Pleural effusion

The pleural cavity is a potential space within the thoracic cavity lined by visceral and parietal pleural membranes

Normally, the pleural apace contains a small physiologic amount of pleural fluid (0.1 mL per kg). An imbalance between normal pleural fluid formation and/or absorption results in accumulation of fluid within the pleural cavity.

The fluid may be either transudative or exudative:

• a transudate results from abnormal accumulation of pleural fluid due to high capillary and interstitial hydrostatic pressures (e.g. - in heart failure) or abnormally decreased capillary oncotic pressure (e.g. - in nephrotic syndrome).
• an exudate results from an inflammatory and malignant processes which alter the permeability of the local capillary and pleural membrane permeability or causes lymphatic blockage

It is thought to be a common condition with an estimated 1-1.5 million new cases in the United States and 200,000-250,000 in the United Kingdom each year.

An effusion as small as 50 mL can be seen on the lateral film as costophrenic blunting, and more than a few hundred millilitres will be visible on the postero-anterior film. (2)

References:

1. Roberts ME, Rahman NM, Maskell NA, et al. British Thoracic Society guideline for pleural disease. Thorax. 2023 Jul;78(suppl 3):s1-42.
2. Light RW. Pleural diseases. 3rd ed. Baltimore, MD: Williams and Wilkins; 1995.