Centrilobular emphysema and paraseptal emphysema are two distinct patterns of lung tissue destruction that differ in where they strike within the lung’s smallest functional units, what drives them, and how much clinical trouble they cause. Centrilobular emphysema, the more common and generally more consequential form, destroys tissue around the tiny airways at the center of each lung lobule and is tightly linked to smoking. Paraseptal emphysema attacks the outer edges of the lobule, near the lung’s surface and the walls dividing one lobule from the next. The two can coexist in the same person, and understanding what separates them matters for gauging disease severity, cancer risk, and the likelihood of complications like a collapsed lung.
Where Each Type Destroys Tissue
Your lungs are organized into thousands of small units called secondary pulmonary lobules. Each lobule has a center, where a terminal bronchiole feeds air into the gas-exchange region, and a periphery, bordered by thin connective tissue walls (interlobular septa) and the pleural membrane lining the lung surface. Centrilobular emphysema targets the center of the lobule, destroying the walls of the respiratory bronchioles and the surrounding alveoli closest to them. Paraseptal emphysema, by contrast, involves the distal part of the lobule adjacent to the pleura and the interlobular septa, leaving the center relatively intact.
On high-resolution CT scans, the difference is usually easy to spot. Early centrilobular emphysema shows up as tiny, evenly distributed areas of low density with ill-defined borders scattered through the upper lobes. Paraseptal emphysema appears as well-defined cystic spaces lined up along the pleural surface, often in a single row just under the lung’s outer edge.
Causes and Risk Factors
Smoking is the dominant cause of centrilobular emphysema. In a study of current smokers who still had normal airflow on standard lung function tests, about 27% already showed centrilobular changes on CT.
Paraseptal emphysema also occurs in smokers, but its relationship with smoking is less straightforward. It tends to appear either in isolation or combined with centrilobular or panlobular emphysema in people with smoking-related COPD. But paraseptal emphysema is also the predominant pattern seen in certain connective tissue disorders. Marfan syndrome, for instance, is associated primarily with paraseptal emphysema rather than the centrilobular type typically found in smokers. Over time, the subpleural cystic spaces of paraseptal emphysema can merge into large bullae, placing people with Marfan syndrome at heightened risk for secondary pneumothorax.
Beyond smoking and connective tissue disease, occupational exposures play a role that is sometimes overlooked. A large cross-sectional study of more than 27,000 people found that workplace exposure to inorganic dust was associated with emphysema even among never-smokers. The association was strongest when emphysema appeared alongside reduced gas-transfer capacity, suggesting the dust was damaging the alveolar surfaces where oxygen crosses into the blood.
How Common Each Subtype Is
Among smokers, centrilobular emphysema is the most frequently encountered pattern. One population-based study estimated that among smokers, the overall prevalence of emphysema was roughly 27%, with centrilobular-predominant emphysema accounting for about 14%, paraseptal-predominant for about 9%, and panlobular-predominant for around 4%. Strikingly, 17% of smokers who did not meet spirometric criteria for COPD already had emphysema visible on CT, and that emphysema was independently linked to reduced walking distance. This finding underscores that emphysema can be present and functionally meaningful well before standard breathing tests flag a problem.
The Mechanism Behind Centrilobular Damage
The tissue destruction in centrilobular emphysema traces back to an imbalance between enzymes that break down proteins (proteases) and the molecules that normally keep those enzymes in check (antiproteases). Cigarette smoke tips this balance by ramping up protease activity in the small airways. Research using animal models of chronic smoke exposure identified a key protective molecule called LEKTI, an anti-serine protease produced in the small airways. In smokers with centrilobular emphysema, LEKTI levels were markedly reduced compared to smokers without emphysema, and in mice, daily antiprotease treatment reversed many of the damaging changes. The implication is that centrilobular emphysema begins when the small airways lose their ability to neutralize the proteases unleashed by smoking.
