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 (1)

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 (3).

A pleural effusion will only be detected:

• on a chest radiograph when the volume of the effusion exceeds 300 ml
• clinically when the volume exceeds 500 ml

Reference:

• (1) Saguil A, Wyrick K, Hallgren J. Diagnostic approach to pleural effusion. Am Fam Physician. 2014;90(2):99-104
• (2) Hooper C et al. Investigation of a unilateral pleural effusion in adults: British Thoracic Society Pleural Disease Guideline 2010. Thorax. 2010;65 Suppl 2:ii4-17
• (3) Bhatnagar R, Maskell N. The modern diagnosis and management of pleural effusions. BMJ. 2015;351:h4520.