Resistant syndrome of inappropriate antidiuretic hormone (SIADH) secondary to atonic bladder in an older woman

Syndrome of inappropriate antidiuretic hormone (SIADH) is the most common cause of hyponatraemia. There are many causes of SIADH, but investigation tends to focus around the most common causes-particularly diseases of the brain and lung, malignancy and medication-induced SIADH [Ellison and Berl (2007, The Syndrome of Inappropriate Antidiuresis. N Engl J Med., 356, 2064-72]. We describe a case of SIADH secondary to atonic bladder in an 83-year old woman, which was discovered on MRI of the abdomen, performed for further characterisation of a known pancreatic lesion. Insertion of a urinary catheter alleviated retention and resulted in prompt resolution of hyponatraemia. This is an under-recognised cause of this common condition, with important implications for investigation and management.