At a structural level, micro-CT comparisons of lung tissue show that the tiny airways feeding each lobule become thicker-walled and narrower in centrilobular emphysema. The number of alveolar attachments, the tethering points that help hold airways open, drops. The narrowing is most severe in the middle of these airways and is more uneven than in panlobular emphysema, which helps explain why centrilobular disease tends to produce such patchy, heterogeneous destruction on imaging.
Impact on Lung Function and Survival
The two subtypes do not carry equal clinical weight. Research consistently shows that centrilobular emphysema has a stronger association with declining lung function and higher mortality than paraseptal emphysema does. In a study tracking patients with COPD over time, moderate-or-worse centrilobular emphysema was linked to steeper declines in gas-transfer capacity across multiple disease stages, as well as additional loss of airflow and higher ten-year mortality among those with more advanced COPD. Paraseptal emphysema, when present, did not show these same associations.
A separate study of smokers with an unusual spirometric pattern called preserved-ratio impaired spirometry found that centrilobular emphysema was associated with lung hyperinflation (the lungs trapping extra air), while paraseptal emphysema was not. The pattern held even after accounting for age, sex, body size, and smoking history. Taken together, the evidence suggests that centrilobular emphysema is the subtype that most reliably tracks with worsening breathlessness, falling lung function numbers, and risk of death.
Lung Cancer Risk
One of the more striking differences between the two subtypes is their relationship to lung cancer. A systematic review and meta-analysis pooling data from multiple studies found that only centrilobular emphysema was significantly associated with lung cancer, with a pooled odds ratio of about 2.2 compared to people without emphysema. Paraseptal emphysema did not show a clear independent association.
Individual studies have put the numbers in even starker terms. One analysis found that the risk of lung cancer in patients with centrilobular emphysema alone was dramatically elevated compared to patients without emphysema, while paraseptal emphysema alone did not reach statistical significance for increased cancer risk. When the two subtypes coexisted, the risk was intermediate. In regression models, the qualitative severity of centrilobular emphysema, along with pack-years of smoking and age, emerged as significant factors correlated with lung cancer occurrence.
This does not mean paraseptal emphysema is harmless from a cancer standpoint, but the evidence to date points to centrilobular emphysema as the subtype that should raise the most concern during screening discussions.
Paraseptal Emphysema and Spontaneous Pneumothorax
Where paraseptal emphysema does carry its own distinct risk is in the realm of pneumothorax. Primary spontaneous pneumothorax, the sudden collapse of part of the lung without any obvious injury, is mainly caused by the rupture of small subpleural blebs or paraseptal bullae. Because paraseptal emphysema by definition sits right along the lung’s surface, its cystic spaces are in the worst possible position to rupture into the pleural space.
In patients with so-called vanishing lung syndrome (idiopathic giant bullous emphysema), paraseptal emphysema and subpleural bullae were the predominant findings across all cases examined, with individual bullae ranging from 1 to 20 centimeters in diameter. Most fell in the 2 to 8 centimeter range. These giant bullae compress the surrounding normal lung tissue, producing severe breathlessness even though the underlying lung parenchyma may be relatively preserved.
When Both Types Appear Together
Many patients do not have a single pure subtype. Paraseptal emphysema frequently coexists with centrilobular or panlobular emphysema, particularly in long-term smokers with established COPD. When it occurs alongside centrilobular emphysema, the combination tends to look and behave more like centrilobular-predominant disease on lung function testing, but the paraseptal component adds its own layer of risk for bullae formation and pneumothorax.
An interesting finding from research into combined pulmonary fibrosis and emphysema suggests the subtype of emphysema matters for the pattern of scarring as well. In patients with both fibrosis and emphysema, the predominant type of emphysema correlated with the predominant type of fibrosis. The paraseptal group tended to have more extensive fibrosis, more honeycombing, and coarser fibrotic patterns compared to the centrilobular group, which had more emphysema overall but less severe fibrotic changes. This suggests the two subtypes may interact differently with the inflammatory and scarring processes that drive interstitial lung disease.
