Pleural effusion, pneumothorax, hemothorax and atelectasis: Pathology review

During a night shift in the Emergency Department, you see George, a 21 year old, who complains of sharp, stabbing left-sided chest pain and shortness of breath, both of which appeared suddenly while he was playing video games. He is a smoker and mentions his younger brother suffered a pneumothorax last year. On examination, the left side of his chest is slightly more expanded than the right side and his breathing is very shallow. The affected side is also hyperresonant on percussion and there’s diminished breath sounds on auscultation. Then there’s Anna, a 58-year-old with progressively worsening dyspnea, pleuritic chest pain, and fever. She has a history of diabetes and cirrhosis. On examination, her blood pressure and heart rate are both increased, and jugular venous pressure is elevated. Also, the base of her left lung field is dull to percussion with decreased breath sounds. Her lower extremities have pitting edema up to the knee. X-rays were performed in both individuals. In George’s case it showed a retracted visceral pleural edge with a decrease in the left lung volume. In Anna, it showed left sided costophrenic angle blurring. Now, they both seem to present with conditions affecting the pleura. But to understand the pathophysiology, it’s a good idea to review the anatomy and physiology first. Ok, so the pleura covers the lungs and consists of the parietal pleura, which is stuck to the chest wall, and the visceral pleura, which is stuck to the lungs. It extends all the way up to the clavicle and first rib, which is something that will help you understand why neck injuries can cause tension pneumothorax. Also know the inferior limit of the pleura is the 7th rib on the midclavicular line, the 10th rib on the mid axillary line, and the 12th rib on the paravertebral line. Between the layers of the pleura is the pleural space, containing 10 to 20 milliliters of a lubricating fluid that helps reduce friction as the lungs expand and contract. There’s also pressure within the pleural space, established by two main opposing forces. One is the muscle tension of the diaphragm and chest wall, which contract and expand the thoracic cavity outwards. The other is the elastic recoil of the lungs, which is the lungs’ tendency to collapse inward. The two forces pulling in opposite directions creates a slight vacuum in the pleural space with a pressure of -5 centimeters of water, relative to 0 centimeters of water in both the thoracic cavity and the lungs. The difference in pressure aids air exchange during inspiration and expiration to help the lungs keep their shape and not collapse.

Ok, now let’s look at the disorders that affect the pleura, starting with pneumothorax. A pneumothorax forms when the seal of the pleural space is punctured, letting air in and equalizing the pressure within the pleural space, lung and chest cavity. As a result, the negative pleural pressure is lost and the equilibrium between the two opposing forces disappears. The result is the affected lung simply pulls inwards and collapses. Now, there are many types of pneumothorax. A primary spontaneous pneumothorax is one that develops in the absence of an underlying condition. And a clue to help you out is that these individuals tend to be thin, tall, young males who smoke. Another high yield fact is that they often have a family history of pneumothorax. The cause is unknown, but they are believed to be due to apical subpleural blebs that rupture without warning.

A secondary spontaneous pneumothorax is one that develops in someone with an underlying lung disease, where the lung or pleura structure is altered. One example you might encounter is emphysema, where bullae, which are air pockets, form on the surface of the lungs and then rupture. This creates a large hole in the visceral pleura and air can go from the airway directly into the pleural space.

There’s also a traumatic pneumothorax which is when trauma, like a gunshot or stab wound or sometimes even iatrogenic causes like lung biopsy or barotrauma due to mechanical ventilation, rips through the parietal pleura, allowing air to enter into the pleural space. Finally, there’s tension pneumothorax, which can develop similarly to a spontaneous pneumothorax. One difference to keep in mind is that a flap of tissue near the leak can create a one-way valve for air to flow into the pleural space. In other words, air can enter, but cannot leave. So, air accumulates in the chest cavity and it pushes the organs in the mediastinum away from the affected side, which is called a mediastinal shift. This also causes the compression of the organs, especially the venous system. This prevents blood from returning to the heart which will reduce cardiac output, and ultimately lead to obstructive shock.

The symptoms of a spontaneous pneumothorax might only consist of shortness of breath and sudden unilateral pleuritic chest pain. This is a sharp knife-like pain that worsens with deep inspiration, coughing, sneezing, or laughing due to irritation of the parietal pleura by the expanding lung. Clinical examination begins with vitals, which are usually normal, except for rapid breathing or tachypnea. Now, on inspection, because the chest wall springs outward a bit as the lung collapses, the chest wall on the affected side might appear bigger than the normal side. On chest palpation, chest expansion is uneven due to decreased chest wall movement of the affected side and tactile fremitus is decreased. Tactile fremitus can be felt when the hand is placed on the chest wall. When a person says “99” the chest wall vibrates. If there’s excess air in the pleural space, it absorbs some of this vibration energy; thus, the vibrations weaken. Now, when the chest is percussed, the extra air in the pleural space makes the resonant sound get louder, and this is something that can help you differentiate it from a hemothorax, where the chest percussion is usually dull. Also, the extra air in the pleural space changes sound transmission in the chest, so on auscultation, normal breath sounds are diminished.

Now, in a tension pneumothorax, because air builds up, increasing the pressure inside the chest, the air starts compressing the organs in there. It’s important to know that if it compresses the heart, it can lead to decreased cardiac output, low arterial blood pressure, distended neck veins, and tachycardia. Moreover, compression of the inferior vena cava, decreases venous return, which leads to edema and even cardiac arrest. There’s also deviation of the trachea towards the unaffected side. Finally the normal lung can also be compressed, further limiting air exchange, resulting in cyanosis and respiratory failure.

Typically, an X-ray or CT is needed for diagnosis, where you should look for a retracted visceral pleural edge, usually seen as a very thin, sharp white line, alongside a decreased lung volume due to the lung collapsing. The space beyond the visceral line is mostly black, because the space where the lung should be is filled with air. Commonly, in a tension pneumothorax, an X-ray or CT might show a mediastinal shift, which is seen as a tracheal deviation and displacement of chest structures away from the affected side.

Now treatment is not usually required in spontaneous pneumothorax, as they are usually small, and the pleura will heal over time. In a larger pneumothorax that’s causing severe symptoms, the air needs to be removed with a syringe or a chest tube connected to a one-way valve system. In a tension pneumothorax, the air needs to be removed right away, sometimes even before the diagnosis is confirmed by imaging. Remember that this is usually done by inserting a needle or a chest tube in the space between the second and third rib of the affected side, on the midclavicular line, which decompresses the chest, and provides an escape route for the trapped air.

Next we have pleural effusion, which is excess fluid in the pleural space. This can happen due to either a transudative, exudative or lymphatic effusion. Ok, so a transudative pleural effusion is when the liquid inside the pleural space is a transudate. It occurs when too much fluid starts to leave the capillaries either because of increased hydrostatic pressure or decreased oncotic pressure in the blood vessels, usually in the context of conditions like heart failure, cirrhosis, and nephrotic syndrome. You’ll be able to differentiate it by the fact that a transudate is low in protein. This can be due to decreased protein production caused by cirrhosis, or because the proteins are lost in urine like when there’s nephrotic syndrome. Transudate also has a low lactate dehydrogenase content, which is a substance released whenever there’s tissue damage, and it appears clear most of the time.