Pleuritis (also known as pleurisy) is defined as inflammation of the parietal pleura which lines the cavity surrounding the lungs. In young and healthy people, pleuritis is most commonly due to viral infections. Other causes include lung infections, cancer, surgery, and chest trauma.
Pleuritis can result in chest pain while breathing which can become quite a bit worse with deep breathing. Coughing, sneezing, and laughing can be very painful. Many patients will hold a pillow tightly if they have to sneeze or cough to try and help the pain. Treatment may include anti-inflammatories, pain relievers, and rest.
A pleural effusion is defined as an abnormal collection of fluid in the pleural cavity (the space between the parietal and visceral pleural membranes), not in the lung itself. Normally, fluid enters the pleural cavity from capillaries in the parietal pleura and exits through lymphatic vessels. With a pleural effusion, this exchange is disrupted and fluid accumulates in the pleural cavity. This collection of fluid may compromise one’s ability to expand the lungs and cause shortness of breath. Patients may also report a feeling of fullness or chest pain. However, some patients are asymptomatic.
Pleural effusion can be classified by the characteristics of fluid found in the pleural cavity. The two types of fluid are transudative (watery fluid) and exudative (protein-rich fluid).
1. Transudate (hydrothorax) is found with pathological processes that change the forces involved with capillary exchange so that more fluid moves out of the capillary and into the interstitial spaces than returns to the blood. Some common causes of transudate include heart failure, malignancy, liver failure, or kidney failure. Right-sided heart failure causes ascites, which is a collection of fluid in the peritoneal cavity that may leak through small holes in the diaphragm and cause a pleural effusion. Left-sided heart failure leads to fluid backup in pulmonary circulation which increases the hydrostatic pressure on the capillary venous end so more fluid enters the pleural cavity. Malignancy or cancer may cause a blockage of lymph flow. Liver failure leads to a decrease in albumin production which decreases blood colloid osmotic pressure (BCOP) and causes more fluid to leave the capillaries than return. Kidney failure leads to a loss of proteins like albumin in the urine which also leads to osmotic forces favoring fluid exiting the capillaries.
2. Exudate is found most often with a pathological process that has caused inflammation and increased pleural membrane permeability. Common causes include pneumonia, tuberculosis, cancer, viral infections, and inflammatory diseases. When a tissue is inflamed, unhealthy cells die and release their contents which include proteins that then attract water. For this reason, exudate has higher amounts of protein, cell fragments, and onsite inflammatory cells than transudate. Lactate dehydrogenase (LDH) is a cytosolic enzyme that catalyzes the reversible conversion of lactate to pyruvic acid and NAD+ to NADH. LDH is not generally found as part of the extracellular fluid and can therefore be used to assess whether an effusion is transudative or exudative. Lipids like cholesterol and triglycerides are also released from inflamed tissues and can be measured in exudate.
Image by Becky T. BYU-I S20
Pleural effusions may also be categorized by the origin of the fluid. With this categorization, we can identify hydrothorax, hemothorax, pyothorax (empyema), chylothorax, and urinothorax.
· A hydrothorax is a collection of serous fluid in the pleural space and is really transudate. The causes of a hydrothorax are listed above in the description of a transudative pleural effusion.
· A hemothorax is a collection of blood in the pleural space. Causes include cancer and blood clotting disorders that lead to uncontrolled hemorrhage in the lungs. Trauma to blood vessels near the chest wall or lung tissue can also cause bleeding into the pleural space.
· A pyothorax (or empyema) is defined as the collection of pus in the pleural cavity. It is usually caused by an infection in tissues that are near the pleural cavity. This infection can lead to inflammation and the onset of an exudative pleural effusion. Infections can also sometimes lead to an abscess that contains pus. Sometimes an abscess can rupture and leak pus into the pleural space. Possible causes for empyema include invasion by bacteria into the lung tissue or even into tissues below the diaphragm (pus-like material can leak through the diaphragm and into the pleural space). The pus is thick and usually must be drained using a chest tube.
· A chylothorax is a pleural effusion that contains a milky fluid consisting of chylomicron-rich lymph. This lymph will contain fat droplets (chylomicrons) because the lymph will have just come from the lacteals of the small intestine where fat was absorbed and chylomicrons were made. The most common causes of a chylothorax include trauma and cancer, both of which can damage lymphatic vessels and create obstruction to lymph flow. As lymph backs up and leaks out of the lymphatic vessels, it can move through the relatively permeable pleural membranes and into the pleural space. Trauma in particular may disrupt the thoracic duct connection in a way that lymph fluid can gradually leak from the duct and into the pleural space.
· A urinothorax involves the movement of actual urine into the pleural space. This is a relatively rare cause of pleural effusion, but it can happen when urine flow is obstructed and an opening (usually due to trauma to the ureters) allows urine to leak into the peritoneal cavity. Over time, that urine will leak through the permeable diaphragm and parietal membrane and into the pleural space. Because transudate can also be a pale yellow, it is important to test any pleural effusion fluid that has a yellow color for urea or creatinine which are found in urine.
Fluid from a pleural effusion is tested to help determine the cause of the effusion. This fluid is collected through thoracentesis, a procedure which involves inserting a needle into the pleural space to remove fluid that can then be tested for protein (including LDH) and lipids. If the effusion is large, removal of fluid through thoracentesis will also allow the lung to re-expand. An ultrasound probe can be used to determine the location of the accumulated fluid and where the needle for thoracentesis should be inserted. Underneath each rib there is a vascular bundle that is composed of a vein, artery, and nerve. To avoid these vital structures, it’s important to always insert the needle just over the rib. After needle insertion into the pleural space, a catheter can be advanced into the fluid to which a syringe is attached to suck out the fluid. It’s important to avoid inserting the needle too far because this could penetrate the visceral layer (surface) of the lung. Air could then enter the pleural space from the lung and cause a pneumothorax.