Apical pleural parenchymal scarring (APPS) describes the formation of non-functional fibrous tissue found exclusively at the very top, or apex, of the lungs. This finding represents a localized area of healing within the respiratory system, a common reaction to past injury or inflammation. The term refers to a scar involving both the lung surface (pleura) and the underlying tissue (parenchyma) in the uppermost region of the chest. Understanding the anatomical components of this scarring provides context for exploring its various origins and clinical significance.
Defining the Location and Damage
The name, apical pleural parenchymal scarring, identifies the anatomical site and the nature of the damage. “Apical” refers to the apex, the superior-most, dome-shaped portion of the lung that extends slightly above the clavicle. This specific location is subject to different mechanical and circulatory forces compared to the rest of the lung.
The scar involves two distinct tissues. It begins with the pleura, the thin, double-layered membrane lining the outside of the lungs and the inside of the chest wall. This outer component is referred to as pleural thickening or fibrosis. Beneath this lining, the scar tissue extends into the parenchyma, the functional, spongy tissue of the lung where gas exchange occurs.
The “scarring” is a process called fibrosis, where the body replaces healthy, elastic lung tissue with dense, non-functional connective tissue. This fibrous tissue is a permanent patch that forms after the body has contained and healed an insult, such as an infection or injury. The resulting scar is visible on X-rays or computed tomography (CT) scans as a wedge-shaped or cap-like opacity at the lung’s peak, sometimes referred to as an “apical cap.”
Primary Causes of Apical Scarring
Apical pleural parenchymal scarring results from past events that triggered a localized healing response in the upper lung. A common and historically significant cause of APPS is a previous infection, most notably healed tuberculosis (TB). The upper lobes are often the first sites of TB infection due to higher oxygen concentration, and the resulting immune response leaves behind fibrotic scars after the infection is cleared.
Severe or chronic pneumonia affecting the top of the lung can also resolve with residual scarring in the pleural and parenchymal layers. These post-infectious scars represent inactive, stable tissue marking the location of a healed infection. When the scarring is found at the apex of both lungs, it is termed biapical scarring, suggesting a widespread or bilateral inflammatory history.
A frequent cause of APPS is considered benign or idiopathic, meaning the precise cause is unknown, and it is attributed to aging. This type of scarring, commonly called an apical cap, increases in frequency with advancing age. It is theorized to be caused by chronic, low-grade ischemia, or reduced blood flow, at the lung’s apex. The superior position of the apex results in lower perfusion and ventilation rates, which may predispose the tissue to minor inflammatory changes that accumulate over decades.
While less common, direct physical trauma to the upper chest can also initiate the scarring process. Prior chest wall injuries, such as a fractured first rib or a severe blow to the shoulder region, can lead to localized bleeding and inflammation that heal with APPS. Rare forms of interstitial lung disease, such as pleuroparenchymal fibroelastosis, are also characterized by fibrosis of the pleura and underlying parenchyma. These diseases predominantly affect the upper lobes and mimic the appearance of an apical cap.
Symptoms and Clinical Implications
For the vast majority of individuals, apical pleural parenchymal scarring is asymptomatic and causes no noticeable health issues. The scarring is most frequently discovered incidentally when a patient undergoes a chest X-ray or CT scan for an unrelated reason, such as a routine screening. Because the scar is usually small and localized, it does not significantly impact overall lung function or breathing capacity.
When symptoms do occur, they are minor and are associated only with cases where the underlying cause was extensive. If the fibrosis is widespread or particularly dense, it can cause a slight restrictive ventilatory defect, meaning the lungs cannot fully expand. In these situations, the patient might experience mild shortness of breath during strenuous activity, but this is uncommon for the isolated apical scar.
The primary clinical implication of finding APPS is ensuring the scar is inactive and benign. A physician evaluates the appearance of the scar on imaging to confirm it is not an active infection or a malignancy. If the scarring appears new, is rapidly changing, or is accompanied by systemic symptoms like fever or weight loss, further investigation is warranted to rule out an active disease process, such as reactivated tuberculosis. Once confirmed as a stable, healed scar, APPS is considered non-progressive, reassuring patients about their long-term respiratory prognosis.