Spontaneous Pneumothorax: A Review of Underlying Etiologies and Diagnostic Imaging Modalities

Primary spontaneous pneumothorax (PSP) occurs without clinically apparent lung disease, though underlying apical blebs or bullae are often identified on CT. The primary risk factor is tobacco use, which can increase the risk up to twentyfold, with a taller, thinner body habitus also being a predisposing factor [ 17 , 18 , 19 ] ( Figure 2 ). SSP arises from numerous underlying lung conditions, many of which are detailed in this review. When an inherited syndrome is suspected, genetic evaluation is clinically valuable. Identifying specific gene mutations can confirm the diagnosis, guide management, estimate recurrence risks, influence treatment strategies, and enable cascade screening for at-risk relatives [ 20 ]. The etiologies of SSP are vast, spanning a wide spectrum of conditions that range from common to exceedingly rare. To provide a clinical framework and address the relative risks of these underlying causes, Table 1 stratifies the key etiologies of SSP based on their approximate frequency, demographics, and recurrence risk.

Imaging modalities for pneumothorax (see Table 2 for summary). The key radiographic sign is a visible visceral pleural line separated from the chest wall by a lucent space without lung markings. On supine radiographs, a “deep sulcus sign” may be the only indicator. Pleural adhesions can cause loculation, requiring careful evaluation of the entire hemithorax. Supplemental views, such as expiratory or lateral decubitus images, can increase the conspicuity of small pneumothoraces [ 21 , 22 ] ( Figure 3 and Figure 4 ). In emergency and critical care settings, bedside ultrasonography offers a rapid, radiation-free method for diagnosing pneumothorax. The primary finding is the absence of “lung sliding,” which is the normal shimmering movement of the visceral pleura against the parietal pleura. On M-mode imaging, the absence of lung sliding manifests as the “barcode sign” (a series of horizontal lines), which replaces the normal “seashore sign” seen with a healthy, moving lung ( Figure 5 ). The most specific sign for pneumothorax is the “lung point,” a transition zone on the chest wall where lung sliding disappears, representing the edge of the collapsed lung. With a reported sensitivity of over 90%, ultrasound is particularly valuable for critically ill patients or children, often outperforming supine chest radiography [ 22 ] ( Figure 5 ). CT is the definitive imaging modality for the diagnosis of SP and, crucially, for the characterization of its underlying causes. Energy-Integrating Detector (EID)-CT: The standard protocol is a non-contrast, thin-slice (1.0–1.25 mm) acquisition reconstructed with a lung algorithm. Multiplanar reformats (MPRs) are essential and superior to radiography for delineating the visceral pleura, quantifying pneumothorax size, and detecting subtle etiologies like subpleural blebs. CT clearly distinguishes pleural gas from the confined gas of a pneumomediastinum. Photon-Counting Detector (PCD)-CT: While EID-CT remains the standard, PCD-CT is an emerging technology offering inherently higher spatial resolution and lower image noise ( Figure 6 ). These technical advantages, demonstrated in early studies, could theoretically improve the detection of small blebs and allow for reduced radiation dose. However, robust clinical evidence from large-scale prospective studies is still needed to determine if these improvements translate to better patient outcomes or changes in management [ 23 , 24 , 25 ]. MRI is not typically used for the primary diagnosis of spontaneous pneumothorax (SP) because of its longer scan times, lower spatial resolution for lung tissue, and susceptibility to motion artifacts. Its main role is limited to a problem-solving tool in specific cases, such as when SP is found incidentally during a scan for other reasons (e.g., cardiac or spine evaluation), or when CT is contraindicated (e.g., in pregnancy) ( Figure 7 ). MRI can visualize pleural surfaces and fluid without ionizing radiation. Therefore, its routine use for SP diagnosis is not recommended [ 21 ].

