Pleural Effusion

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Pleural effusion is the accumulation of excessive amounts of pleural fluid in the pleural space/cavity. This may be caused by pulmonary, pleural or extrapulmonary disease.

Normal pleural fluid is formed by the filtration from systemic pleural microvessels (primarily parietal). Its volume is about 0.2-1 ml.

Visceral and parietal pleural surfaces consist of a mesothelial layer and three connective tissue layers. Visceral pleura is thicker than parietal pleura. Together, the visceral and parietal pleural layers and the lubricating liquid in the interposed pleural space have a combined thickness of 0.2 to 0.4 mm, while the width of the pleural space is 10 to 20 micrometers.

Pleural effusion may be exudative or transudative. If the pleural fluid protein (normal serum protein) is less than 2.5mg/dl, the effusion is transudative, while if it is greater than 3.5mg/dl, the effusion is exudative.
If pleural fluid protein is between 2.5-3.5 than Light’s criteria should be used.

Light’s Criteria

Pleural fluid / Serum protein > 0.5
Pleural fluid LDH / Serum LDH > 0.6
Pleural fluid LDH more than 2/3rd of upper limit of normal serum LDH

Atleast one of the three criteria must be present for an exudate; if none is present, the fluid is virtually always a transudate.

Transudative Effusions
Transudative effusions form primarily when the balance of hydrostatic forces influencing the formation and absorption of pleural fluid is altered to favor fluid accumulation. The permeability of capillaries to proteins is normal. Movement of fluid from the peritoneal or retroperitoneal spaces takes place. Iatrogenic causes include crystalloid infusion into a central venous catheter that has migrated

Exudative Effusions
Exudative effusions develop secondary to pleural and lung inflammation (resulting in a capillary protein leak) or impaired lymphatic drainage of the pleural space (resulting in decreased removal of protein from the pleural space) or movement of fluid from the peritoneal space, as seen with acute or chronic pancreatitis, chylous ascites, and peritoneal carcinomatosis

Common Causes of exudative effusions include:

Tuberculosis
Malignancy
Parapneumonic Effusions

Less Common Causes of exudative effusions include:
Pulmonary infarction
Rheumatoid arthritis
Autoimmune diseases
Benign asbestos effusion
Pancreatitis
Post-myocardial infarction syndrome

Diagnostic Appropach

Conventional Radiography
75 mL fluid obliterates the posterior costophrenic sulcus
175 mL is necessary to obscure the lateral costophrenic sulcus on an upright chest radiograph
500 mL will obscure the diaphragmatic contour on an upright chest radiograph

Chest X-rays
PA view is abnormal in the presence of 200ml fluid. Only 50ml can produce posterior costophrenic blunting on lateral chest x ray.

Lateral Decubitus Film
Small effusions are thinner than 1.5 cm
Moderate effusions are 1.5 to 4.5 cm thick
Large effusions exceed 4.5 cm
Effusions thicker than 1cm are usually large enough for sampling by thoracentesis, since at least 200 mL of liquid are already present.

Ultrasonography
Ultrasonography is more accurate in estimating fluid volume. It can be used to guide thoracoentesis (esp. for small or loculated effusions). Fibrinous septations are better visualized on ultrasound than on CT scan

CT Scan
For investigating effusions, contrast enhanced CT scan should be done before full drainage of fluid. CT scan facilitates measurement of pleural thickness, distinguishing an empyema from a lung abscess. Visualization of small pneumothoraces in supine patients and underlying lung parenchymal processes that are obscured on chest radiographs by large pleural effusions can be done. Determination of the exact location of pleural masses and characterization of their composition may be accomplished. Occasionally peripheral bronchopleural fistulae may be identified.

Diagnostic Tap
Diagnostic tap should not be performed for bilateral effusions in a clinical setting strongly suggestive of pleural transudate, unless there are atypical features including:

Unilateral effusion
Febrile patient
Pleuritic chest pain
Absence of cardiomegaly
…or a failure to respond to therapy

Diagnostic Sample
Protein, LDH, pH should be measured, Gram stain, AFB stain performed, cytology, microbiological culture should be done in addition. The yield for culture is greater if the sample is also sent in blood culture bottles, especially for anaerobic organisms.

pH
pH should be performed in all non-purulent exudative effusions. Effusions with a low pH:

Collagen vascular diseases (particularly RA)
Esophageal rupture
Malignancy

Glucose
Glucose level < 30mg/dl

Empyema
Rheumatoid Pleurisy

Glucose level <60mg/dl

Parapneumonic effusion
Malignancy
Tuberculosis
Connective Tissue Diseases
Paragonimiasis
Churg-Strauss Syndrome
Esophageal rupture

Fluid Cytology
Fresh samples are better for cytology. Samples can be stored at 4°C for 4 days. If part of the sample is clotted it should be stained and treated as a histological section as this will improve the yield. Sending samples in citrated bottles is preferred by some cytologists.
Malignancy can be diagnosed by cytology alone in 60% of cases. If 1st sample is negative it should be repeated. Yield is higher for adenocarcinoma as compared to mesothelioma, squamous cell carcinoma, lymphoma and sarcoma

Percutaneous pleural biopsy
Percutaneous pleural biopsy is performed in undiagnosed exudative effusions, with non-diagnostic cytology. It is usually done for clinical suspicion of tuberculosis or malignancy. 40% of mesothelioma patients have tumor seeding after biopsy. Biopsy should be sent for histopathology and mycobacterium culture.

Thoracoscopy
Thoracoscopy should be considered when less invasive tests have failed. Diagnostic sensitivity above 95%. Complications include hemorrhage, subcutaneous emphysema or arrhythmias.

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The writer enjoys medical education and has special interest in community medicine.