SIADH and Lung Small Cell Carcinoma: A Clinical Case.

Hyponatremia is the most common electrolyte disorder found in clinical practice. One of the causes is the syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH is characterized by hypotonic and euvolemic hyponatremia with elevated urinary osmolarity and increased ADH secretion without a triggering stimulus. This pathology is often associated with neoplastic conditions, namely lung neoplasms. We report the case of a 54-year-old woman admitted with a history of vertigo and uncontrollable nausea with hyponatremia of 100meq/L at admission and diagnosed with paraneoplastic SIADH secondary to small cell lung carcinoma. After establishing the diagnosis, the patient was transferred to the Portuguese Oncology Institute of Lisbon (IPO), where she began chemotherapy. This case highlights the challenge of controlling sodium levels before initiating oncologic treatment and illustrates how chronic hyponatremia may allow for some degree of neurological adaptation to markedly low sodium concentrations.