Accurate identification of pneumothorax is crucial, but several entities can mimic its appearance on imaging, leading to potential diagnostic errors. Radiologists should be aware of these common pitfalls on both chest radiography (CXR) and computed tomography (CT) ( Figure 8 illustrates some of these common mimics and their differentiating features). Skin Folds: These can create a pseudo-pleural line. They are differentiated by often extending beyond the thoracic cavity, appearing thicker or less sharp than a true pleural line, and having visible lung markings peripheral to them [ 26 ]. Bullous Disease/Large Cysts: Apical bullae are often misinterpreted as a loculated pneumothorax. A bulla is favored by a convex inner margin and the presence of faint lung markings or septations within the lucency [ 26 ]. Medical Equipment: Overlying lines from catheters, ECG leads, or patient gowns can create confusing linear opacities and require careful correlation with equipment positioning [ 26 ]. Artifacts: Motion or image processing can occasionally create spurious lines that mimic a pleural edge. On CT, care must be taken to differentiate pneumothorax from the following mimics: Large Bulla: Differentiating a large bulla from a loculated pneumothorax can be challenging. A bulla is favored by its intraparenchymal location (surrounded by attenuated lung), a convex inner margin, and the absence of a distinct visceral pleural line separate from the bulla wall. Multiplanar reformats (MPRs) are invaluable for this assessment [ 26 ]. Pneumomediastinum: Extensive pneumomediastinum can dissect along fascial planes adjacent to the pleura, mimicking a paramediastinal pneumothorax. CT will clearly show the gas to be confined within mediastinal planes rather than free in the pleural space. Congenital Pulmonary Airway Malformations (CPAMs): In pediatric patients, large cystic CPAMs can be confused with pneumothorax. The presence of discernible cystic walls and internal septations helps differentiate a CPAM [ 27 ]. Peripheral Cavitary Lesions: A large, peripheral cavitating pneumonia or neoplasm can mimic a hydropneumothorax. A thicker, irregular wall and associated parenchymal abnormalities are clues to the correct diagnosis [ 26 ]. Panlobular Emphysema: Conditions like alpha-1 antitrypsin deficiency can cause diffuse hyperlucency that may obscure a small coexisting pneumothorax ( Figure 9 ). Careful inspection for the visceral pleural line is key [ 28 ].

Pulmonary infections are a well-established cause of SSP, typically resulting from parenchymal necrosis, cyst formation, and subsequent visceral pleural rupture. Key infectious etiologies include tuberculosis, COVID-19, and opportunistic infections like Pneumocystis jirovecii pneumonia. In endemic regions, tuberculosis is a leading cause of SSP, occurring in approximately 1% of patients with active disease, often in its advanced stages [ 21 , 29 ]. The mechanism involves the rupture of a tuberculous cavity or bleb into the pleural space ( Figure 10 ). Common complications in these patients can include the formation of a bronchopleural fistula and empyema ( Figure 10 ). SP emerged as a notable complication during the COVID-19 pandemic, with a reported incidence of 1–1.4% in hospitalized patients. It is thought to result from diffuse alveolar damage, cystic lung changes, or barotrauma, particularly in patients requiring positive pressure ventilation [ 30 ]. The most common underlying CT findings of COVID-19 pneumonia are bilateral, peripheral ground-glass opacities (GGOs) or mixed consolidation and GGO ( Figure 11 ). Nodular opacities with a peribronchovascular distribution are also seen [ 31 ]. PJP, an opportunistic infection common in individuals with Acquired Immunodeficiency Syndrome (AIDS), carries a significant risk of SSP [ 32 ]. The primary mechanism is the rupture of thin-walled intrapulmonary cysts or pneumatoceles, a finding that can lead to pneumothorax in up to one-third of affected patients [ 21 , 32 ]. The classic imaging feature of PJP is diffuse, bilateral GGO, though consolidation, cysts, and nodules can also develop [ 33 ]. Necrotizing bacterial pneumonias caused by organisms such as Staphylococcus aureus , Klebsiella pneumoniae , and Pseudomonas aeruginosa can also lead