Reassessing Discordant Exudative Pleural Effusion in Heart Failure: A Rare Case of Occult Malignancy Uncovered by Post-Thoracentesis Tension Pneumothorax.

Pleural effusions can be manifestations of heart failure, pulmonary embolism, infection, and malignancy. Symptom assessment, imaging, and pleural fluid analysis serve as key determinants in guiding clinical management. We present the case of a 75-year-old woman with a history of heart failure who developed worsening dyspnea and lower extremity edema. Initial imaging revealed a large unilateral pleural effusion, initially presumed to be secondary to heart failure exacerbation. Pleural fluid analysis demonstrated a discordant exudative effusion by Light's criteria. Following thoracentesis, the development of a tension pneumothorax improved radiographic clarity, revealing previously obscured metastatic osseous lesions that had been masked by the pleural effusion and adjacent lung parenchyma on prior imaging. This prompted further advanced cytologic evaluation which identified dysplastic breast ductal epithelial cells consistent with a malignant pleural effusion. This case underscores the importance of maintaining a broad differential diagnosis, even when pleural fluid analysis is only marginally exudative. Moreover, the occurrence of tension pneumothorax facilitated the identification of occult metastatic lesions, highlighting the essential role of post-procedural imaging in detecting concealed malignancy. This case further emphasizes that even mildly positive exudative effusions necessitate comprehensive investigation for malignancy, as subtle presentations may still indicate significant underlying pathology.