First Things First

• SIADH is a disorder of excess and inappropriate ADH, leading to retention of excess water in the body, usually creating the cardinal combination of hyponatremia with volume overload. All disorders of sodium MUST be evaluated in terms of both water and sodium.
• SIADH is usually not life-threatening unless significant metabolic encephalopathy and/or seizures develop. If these neurologic complications are present, more aggressive correction of serum sodium to 125 mEq/L may be appropriate.
• Na deficit (mEq) = 0.6 × ideal body weight × (125 - current plasma sodium concentration)
• Another diagnosis that closely mimics SIADH is the so-called “cerebral salt wasting” syndrome (CSW), which commonly found in neurosurgical disorders, including subarachnoid hemorrhage.
• CSW is inappropriate urinary excretion of sodium, usually accompanied with volume loss, and thus should be considered in patients with hyponatremia and evidence of volume depletion.