Review Question - QID 106822

This patient's presentation is most suggestive of lung cancer resulting in a paraneoplastic syndrome of inappropriate antidiuretic hormone secretion (SIADH). A mass can be seen on the CT scan, and the patient has a significant history of smoking with weight loss and persistent cough suggesting small cell lung cancer. SIADH is characterized by serum Na < 135 mEq/L (hyponatremia), plasma osmolality < 275 mOsm/kg, urine osmolality > 100 mOsm/kg, and urine Na > 40 mEq/L.

The criteria for laboratory diagnosis of SIADH include the following: low plasma sodium, low plasma osmolality, inappropriately concentrated urine, increased urinary sodium, and normal body fluid status (euvolemia). 10-15% of small cell lung cancers present with SIADH; however, other causes of SIADH beyond paraneoplastic include CNS disturbances such as infection, stroke, hydrocephalus, or trauma as well as a medication side-effect from NSAIDs, antidepressants, chemotherapy agents, clofibrate, carbamazepine, and narcotics. Other paraneoplastic syndromes seen with small cell lung cancer are increased ACTH secretion and Eaton-Lambert syndrome.

Incorrect Answers:
Answer 1: Psychogenic polydipsia can present with hyponatremia, however the significant smoking history, lung mass and lack of history for polydipsia point away from this diagnosis.
Answer 2: If this patient were having glucose in their urine this could suggest diabetic ketoacidosis or hyperosmolar hyperglycemic coma however the history does not suggest this.
Answer 3: This answer is consistent with nephrogenic diabetes insipidus, but this is not the mechanism found in small cell lung cancer.
Answer 5: Decreased ADH can be seen in central diabetes insipidus in which you would see very dilute urine and an increased serum sodium rather than what is presented.