Question 30

A 52-year-old patient is admitted to your ICU with a World Federation of Neurosurgeons (WFNS) Grade IV subarachnoid hemorrhage following a cerebral aneurysmal bleed. On day four of admission, you note the patient has become hyponatraemic (Na+ 126 mmol/L).

a) List six differential diagnoses of the acute hyponatraemia. Indicate the most likely diagnosis. (30% marks)

b) Outline your principles of management for this level of hyponatraemia, and your specific management based on the most likely diagnoses. (70% marks)

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College answer

Not available.

Discussion

The possibilities are:

SIADH
Cerebral salt wasting
Hypoadrenalism (due to pituitary injury)
Hypothyroidism (due to pituitary injury)
Hypovolemic hyponatremia (due to osmotic diuresis with mannitol)
Hypervolemic hyponatremia (due to huge fluid volumes)
Inappropriate IV fluid replacement (eg. 5% dextrose)

Of these, SIADH is probably the most common, and therefore the most likely. Kao et al (2009) looked at the records of about 300 SAH patients with hyponatremia, and found that 34.5% had SIADH, whereas only 23% had cerebral salt wasting. Marupudi & Mittal (2015) mention that this inappropriate ADH excess is usually the result of direct stimulation of the hypothalamus by the subarachnoid blood.

What would you do about this?

Well: you can't just leave it and monitor it as you might in a stable ward patient, as this is somebody with SAH, and for these people, hyponatremia is associated with a poor outcome. The problem is, whereas normally you would fluid-restrict these patients, in this scenario you wouldn't want to do that, because hypovolemia promotes vasospasm. Thus, you're forced to replace the sodium intravenously, while maintaining a high or normal fluid balance.

Treatment options
- Hypertonic saline infusion to increase sodium to the desired concentration (the endpoint is to get above 135 mmol/L)
- Slow increase of sodium concentration into the normal range over the course of at least 24 hours (that would be a rise of 9 mmol/L)
- Options to maintain normal sodium levels include:
  - fludrocortisone to increase sodium retention
  - vaptan drugs such as tolvaptan or conivaptan to block renal ADH receptors
  - Urea, to induce osmotic water elimination and sodium resorption
Monitoring
- Check serum sodium every 4-6 hours
- Monitor bedside fluid balance to ensure it remains neutral or increased
Support
- Maintain normovolemia with isotonic crystalloid rich in sodium: normal saline or Plasmalyte would be appropriate
- Avoid large volumes of hypotonic fluids

Interestingly, this fill-and-season strategy is also exactly what you would do for cerebral salt wasting anyway, as it is a syndrome characterised by volume loss as well as sodium loss, which means you really don't need to choose between these differentials in the setting of SAH.

References

Marupudi, Neena I., and Sandeep Mittal. "Diagnosis and management of hyponatremia in patients with aneurysmal subarachnoid hemorrhage." Journal of clinical medicine 4.4 (2015): 756-767.

Kao, Lily, et al. "Prevalence and clinical demographics of cerebral salt wasting in patients with aneurysmal subarachnoid hemorrhage." Pituitary 12.4 (2009): 347-351.

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