to SSP through parenchymal destruction and fistula formation ( Figure 12 ). Additionally, the rupture of pulmonary hydatid cysts is a rare infectious cause of pneumothorax [ 21 ]. Chronic obstructive pulmonary disease (COPD) is the leading cause of SSP, resulting from the rupture of apical blebs or bullae. CT confirms the pneumothorax and demonstrates the characteristic underlying emphysema, bronchial wall thickening, and bullae [ 6 , 34 ] ( Figure 13 ). In cystic fibrosis (CF), SP has a high incidence and recurrence rate, caused by the rupture of apical subpleural cysts. Thoracic CT is characteristic, revealing the culprit cysts along with upper-lobe predominant bronchiectasis, bronchial wall thickening, and mucoid impaction [ 35 , 36 , 37 ] ( Figure 14 ). SP is an infrequent complication of asthma, typically resulting from barotrauma. During a severe exacerbation, intense coughing or air trapping can lead to alveolar rupture. While chest imaging may be normal between attacks, CT performed during an acute episode often demonstrates the underlying bronchial wall thickening, mucous plugging, and air trapping that contribute to this risk [ 38 , 39 ] ( Figure 15 ). Malignancy accounts for up to 10% of SSP cases and may be the initial manifestation of an occult cancer [ 40 , 41 ]. The pneumothorax typically results from necrosis of a subpleural lesion, cavitation, or as a complication of therapy [ 40 , 42 , 43 ]. While most associated with sarcomas and primary lung cancer, cavitary metastases from other tumors can also be the cause [ 44 , 45 , 46 ]. CT is critical for identifying the responsible underlying nodules, masses, or cavitary lesions [ 42 ] ( Figure 16 and Figure 17 ). Kaposi sarcoma (KS), a Human Herpesvirus 8 (HHV-8) associated malignancy seen in patients with acquired immunodeficiency syndrome (AIDS), can also lead to SSP. While pulmonary KS has many imaging manifestations, pneumothorax is a less common but important complication. It is thought to arise from the necrosis of a subpleural KS nodule or from bulla formation secondary to parenchymal destruction by the tumor. CT will demonstrate the underlying ill-defined pulmonary nodules distributed along the bronchovascular bundles, in addition to the pneumothorax itself [ 47 ] ( Figure 18 ). Interstitial lung diseases (ILDs) encompass a diverse group of disorders characterized by inflammation and fibrosis of the lung parenchyma. Spontaneous pneumothorax is a well-recognized complication, often portending a poor prognosis. While some forms are idiopathic (e.g., IPF) or associated with systemic granulomatous disease (e.g., sarcoidosis), ILD is also a frequent and significant manifestation of several connective tissue diseases. Idiopathic pulmonary fibrosis (IPF) is the second-most-common cause of SSP, occurring in 2–20% of patients and portending a poor prognosis [ 48 ]. The pneumothorax is typically caused by the rupture of subpleural honeycombed cysts or fibrotic blebs. CT is essential for diagnosis, revealing the characteristic usual interstitial pneumonia (UIP) pattern of peripheral and basilar-predominant interstitial fibrosis, traction bronchiectasis, and honeycombing that underlies the pneumothorax ( Figure 19 ). SP is a reported, albeit uncommon, complication of lymphoid interstitial pneumonia (LIP), an interstitial lung disease associated with autoimmune disorders such as Sjogren’s syndrome and AIDS. The pneumothorax is thought to result from the rupture of the thin-walled perivascular cysts that are a key feature of the disease. CT typically demonstrates these cysts, which are often accompanied by ground-glass opacities and centrilobular nodules, predominantly in the lower lung zones [ 49 , 50 , 51 ]. Sarcoidosis is a rare cause of SP (~2% of patients) but is an important consideration in young non-smokers. The mechanism is typically rupture of a subpleural bleb or necrosis of a granuloma, usually in late-stage fibrotic disease [ 52 ]. CT reveals the underlying cause by demonstrating the characteristic upper- and mid-lung zone predominant fibrosis, architectural distortion, and honeycombing [ 53 ] ( Figure 20 ). The diffuse cystic lung diseases are significant causes of secondary spontaneous pneumothorax (SSP). In these conditions, the common underlying