Syndrome of inappropriate antidiuretic hormone secretion

Syndrome of inappropriate antidiuretic hormone secretion is a disorder in which antidiuretic hormone activity causes water retention despite low plasma osmolality, typically producing hypotonic hyponatremia.

Antidiuretic hormone: A peptide hormone that increases kidney water reabsorption and helps regulate plasma osmolality. Its sustained activity drives the water retention that defines this syndrome.
Euvolemic hyponatremia: Hyponatremia without clinically apparent edema or volume depletion. This is the typical clinical volume pattern in the syndrome.
Small-cell lung cancer: An aggressive lung cancer that can produce hormones ectopically. Some tumors secrete antidiuretic hormone and trigger the syndrome.
Cerebral edema: Excess fluid accumulation in brain tissue that can raise pressure and disrupt function. Acute hypotonicity can move water into brain cells and cause swelling.
Vasopressin V2 receptor: A kidney-cell receptor through which vasopressin promotes water reabsorption. V2 signaling inserts water channels into collecting-duct cells despite the body's low osmolality.
Cerebral salt wasting: A renal sodium-loss syndrome associated with reduced circulating volume, often after brain injury. Both can follow neurologic disease, but salt wasting causes volume depletion rather than water retention.
Pulmonary disease: Disease affecting the lungs or respiratory system. Pneumonia and other pulmonary disorders can stimulate inappropriate antidiuretic activity.
Osmotic demyelination syndrome: A neurologic disorder caused by injury to myelin after rapid correction of chronic hyponatremia. Overly rapid sodium correction can cause this serious treatment complication.
Aquaporin 2: A water-channel protein regulated by vasopressin in kidney collecting-duct cells. Its increased presence at the cell surface allows retained water to enter the circulation.
Adrenal insufficiency: Inadequate production of adrenal hormones, including cortisol and sometimes aldosterone. It can mimic this syndrome by impairing water excretion and must be excluded.

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