Pleural thickening (PT) is not a specific disease but a physical manifestation of an underlying health problem that has caused injury or inflammation to the lungs’ lining. The pleura is a thin, double-layered membrane surrounding the lungs and lining the chest cavity, allowing smooth movement during respiration. When irritated, the body repairs the damage by producing fibrous scar tissue, making the pleura abnormally dense and stiff. This scarring compromises the pleura’s elasticity, potentially restricting how fully the lungs can expand and contract during breathing.
Occupational and Environmental Exposure
The most widely recognized environmental cause of this condition is the inhalation of asbestos fibers, which leads to specific forms of pleural scarring. These microscopic fibers lodge in the lung tissue and migrate to the pleura, triggering a chronic inflammatory response that results in the formation of dense, fibrous tissue. Asbestos-related pleural thickening often manifests in two distinct ways: diffuse pleural thickening (DPT) and pleural plaques.
DPT is widespread scarring that affects large areas of the pleura, sometimes encasing the lung and significantly impairing respiratory function. In contrast, pleural plaques are localized, benign patches of scarring typically found on the parietal pleura (the layer lining the chest wall). Asbestos-related pleural changes have a long latency period, often not becoming apparent until decades after the initial exposure. While asbestos is the dominant concern, exposure to other inorganic dusts, such as silica or talc, can also induce a fibrotic reaction in the pleura.
Infectious and Inflammatory Causes
Internal disease processes causing fluid buildup or intense inflammation within the chest cavity are frequent drivers of pleural thickening. Infections resulting in a significant pleural effusion (accumulation of fluid between the pleural layers) often precede scar tissue development. If the body fails to completely clear this fluid, the inflammatory cells and debris organize into fibrin, which eventually matures into permanent, dense scar tissue.
Bacterial pneumonia is a common example, particularly when it progresses to empyema, where the pleural space fills with pus. Historically, tuberculosis (TB) pleuritis was a major cause, as the immune system’s intense reaction frequently generated effusions that scarred over time. Various fungal infections can also trigger this sequence, leading to a profound inflammatory reaction and subsequent fibrosis of the pleural membrane. The severity of the initial infection and the resulting inflammatory response directly influence the extent of the residual pleural scarring.
Physical Trauma and Medical Intervention
Pleural thickening can arise as a direct consequence of physical injury or necessary surgical procedures that disrupt the chest wall or lungs. Traumatic injuries causing bleeding into the pleural space, known as hemothorax, are a notable cause. If the blood is not rapidly reabsorbed, the fibrin in the blood clot acts as a scaffold, which is then replaced by permanent scar tissue, leading to localized or diffuse thickening.
Medical treatments can intentionally or unintentionally induce pleural scarring as a form of healing or therapy. Procedures like thoracic surgery, which involves opening the chest cavity, can result in scarring as the surgical site heals. Pleurodesis intentionally creates inflammation and scarring between the pleural layers to prevent recurrent fluid accumulation or lung collapse. During this intervention, a chemical irritant like sterile talc is introduced into the pleural space to fuse the two layers together, causing a controlled form of pleural thickening.
Malignancy and Systemic Disease
The most serious underlying conditions causing pleural thickening involve cancerous growth or chronic systemic inflammation. Malignant Pleural Mesothelioma is a rare but aggressive cancer of the pleura, almost always linked to past asbestos exposure, where the tumor appears as nodular or diffuse thickening. The cancerous cells proliferate, leading to extensive, irregular, and often rapid thickening that can fully encase the lung.
Pleural thickening may also signal metastatic cancer, occurring when a tumor originating elsewhere (such as the breast or lung) spreads to the pleura. These secondary tumors induce inflammation and often cause malignant effusions that scar into thick plaques. Chronic systemic inflammatory conditions, distinct from acute infection-driven causes, can also affect the pleura. Diseases like Rheumatoid Arthritis or Systemic Lupus Erythematosus (SLE) involve immune system dysfunction that targets the pleura, leading to persistent inflammation and the development of pleural fibrosis.