Pembrolizumab-Induced Capillary Leak Syndrome: A Rare but Life-Threatening Immune-Related Adverse Event

Capillary leak syndrome (CLS) is an exceptionally rare and potentially life-threatening adverse event characterized by the abrupt shift of intravascular fluid into the extravascular compartment, leading to profound hypotension, edema, and potential multi-organ dysfunction. This condition has been associated with immune checkpoint inhibitors (ICIs) such as pembrolizumab. We present the case of a 67-year-old woman with a history of squamous cell lung carcinoma who underwent four cycles of pembrolizumab and presented multiple times with progressive shortness of breath, anasarca, and dyspnea on exertion. Clinical evaluation ruled out common causes of respiratory compromise and fluid overload, prompting consideration of immune-mediated pathology. Diagnostic studies and clinical evolution supported a diagnosis of CLS, underscoring the importance of early recognition and intervention. This case highlights CLS as a severe but treatable toxicity of pembrolizumab, emphasizing that timely detection and appropriate supportive care are essential for favorable clinical outcomes.