A 59-year-old female with no past medical history is admitted to the neurointensive care unit following a large right middle cerebral artery ischemic stroke. She was not a candidate for intravenous tPA nor intra-arterial therapy. She has had progressive somnolence and anisocoria with a right larger than left pupil that was not responsive to direct or consensual light testing. She was started on hyperosmolar therapy followed by a decompressive hemicraniectomy. She is now poststroke day 5 and continues to have malignant cerebral edema. Prior to her next dose of mannitol her lab values are as follows:
What is her osmolar gap?
A. 0Correct Answer: A
The osmolar gap estimates the unknown osmotic agent in the blood, which in this patient’s case is mannitol. To calculate the osmolar gap we need to calculate the calculated osms.
Calculated osm = (2 x Na) + (BUN / 2.8) + (Glu / 10)
Osm Gap = Measured Osm - Calculated Osm
Mannitol is a hypertonic solution of sugar that is used to treat cerebral edema. The medication works as an osmotic diuretic which causes large volume urinary output due to high concentrated urine within the distal collecting duct and allows for extraction of extracellular fluid into the bloodstream due to the osmotic gradient between the intravascular and extravascular, extracellular compartment. There are a number of complications that can occur with mannitol therapy including volume depletion, electrolytes imbalance such as hyponatremia, and metabolic acidosis. Acute kidney injury can also occur secondary to dehydration and mannitol accumulation. Therefore, mannitol should only be given within specific parameters including osmolar gap <12 (the gap value varies between institution). In our patient’s case her osmolar gap is 0 and therefore, she should receive mannitol 0.25 to 1 g/kg every 6 to 8 hours.
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