mechanism is the rupture of the characteristic thin-walled cysts, leading to air leakage into the pleural space PLCH is a rare, smoking-related disease where SP is a common presenting feature, particularly in young adults [ 54 , 55 ]. Since pneumothorax can be the initial manifestation of the disease, CT evaluation is crucial. The characteristic CT findings are a combination of nodules and bizarrely shaped cysts, with a distinct predominance for the upper and mid-lung zones and relative sparing of the lung bases [ 54 , 55 , 56 ] ( Figure 21 ). Spontaneous pneumothorax (SP) is a hallmark of lymphangioleiomyomatosis (LAM), a cystic lung disease that occurs almost exclusively in women of reproductive age, either sporadically or in association with tuberous sclerosis complex (TSC). CT is diagnostic, revealing numerous thin-walled, round cysts distributed diffusely throughout the lungs, surrounded by otherwise normal-appearing lung parenchyma [ 57 , 58 , 59 ] ( Figure 22 ). Birt–Hogg–Dubé syndrome (BHDS) is an autosomal dominant disorder characterized by a triad of lung cysts, renal neoplasms, and skin lesions [ 60 ]. SP is a major feature of the syndrome, affecting 22–41% of individuals due to the rupture of the characteristic thin-walled pulmonary cysts [ 61 , 62 ]. On CT, these cysts are a key diagnostic clue, typically appearing as elliptical or lentiform shapes with a notable basilar and subpleural distribution [ 17 , 63 ] ( Figure 23 ). SP in rheumatoid arthritis (RA) results from ruptured necrobiotic nodules or blebs in the setting of RA-ILD. CT identifies the pneumothorax and underlying cavitary nodules or fibrotic changes [ 64 , 65 , 66 ] ( Figure 24 ). A rare complication of systemic sclerosis-associated ILD, SP is caused by the rupture of bullae or honeycombed cysts. Key CT findings include the underlying fibrotic ILD and an often strikingly dilated esophagus [ 67 , 68 ] ( Figure 25 ). SP is a rare manifestation, resulting from the formation and rupture of apical bullae in the context of upper-lobe fibrosis [ 69 , 70 ]. SP is a key diagnostic feature of Marfan’s syndrome, caused by the rupture of apical bullae from an inherent collagen defect. Radiologists may suggest the diagnosis when a pneumothorax is seen with a dilated aortic root, scoliosis, or pectus deformity [ 17 , 71 , 72 , 73 ] ( Figure 26 ). In vascular EDS (vEDS), poor tissue integrity leads to a high prevalence of SP, often with concomitant hemothorax. Associated life-threatening arterial aneurysms or dissections may also be evident on chest imaging [ 17 , 74 ] ( Figure 27 ). Although rare overall, catamenial pneumothorax is the most frequent cause of recurrent spontaneous pneumothorax in women of reproductive age and therefore warrants specific consideration. As noted in Table 1 , while rare, it carries an extremely high risk of recurrence tied to the menstrual cycle. It is defined by the occurrence of pneumothorax in close temporal relationship with menstruation (typically within 72 h of onset). The underlying cause is thoracic endometriosis, where endometrial implants are most often found on the right hemidiaphragm and pleura [ 75 ]. While pneumothorax is the most common manifestation, imaging may also reveal pleural effusion, hemothorax, or diaphragmatic nodules. CT is the primary modality for identifying these associated findings. MRI is particularly valuable as it can identify the high T1 signal characteristic of subacute hemorrhage within endometriotic implants on the pleura or diaphragm, which may not be apparent on CT [ 75 , 76 , 77 , 78 , 79 ] ( Figure 28 ). Spontaneous pneumothorax is a recognized complication of electronic cigarette or vaping use-associated lung injury (EVALI). The mechanism is not fully elucidated but is thought to involve intense airway inflammation and alveolar damage from inhaled toxins, leading to the formation of subpleural blebs that are prone to rupture. While the underlying CT findings of EVALI are non-specific and can mimic organizing pneumonia or hypersensitivity pneumonitis, the presence of apical paraseptal emphysema and blebs should raise suspicion in this clinical context [ 80 , 81 ] ( Figure 29 ). Other acquired causes of SP include drug-induced pneumothorax, a rare complication of medications