Introduction

There is a wide variation in how the lung appears based on age and body condition in normal animals. The altered opacity of the lung may be either increased (more opaque) or decreased (more lucent), but the majority of pulmonary diseases in dogs and cats produce an increased opacity. Not all diseases lead to alteration of opacity due to the pathogenesis of that particular disease, disease severity, or stage of the disease. Furthermore, alterations in opacity are often not specific for the type of lung disease. This additionally adds to frustration in interpretation.

One of the most important signs to help further classify the pattern of lung disease is the size (expansion) of the lungs or lobes, which may be decreased, normal, or increased. Lungs that have decreased size are described as incompletely expanded. Lungs that have normal or increased size are fully expanded. An incompletely expanded lung that has an increased opacity that completely or partially obscures the margins of pulmonary blood vessels and airway walls is called collapse or atelectasis.

Atelectasis is reduced inflation of all or part of the lung. It can be mistaken for alveolar disease. The radiographic signs that alert the viewer to reduced lung volume are a mediastinal shift toward the abnormal-appearing lung, crowding and reorientation of pulmonary blood vessels, crowding of ribs, compensatory hyperinflation of other lung lobes, bronchial rearrangement, cardiac rotation, displacement of interlobar fissures, displacement of the diaphragm, change in location of abnormal structures, and rounded pulmonary margins.

Generally, a fully expanded lung that has altered opacity is abnormal and indicates pulmonary disease. A fully expanded lung that has a homogeneous increased opacity that obscures the margins of pulmonary blood vessels and airway walls is called consolidation; air bronchograms may or may not be present.

Consolidation can be caused by an exudate or other disease that replaces alveolar air. A fully expanded lung that has a hazy increased opacity that only partially obscures the margins of pulmonary blood vessels and airway walls is generally called an interstitial opacity. This is caused by partial filling of air spaces, interstitial thickening (due to fluid, cells, or fibrosis), increased capillary blood volume, or a combination of these. The differential diagnosis for increased opacity in a fully expanded lung includes such diseases as pneumonia, neoplasia, hemorrhage, pulmonary edema, and immune-mediated diseases.

A fully expanded lung that has a decreased opacity may be due to retention of air in the lung downstream to the obstruction (i.e., air trapping), reduced pulmonary blood volume (i.e., oligemia or hypoperfusion), or permanently enlarged air spaces downstream to the terminal bronchiole with destruction of the alveolar walls (i.e., emphysema).

Once an increased opacity is recognized, the distribution of the changes should be categorized as one of the following: cranioventral, caudodorsal, diffuse, lobar, focal, locally extensive, multifocal, and asymmetric. The cranioventral distribution generally conforms to the region of the left-cranial, right-cranial, and right-middle lung lobes. This designation tends to imply that gravity has an effect on the distribution of the lesion. It is important to note that on the lateral view, the cranioventral lung field actually extends into the caudoventral portion of the thorax and is superimposed on the heart. The caudodorsal distribution generally conforms to the region of the left-caudal, right-caudal, and accessory lung lobes. Diffuse distributions imply that the lesion is distributed by a hematogenous route or by the airways.

Action Plan Oriented Approach

In a patient with respiratory disease, it is imperative to develop an action plan based on a combination of the clinical signs and location of any disease visible in the lung radiographically. If there is an increased soft tissue opacity, the reader needs to determine if it is in the airspace, airway, or interstitium in order to move to the next diagnostic step.

 Are the pulmonary vessels well visible and sharply bordered?

 If yes: this is less likely to be airspace (alveolar) disease or interstitial.

 Scrutinize the airways for wall opacity, shape and lumen size, rings and lines.

 If no: airspace or interstitial disease is likely present.

 Consider most likely differentials such as pneumonia, hemorrhage, edema, infection (all organisms, sepsis), ARDS, neoplasia.

 Are the airways more prominent with increased numbers of visible rings and lines in the periphery of the lung?

 If yes, correlate to clinical signs: coughing! These types of changes are typically chronic, and clinical signs can wax and wane.

 If coughing, next step is likely airway examination, bronchoscopy, airway sampling, etc.

Airspace

Disease that fills the airspace creates an increased opacity that silhouettes with the air, effaces the borders of the heart, pulmonary vessels, and depending on severity may result in air bronchograms and lobar sign. Airspace disease is typically due to infection, edema, hemorrhage, or neoplasia, and generally in that order of likelihood. The action plan is to rule out each based on the clinical history and physical examination findings, as well as abdominal screening with radiography and ultrasound once pneumonia and edema are ruled out, as well as a coagulopathy.

