Hyponatremia

Question 1

How do you rule out psudo-hyponatraemia and what are some causes?

Answer 1

• Measure serum osmolality (mOsm/Kg) and then compare it to calculated osmolarity (mOsm/L). If there are similar then it is a true hyponatraemia. If very different then it is a psudo-hyponatraemia. Causes include hyperlipidaemia or taking bloods from an arm with a drop.

Question 2

How do you calculate osmolarity?

Answer 2

2xNa + urea = Glucose

Question 3

What is extracellular hypertonicity?

Answer 3

It is hyponatremia due to the osmotic effects of hyperglycaemia. So often the hyponatraemia does not require treatment

Question 4

How do you measure the corrected sodium in extracellular hypertonicity?

Answer 4

Glucose/4 + measured sodium.

Question 5

How do you assess the volume status of a patient?

Answer 5

• pulse
• Postural BP
• Urine output/fluid status,
• JVP,
• Ascites,
• Oedema

Question 6

What are some causes of hypovolaemic hyponatraemia?

Answer 6

• Burns,
• Sweating,
• Diarrhoea,
• Vomiting,
• Fistula,
• Addison’s disease

Question 7

What are some causes of Euvolaemic hyponatraemia?

Answer 7

• SIADH
• Hypothyroidism,

Question 8

What are some causes of hypervolaemic hyponatraemia?

Answer 8

• Renal failure,
• Heart failure,
• Liver failure,
• Nephrotic sundrome

Question 9

Once the fluid status of a hyponatraemic patient is established, what is the next step?

Answer 9

Measure urine sodium (losing sodium or retaining sodium) and urine osmolality (how concentrated the urine is)

Question 10

What is the cause of a hypervolaemic hyponatraemic with a urine sodium over 20 and under 20?

Answer 10

If urine Na is > 20 then acute/chronic renal failure.
If urine Na is <20 then hepatic failure or nephrotic syndrome

Question 11

What are the causes of a euvolaemic hyponatremic patient with urine Na >20?

Answer 11

If osmolality is >100 then SIADH is indicated.
If osmolality is <100 then it suggests H20 intoxication.
- Could also be due to hypothyroidism

Question 12

What is the cause of a hypovolaemic hyponatraemic with a urine sodium over 20 and under 20?

Answer 12

If urine Na > 20 then renal losses, diuretic excess, mineralocorticoid def (Addison’s) or salt loosing nephropathy.
If urine Na < 20 then extra renal losses eg, vomiting, burns or diarrhoea

Question 13

What is the difference between osmolarity and osmolality?

Answer 13

Osmolarity is the number of solute particles per liter (changes with temp so osmolality is preferred)
Osmolality is the number of solute particles per Kg.

Question 14

Describe features of SIADH

Answer 14

• Common cause of low Na but euvolaemia.
• High osmolality and high urine sodium/
• Diagnosis of exclusion of renal disease, thyroid disease, adrenal disease and not on diuretics

Question 15

What are some causes of SIADH?

Answer 15

• Pulmonary infections and lesions,
• Carcinoma
• CNS disorders,
• AIDS,
• Post op pain/ stress,
• Vomiting
• Drugs eg, amitriptyline and other tricyclic antidepressants or fluoxeine (can include lots others two but these are the most commons)

Question 16

What is the treatment for hyponatraemia?

Answer 16

If hypovolaemic - Isotonic saline.
If hypervolaemic - Salt and fluid restriction +/- loop diuretics,
If Euvolaemic - Water restriction. Try 1L per 24h
Aim to increase Na gradually by 1mmol/L/hour to 120mmol/L then treat conservatively

Question 17

What is the pharmacological management of hyponatraemia?

Answer 17

• Demeclocycline (antagonist of vasopressin)
• Aquaretics (not recommended)