Pathological processes involving the pleura

The pathological processes involving the pleura may be conveniently classified into neoplastic and non‐neoplastic, the latter including inflammatory and tumour‐like conditions. Primary diseases of the pleura of any type are rare (e.g. solitary fibrous tumour or malignant mesothelioma), and the pleura is usually involved secondarily by disease processes located elsewhere (e.g. bacterial pneumonia leading to empyema, or lung carcinoma with metastases to the pleura).

Pleural effusion (accumulation of fluid in the pleural space) is a common manifestation of many of these primary and secondary pleural disorders and most are related to infections, malignancy, heart failure or pulmonary emboli. Normally, there is less than 1 mL of fluid within the pleural cavity, this being maintained by a balance between hydrostatic and oncotic pressure within parietal and visceral pleural vessels, and by an extensive network of lymphatic channels which drain fluid away. Inflammatory disorders result in increased vascular permeability and therefore leakage of protein‐rich serous or haemorrhagic exudate into the pleural cavity causing an exudative pleural effusion. Similarly, malignancies cause vascular alterations and obstruction of lymphatics resulting in an exudate forming within the pleural space.

A non‐inflammatory or transudative pleural effusion, also referred to as hydrothorax, is typically bilateral, low in protein and usually results from increased capillary hydrostatic pressure (such as in heart failure) or reduced colloid osmotic pressure (such as in hypo‐albuminaemia caused by cirrhosis or nephrotic syndrome).

Measurement of pH, glucose, protein and lactate dehydrogenase (LDH) concentrations in pleural fluid and blood may aid the distinction between exudates and transudates, but more specialised pleural fluid assays (e.g. natriuretic peptides in heart failure or chylomicrons in chylothorax) may be employed in certain cases.

Chylothorax (or chylous pleural effusion) is a special type of effusion (usually exudative) referring to accumulation of lymphocytes, immunoglobulin and lipid‐rich milky fluid in the pleural cavity. Chylothorax may be secondary to traumatic rupture of major lymphatic ducts or obstruction thereof by tumour, most of which are malignant lymphomas. The chylous fluid forms a creamy or fatty supernatant when left to stand and contains a high concentration of chylomicrons and triglyceride (although the latter may be low). Pseudochylothorax is caused by cholesterol crystal accumulation in any chronic pleural effusion (especially those related to tuberculosis, rheumatoid disease or chronic haemothorax) and is usually neutrophil‐rich and lacking chylomicrons. Urinothorax is a rare cause of pleural effusion which may be considered in patients with undiagnosed pleural effusion associated with obstructive uropathy, urological trauma (including surgical injury) or kidney biopsy.