Pleural Effusion – causes, symptoms, diagnosis, treatment, pathology

“Pleural” refers to the space between
the chest cavity and the lungs, and “effusion” refers to a collection of fluid, so a pleural
effusion is when a disease process causes fluid to start to collect in the pleural space,
which can sometimes restrict lung expansion. The pleural cavity or pleural space lies between
the parietal pleura which is stuck to the chest wall and the visceral pleura which is
stuck to the lungs. Because the lungs fit snugly inside the chest
cavity, the visceral and parietal pleura lie right next to each other, and the very very
thin space between them contains a layer of fluid that acts as lubrication to allow the
lungs to slide back and forth as they expand and contract. This pleural fluid is similar to interstitial
fluid and is made slippery by proteins like albumin. It’s so similar to interstitial fluid because
it–essentially–is interstitial fluid. There is always a tiny bit of plasma that
leaks out of capillaries and gets into the interstitial space, and since these capillaries
are so close to the edge of the pleural space, that fluid makes its way into that space and
collects there. If there were no way out of the pleural space,
then it would fill up with fluid, but fortunately, there are lymphatic vessels in the pleura
then drain the fluid away and deliver it back into the circulatory system. A pleural effusion is when there’s excess
fluid in the pleural space either because too much pleural fluid is produced by the
body., which can be due to either a transudative or exudative effusion or because the lymphatics
can’t effectively drain away the fluid, called a lymphatic effusion. A transudative pleural effusion occurs when
too much fluid starts to leave the capillaries either because of increased hydrostatic pressure
or decreased oncotic pressure in the blood vessels. Hydrostatic pressure is what we normally think
of as blood pressure; it is the force that blood exerts on the walls of the blood vessel,
and can be thought of as a pushing force. A common cause of increased hydrostatic pressure
in the lung capillaries is heart failure. That’s because when the heart can’t effectively
pump blood out to the body, it backs up into the pulmonary vessels and causes the blood
pressure in those vessels to rise. The high pressure forces fluid out of the
capillaries and into the pleural space. Oncotic pressure results from the the inability
of solutes like large proteins – albumin for example – to move across through the capillary. By the process of osmosis – the process, not
the company – fluid moves from areas of low solute concentration to high solute concentration. Fluid therefore flows out of capillaries and
leaks into the pleural space when there is decreased oncotic pressure in the blood vessels. Two causes of low oncotic pressure are cirrhosis,
where the liver makes fewer proteins and nephrotic syndrome, where proteins are lost through
the urine. An exudative pleural effusions is due to inflammation
of the pulmonary capillaries which makes them much more leaky. The larger spaces between endothelial cells
allows fluid, immune cells and large proteins like lactate dehydrogenase (LDH) –which is
found in all cells, to leak out of the capillaries. The causes can vary – trauma, malignancy,
an inflammatory condition like lupus, or an infection like pneumonia. If the underlying reason is an infection,
like a bacterial or mycobacterial infection, then it’s also possible for that infection
to spread into the pleural space which is a walled off space – a bit like an enormous
abscess. Just like an abscess, the infected pleural
space can develop fibrinous walls and have loculations. Finally, there can be a lymphatic pleural
effusion, called a chylothorax. In a chylothorax, the thoracic duct is disrupted,
and lymphatic fluid accumulates in the pleural space. The most common cause of chylothorax is when
the thoracic duct accidently gets damaged during a thoracic surgery, but it can also
be caused by tumors in the mediastinum that press up against the thoracic duct and compress
it shut. Symptoms of a pleural effusions mostly depend
on it’s size. A small effusion might go unnoticed, whereas
a large one might cause pain while inhaling–called pleurisy–or shortness of breath and might
be more obvious only when lying down flat. Classically, a pleural effusion will cause
decreased breath sounds, dullness to percussion, which is tapping, on the back, and decreased
tactile fremitus. Tactile fremitus is a normal finding – it’s
when the chest wall can be felt vibrating when a person speaks. If there’s excess fluid in the pleural space
it absorbs some of this vibration energy, and the vibrations can’t be felt as strongly. Finally, if the pleural effusion is large
enough, it can start to push against the lung not letting it fully aerate and even causing
the trachea to shift away from the side of the effusion–called tracheal deviation. On an Xray taken of someone standing upright,
the pleural effusion fluid can settle into the costophrenic angle – which is where the
diaphragm meets the chest wall – and cause blurring of the angle as fluid displaces the
air that is usually there. On an Xray taken of someone lying down, gravity
will cause the freely mobile pleural effusion to settle along the chest wall, creating a
layering effect. To remove pleural effusion fluid both to relieve
symptoms but also to find out the cause, a thoracentesis can be done. In a thoracentesis, a hollow needle to drain
the fluid is carefully inserted over the top of a rib, to avoid injuring the neurovascular
bundle running along the underside of each rib. Transudative fluid looks clear, exudative
effusions are full of immune cells and therefore look cloudy, and lymphatic fluid is filled
with fats and looks like milk. Often, the biggest challenge is distinguishing
between a transudative and exudative effusion, and the key difference is the amount of protein
in the effusion – exudates have much more! To help with that distinction there are criteria
called the Light Criteria. A pleural effusion is considered exudative
if the ratio of pleural fluid protein to the serum protein is greater than 0.5; the ratio
of pleural fluid LDH to serum LDH is greater than 0.6; or if the LDH in the pleural fluid
is more than two-thirds the normal upper limit of normal levels in the serum. Although, it’s not part of Light criteria,
another feature of exudates is that they typically have a pleural fluid cholesterol level of
over 45 mg/dL. Treating a pleural effusion typically means
removing the fluid as well as treating the underlying cause – and the approaches can
vary quite a lot. Small transudative pleural effusions resulting
from heart failure may be treated with diuretics and sodium restriction, whereas larger pleural
effusions due to an oncologic process might require draining with a tube. Finally, there might a large, loculated pleural
effusion caused by bacterial pneumonia or tuberculosis which may require surgery because
it can thicken into a paste-like substance called an empyema which wouldn’t drain easily
through a tube. Alright, as a quick recap – a pleural effusion
is when fluid collects in the pleural space around the lungs, restricting expansion and
causing pain and difficulty breathing. Pleural effusions can be due to excess fluid
collection, transudative and exudative, or due to blockage of lymphatic drainage. Diagnosis is usually done with a thoracentesis
which can help alleviate symptoms and can be used to identify the underlying cause. Thanks for watching, you can help support
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