Diagnostic Criteria and Classification of SIADH

Essential Diagnostic Criteria

The American College of Physicians recommends that SIADH is diagnosed when five cardinal criteria are met: hypotonic hyponatremia (serum sodium <134 mEq/L), plasma osmolality <275 mosm/kg, inappropriately concentrated urine (osmolality >500 mosm/kg), urinary sodium >20 mEq/L, and clinical euvolemia in the absence of hypothyroidism, adrenal insufficiency, or volume depletion 1
The diagnosis requires simultaneous demonstration of hyponatremia with hypo-osmolality: Serum sodium <134-135 mEq/L with plasma osmolality <275 mosm/kg 1
Inappropriately concentrated urine: Urine osmolality >100 mosm/kg (typically >500 mosm/kg) despite low plasma osmolality 1
Elevated urinary sodium: Urine sodium concentration >20-40 mEq/L, indicating continued natriuresis 1
Clinical euvolemia: Absence of edema, orthostatic hypotension, dry mucous membranes, or signs of volume overload 1
Normal organ function: Normal renal, adrenal, and thyroid function must be confirmed 1
Volume status assessment is paramount—SIADH patients are euvolemic, not hypovolemic or hypervolemic, with no orthostatic hypotension or tachycardia, and no peripheral edema, ascites, or jugular venous distention 1, 2

Small cell lung cancer is the most common malignant cause of SIADH, occurring in 1-5% of cases, according to the American Society of Clinical Oncology 3, 1
CNS infections, space-occupying lesions, subarachnoid hemorrhage, and head trauma can disrupt hypothalamic-pituitary function and cause SIADH, as stated by the American Academy of Neurology 4, 1
High-risk medications for SIADH include SSRIs, carbamazepine, and oxcarbazepine, as warned by the American Geriatrics Society 1

The core mechanism of SIADH involves persistent or elevated plasma AVP despite hyponatremia and low plasma osmolality, leading to water retention and physiologic natriuresis, as described by the American Physiological Society 1
Nonosmotic stimuli, such as pain, nausea, and stress, can override normal osmotic regulation and cause AVP excess, impairing free-water excretion 1

Confusing SIADH with cerebral salt wasting (CSW) in neurosurgical patients can lead to incorrect treatment, as SIADH requires fluid restriction and CSW requires volume and sodium replacement, according to the American Association of Neurological Surgeons 1, 2, 4
Physical examination alone has poor accuracy for volume assessment, and CVP should be used when available in neurosurgical patients, as recommended by the Society of Critical Care Medicine 1, 2

Symptoms of hyponatremia correlate with both the absolute sodium level and the rate of decline, particularly when falling faster than 0.5 mmol/L/hour, as stated by the American Heart Association 1
Acute hyponatremia (<48 hours) produces more severe symptoms at higher sodium levels than chronic hyponatremia due to inadequate cerebral adaptation, according to the European Society of Intensive Care Medicine 1

Initial screening should include serum sodium, plasma osmolality, urine osmolality, and urine sodium simultaneously, as recommended by the American Society for Clinical Pathology 1, 4
Exclude pseudohyponatremia by checking serum glucose, and assess organ function with TSH, cortisol, and serum creatinine, as stated by the Endocrine Society 1, 2