Emphysema represents a condition of the lungs characterized by “permanent, abnormal enlargement of air spaces (the areas of the pulmonary acini – the respiratory units of the lungs), distal to the terminal bronchiole (the smallest airway without respiratory components in its walls), accompanied by the destruction of their (the respiratory unit’s) walls”. Although some have indicated that this definition includes the concept that these changes are “without obvious fibrosis”, the fact is, that paracicatricial emphysema, as well as some paraseptal and centrilobular emphysematous changes are associated with fibrosis and thus, that aspect of the definition has been dropped. There is however, no universal agreement on what constitutes the most reliable method to assess “abnormal enlargement” of the air spaces distal to the terminal bronchial and to quantify the “destruction” of their walls.
Four primary types of emphysema: centrilobular, paraseptal, panlobular and paracicatricial, are described based upon microscopic/histiologic criteria, which although not seen on imaging studies, is helpful in trying to give a general classification to the system. There are, in addition to the four main categories, several sub-classifications and abnormal findings associated with emphysema. Emphysema is also graded pathologically as mild, moderate or severe. Classifications of emphysema overlap visually and thus, the significance and type of emphysema observed is made based upon the patient’s history, including whether he or she smoked, the clinical examination, pulmonary function testing and imaging findings, taken altogether.
Understanding the Anatomy:
The airways are made up of hollow tubes that conduct air to and from the lungs. They are lined by pseudostratified, ciliated, columnar epithelium. Cartilage supplies support and prevents collapse of these air tubes. An additional muscle coat and submucosal glands contribute to their structure. Airways are divided into bronchi and bronchioles. The bronchi are the larger tubes having cartilage within their walls and the bronchioles are the smaller tubes that do not have cartilage within their walls. However, cartilage within the bronchi becomes sparser towards the periphery, where it only is present at bifurcations. The mainstem bronchus bifurcates into the segmental bronchi, which bifurcate into the large subsegmental bronchi, which bifurcate into the small bronchi, which bifurcate into the terminal bronchioles (the last air conducting tubes, which do not have cartilage in their walls and do not have respiratory or air exchanging components associated with them), which bifurcate into the acini (the respiratory units of the lungs) consisting of respiratory bronchioles (small air tubes with alveoli or gas exchanging components directly entering into them), alveolar ducts, alveolar sacs and alveoli.
The pulmonary acinus is the respiratory unit of the lung, which is supplied by a terminal bronchial. A secondary pulmonary lobule contains 3 to 5 terminal bronchioles (small air tubes, which do not have respiratory or gas exchanging components associated with them), each of which is connected to the respiratory portion of the lung known as the acinus, containing 3 to 8 generations of respiratory bronchioles with their associated alveolar ducts, alveolar sacs and accompanying alveoli. The secondary pulmonary lobules vary in size and shape and may be up to 1 cm in diameter. Respiratory bronchioles look similar to terminal bronchioles, but have alveoli opening directly into their lumen. Although emphysema is often classified histologically based upon the changes to the pulmonary acinus or respiratory unit of the lung, radiographically it is preferred to describe the findings in relationship to the secondary pulmonary lobules, clusters of 3 to 5 terminal bronchioles with their attached acini.
People with emphysema have obstructive lung disease. This is manifested physiologically by both airflow obstruction and a decrease in diffusion capacity. The airflow obstruction occurs when the airways collapse on expiration due to the destruction of lung tissue and loss of airway tethering and support. Loss of diffusion capacity occurs due to the destruction of both the lung tissue and its pulmonary vascular bed.
Chronic obstructive pulmonary disease (COPD), represents chronic irreversible airway destruction secondary to a combination of emphysema and chronic bronchitis. It is not a non-specific term. Emphysema is a more specific term and the types and locations of emphysema (centrilobular, paraseptal, panlobular and paracicatricial) are even more specific. Smokers can and often have both chronic bronchitis and emphysema simultaneously.
Chest X-Ray Changes in Emphysema:
The vast majority of smokers have no findings on chest x-rays. However, as smoking-caused emphysema develops, in some, there are findings on chest x-rays that can point to such a diagnosis.
There are two primary findings on chest x-rays with emphysema, which are:
1. Increased lung volumes, i.e., hyperaeration or over or hyperinflation.
2. Lung destruction (bullae or decreased vascularity, which gives increased lung lucency and decreased vessel size and presence).
In end-stage disease, emphysema and chronic obstructive pulmonary disease can result in pulmonary arterial hypertension (PAH) – which causes an increase in the pulmonary arterial size and eventually, cor pulmonale (cp) – which has right ventricular enlargement and resulting cardiomegaly.
