Biochemistry - hyponatraemia

Question 1

What are the two causes of hyponatraemia?

Answer 1

Loss of sodium or water retention

Question 2

What are the causes of hyponatraemia due to water retention?

Answer 2

• Oedematous:
  
  • Decreased water excretion e.g. CHF, nephrotic syndrome
  • Increased water intake e.g. inappropriate IV dextrose
• Non-oedematous:
  
  • Decreased water excretion e.g. SIADH, renal faiure
  • Increased water intake e.g. compulsive water drinking

Question 3

What are the causes of hyponatraemia due to loss of sodium?

Answer 3

• Increased sodium loss e.g. from gut (vomiting), kidneys (mineralocorticoid deficiency or spironolactone) or skin
• Decreased sodium intake - extremely rare

Question 4

What are the diagnostic criteria for SIADH?

Answer 4

• Hyponatraemia with corresponding hyo-osmolarity (<280mmol/kg)
• Continued renal excretion of Na
• Urine less than maximally dilute
• No clinical evidence of volume depletion (patient typically euvolaemic)

Question 5

What is the pathogenesis of SIADH?

Answer 5

• Non-osmotic stimuli of ADH secretion override its osmotic regulation.
• High ADH drives the serum osmolarity down and the urine osmolarity up; the patient is usually euvolaemic

Question 6

What are some symptoms of hyponatraemia?

Answer 6

• Nausea, malaise, headache, lethargy, weakness, dizziness, reduced level of consciousness
• Seizures, coma and focal neurological signs are seen when there is severe hyponatraemia (<115mmol/L)

Question 7

What are the clinical signs of sodium/volume depletion?

Answer 7

Tachycardia, postural hypotension, dry mucuous membranes, soft/sunken eyeballs, decreased consciousness, decreased skin turgor, decreased urine output

Question 8

Why do oedematous patients become hyponatraemic?

Answer 8

• Reduced circulating blood volume (due to HF for example) causes increased fluid in the interstitial compartment.
• This causes secondary hyperaldosteronism, resulting in sodium and water retention, thus expanding the ECF volume.
• The effective hypovolaemia also stimulates ADH secretion, resulting in additional pure water retention –> hyponatraemia

Question 9

How are patients with hyponatraemia due to sodium depletion treated?

Answer 9

Sodium replacement

Question 10

How are patients with hyponatraemia who are normovolaemic treated?

Answer 10

Fluid restriction

Question 11

How are oedematous patients with hypovolaemia treated?

Answer 11

Diuretics