Pleural effusion: Clinical practice

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.

The transudative component is thought to occur due to obstruction of the pulmonary circulation, causing an increased hydrostatic pressure in that area.

The exudative component occurs due to release of vasoactive mediators from the platelet-rich clots which increase capillary permeability.

Now, regardless of the cause, the symptoms of a pleural effusion include progressively worsening shortness of breath, due to limited expansion of the lungs, and pleuritic chest pain, which is a sharp pain that worsens with inspiration due to irritation of the somatically innervated parietal pleura.

However, much like Sherlock Holmes, you should look for associated symptoms that may clue you to identifying the potential etiology.

For example, fever, chills and a productive cough may indicate pneumonia.

Unintentional weight loss and loss of appetite may indicate a malignancy.

Night sweats, travel to an endemic area, and hemoptysis can indicate tuberculosis.

Orthopnea and paroxysmal nocturnal dyspnea can indicate congestive heart failure.

History of occupational exposure to asbestosis may point towards mesothelioma.

Joint pain or a rash can indicate autoimmune disease, and so on.

Normally, on physical exam there are diminished breath sounds on auscultation, and dullness when percussing the affected area.

Additional physical exam signs that may help narrow the differential diagnosis include peripheral edema, crackles and an S3 heart sound, which may indicate heart failure.

Stigmata of chronic liver disease like jaundice and dilated umbilical veins may indicate cirrhosis.

A joint effusion along with a pleural effusion may indicate an autoimmune disease. “If you’re effusing in two, think autoimmune”.

Alright, now to actually see an effusion, a chest x-ray is done. Uniquely, it’s obtained both while the individual is upright and lying down on their side, called the lateral decubitus position.

Now, the goal of a chest x-ray is three-fold: diagnose the presence of a pleural effusion, then determine if it’s freely flowing in the pleural space, which may clue towards a transudative effusion, or if it’s stuck and loculated, which may indicate an exudative effusion, and finally, determine if we can obtain a sample of the pleural fluid.

Now when the individual is upright, it takes at least 250 milliliters of fluid to see a pleural effusion.

Now normally, the costophrenic angle; which is the angle between the pleura covering and ribs and the pleura covering the diaphragm is sharp and well-demarcated.

In a pleural effusion however, this angle is obliterated by the radiopaque, or “white” fluid.

Larger effusions can also obscure the diaphragmatic contour and heart borders, form an air-fluid level similar to a half-filled cup of water, or they can form a concave upper surface that looks like a moon crescent, and is called the “meniscus sign” - because meniscus is greek for crescent.

Also, massive effusions can push the entire mediastinum to the opposite side, which can cause the trachea to shift from the midline.

Now, since transudative effusions are often caused by a systemic disease, there are usually bilateral effusions of similar size.

In contrast, exudative effusions are usually caused by localized disease, and therefore are usually unilateral or of different sizes, but there are plenty of exceptions to this rule.

For example, early on in congestive heart failure there may only be an effusion on one side, and conversely, a large pneumonia might lead to bilateral effusions.

Alright, now, on the other hand, lateral decubitus films can detect effusions that are as little as 50 milliliters, and also help tell us if the effusion is free flowing, which is where the fluid layers on top of the chest wall, or loculated, in which the fluid doesn’t layer.

An alternative to chest x-ray, is a bedside ultrasound and it can help determine the amount and nature of the effusion.

A homogenous “black” effusion means that it’s free flowing, and probably transudative.

Whereas, a heterogenous effusion with “white” septations indicates that it’s loculated, and probably exudative.

Now, on lateral decubitus chest x-ray we look at the distance between the chest wall and the top of the fluid level. Kind of like measuring the depth of an ocean, the chest wall being the seabed, and the fluid level being the surface of the sea.

If this distance is at least 1 centimeter, then that’s enough fluid to proceed to the next step - a diagnostic thoracentesis; which is inserting a thin needle through the chest wall to obtain a sample of the effusion.

This is different from a therapeutic thoracentesis, which is aspirating at least one liter of pleural fluid, providing symptomatic relief for massive effusions causing shortness of breath.

The bedside ultrasound can be used to visually guide the needle through the chest wall, which prevents damage to nearby structures like the liver.