A previously fit 36-year-old patient has been admitted to your Intensive Care Unit with an isolated severe head injury. 18 hrs after admission he develops polyuria. Outline the way in which you would evaluate this polyuria.
The causes of polyuria in this patient include
a) Diabetes insipidus
b) Mannitol or other diuretics
c) Use of hypertonic saline
d) Cerebral salt wasting syndrome
e) Effects of ingested alcohol prior to the trauma
Evaluation and treatment
a) DI : Serum and urine osmolality and Na measurements,
b) If mannitol: check serum osmolar gap, and ensure it is < 320 mOsm/kg to prevent renal toxicity
c) Hypertonic saline: Large urine outputs are a feature of hypertonic saline therapy and this can be confirmed by high serum and high urine Na.
d) CSW: Low intravasc volume, low serum Na, high urine Na.
e) Residual alcohol: based on index of suspicion, check serum osmolar gap and ethanol levels.
To paraphrase the already well-written college answer, the causes of polyuria following a head injury include the following:
- Diabetes insipidus
- Cerebral salt wasting
- Appropriate post-resuscitation diuresis
- Appropriate natriuresis following hypertonic saline infusion
- Mannitol-induced diuresis
- Hypothermic diuresis due to therapeutic cooling
A more detailed tabulated answer would look like this:
Risk factors for diabetes insipidus in traumatic brain injury:
An approach to the investigations and management of polyuria
Once the polyuria is discovered:
After 3 hours of obervation:
If the serum sodium continues to rise, with low urinary sodium and urine osmolality under 300mOsm/kg,
consider 0.5μg of DDAVP.
One should hope that the ADH-inhibiting effects of alcohol would have worn off after 18 hours, or else this gentleman had a minimum blood alcohol level of around 0.36% at the time of his injury (which is not entirely unreasonable).
The college did not ask for treatment options in their question, and so I have offered none.