Hyponatraemia and syndrome of inappropriate ADH secretion Flashcards
What is hyponatraemia commonly defined as
a serum sodium concentration of less than 135mmol/L
What are the majority of causes of hyponatraemia associated with
low plasma osmolality and increased antidiuretic hormone levels
What are most cases of hyponatraemia caused by
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
What mediates the increased secretion of ADH in patients with volume depletion (hypovolaemia)
Carotid sinus baroreceptors which sense the reduced pressure
What are the most common cause of hyponatraemia in adults
Thiazide diuretics
In hypothyroidism, what might cause increased plasma ADH levels
reduced cardiac output and activation of the carotid sinus baroreceptors
In adrenal insufficiency, what causes an increase in plasma ADH levels
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
What might cause SIADH (Syndrome of inappropriate ADH secretion)
CNS pathology
pulmonary pathology
malignancy (ADH secreted by the tumour)
drugs
What detects an elevated level of fluid
The renal juxtaglomerular cells
What do the juxtaglomerular cells do if they detect an elevated fluid level
Cause a reduction in renin and aldosterone levels causing an increased sodium excretion and thus preventing fluid overload
What patients may have an excessive water intake (more than 10L per day)
psychiatric patients with polydipsia
following ecstasy
marathon runners
When might the aqueous fraction of the plasma volume be reduced
Patients with hyperlipidaemia (uncontrolled diabetes) or hyperproteinaemia (multiple myeloma)
If the aqueous fraction of the plasma volume is reduced, what happens to the sodium concentration
it is reduced
How can you calculate osmolality
2x (Na+ + K+) + urea + glucose)
What is sometimes seen in patients with hyperglycaemia
Why ?
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
What is sometimes seen in renal failure
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
How do most patients present with a serum sodium concentration of more than 125mmol/L
asymptomatic
What are some symptoms if the decrease in serum sodium is large or happened suddenly
Headache anorexia, nausea, vomiting lethargy muscle cramps depressed reflexes confusion, disorientation seizures coma, death
What does hypotonic hyponatraemia cause
entry of water into the brain, resulting in cerebral oedema and intracranial hypertension
What must the correction of Na concentration not exceed
10nmol/L in the first 24 hours
28mmol/L in the first 48 hours
What can rapid correction of hyponatraemia cause
Shrinkage of the braine –> demyelination of the pontine and extrapontine neurones
What is the osmotic demyelination also know as
cerebral pontine myelinolysis
In what patients is the risk of osmotic demyelination even higher in
alcohoics
malnourished
liver failure
potassium depletion
In hyponatraemia, what should also be measured and why
lipids and protein to rule out pseudohyponatraemia
Blood glucose
TSH, T4 and cortisol - hypothyroidism and adrenal insufficiency
What patients will have a high spot urinary sodium
Hypovolaemic patients with a renal cause of fluid and sodium loss (thiazide diuretics)
What patients will have a low spot urinary sodium
Those with an external cause of fluid and sodium loss (diarrhoea or vomiting
What are the key test for investigating SIADH
paired plasma osmolality and urine osmolality
sodium concentration
What would be the results if a patient was positive for SIADH
low plasma osmolality
high urine osmolality
How should patients be investigated further if they have SIADH
brain imaging - CT contrast
MRI
CT chest
What is the treatment of hyponatraemia
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
How do we treat a patient with hypovolaemia
rehydration with isotonic saline (0.9%)
This causes ADH release and allows excretion of the excess water
How do we treat hypervolaemic patient
Fluid restriction and drug review
What is the treatment for mild-moderate hyponatraemia in euvolaemic patients with SIADH
Fluid restriction or increased dietary intake of salt
If not tolerated, then demeclocycline
How does demeclocycline work
it reduces the responsiveness of the collecting tubule cells to ADH and therefore increases water excretion
How do we treat patients with severe hyponatraemia in euvolaemic patients with SIADH
Hypertonic saline with extreme caution
Loop diuretic - inhibits sodium chloride reabsorption in the thick ascending limb of the loop of Henle
What can giving potassium do
Raise the plasma sodium concentration
How do vaspressin recepto antagonists work
they cause a selective water diuresis without affecting sodium and potassium excretion
Who are most likely to benefit from vasopressin receptor antagonists
Moderate chronic hyponatraemia if water restriction is insufficient