Genetic Differences Between the Subtypes
The protease-antiprotease imbalance that underlies emphysema has a genetic component, and intriguingly, different gene variants appear to predispose to different subtypes. A study examining genes involved in this balance found that variants in MMP9 and TGFB1 were associated with centrilobular emphysema, while variants in TIMP2 and TNF were linked to paraseptal emphysema and airflow obstruction. People carrying at least one copy of the TIMP2 or TNF risk variants had roughly double the odds of developing paraseptal changes, whereas certain MMP9 and TGFB1 variants actually halved the risk of centrilobular changes in carriers.
These findings fit with the broader understanding that emphysema subtypes are not just different locations of the same disease but may represent partially distinct biological pathways. The genes that influence which type of emphysema develops involve different arms of the tissue-breakdown and tissue-repair machinery, which could eventually matter for targeted treatments.
How Radiologists Grade Severity
Diagnosing and grading emphysema subtypes relies heavily on CT imaging, but the process is less straightforward than it might seem. Visual scoring by radiologists, where a reader looks at the scan and grades the severity and distribution of emphysema, remains the standard for subtyping. Experienced radiologists generally agree well with each other when scoring emphysema, but the story is less reassuring when you look more closely. In the National Emphysema Treatment Trial, even trained chest radiologists showed significant variation in how they quantified emphysema, both between readers and when the same reader scored the same scan twice.
Automated quantification methods, which measure the percentage of lung volume below a density threshold on CT, can reliably detect emphysema in the aggregate but struggle to distinguish subtypes. Deep learning algorithms have shown promise in classifying centrilobular emphysema severity, outperforming older rule-based approaches, though agreement with visual scores for paraseptal emphysema remains only moderate. Newer photon-counting detector CT technology appears to maintain good agreement with standard low-dose CT for detecting anything beyond trace emphysema, and deep learning models applied to these scans show stronger correlation with visual scoring than older density-threshold methods.
For the individual patient, the practical takeaway is that a CT report saying “mild paraseptal emphysema” or “moderate centrilobular emphysema” carries some inherent subjectivity. If the grading matters for a clinical decision, like whether to pursue lung volume reduction, getting a read from a radiologist experienced in emphysema subtyping is worth the effort.
CT Classification of Centrilobular Lesions
Within centrilobular emphysema itself, researchers have described a progression of damage visible on CT. The earliest stage appears as small, round or oval areas of low density with well-defined borders, typically under 5 millimeters, corresponding to dilation of the terminal or respiratory bronchioles. As destruction progresses, the low-density areas become polygonal or irregular with ill-defined borders, reflecting destruction of the proximal alveolar ducts. In the most advanced form, these irregular areas coalesce into larger zones exceeding 5 millimeters, with increasingly blurred boundaries between destroyed and preserved tissue.
This classification, based on careful CT-to-pathology correlation, is useful because it maps the visual appearance on a scan to a specific stage of anatomical destruction. A report describing “confluent centrilobular emphysema” signals a fundamentally different degree of tissue loss than one noting a few scattered centrilobular lucencies, even though both carry the same subtype label.
Emphysema in People Who Have Never Smoked
While smoking dominates the conversation around emphysema, a meaningful minority of cases occur in never-smokers. The large SCAPIS cohort study, which included nearly 14,000 never-smokers, found that occupational exposure to inorganic dust was associated with emphysema in this group, with about a 46% increase in odds. Alpha-1-antitrypsin deficiency, a genetic condition, is the classic non-smoking cause and tends to produce panlobular rather than centrilobular emphysema. Connective tissue disorders like Marfan syndrome preferentially cause paraseptal emphysema. And some individuals develop paraseptal emphysema without any identifiable cause at all, a situation that tends to be clinically mild but occasionally leads to giant bullae or pneumothorax.
For a never-smoker who receives a CT report mentioning emphysema, the subtype provides a clue about what might be driving it. Paraseptal emphysema in a tall, thin young person raises the question of a connective tissue disorder. Panlobular emphysema, especially in the lower lobes, suggests alpha-1-antitrypsin deficiency and warrants blood testing. Centrilobular emphysema in a never-smoker, while uncommon, should prompt questions about occupational dust exposure, secondhand smoke, or biomass fuel use.