SIADH (Syndrome of Inappropriate ADH)

SIADH occurs when antidiuretic hormone (ADH, or vasopressin) remains active when the body should be conserving less water. The renal collecting ducts reabsorb excess water, so plasma sodium becomes diluted even though the patient has not primarily lost sodium. Total body sodium may be normal; the problem is too much water relative to that sodium. The patient is usually clinically euvolaemic. There may be a small increase in total body water, but not the marked peripheral oedema of hypervolaemia or the obvious orthostasis and dry mucous membranes of significant volume depletion. The kidneys may excrete some sodium as the retained water expands the circulation, helping the patient appear “normally hydrated” at the bedside. SIADH can accompany pulmonary or central nervous system disease, malignancy, postoperative pain or nausea, and some medications. The cause matters because correcting the trigger may be as important as restricting water. Review the history for new drugs, recent surgery, lung or neurological illness, and symptoms suggesting an underlying disease. The danger is not the sodium value alone. Low plasma tonicity draws water into brain cells. A gradual...