Pleural effusion, often called “water on the lungs,” is a condition where excessive fluid accumulates in the pleural space, the narrow area between the lungs and the inner chest wall. This space is lined by the pleura, two thin membranes that normally contain only a few milliliters of lubricating fluid. When fluid production exceeds reabsorption, the excess volume puts pressure on the lungs, hindering expansion. This typically causes symptoms like shortness of breath, chest pain, or a cough. Understanding the underlying cause is necessary for effective treatment.
Understanding the Two Main Types of Fluid Buildup
The causes of pleural effusion are broadly categorized based on the fluid’s composition and the physiological mechanism involved. This distinction between transudative and exudative effusions helps medical professionals narrow down the underlying disease.
Transudative effusions occur when fluid leaks into the pleural space due to systemic factors that alter the balance of pressure within the blood vessels. This fluid is typically thin, watery, and has a low concentration of protein because the pleura is not damaged.
Exudative effusions, in contrast, result from local diseases that directly injure the pleural membrane. This damage causes blood vessels in the pleura to become leaky, allowing protein, inflammatory cells, and larger molecules to pass into the pleural space. Consequently, exudative fluid is thicker, rich in protein, and often cloudy, signaling a localized inflammatory or infectious process.
Causes Related to Fluid Pressure Imbalance
Conditions leading to transudative effusions involve systemic problems that disrupt fluid dynamics, primarily through changes in hydrostatic or oncotic pressure. The most frequent cause overall is congestive heart failure (CHF). In CHF, the heart struggles to pump blood effectively, leading to a backup of pressure in the blood vessels of the lungs and chest wall, which forces fluid to filter out into the pleural space.
Liver disease, most commonly advanced cirrhosis, is another significant cause, often resulting in hepatic hydrothorax. Cirrhosis impairs the liver’s ability to produce albumin, a protein that helps keep fluid within the blood vessels by maintaining oncotic pressure. The combination of low albumin and high pressure in the liver’s circulatory system can cause fluid to accumulate in the abdomen, which may then move through small defects in the diaphragm into the chest cavity.
Kidney diseases, such as nephrotic syndrome, also cause transudative effusions by leading to severe protein loss in the urine. This loss results in low blood protein levels, which decreases the oncotic pressure necessary to draw fluid back into the blood vessels. As a result, fluid leaks out of the capillaries and accumulates in various body spaces, including the pleura.
Causes Related to Inflammation and Injury
Exudative effusions arise from localized processes that cause direct damage to the pleura, triggering an inflammatory response. Infections are a leading cause, especially bacterial pneumonia, which can result in a parapneumonic effusion. The infection causes inflammation in the adjacent lung tissue, increasing the permeability of the pleural capillaries and allowing inflammatory fluid and cells to leak out. If the fluid becomes overtly infected with bacteria, it is termed empyema, requiring specific treatment.
Malignancy is another major cause of exudative effusion, often referred to as malignant pleural effusion. Cancers (particularly lung, breast, and lymphoma) can cause effusion in several ways, including direct invasion of the pleura by tumor cells. Tumors can also block the lymphatic channels responsible for draining fluid from the pleural space, leading to unchecked accumulation.
A pulmonary embolism (PE), a blood clot in the lung, can also trigger an exudative effusion. The clot causes localized inflammation and sometimes tissue death, which increases the permeability of the surrounding vessels. Autoimmune conditions, such as systemic lupus erythematosus or rheumatoid arthritis, are additional causes where widespread inflammation affects the pleura. This systemic inflammation can irritate the pleural membranes, creating an exudative fluid rich in inflammatory cells.
Less Frequent Underlying Causes
While pressure and inflammation account for the majority of cases, other distinct conditions can cause fluid accumulation in the pleura. Trauma, such as a severe chest injury or post-surgical complications, can directly damage the blood or lymphatic vessels, leading to fluid or blood leakage into the pleural space. An esophageal rupture, a rare but serious event, allows stomach contents to leak into the chest, causing a severe inflammatory exudative effusion.
Infections below the diaphragm, such as a subphrenic abscess, can cause an effusion due to the inflammatory fluid irritating the pleura from below. Certain medications are also documented to cause pleural reactions, which are often immune-mediated and result in an exudative picture. Drugs like amiodarone, methotrexate, and phenytoin are known examples that can induce this side effect.