
Q

What is the treatment of hyponatraemia

A

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

31
Q

How do we treat a patient with hypovolaemia

A

rehydration with isotonic saline (0.9%)

This causes ADH release and allows excretion of the excess water

32
Q

How do we treat hypervolaemic patient

A

Fluid restriction and drug review

33
Q

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

A

Fluid restriction or increased dietary intake of salt

If not tolerated, then demeclocycline

34
Q

How does demeclocycline work

A

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

35
Q

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

A

Hypertonic saline with extreme caution

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

36
Q

What can giving potassium do

A

Raise the plasma sodium concentration

37
Q

How do vaspressin recepto antagonists work

A

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

38
Q

Who are most likely to benefit from vasopressin receptor antagonists

A

Moderate chronic hyponatraemia if water restriction is insufficient