Direct X-Ray Signs of Emphysema:
1. Bullae with defined walls.
2. Irregular areas of radiolucency.
Indirect X-Ray Signs of Emphysema:
1. Hyperaeration or Over/Hyperinflation:
Increased lung volumes and/or overinflation is the most common finding on chest x-rays, however, it is non-specific and can often occur in non-smokers such as persons capable of and taking a good inspiratory effort; persons with greater overall lung capacity, i.e., runners, divers or people living at high altitudes; persons with elongated body shapes as an anatomic variation or due to disease such as with Marfan’s Syndrome (Abraham Lincoln’s disease) or those with asthma. Hyperaeration or over/ hyperinflation is visualized by several variables on chest x-rays, including measurements used by some authors:
a) Increased Lung Height: When the measurement of the dome of the right hemi-diaphragm to the tubercle of the first rib measures greater than or equal to 30 cm, this is considered overinflation.
b) Flattening of the Diaphragm: Measured by drawing a line on the PA view between the costophrenic and cardiophrenic angles and determining whether or not the diaphragm is less than 1.5 cm above that line. If it is, then the diaphragm is flattened. This can also be determined on the lateral view by drawing a line from the anterior costophrenic to the posterior costophrenic angles and likewise, determining whether or not the diaphragm is less than 1.5 cm (by some authors to 2.7 cm) above that line. The costophrenic angle can also be called the vertebrophrenic angle and the anterior costophrenic angle on the lateral view can be known as the sternophrenic angle.
c) Increased Retrosternal Air Space: When the horizontal distance between the posterior margin of the sternum and the anterior margin of the ascending aorta measures greater than 2.5 cm (some say as much as 4.5 cm) in distance and where the horizontal distances are measured at 3 cm beneath the angle of Louis (sternomanubrial junction), increased retrosternal air space enlargement is considered.
d) A Low Position of the Diaphragm: When the dome of the diaphragm is at or below the anterior end of the right seventh rib, this is considered overinflation.
e) Air Space Below the Cardiac Contour: Overinflation is associated with visualization of lungs extending beneath the cardiac silhouette.
f) Narrowing of the Heart Diameter: The narrow heart width of less than 11.5 cm is often due to overinflation. This can also be seen in other conditions including Addison’s disease.
g) Visualized Diaphragmatic Muscle Slips: Is a sign of overinflation.
h) Obtuse Costophrenic Angles: Which appear “pseudo-blunted” are also result of overinflation.
2. Alternations in the Vascular Pattern:
These findings can present on chest x-rays in several ways including:
a) Local Avascular/Translucent Areas: Caused by destroyed lung tissue from emphysema and bullae formation.
b) Decreased Peripheral Vascular Markings: Given bullae and emphysematous holes, destroyed lung tissue receives and needs a lesser vascular supply, resulting in decreased peripheral markings.
c) Distortion of Vessels: Straightened, stretched, elongated, curved and pruned vessel appearances may be identified.
d) Increased Branching Angles: This is caused by vascular bifurcations stretched about the bullae, which widen the vascular bifurcation angles, distorting them.
In the days prior to CT, a positive chest x-ray study for emphysema was considered present, when two of the four below categories were identified:
Flattening of the diaphragms on the PA view.
Irregular areas of radiolucency on the PA view.
An increased retrosternal air space on the lateral view.
Flattening of the diaphragms on the lateral view.
CT and HRCT Findings In Emphysema:
CT and HRCT utilize cross-sectional imaging, removing overlapping and volume averaged structures as seen on chest x-rays and especially when utilizing the high resolution lung processing algorithm, markedly improves the imaging visualization of changes of emphysema. Supine HRCT images with thinner slices can be reconstructed from the standard supine spiral CT, to give better visualization of emphysema, compared to CT and further improve harder to see emphysema presentations. But, understand, that at best, HRCT_looks like a gross cut specimen in an autopsy laboratory and is not a microscopic examination. As a result, CT and HRCT can underestimate or miss mild emphysema or alternatively, the visual assessment of apparent emphysema can overestimate the extent of disease. An additional way to improve visualization of emphysema by imaging, is the use of the Minimal-Intensity Projection Reformations (MinIP). This technique suppresses visualization of the pulmonary vessels and optimizes visualization of the low attenuation areas of emphysema.
