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.
Higdon et al. discusses the management of oncologic emergencies, including paraneoplastic SIADH. SIADH should be suspected if a cancer patient presents with euvolemic hyponatremia. Management should include strict fluid restriction and furosemide.
Bordi et al. discusses recent advances in the treatment of SIADH. Vasopressin V2-receptor antagonists, such as tolvaptan, are a valuable treatment option for managing paraneoplastic SIADH. Although the long-term safety and efficacy of these drugs needs continued investigation, initial results are promising.
Illustration A summarizes the laboratory work-up of hyponatremia. Illustration B shows endocrinopathies manifesting from paraneoplastic syndromes.
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.
Higdon ML, Higdon JA. Treatment of oncologic emergencies. Am Fam Physician. 2006 Dec 1;74(11):1873-80.
PMID:17168344 (Link to Abstract)
Bordi P, Tiseo M, Buti S, Regolisti G, Ardizzoni A. Efficacy and safety of long-term tolvaptan treatment in a patient with SCLC and SIADH. Tumori. 2015 Feb 20;0(0):0.
PMID:25702667 (Link to Abstract)