Syndrome of Inappropriate Antidiuretic Hormone Secretion secondary to Small Cell Lung Cancer
Abstract
SIADH is a paraneoplastic syndrome of impaired water excretion caused by the inability to suppress the secretion of ectopic antidiuretic hormone by small cell lung cancer tumor. SIADH is a hypo-osmolar euvolemic hyponatremia.
Patient is a 76-year-old female who presented to the emergency department on 10/5 due to abnormal lab values on the basic metabolic panel ordered by her primary care physician. Basic metabolic panel showed hyponatremia at 119 mmol/L (137-145 mmol/L) that was measured on 10/4. Serum osmolality completed on the same day showed 253 mOsmol/kg (range 280-301). Reports dizziness, visual changes, SOB, and non-productive cough. Denies nausea, vomiting, diarrhea, headache, or chest pain. Denies use of diuretics or polydipsia. Current every day smoker with a 58-year pack history. Lung cancer screening on 7/7/22 showed new tubular opacity of the lateral segment of the right middle lobe. Patient was previously seen in the ED and admitted on 9/30 for possible pneumonia and discharged with Tessalon Perles. Chest X-Ray on 9/30 showed fullness in the right hilum but an enlarged lymph node or mass is difficult to exclude. Chest CT on 9/30 revealed complete right middle lobe collapse and occlusion of the right middle lobe bronchus are suspicious for malignancy given 2 cm enlarged subcarinal lymph node. BMP on 9/30 showed sodium level of 122 mmol/L. Complete metabolic panel on 10/5 was 121 mmol/L. Diagnosis is hypo-osmolality and hyponatremia secondary to SIADH; nonspecific abnormal finding of lung field.
Patient was admitted for correction of hyponatremia and referred for further investigation of unspecified lung mass via outpatient. While admitted, the patient was treated with Tolvaptan 15 mg PO on 10/5. A complete metabolic panel was ordered on 10/6 that showed her sodium increased to 132 mmol/L. Her discharge instructions stated she was to continue Tolvaptan 15 mg PO at bedtime, fluid restriction of 1.5 L per day, and to repeat BMP on 10/10. A bronchoscopy with endobronchial US-directed biopsy was completed outpatient, results showed small cell lung cancer. Patient’s follow-up basic metabolic panel or current status is unknown. Patient’s small cell lung cancer was staged at IIIA based on the Tumor-Node-Metastases classification. Tumor >2 cm but ≤3 cm in greatest dimension and does not invade main bronchus, metastasis in ipsilateral mediastinal and/or subcarinal lymph nodes, and no distant metastasis. Patient’s small cell lung cancer treatment is unknown. Per UpToDate, recommended treatment for stage IIIA small cell lung cancer is chemotherapy drug regimen consisting of Etoposide plus Cisplatin and radiation therapy.
SIADH is a common paraneoplastic syndrome in small cell lung cancer patients. A patient presenting with hyponatremia with an extensive history of tobacco use should be thoroughly evaluated for SIADH secondary to SCLC. SCLC has a poor prognosis with the survival time from the time of diagnosis for limited and extensive disease are 15-20 months and 8-13 months. United States Preventive Services Taskforce recommends annual low-dose CT screening for adults aged 50-80 years who have a 20 pack-year smoking history and currently smoking or have quit within the past 15 years. The Nelson trials have recently reported a 24% reduction in mortality when comparing patients who received low-dose CT screening to patient’s who did not receive the screening. The patient had her annual lung cancer screening performed, but never followed up due to personal reasons. For the treatment of her hyponatremia, she was treated with Tolvaptan and fluid restriction. In one study, doses from 3.75 mg to 15 mg of Tolvaptan were used to correct hyponatremia in patients with SCLC and results proved less than 15 mg of Tolvaptan is enough to stabilize sodium levels. Although there is a risk of overcorrection with the use of 15 mg of Tolvaptan, there has been no consensus reached on initial dosing of Tolvaptan and decision must be made with consideration for all patients factors.