Question 10.1

The following results were obtained from a 62-year-old female one week following a subarachnoid haemorrhage with increasing confusion:

Parameter

Patient Value

Adult Normal Range

Sodium

130 mmol/L*

135 – 145

Potassium

4.0 mmol/L

3.5 – 5.0

Chloride

96 mmol/L

95 – 105

Bicarbonate

26.5 mmol/L*

22.0 – 26.0

Glucose

5.5 mmol/L

3.5 – 6.0

Urea

2.5 mmol/L*

3.0 – 8.0

Creatinine

37 μmol/L*

45 – 90

Magnesium

0.87 mmol/L

0.75 – 0.95

Albumin

33 g/L*

35 – 50

Protein

74 g/L

60 – 80

Total bilirubin

10 μmol/L

< 26

Alanine transferase          

26 U/L

< 35

Serum Osmolality

274 mosm/kg*        

285 – 295

Ionised calcium

1.19 mmol/L

1.10 – 1.35

Calcium corrected

2.34 mmol/L

2.12 – 2.62

Phosphate

0.97 mmol/L

0.80 – 1.50

a) What are the two most likely causes for this biochemical profile? (10% marks)

b) How would you distinguish between the two biochemically and clinically? (30% marks)

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College answer

Not available.

Discussion

What we are seeing here is a patient with recent CNS pathology who has now developed

  • Hypoosmolar hyponatremia
  • Mild metabolic alkalosis
  • A low urea (suggestive of low protein metabolism, high urine output or poor protein intake)
  • New confusion

The two most likely causes of this biochemical profile would surely have to be endocrine, and related to the SAH. Of these the two natural possibilities are:

  • SIADH
  • Cerebral salt wasting

Hypoadrenalism (due to pituitary dysfunction) is an alternative, and one could also make the argument that any pituitary injury from SAH that is bad enough to cause hypoadrenalism would probably also cause hypothyroidism. However SIADH and CSW are by far more likely. The way to discriminate between these lies in being able to demonstrate that the body fluid volume is decreased.

  • Both will have natriuresis
  • Only CSW will have polyuria (in SIADH the urine output will be normal or decreased)
  • Only CSW will have hypovolemia (and its biochemical consequences, eg. a raised urea)

References