Obstructive Jaundice as the Initial Presentation of Lung Squamous Cell Carcinoma Complicated by High-Output External Biliary Drainage and Severe Hyponatremia

Malignant biliary obstruction most commonly arises from primary pancreatic or biliary malignancies. Pancreatic metastases from lung cancer are rare and may mimic primary pancreatic cancer on imaging. External biliary drainage is often performed when endoscopic retrograde cholangiopancreatography fails; however, prolonged external diversion of bile can result in significant electrolyte disturbances due to the sodium-rich composition of bile. We report a 66-year-old man presenting with painless jaundice and significant weight loss who was found to have a pancreatic head mass causing biliary obstruction and a synchronous left hilar lung mass. Following unsuccessful endoscopic retrograde cholangiopancreatography due to distal common bile duct compression, percutaneous transhepatic biliary drainage was performed. Within 24 hours, biliary output exceeded 12 L, resulting in severe hyponatremia, acute kidney injury, and hypotensive shock requiring vasopressor support. Management included aggressive fluid resuscitation, electrolyte replacement, octreotide therapy, and eventual internal biliary stenting. Histopathology from pancreatic tissue and a subcutaneous pelvic metastasis demonstrated squamous cell carcinoma consistent with pulmonary origin. Following conversion to internal biliary drainage, electrolyte abnormalities resolved and renal function returned to baseline. This case highlights pancreatic metastasis as a rare manifestation of lung squamous cell carcinoma and underscores the potentially life-threatening metabolic consequences of high-output external biliary drainage.