A metabolic study of patients with lung cancer and hyponatremia of malignancy

One-third of patients with lung cancer and hyponatremia have no evidence of ectopic arginine vasopressin (AVP) production and the cause of their hyponatremia is not conclusively established. We sought to distinguish patients with hyponatremia caused by elevated AVP versus those with ectopic atrial natriuretic peptide (ANP) via this detailed metabolic study.

We enrolled 24 patients recently diagnosed with lung cancer in a metabolic study in which patients were placed on sodium and fluid restriction for 4 days. Serum electrolytes, osmolality, urine electrolytes and osmolality, plasma AVP, ANP, aldosterone, urinary cyclic AMP and cyclic guanosine 3',5'-monophosphate were measured daily and tumor tissue was obtained to measure ectopic hormone production. We attempted to characterize the pathophysiology of hyponatremia caused by ectopic ANP production in patients with small cell lung cancer (SCLC) and to determine its effect on the aldosterone axis.

Seven of the nine patients with SCLC presented with hyponatremia and three had elevated ANP levels at presentation without elevation of AVP. All three patients who presented with hyponatremia and elevated ANP showed a decline in serum sodium following fluid restriction, whereas two patients with SCLC and elevated AVP had normalized serum sodium levels. The combination of hyponatremia and elevated ANP was associated with a persistent natriuresis and inappropriately low aldosterone levels despite sodium restriction, suggesting ANP suppression of the aldosterone axis.

Management of patients with hyponatremia and SCLC should be guided by the knowledge that some patients with SCLC have ectopic production of ANP as the cause of their hyponatremia.