Pulmonary patterns attempt to assign an anatomical location of the lesion based on the morphologic changes expected grossly and histopathologically within the lung. It is an attempt to correlate radiology and histopathology. The anatomical locations are grouped into four categories: vascular, bronchial, interstitial, and alveolar. Once the anatomical disease is assigned to one of these locations, the route of entry (aerogenous, hematogenous, or direct extension) and the underlying etiology (e.g., bacteria, virus, fungal, inflammation) can better predicted so that appropriate testing and sampling can be performed and definitive diagnosis obtained.¹

Histological

• Type of pneumonia—Port of entry and location of lesion
• Broncho-pneumonia—Aerogenous: bronchi, then alveoli with interstitial hyperemia
• Interstitial pneumonia—Aerogenous or hematogenous: alveolar walls and interstitium
• Granulomatous pneumonia—Aerogenous or hematogenous: randomly distributed
• Embolic pneumonia—Hematogenous: centered in arterioles and capillaries

Additionally, specific diseases are often not limited to a single, “typical” radiographic pulmonary pattern. For example, edema can range from having an interstitial, alveolar, or even peribronchial pulmonary pattern as fluid shifts and severity of disease progresses or resolves, which reduces the value of using patterns solely as a means of differentiating disease. Pulmonary patterns can be a continuum of each other.

Pulmonary Patterns Defined

Regardless of the deficiencies of the patterned approach to pulmonary description, it remains relevant for differentiating lung disease and remains in the literature. It should be understood that the anatomical implication of each pattern is inaccurate. As mentioned, the pulmonary patterns are alveolar, bronchial, interstitial, and vascular.

Alveolar Pattern

The alveolar pulmonary pattern was defined to describe diseases that are affecting the alveolar lung spaces only.² It is now known that abnormalities are not limited to just the alveolar lung space, but can affect the interstitium, bronchial, and vascular spaces, as well. However, the term persists as a more appropriate term has not been agreed upon.³ Regardless of what the pattern is called, the alveolar lung pattern has a specific set of criteria that must be met in order for pulmonary diseases to be categorized as such. A set of differential diagnoses for the pattern can be narrowed down when taking other descriptors into consideration such as regional distribution and volume of affected lung.

To be classified as an alveolar pattern, the first necessary criterion is that the lung must be more opaque than normal. Alveolar lung disease is one of the most intense/opaque per unit area of the pulmonary patterns. Options for this decrease in aeration are that the alveoli are simply collapsed and do not contain air (atelectasis) or the air is replaced by soft tissue opaque material: fluid, cells, hemorrhage, or pus. This material accumulates in the smaller airways initially and can progress to include lobar bronchi.

Three descriptive hallmarks are seen with alveolar disease of any cause: air bronchogram, lobar sign, and border effacement. Having any one of these three would classify the pulmonary pattern as alveolar—often not all hallmarks are present. An air bronchogram occurs when alveoli are not aerated; i.e., they contain fluid (edema, hemorrhage, pus) or are collapsed while the bronchi remain aerated. Its appearance is likened to a “tree in a snowstorm,” where the branching lucent bronchus is evident against a white backdrop. A lobar sign occurs when alveolar disease is peripheral relative to lobar margins and inconsistent among lung lobes. This allows the margin of the lung lobe to be evident as a relatively distinct line between an affected lobe that is adjacent to a more aerated lobe. Border effacement is the most consistently present hallmark of alveolar disease, and often times, one of the most forgotten. When objects are effaced, their margins can no longer be delineated. In order for this to occur, the objects have to be identical in opacity (usually both are soft tissue/fluid opaque) and they have to be in direct physical contact with each other. In the case of alveolar disease, fluid within alveoli, which is identical in opacity to the heart, pulmonary vasculature, and bronchial walls, contacts any of these structures such that their margins are no longer distinguishable.

When not caused by atelectasis, alveolar disease is typically caused by fluid accumulation (edema, hemorrhage, or pus) within alveoli.² The regional distribution is the most effective means of distinguishing these as shown in the following table.

