Pleural Effusion - Mechanism and Causes
PKGhatak, MD
A small amount of pleural fluid is produced by the parietal pleura (a thin layer covering the inside of the chest wall, diaphragm and hilum), just enough to lubricate both layers of pleura so that during breathing the two surfaces slide smoothly one over the other without friction. The fluid does not accumulate because just as the fluid is produced continuously, the fluid is drained continuously by the lymphatic channels of the parietal pleura. Pleural effusion develops when excess fluid is produced or removal is delayed.
Clinical Types of Pleural Effusion:
It is useful to classify pleural effusion as Transudate and Exudate.
Transudate fluid is thin, has a low protein concentration, low LDH, and has none to two WBCs.
Transudate pleural effusion takes place because the fluid oozes out of lung alveoli due to high capillary pressure. The most common cause of it is congestive heart failure, Nephrotic syndrome, cirrhosis of the liver, hypoalbuminemia from malabsorption syndrome, protein losing enteropathy and acute left ventricular failure, Pulmonary edema, and massive pulmonary embolism.
Mechanism of Transudate. Like liver nodules, the alveoli of the lung have a dual blood supply. The pulmonary artery brings in venous blood to the alveolar capillary. This is the source of most blood in the lung. In overloaded heart chambers in congestive heart failure, the pressure rises in heart chambers sharply. The back pressure is felt in the capillaries of the alveoli. When the pressure exceeds the oncotic pressure of blood, the water and dissolved solutes leak out in the interalveolar and pleural spaces resulting in pleural transudative pleural effusion.
Mechanism of Exudate: Inflammation of pleura by bacteria, mycobacteria tuberculosis, and fungus is generally due to the extension of lung infection. The pleural fluid is characterized by the presence of over 50% serum albumin concentration and over 60% LDH of the serum. The sugar concentration of the fluid in pneumococcal pleural effusion is low and has a high WBC count.
Clinical subtypes of pleural effusion.
Empyema. In certain infections, the WBC numbers are so numerous that the pleural fluid becomes white cloudy- called empyema. Common causes of empyema are lung abscess rupturing in the pleural space, infected stab wounds, septicemia, post operative chest wounds. contaminated traumatic chest wall injuries.
Hemorrhagic Pleural Effusion. Traumatic chest wall injuries, contusion, laceration and infarction of the lung produce hemorrhage in the pleural space. Coagulation abnormalities either secondary to medical therapy, thrombocytopenia (platelet count 50,000 or less), deficiencies of coagulation factors or circulating anticoagulants may produce hemorrhagic pleural effusion.
Malignant Pleural Effusion. Malignant tumors of pleura are rather rare, only mesothelioma was an exception. In general, malignant pleural effusion is secondary to known lung malignancy, common in metastatic breast, ovary, colon cancers and primary lung cancer, lymphoma, and leukemias.
Chylous Pleural Effusion. The pleural fluid appears milky due to the presence of chylomicron. Chylomicrons contain long chain fatty acids, cholesterol esters, phospholipids. This condition results from obstruction or laceration of the thoracic duct. Cancer of the apical section of the lung. Pancoast tumors, lymphoma, and tuberculosis and filariasis cause obstruction of the thoracic duct. Fractures of 1st rib, sternum and upper thoracic vertebrae may cause laceration of the thoracic duct.
Bilateral Pleural Effusion. It is due to systemic diseases like CHF, nephrotic syndrome, cirrhosis, etc.
Unilateral Pleural Effusion. Unilateral effusion is due to one sided pleural disease but subsequently may be bilateral, if the disease spread to the other side. Example- pneumonia.
Localized Pleural effusion. The accumulated pleural fluid is walled off by fibrin and coagulated proteins.
Pleural Thickening. In tubercular pleural effusion, if the pleural fluid is not by evacuated by thoracentesis or drug therapy, layers of fibrous tissues are deposited over the surface of the lung like a straitjacket and prevent the expansion of the lung during breathing.
Cryptogenic or Unknown Causes of Pleural effusion. The effusion may start as unilateral or bilateral, and transudate effusion may turn exudate. In long term follow up and repeated pleural biopsies detect about 10% due to mesothelioma or carcinoma of the lung. The cause of the remaining cases remains unknown.
Chronic Pleural Effusion. This condition is usually associated with end organ failure.
Rare causes of Pleural Effusion.
Meig's syndrome. Pleural effusion and ascites are associated with a benign ovarian tumor.
Ovarian Overstimulation Syndrome. Injectable fertility drugs to generate eggs in ovaries may produce capillary leaks and ascites and pleural effusion may develop.
Post Radiation Therapy also for the same reason produces this complication.
Allergic drug reactions, Collagen vascular diseases and Autoimmune diseases can cause a mild form of vasculitis and pleural effusion.
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