Paraneoplastic syndromes represent a constellation of clinical findings and effects from a primary malignancy, though they are not related to the malignancy’s direct invasion or metastases. These syndromes are mediated by hormones secreted by the malignant cells and/or the immune response the hormones trigger.1

The clinical manifestations of paraneoplastic syndromes may represent the very early or late stages of disease and have no direct correlation on the extent or prognosis of disease.2 Up to 10% of patients with lung cancer will develop paraneoplastic syndromes, many of which are dependent on the subtype of lung cancer.3

One of the most common paraneoplastic syndromes associated with lung cancer is the syndrome of inappropriate antidiuretic hormone (SIADH) secretion. Up to 16% of patients with small cell lung cancer (SCLC) will develop SIADH.4

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An overwhelming majority (70%) of paraneoplastic SIADH cases are diagnosed in patients with SCLC. SIADH is a euvolemic hypoosmolar hyponatremic state in which the patient can present with nonspecific symptoms such as nausea, vomiting, decreased oral intake, and fatigue. Patients can develop hyponatremia at varying degrees below a “normal” value of 135 mEq/L: mild (130 to 134), moderate (125 to 129) and severe (less than 125). Not only is it important to assess the “absolute” sodium level, but also the time period over which the sodium decreased.

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There is a vast differential diagnosis for patients with hyponatremia, especially those with malignancy. A careful history and physical examination must therefore be performed to rule out other potential causes, as the treatment algorithms will vary based on type of hyponatremia.