The Main CT/HRCT Classifications of Emphysema:
1. Centrilobular Emphysema (also known as proximal acinar or centriacinar emphysema). Centrilobular emphysema affects the proximal aspects of the lung acinus, primarily in the areas of the respiratory bronchioles. It is considered secondary to smoking and initially involves the right and then later the left upper lung zones. Its CT/HRCT appearance is one of focal areas of low attenuation holes without walls, measuring millimeters to 1 cm in size in the central portions of the lung, away from the periphery or fissures. It may have a “moth-eaten” or “Swiss-cheese” appearance. A distinguishing feature is that pulmonary vessels can often be seen within the areas of low attenuation. Centrilobular emphysema is most pronounced in the right greater than left upper, greater than middle, greater than lower lung zones. With progression, bullae developed, which do have walls and result in visualized paraseptal emphysema and subpleural bullae. Severe centrilobular emphysema results in confluent holes that look like panlobular emphysema, but still, some centrilobular disease in the apical areas are often identified. Classic panlobular disease has an uneven distribution of confluent bullae. Because the secondary pulmonary lobule contains multiple (3 to 5) acini, centrilobular emphysematous foci are scattered in the lobule and although appearing to be centrally located, may not be always be histologically.
2. Paraseptal Emphysema (also known as distal acinar emphysema). Paraseptal emphysema affects the distal aspects of the lung acinus – the alveolar ducts and sacs in the lung periphery, with areas of destruction often marginated by the interlobular septa. It is either idiopathic or secondary to smoking. The CT/HRCT appearance is easily recognizable by the emphysematous low attenuation spaces seen adjacent to the pleura and interlobular septa with well-defined walls having a “saw-tooth” appearance. It is most frequently seen along the peripheral chest wall, adjacent the midline upper mediastinum and azygoesophageal recess and along the fissures. It also can cause the interlobular septa to become prominent, mimicking lymphangitic carcinomatosis on chest x-rays, but is easily separable on CT/HRCT. Coalescence of disease results in subpleural bullae, especially in the apices of the upper lung zones, which can rupture and result in pneumothorax (px). Although subpleural bullae are commonly associated with paraseptal emphysema, they are seen all types of emphysema as well as in an isolated phenomenon, usually in small numbers in the lung apices, unrelated to emphysema. Apical subpleural bullae are present in 80% to 90% of patients with spontaneous pneumothorax. Paraseptal emphysema can at times be difficult to differentiate from honeycombing. Ways to differentiate the two, which do not always work, include the paraseptal holes forming a single row of holes adjacent to the chest wall, being most severe in the apices with occasional holes (often called “cystic” spaces) greater than 1 cm in diameter. Honeycombing often forms multiple rows adjacent the chest wall, is most severe at the lung bases rather than the apices and usually has holes (“cystic“ spaces) less than 1 cm in diameter.
3. Panlobular Emphysema (also known as panacinar emphysema). Panlobular emphysema affects all components of the lung acinus and involves the entire secondary pulmonary lobule containing 3 to 5 pulmonary acini. It is most frequently secondary to alpha-1-anti-trypsin deficiency (also called alpha-1-protease inhibitor deficiency). It is classically considered the emphysema of non-smokers. However, it can occur in elderly individuals with distal bronchiole and bronchiolar obliteration, in IV drug addicts with talcosis, and patients with Swyer James (MacLeod’s) Syndrome and even occasionally in smokers. This has a lower lung zone predominance, greatest in the left lower lung zone. There are widespread areas of abnormal low attenuation areas with decreased vascularity. Thus, it can be easily overlooked, since it is diffuse in lacking juxtaposed contrasting densities. Left lower lung zone is more involved than the middle and upper lung zones. Patterns of panlobular and centrilobular emphysema frequently co-exist in the same individual. Usually with alpha-1-anti-trypsin deficiency, there is greater loss of lung caused by infection than seen in the general population and these patients can develop bronchiectasis and bronchial wall thickening. Bullae are less frequent in this type of emphysema compared to other types.
4. Paracicatricial Emphysema (also known as cicatricial or irregular emphysema). This emphysema is most frequently associated with patients having complicated types of pneumoconioses with progressive massive fibrosis (PMF), but can occur adjacent to a localized parenchymal scar, with diffuse pulmonary fibrosis or after an episode of pneumonia. It can be called irregular air space enlargement. It is not localizable to a specific site in the acinus and is associated with fibrosis.
