Pleural effusion

Pleural effusion, also called hydrothorax, refers to the presence of fluid in the pleural cavity. If visible on a chest radiograph, there are at least 100 mL of fluid.

If there is simultaneous pneumothorax, the condition is called hydropneumothorax.

Etiology and types

• Types according to composition and cause
  • Transudates (hydrothorax/HTX)
    • Heart failure
    • Hypoalbuminaemia
      • Cirrhosis
      • Nephrotic syndrome
    • Atelectasis
  • Exudates
    • Malignancy (lung cancer, mediastinal cancer)
    • Pneumonia (parapneumonic effusion or thoracic empyema)
    • Haemothorax
    • Chylothorax
    • Constrictive pericarditis
    • Pulmonary embolism
    • COVID-19
• Types according to laterality
  • Bilateral effusions
    • Heart failure
  • Unilateral effusions
    • Pulmonary embolism
    • Haemothorax
    • Tuberculosis
    • Acute pancreatitis (only left side)

Clinical features

The pleural effusion itself may cause dyspnoea and hypoxaemia. Symptoms of the underlying cause is often present.

Decreased lung sounds and increased dullness on percussion may be felt over the affected area.

Diagnosis and evaluation

A lot can be gathered from the gross appearance of the fluid alone. Turbid fluid is suggestive of empyema or infection. Bloody fluid is suggestive of haemothorax. Milky fluid is suggestive of chylothorax.

Radiological evalation

Chest radiograph is the first choice if pleural effusion is suspected, although it can't detect effusions < 300 mL. It will show blunting of the costophrenic angle and, if large enough, opacification of parts or of the entire lung. There may be a midline shift toward the contralateral side. Pleural effusion filling the entire pleural cavity causes so-called white-out of the hemithorax, but is very rare.

If there is simultaneous pneumothorax, a horizontal fluid level called an air-fluid level (or "niveau") will be present.

A chest radiograph taken with the patient lying on their side and with the effusion-side down may allow visualisation of small amounts of subpulmonal fluid. This is sometimes called a "Frimann-Dahl image" (according to the lecture, probably a Hungarian term).

Ultrasonography and CT may also be used to visualise pleural effusion. CT is rarely necessary, unless the intention is to rule out malignancy as a cause. On ultrasound pleural fluid is seen as hypoechoic or anechoic structures in the costophrenic recess. CT can be used to measure fluid density and therefore assist in the determination of the type of fluid.

Laboratory evaluation

If the type of pleural fluid is uncertain, it should be drained with a needle for laboratory examination. If the fluid needs to be drained anyway (due to symptoms), the fluid should also be sent for laboratory examination. Only a laboratory evaluation can distinguish a transudate from an exudate.

One can analyse "regular" biochemical tests as well as microbiological tests like culture or PCR and others. Routine tests performed on pleural fluid include white blood cell count with differential, protein, LDH, and glucose. If there is an elevated WBC and it's predominantly polymorphonuclear cells, infection is most likely. If it's predominantly lymphocytes, tuberculosis and lymphoma are more likely.

If malignancy is suspected, one should send the fluid to cytology as well. If infection is suspected, Gram stain and culture should be performed.

Distinguishing between exudate and transudate

There are several approaches to distinguishing between pleural exudate and transudate. Here are two options:

UpToDate recommends using the PFO₃ (pleural fluid-only three-test). This is the most simple as it does not require a concomitant blood test. According to the PFO₃ criteria, the fluid is an exudate if at least one of the following is fulfilled:

• The protein concentration of the pleural fluid is > 30 g/L
• The cholesterol concentration of the pleural fluid is > 1.4 mmol/L
• The LDH concentration of the pleural fluid is > 67% of the upper normal limit of serum LDH

We can also use Light's criteria to distinguish between transudates and exudates. According to the criteria, the fluid is an exudate if at least one of the following is fulfilled:

• The protein concentration of the pleural fluid is > 50% of plasma protein concentration
• The LDH concentration of the pleural fluid is > 60% of serum LDH concentration
• The LDH concentration of the pleural fluid is > 67% of the upper normal limit of serum LDH

A pleural fluid pH of < 7.30 means an empyema is most likely. If the pH is 7.30 - 7.45, an exudate is more likely, and if 7.40 - 7.55, transudate is more likely. However, the pH alone is not as accurate as using any of the above criteria.‎