What are common etiologies of hyponatremia after TBI, and why is it problematic?

• A. SIADH and cerebral salt wasting; hyponatremia worsens cerebral edema and increases risk of seizures.
• B. Hypernatremia due to dehydration.
• C. Hypokalemia.
• D. Hyperglycemia.

Answer: (A)

Hyponatremia after brain injury is most often due to two brain-driven disorders that disrupt sodium and water balance: SIADH (syndrome of inappropriate antidiuretic hormone secretion) and cerebral salt wasting. In SIADH, ADH is released inappropriately, causing water retention and a dilutional drop in serum sodium with a normal or slightly high blood volume. In cerebral salt wasting, the injured brain triggers excessive renal loss of sodium (natriuresis) with accompanying water loss, leading to hypovolemia and low sodium levels. Both conditions lower serum osmolality and create an osmotic gradient that draws water into brain tissue, worsening cerebral edema, increasing intracranial pressure, and raising the risk of seizures and herniation. Because hyponatremia can worsen brain injury outcomes, recognizing and treating the underlying mechanism is essential: fluid restriction and addressing ADH excess in SIADH, versus volume and sodium replacement (often isotonic saline, and sometimes hypertonic saline for severe symptoms) in cerebral salt wasting, with careful monitoring to avoid overly rapid correction.

Hypernatremia due to dehydration would raise sodium, not cause hyponatremia, so it isn’t an etiologic contributor to post-TBI hyponatremia. Hypokalemia is a separate electrolyte issue and not a primary cause of low sodium. Hyperglycemia can cause a dilutional appearance of hyponatremia, but it is not the typical etiologic mechanism behind hyponatremia after TBI.