5. Other Sub-Categories of Emphysema:
a) Bullous Emphysema (also known as Bullous Lung Disease): Bullous emphysema is so named when there are multiple large bullae (holes) associated with a compromise in pulmonary function. It is usually associated with concomitant emphysema, although occasionally, it can be familial (thus, the designation of “Bullous Lung Disease” rather than “Bullous Emphysema”). Bullae are thin-walled, sharply-demarcated areas of lung destruction measuring 1 cm or more, with avascularity and thin walls measuring less than 1 mm in thickness. The bullae are commonly subpleural in location and can occur in any of the four types of emphysema, but most commonly in the centrilobular and paraseptal types. The bullous lesions can be uni or multi-locular with a septated space. When looking at an autopsy removed lung, a bullous lesion can be seen as a localized empty sac or bag-like protrusion from the lung surface, because they are most commonly subpleural in location. Bullae can be associated with pneumothorax formation, infection and/or hemorrhage. They can occasionally disappear, especially after infection or hemorrhage, or even on a spontaneous idiopathic basis. Bullous emphysema has three types:
Subpleural Type: These subpleural bullae are the most common types. They contain only gas with no alveolar remnants or blood vessels. They are often located in the apex of the upper lung zone and along the costophrenic rim of the middle lobe and lingula, but may be seen in the vicinity of parenchymal scars.
Superficial Type: The bullae are found along the anterior edge of the upper and middle lung zones or lingula, and over the diaphragms. They contain blood vessels and strands of partially-destroyed lung.
Deep Type: These bullae are found within the lung substance and contain strands of partially-destroyed lung tissue and blood vessels.
b) Giant Bullous Emphysema (“Vanishing Lung Syndrome” or “Primarily Bullous Disease of the Lungs”): This condition is often seen in younger males who show large progressive upper lung zone bullae that are often asymmetric. The giant bullous lesions occupy greater than or equal to one third of the hemithorax. Oftentimes, the adjacent normal lung is compressed, becoming atelectatic, making it difficult for that portion of the lung to appropriately function. Spontaneous pneumothorax is a common consequence of this disease. Other etiologies for Vanishing Lung Syndrome include pulmonary sarcoid or Langerhans cell histiocytosis.
c) Focal Dust Emphysema (also known as Focal Emphysema of Silicosis): This is focal emphysema surrounding silicotic nodules. This is a chemotoxic destruction of lung tissue and not smoking caused, although those with silicosis can also be smokers with emphysema not related to this phenomenon.
d) Congenital Lobar Emphysema: Congenital lobar emphysema (CLE) usually presents in the first month of life, associated with respiratory distress. There is marked overinflation of the lobe – left upper > right middle > right upper lobes. Lower lobe involvement is rare. There is often mediastinal shift away from the overinflated lobe. Etiology is usually, but not always, partial or complete bronchial obstruction from such causes as deficient cartilage within the bronchus, an extrinsic compression such as by an anomalous vessel or bronchogenic cyst or problem involving the lumen itself, such as a mucosal fold. There is overinflation and air trapping resulting in a hyperlucent lung, although retained fetal lung fluid can leave it looking opaque. Resection of the abnormal lobe is often required.
e) Interstitial Emphysema: Air can escape the lungs through injury too and rupture of the alveoli (barotrauma) when mechanical ventilation occurs. Air dissects into the lung tissue/interstitium, as collections of perivascular air or as air bubbles within areas of consolidation, which is seen on CT/HRCT, but not usually seen on chest x-rays. This air continues to dissect and make its way into the mediastinum causing a pneumomediastinum (which can be visible on chest x-ray) and can also continue into the subcutaneous soft tissues of the chest wall as subcutaneous emphysema (also visible on chest x-rays).
My Training and Acknowledgments:
The above diagrams and articles are based upon my education, training and experience as well as from multiple peer-reviewed journal articles, textbooks and lectures.
Medicine is an “art” based upon “science.” We as educators and clinicians are constantly learning and updating our teachings and knowledge base. Certainly, by my presentation I have added additional “art” by providing the visual diagrams (no pun intended) to the “science” with the hope that it will help further clarify the learner’s understanding of what Radiologists look for when observing emphysema.
Unfortunately, many authors use individualized coined terms and some terms with the same name, mean different things to different authors. There is no easy fix. I have tried to standardize the language. The bottom line is, that imaging studies are part of the “Sherlock Holmes” investigation of a patient’s disease – identifying if the anatomy is normal or abnormal and trying to direct one to a specific diagnosis, when possible.
This article is provided as a public service by Daniel Powers, M.D.: B-Reader, Board-Certified Diagnostic Radiologist, Certified by the American Board of Radiology.
If you detect any errors, have additional information to point me to, use other useful terms or have comments, please do e-mail them to me at firstname.lastname@example.org.