Patients predominantly present with breathlessness, but cough and pleuritic chest pain can be a feature. The aetiology of the pleural effusion determines other signs and symptoms.
Postero-anterior chest x-ray will show an effusion of >200 mL of fluid. An ultrasound, chest computed tomography scan, or lateral decubitus study indicates whether the fluid is free-flowing or loculated, and whether or not septations are present.
Aspiration and evaluation of the pleural fluid with biochemistry, cytology, and culture determines the nature of the effusion.
Treatment is based on the nature of the effusion and underlying condition.
Therapeutic thoracentesis with nearly complete removal of the fluid collection alleviates dyspnoea and cough in most circumstances.
A pleural effusion results when fluid collects between the parietal and visceral pleural surfaces of the thorax. A thin layer of fluid is always present in this space for lubrication and ease of movement of the lung during inspiration and expiration. If the normal flow of fluid is disrupted, with either too much fluid produced or not enough removed, then fluid accumulates, resulting in a pleural effusion. 
Consultant Respiratory Physician
Chelsea and Westminster Hospital NHS Foundation Trust
OO declares that he has no competing interests.
Dr Orhan Orhan would like to gratefully acknowledge Drs Clare Ross, Philip W. Ind, Anob M. Chakrabarti, Richard Light, Amber Degryse, Rachel J. Davies, and Peter M. George, previous contributors to this topic.
Consultant Chest Physician and Senior Lecturer
Oxford Centre for Respiratory Medicine and University of Oxford Churchill Hospital
YCGL declares that he has no competing interests.
Pleural Research Registrar
North Bristol Lung Centre
Westbury on Trym
CH declares that she has no competing interests.
Professor of Medicine and Director
Division of Pulmonary/Critical Care/Allergy/Sleep Medicine
Medical University of South Carolina
SS declares that he has no competing interests.
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