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Hyponatremia plagues many neurosurgical patients due to the syndrome of inappropriate secretion of ADH (SIADH) or the cerebral salt wasting syndrome (CSW). Both diseases may appear similar (hyponatremia, increased urine osmolarity, increased urine sodium, normal adrenal, renal and thyroid function), but there is one BIG difference. Patients with SIADH are euvolemic or hypervolemic (excess ADH causes fluid retention) whereas patients with CSW are fluid depleted (impaired renal handling of sodium and water). To differentiate, look for signs of hypovolemia: orthostatics, dry mucus membranes, hemoconcentration, pre-renal azotemia, and/or hemodynamics (IVC collapse anyone?).
Bottom line: Distinguish SIADH from CSW because the treatments are exact opposites:
SIADH: Fluid restrict
CSW: Give water and salt (i.e., 0.9% saline)
References
Cerebral salt wasting syndrome: a review. Harrigan MR
Neurosurgery. 1996 Jan;38(1):152-60.