Severe Paraneoplastic Hyponatremia and Hypoosmolality in a Patient with Small-Cell Lung Carcinoma: Syndrome of Inappropriate Antidiuretic Hormone Secretion Versus Atrial Natriuretic Peptide or Both?

It is well documented in literature that a majority of small-cell lung cancers are associated with paraneoplastic phenomena. We report the case of a 63-year-old man diagnosed with small-cell lung carcinoma, in whom a severe hyponatremia and renal sodium loss with inappropriate antidiuresis were also found during a routine laboratory testing. Syndrome of inappropriate antidiuretic hormone secretion was first suspected in this patient, but another complex pathogenetic mechanism involving atrial natriuretic peptides could be associated, potentiating the deflation of the plasma sodium level. In our patient, the plasma–atrial natriuretic peptide base level, determined with a sensitive radioimmunoassay, was above the normal range (183 pg/mL; normal range, 50 pg/mL, ± 10 pg/mL), and the antidiuretic hormone plasma level had an oscillatory pattern, varying between 5.5 pg/mL and 7 pg/mL (normal range, 0–4.7 pg/mL). We discuss the pathogenesis and clinical aspects of this association and the therapeutic options for these types of patients.