SIADH Clinical Features and Management

Symptoms and Diagnosis

The American Academy of Neurosurgery recommends considering symptoms such as headache, nausea, vomiting, confusion, lethargy, seizures, and coma in patients with SIADH, with severity related to both the absolute serum sodium concentration and its rate of fall 1
Cerebral Salt Wasting (CSW) is characterized by hypovolemia rather than euvolemia, and should be considered in patients with hyponatremia and unquenchable thirst 2, 3
Polydipsia, marked by excessive fluid intake, can cause dilutional hyponatremia and should be considered in patients with unquenchable thirst 2, 4

Extracellular fluid (ECF) status is key to distinguishing between SIADH and other causes of hyponatremia, with SIADH patients typically being euvolemic 2, 3
Central venous pressure (CVP) measurements can help differentiate between SIADH (CVP 6-10 cm H₂O) and CSW (CVP <6 cm H₂O) 2
Laboratory findings that suggest SIADH include serum uric acid <4 mg/dL, urine sodium >20 mEq/L, and urine osmolality inappropriately high (>500 mosm/kg) relative to plasma osmolality 2, 6

Treatment of SIADH typically involves fluid restriction (<800 mL/day), which would be counterproductive and potentially harmful in a patient with true polydipsia 2, 6
If unquenchable thirst is present with hyponatremia, consider measuring serum and urine osmolality and electrolytes to differentiate causes, assessing volume status carefully, and evaluating for other causes of hyponatremia with thirst 2, 4, 6