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 a lateral film as costophrenic blunting, and more than a few hundred millilitres will be visible on a postero-anterior film. (2) This should usually prompt pleural ultrasound - a commonly used point of care test that guides intervention, with strong evidence suggesting this improves safety and can guide management decisions (3)

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. Expert Panel on Thoracic Imaging, Morris MF, Henry TS, et al. ACR Appropriateness Criteria® Workup of pleural effusion or pleural disease. J Am Coll Radiol. 2024 Jun;21(6):S343-52.
3. Stevic R, Colic N, Bascarevic S, et al. Sonographic indicators for treatment choice and follow-up in patients with pleural effusion. Can Respir J. 2018;2018:9761583.