Pleural effusion: Clinical

Our lungs are covered by two layers of pleura, the inner visceral pleura, and the outer parietal pleura.

Sandwiched between these layers is the pleural space, which normally contains about 10 milliliters of fluid and that provides a bit of lubrication, so that the lungs can smoothly expand within the chest cavity without encountering much friction.

Too much fluid in that space results in a pleural effusion, which can actually hinder lung expansion.

Now, pleural effusions can be broadly classified into transudative and exudative effusions.

Transudative effusions are often caused by systemic diseases, and result from either an increase in the intravascular hydrostatic pressure, such as in congestive heart failure, or as a result of a decrease in the intravascular oncotic pressure due to a decrease in serum albumin, like in liver cirrhosis, nephrotic syndrome, or malnutrition.

Exudative effusions, on the other hand, are usually due to local diseases that may cause inflammation, resulting in increased capillary permeability.

These include infections like pneumonia or tuberculosis, primary lung or metastatic malignancy, autoimmune diseases like systemic lupus erythematosus or rheumatoid arthritis, or pancreatitis.

One more specific type of effusion is a chylothorax, and it results from impaired lymphatic drainage of the pleura, which can happen after accidental damage during surgery, trauma, or cancer invasion.

Interestingly, pulmonary embolism can cause both transudative and exudative effusions.