Answer is B (Urinary sodium excretion is low / normal): SIADH is typically characterized by high urinary sodium (Increased rate of excretion of. odium). Syndrome of Inappropriate ADH Secretion (SIADH) is associated with increased secretion of vasopressin (ADH), which leads to increased absorption of water producing dilutional hyponatremia (serum sodium typically < 135 meq/l) along with concentrated or hyperosmolar urine. Excessive retention of water stimulates compensatory mechanisms that enhance Watriuresis'. Natriuresis results in increased urinary sodium excretion rate increased urinary sodium concentration) and is believed to compensate for increased volume from inappropriaye ADH secretion preventing a state of clinical hypervolemia, hypeension or edema. SIADH is associated low serum sodium levels ( Cardinal features of SIADH include: Hyponatremia (dilutional hyponatremia with Na+ < 135 mmo1/1) Decreased plasma osmolality (increased urine osmolality > 150 m osm). High Urine sodium (over 20 meq4) Low Blood urea Nitrogen < 10 mg/L Hypouricemia ( Clinical Euvolemia - Absence of signs of hypervolemia (edema, ascitis) - Absence of signs of hypovolemia (ohostatic hypotension, tachycardia, features of dehydration) Absence of cardiac, liver or renal disease Normal thyroid and adrenal function A high BUN suggests a volume contracted state and excludes a diagnosis of SIADH. Water loading test may be used to help diagnosis of SIADH Water loading test is recognized as a 'supplemental criteria' in establishing a diagnosis of SIADH. The 'Water Loading Test' is of value when there is unceainity regarding the etiology of modest degrees hypo-osmolality in euvolemic patients but it does not add useful information if the plasma osmolality is < 275 mOsm/kg H2O Test Objective / Principle Abnormal Water Loading Test Criteria Fixed quantity of water is given to a patient and the amount of Abnormal water load urine produced and changes in blood/urine osmolality are Inability to excrete at least 90% of a 20m1/kg recorded. water load in 4 hours, and/or Inability to excrete a normal water load (decreased urinary Failure to dilute urine (osm) to < 100 mOsm/kg output) or failure to dilute urine is considered abnormal H-,O. Vaptans are new FDA approved agents for treatment of SIADH `Vaptans' are a new class of drugs that have emerged for treatment of hyponatremia. These medications act as Vasopressin receptors antagonists blocking the action of AVP in renal tubule, pituitary or smooth muscles depending upon receptor selectivity. Conivaptan , itravenous use) Conivaptan is a combined V1/V2 receptor antagonist is FDA approved for sho-term intravenous use fin- treatment of hospitalized patients with SIADH. Tolivaptan (Oval use) Tolivaptan is a V2 receptor antagonist that has received FDA approval far oral use 100 mOsmol/kg H20 with normal renal function) at some level of hypoosmolality. (Urinary concentration must be inappropriate for Plasma Hyposmolality) * Clinical euvolemia, as defined by the absence of signs of hypovolemia (ohostasis, tachycardia, decreased skin rurgor, dry mucous membranes) or hypervolemia (subcutaneous edema, ascites). * Elevated urinary sodium excretion while on normal salt and water intake. * Absence of other potential causes of euvolemic hypoosmolality: hypothyroidism, hypocoisolism (Addison's disease or pituitary adrenocoicotropic hormone insufficiency) and diuretic use. Supplemental * Abnormal water load test (inability to excrete at least 90% of a 20 mL/kg water load in 4 hours and/or failure to dilute Uosm to < 100 mOsmol/kg H-0). * Plasma AVP level inappropriately elevated relative to plasma osmolality. * No significant correction of serum insufficiency) and diuretic use. Supplemental Abnormal water load test (inability to excrete at least 90% of a 20 mL/kg water load in 4 hours and/or failure to dilute Uosm to < 100 mOsmol/kg H-0). Plasma AVP level inappropriately elevated relative to plasma osmolality. No significant correction of serum [Nal with volume expansion but improvement after fluid restriction Assessment of Extracellular Fluid Volume (ECFV) Clinical Findings Type 1, Hypervolemic Type II, Hypervolemic Type III, Hypervolemic SIADH Euvolemic History CHF, cirrhosis, or nephrosis Yes No No No Salt & water loss No Yes No No ACTH- coisol deficiency and/or nausea and vomatiting No No Yes No Physical examination Generalized edema, ascites Yes No No No Postrlial hypotension May be May be May be" No Laboratory BUN, creatinine High-normal High-normal Low-normal Low-normal Uric acid High-normal High-normal Low-normal Low-normal Serum potassium Low-normal Low-normal Normal Normal Serum albumin Low-normal High-normal Normal Normal Serum coisol Normal-high Normal-high Low Normal Plasma renin activity High High Low Low Urinary sodium (Meg unit of time) Low Low High High