The Interstitium

The continuum of connective tissue throughout the lung divided into:

1.  Bronchovascular: surrounds bronchi, arteries, veins

2.  Parenchymal: between alveolar and capillary membranes

3.  Subpleural connective tissue

An increased opacity of the lung does not correlate well to a microscopic anatomic location such as the interstitium because of superimposition, partial filling of the alveoli, parenchymal interstitium thickening, or partial collapse of the lung will all lead to attenuation of the x-ray beam that will look similar radiographically (hazy increased opacity of the lung). This is typically a ground glass appearance. Interstitial disease as a sole pattern is difficult to diagnose, and it is typically part of the bronchial and airspace patterns.

Structured interstitial disease is either in the form of nodules or masses.

 Micronodular or miliary (pinpoint foci): Numerous, diffusely distributed and poorly marginated pinpoint soft tissue opaque foci.

 Nodule: Round soft tissue opacity, well or poorly defined that is less than 3 cm.

 Small nodule: Less than 3 mm. Also called micronodular or miliary.

 Mass: 3 cm or greater soft tissue, shape and demarcation don't matter.

When interstitial disease is suspected after ruling out airspace and airway disease, the logical approach to further investigation is typically either CT or a lung biopsy.

 Pathology: Increased amount of fluid, cellular material or fibrosis in the interstitium. Most common in chronic lung diseases but also in neoplasia, toxic inhalation.

 Two main forms: structured and unstructured increased opacities

 Unstructured interstitial opacities: The lung appears diffusely more opaque or hazy. The contrast between the lung and vessels is diminished, and the vascular markings are poorly marginated. The changes may also have a peribronchial location.

 Fat animals, underexposure and expiration will create the appearance of an interstitial pattern.

 Differential diagnoses: artifact, age-related changes, fibrosis, lymphosarcoma, diffuse metastases, pneumonitis (viral, toxic, metabolic, parasitic), edema, bronchopneumonia, pulmonary embolism, hemorrhage.

 Structured interstitial opacities: Nodules and reticular patterns. Differentials include metastatic neoplasia, chronic infectious disease (bacterial or fungal), lymphosarcoma, PIE (pulmonary infiltrates with eosinophilia), hypersensitivity reactions, parasitic infections, fluid-filled bulla, cyst, abscess. If cavitary nodules are present, differentials include primary lung tumor, metastases, mycosis, paragonimiasis, abscess, cyst, bronchiectasis.

Systematic Approach to Assessing Pulmonary Patterns

Determine the following from the radiograph:

 If the lung has an altered opacity (increased or decreased)

 Degree of expansion of the lung (lung size)

 Distribution of the lesion

 Cranioventral

 Caudodorsal

 Lobar: localized to entire lobe

 Diffuse: no normal lung visible

 Focal: well-defined lesion, a mass or nodule

 Multifocal: more than one lesion, one or more lobes

 Type of opacity: mixed, alveolar (airspace), bronchocentric, nodular interstitial, unstructured interstitial

Practical Approach to Differential Diagnoses and Action Plan

Differential diagnoses may be prioritized by incorporating other information such as signalment, history, and results of other tests.

The location of the opacity helps to differentiate the most likely cause. Location is broken down into the following categories: cranioventral airspace disease, caudodorsal or diffuse airspace disease, diffuse bronchocentric, lobar, focal, and multifocal. Below is a list of differential diagnoses for each of these categories.

Cranioventral airspace pattern (left and right cranial, right middle)

 Gravity has an effect on distribution

 Consolidation is main radiographic feature

 Aspiration or bronchopneumonia

 Hemorrhage

 Neoplasia

Caudodorsal-to-diffuse airspace pattern

 Implies a hematogenous pathway of distribution or via airways

 Congestive left heart failure

 Toxin inhalation

 Viral or parasitic infections

 Strangulation

 Near drowning

 Fibrosis

 Thermal injury

 Septicemia and endotoxemia

 Disseminated intravascular coagulation

 Some cancers (e.g., lymphoma)

References

1.  Scrivani P. Nontraditional interpretation of lung patterns. Veterinary Clinics of North America: Small Animal Practice. 2009;39:719–732.