SIADH. CAUSING OR CAUSED BY PSYCHOSIS? A. Tendler 1 , J. Wiese 1 ; 1 Tulane University, New Orleans, LA (Tracking ID #77076) LEARNING OBJECTIVES: 1. Recognize organic psychosis as a correctable cause of SIADH 2. Distinguish SIADH from from psychogenic polydypsia. 3. Know other causes of SIADH in the psychiatric patient. CASE INFORMATION: A 56 year-old woman with a history of schizophrenia was admitted to the inpatient psychiatry service with paranoid delusions of her brother trying to kill herself and all religious ministers. Her physical examination was normal, including normal neurologic and pulmonary examinations. Her serum sodium was 124 mmol/L and her measured osmolality was 261 mosm/kg. The urine osmolality was 522 mosm/kg. She was treated with risperidone and free water was restricted. She was allowed a liberal salt-containing diet. Her psychosis improved over the next week, with normalization of her serum sodium. IMPLICATIONS/DISCUSSION: Hyponatremia in the schizophrenic patient should not be reflexively attributed to psychogenic polydyspia. Because ADH is liberated from the hypothalamus through the posterior pituitary, anti-psychotic medications acting at surrounding structures may cause SIADH, as may primary psychosis. The urine osmolarity is the best test to distinguish SIADH from psychogenic water drinking, as the concentrating effects of ADH should increase the urine osmolarity. Hyponatremia may cause neurologic impairment, but this impairment is usually in the form of seizures as opposed to psychosis. Treatment of psychosis-induced SIADH is with neuroleptics designed to control the psychosis. Medical consultation for hyponatremia in the psychiatric ward is a common occurrence . Physicians should be aware of this underdiagnosed cause of hyponatremia, and of its successful resolution with anti-psychotic therapy. received no relief, however, and he re-presented with persistent back pain and fevers. Further questioning revealed he had a past history of mitral regurgitation and had had a tooth extraction six months prior. He had a 3/6 systolic murmur at the apex and point tenderness over the lumbar spine. His blood cultures grew Streptococcus viridans. An MRI of the lumbar spine showed edematous changes at T12-L1 with loss of disc space, consistent with discitis. An echocardiogram confirmed endocarditis. IMPLICATIONS/DISCUSSION: Although the diagnosis of urinary tract infection and lower back strain adequately explained his symptoms, each of the two diagnoses was insufficient in explaining all of the symptoms. William of Occam suggested that,``one should not increase, beyond what is necessary, the number of entities required to explain anything.'' Known as Occam's Razor, this principle guides physicians to search for one diagnosis to explain all symptoms, instead of invoking multiple diagnoses. The fever, bacteremia, discitis and immunemediated glomerulonephritis in a patient with valvular abnormality and poor dentition can all be attributed to infective endocarditis.