Case Report: Hyponatremia of malignancy – An alternative mechanism? Syndrome of inappropriate atrial natriuretic peptide (SIANP)

Euvolemic hyponatremia in the setting of lung cancer is most commonly due to the syndrome of inappropriate anti-diuretic hormone secretion (SIADH). However, a subset of patients with small cell carcinoma and hyponatremia also have elevated levels of atrial natriuretic peptide (ANP), which is produced by some small cell tumors. We report the case of a 64-year-old man with a limited-stage small cell carcinoma of the lung undergoing chemoradiation therapy, who was admitted to hospital with a pulmonary embolism. Two months earlier, at the time of diagnosis with lung cancer, he had a hypotonic, euvolemic hyponatremia, presumed to be caused by SIADH. At that time, his serum sodium readily normalized with fluid restriction and ADH-antagonist therapy with demeclocycline. However, during his second admission, the hyponatremia was refractory to treatment: his sodium level slowly declined from 138 mmol/L to a nadir of 118 mmol/L, despite early initiation of fluid restriction and maximal doses of demeclocycline. Laboratory values revealed a low but inappropriately non-zero level of ADH but also an inappropriately low level of aldosterone and an elevated level of ANP, suggesting that SIADH might not be the only contributor to the hyponatremia. While a causal link between ectopic ANP production and hyponatremia has never been established, an inappropriately high level of ANP can directly decrease sodium re-absorption in the proximal convoluted tubule of the kidney and increase glomerular filtration rate (GFR), resulting in greater excretion of sodium and water. In addition, high circulating levels of ANP can inhibit aldosterone secretion, potentially resulting in further sodium wasting. Here, the elevated ANP and inappropriately low aldosterone suggested the possibility that the refractory hyponatremia, although initiated by SIADH, was further worsened by an ANP-mediated natriuresis and suppression of aldosterone response.