like bleomycin or methotrexate that cause parenchymal fragility [ 82 ]. Barotrauma from mechanical ventilation, diving, or altitude changes can lead to alveolar rupture, often via the Macklin effect [ 83 , 84 , 85 ]. Finally, severe metabolic disturbances, most notably anorexia nervosa, are known to cause emphysema-like changes that increase alveolar fragility from profound malnutrition [ 86 , 87 ]. SP occurs in 5–10% of patients with NF1 due to the rupture of apical bullae associated with NF1-related cystic lung disease. The diagnosis should be suspected when a pneumothorax is seen with thoracic stigmata like neurofibromas or ribbon-like rib deformities [ 17 , 88 , 89 , 90 , 91 , 92 ] ( Figure 30 ). SP is an exceedingly rare complication of IgA vasculitis (formerly Henoch–Schönlein purpura, HSP), with a proposed mechanism involving pulmonary capillaritis and subsequent subpleural bleb formation [ 93 ] ( Figure 31 ). In Loeys–Dietz syndrome, a genetic connective tissue disorder, SP can be an occasional presenting feature. The diagnosis is suggested by associated vascular findings (arterial tortuosity, aneurysms, dissections) and skeletal abnormalities (pectus deformities, scoliosis) [ 94 , 95 ].

The etiologies of SP vary significantly at the extremes of age. In children and neonates, while over half of cases are primary (PSP), secondary causes related to infections, cystic fibrosis, or asthma are common. Pulmonary interstitial emphysema is a key factor in neonates with respiratory distress syndrome [ 96 , 97 ]. In the elderly, SP is overwhelmingly secondary (SSP), most commonly from COPD, and is associated with significantly higher morbidity and mortality, especially when related to underlying interstitial pneumonia [ 98 ].

Pneumothorax signifies intrapleural gas accumulation between the visceral and parietal layers. It can be broadly classified as traumatic/iatrogenic or spontaneous. This review will focus on spontaneous pneumothorax (SP), which is further subdivided into two distinct categories. Primary spontaneous pneumothorax (PSP) occurs without any clinically apparent underlying lung disease, whereas secondary spontaneous pneumothorax (SSP) develops as a complication of pre-existing pulmonary conditions. While PSP is a well-defined clinical entity, the causes of SSP are numerous and varied, often posing a diagnostic challenge. The purpose of this pictorial review is to provide a comprehensive overview of the diverse etiologies of spontaneous pneumothorax, with a particular focus on SSP, and to highlight the crucial role of modern diagnostic imaging in uncovering these underlying causes. Understanding the clinical context and imaging findings is paramount, as the clinical sequelae of pneumothorax range widely from incidental detection to life-threatening circulatory collapse [ 1 , 2 , 3 , 4 , 5 , 6 ] ( Figure 1 ). The estimated annual incidence of SP in males ranges from 4.2 to 16.8 per 100,000, whereas in females it ranges from 3.8 to 9.8 per 100,000 annually, with documented geographic variability [ 7 , 8 , 9 ]. SP has a varied age distribution, with PSP predominantly occurring in adolescents and young adults (15–34 years) and SSP predominantly occurring in children or adults over 55 years of age [ 7 , 10 ]. SSP has a higher recurrence rate ranging from 40% to 56% as compared to 16% to 54% in PSP [ 11 , 12 , 13 , 14 , 15 , 16 ]. In this article, we review the categorization, etiologies, and imaging modalities for SP, with the main emphasis on familiarizing radiologists with the various disorders and associated key imaging features that may indicate potential underlying culprit etiologies for SP.

Secondary spontaneous pneumothorax (SSP) presents a diagnostic challenge due to its vast array of underlying causes beyond COPD. Identifying the specific etiology is crucial for management and predicting recurrence, making the radiologist’s role vital. While chest radiography is the first step, CT remains superior for detecting subtle pathologies. A thorough understanding of the diverse causes of SSP, paired with appropriate imaging, is essential for improving patient outcomes. Future research should focus on the clinical impact of emerging technologies like photon-counting CT and the integration of genetic testing.