Histological

• Type of pneumonia—Port of entry and location of lesion
• Lesion distribution
• Broncho-pneumonia—Aerogenous: bronchi, then alveoli with interstitial hyperemia—Ventral
• Interstitial pneumonia—Aerogenous or hematogenous: alveolar walls and interstitium—Diffuse
• Granulomatous pneumonia—Aerogenous or hematogenous: randomly distributed—Multifocal
• Embolic pneumonia—Hematogenous: centered in arterioles and capillaries—Multifocal

Interstitial Pattern

The designation of pulmonary abnormalities into this category implies that the fluid, cells, or fibrosis exists in the supporting connective tissue surrounding vasculature and bronchi. The location of the disease likely includes regions other than the interstitium and, as severity progresses, the abnormality can appear alveolar, radiographically. The interstitial pulmonary pattern is subcategorized into structured and unstructured. The unstructured interstitial pulmonary pattern is notoriously difficult to diagnose as the appearance of the interstitium is greatly influenced by technical factors; additionally, it may indicate active or past disease.⁴ An unstructured interstitial pulmonary pattern is described by an increase in pulmonary opacity that is less intense per unit area when compared to alveolar disease such that margins of vessels are less delineated, but not totally effaced. Any disease resulting in an alveolar pulmonary pattern can appear as an unstructured interstitial pulmonary pattern with a similar regional distribution. Additionally, in cases of an unstructured pulmonary pattern that is distributed diffusely, fibrosis, neoplasia (lymphoma, hemangiosarcoma), or pneumonitis (infectious/non-infectious inflammatory) should be considered.

The structured pulmonary pattern is reserved for any nodular or mass lesion(s) seen within the lung. This pattern can be relatively intense per unit area, depending on size of the nodule. The difference between a nodule and a mass is strictly size, where a mass is 3 cm or greater. Masses and nodules are best delineated from other lesions of high intensity per unit area (i.e., alveolar lung disease) by the rounded, often well-defined margins seen with structured interstitial disease.

Bronchial Pattern

The bronchial pulmonary pattern implies disease limited to the bronchi. In most cases, the ailment affecting the bronchus extends into surrounding peribronchial interstitium. It is a common mistake for beginning interpreters to assign a bronchial pattern when bronchi are noticed.

Of course, this is inappropriate as bronchi must be present in order for air conduction to occur. Seeing bronchi does not make the study abnormal. Disease can be assumed, however, when the bronchi are thickened, irregular, and/or indistinct in margination. As with many interpretations, there can be pitfalls when the normal radiographic appearance of the lung is not well recognized. Interpreters often mistake small pulmonary arteries and veins as bronchial walls and misinterpret the appearance as bronchial disease. All suspected cases of bronchial pattern should be scrutinized at a distance for a general increase in opacity, then close-up to determine where the true bronchial wall is located. A bronchus, in most cases unless bronchiectasis is present, should taper as is approaches the periphery. Vascular pairs will diverge. Additionally, a bronchial wall that is thickened enough to be of similar size to peripheral vasculature will often be much more irregular.

Vascular Pattern

This pattern is reserved for cases where pulmonary vasculature are too big or too small. Many do not consider this a true pulmonary pattern. If both arteries and/or veins are enlarged, causes of cardiovascular disease should be pursued.

Conclusion

Differentiation of pulmonary disease is best accomplished by incorporation of pulmonary patterns, regional distribution, and recognition of changes in lung volume into formation of an appropriate list of differential diagnoses. The pulmonary pattern is not the only descriptor that will assist with diagnosis, and to some degree, it is not even the most important. At the very least, recognizing whether abnormalities are associated with airway or not will help the interpreter determine the most useful sampling technique (airway wash v fine needle aspiration) to diagnose and treat.

References

1.  Lopez A. Respiratory System, Mediastinum, and Pleurae. In: Zachary JF, McGavin MD, editors. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, Mo.: Elsevier Mosby; 2012:458–538.

2.  Myer W. Radiography review: the alveolar pattern of pulmonary disease. Vet Radiol. 1979;20(1):10.

3.  Felson B. New look at pattern-recognition of diffuse pulmonary-disease. Am J Roentgenol. 1979;133(2):183–4.

4.  Myer W. Radiography review: the interstitial pattern of pulmonary disease. Vet Radiol. 1980;21(1):18–23.