Battle at the Nephron: A Case of Metastatic Small Cell Lung Cancer Complicated by AVP-D and SIADH

Introduction: Syndrome of inappropriate antidiuretic hormone (SIADH) is characterized by hyponatremia and low urine output. Conversely, arginine vasopressin deficiency (AVP-D) is characterized by hypernatremia and polyuria. This case involves an active smoker with suspected COPD who had a low dose lung CT in 2021 revealing suspicious lung nodules.


Description: A 60 year old female with suspected COPD, HTN, and hypothyroidism presented with shortness of breath and was admitted for acute hypoxic respiratory failure. CT thorax showed a large hilar mass compressing the R mainstem bronchus and SVC, and a large R pleural effusion. She was admitted to the medical ICU due to significant oxygen requirements and underwent thoracentesis with 1.4 L fluid removed. She improved with high-dose steroids and empiric antibiotics for COPD exacerbation and possible pneumonia. Workup revealed SCLC with diffuse metastasis. Oncology was consulted and she started carboplatin and etoposide, and had palliative radiation to the brain and lung.
On hospital day 28, the patient developed polyuria with Uosm 157, Posm 295, PNa 146, and copeptin < 2.8. Brain MRI showed metastasis to the dorsum sella with pituitary encroachment. Nephrology was consulted for management of AVP-D and the patient was started on ddAVP. There was initial improvement in urine output and sodium levels, however she subsequently developed hyponatremia so ddAVP was stopped. Despite discontinuation, she remained hyponatremic with elevated urine osmolality, consistent with SIADH. Throughout her hospitalization, she continued to have alternating hypernatremia and hyponatremia and was ultimately discharged on 50 mcg ddAVP daily.


Discussion: AVP-D is rarely caused by pituitary metastasis, with most cases being asymptomatic. Presentations vary based on lesion size/location, magnitude of trauma, and degree of neurohypophysial destruction.
This case illustrates a rare but clinically significant scenario in which a patient with metastatic SCLC developed both AVP-D due to pituitary metastasis and SIADH, a frequent paraneoplastic complication of SCLC. It provides a valuable opportunity to review the pathophysiology of sodium and fluid balance as well as highlights the importance of involving specialists to aid in the diagnosis and management strategies for complex cases.