I. What is pleural effusion We often talk about pleural effusion, which is actually pleural fluid. In normal people, there is 3-15ml of fluid in the pleural cavity, which plays a lubricating role during respiratory movement, and its filtration and absorption are in a dynamic balance. If the dynamic balance is disrupted by systemic or local lesions, resulting in too rapid formation or too slow absorption of fluid in the pleural cavity, pleural effusion (referred to as pleural fluid or pleural water) will be produced clinically. Second, what are the causes of pleural effusion? There are various causes of pleural effusion, which can be caused by local diseases in the chest or a clinical manifestation of systemic diseases. Pleural effusion is a manifestation of many diseases in the pleura. The qualitative diagnosis of pleural effusion is very important, and its correct diagnosis directly affects whether the patient can receive timely treatment. What are the examination methods of pleural effusion? The majority of patients can be diagnosed by clinical symptoms, signs, chest CT, various laboratory tests (including biochemical, routine, cytological, bacteriological, CEA, flow cytological tests, etc.) and cytological examination of pleural effusion and pleural biopsy. However, after these methods, about 20%-30% of patients still cannot get a definite diagnosis. What is “unexplained pleural effusion” These 20-30% of pleural effusions cannot be diagnosed with certainty, which is called “unexplained pleural effusion” or intractable pleural effusion. The common causes of unexplained pleural effusion are mainly tuberculosis and pleural metastases. Pleural metastases are most common in lung adenocarcinoma, followed by metastases from pleural askew tumors, thyroid and breast tumors, and lymphoma infiltration. V. How to diagnose and treat “unexplained pleural effusion” Clinically, for patients with pleural effusion that cannot be diagnosed by routine pleural fluid, routine pleural fluid biochemistry, cytological examination of pleural effusion and percutaneous closed pleural biopsy, early thoracoscopy should be performed to clarify the cause, reduce misdiagnosis and leakage, and provide timely treatment. What are the advantages of thoracoscopy in the diagnosis of “unexplained pleural effusion”? Thoracoscopy can examine the mural and visceral pleura, diaphragm and mediastinal surface comprehensively from top to bottom, from near to far, and if abnormalities are found, determine the location, size, number, invasion range and hardness of the abnormal tissues. In addition, multi-site biopsy can be performed to significantly increase the positive rate of pleural disease diagnosis. In addition, thoracoscopic surgery can also be used to spray talcum powder for thoracic mold fixation to completely eliminate pleural effusion. Seven, what are the risks of thoracoscopy The possible complications of thoracoscopic surgery are pneumothorax, air embolism, subcutaneous emphysema, postoperative fever, and damage to important organs during the operation. (1) For patients with pleural effusion who cannot be diagnosed by routine pleural fluid, routine pleural fluid biochemistry, cytological examination of pleural effusion and percutaneous closed pleural biopsy, early thoracoscopy should be performed to clarify the etiology; (2) When a malignant lesion is highly suspected under thoracoscopy but the pathology (2) When thoracoscopy is highly suspicious of malignant lesions but the pathology does not support it, another biopsy should be performed.