School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
Introduction
After performing a systematic review of the entire thorax, careful scrutiny of the lung can be one of the most challenging parts of analyzing a thoracic radiograph. Using a practical approach to determine if the lung is abnormal or not, then making a decision on whether the air space, the airway is important. Also important to realize is that most all diseases cross the borders of all parts of the lung.
Are Patterns Important?
The principles of the pulmonary pattern are important, but a lot of stress can be associated with trying to determine the correct pattern so that one can make the correct diagnosis. A better approach is to determine if the main opacity of the lung is in the air space or the airway. Most diseases are in one or the other. In between is the interstitium and the interstitium is often involved anyway. Pure interstitial disease is really only sure when structured nodules are present. Unstructured interstitial disease is often very difficult to diagnose as a sole abnormality. Determining if disease is in the air space or airway is the best way to set a course of action. By creating a best course of action based on which compartment is most affected and then re-assessing the radiograph at an appropriate time point following treatment is good medical practice.
The Normal Lung
The normal feline lung is rather lucent with few linear markings, mostly visible centrally. The periphery of the lung is almost devoid of vascular markings. Thinner body condition makes the lung look hyperlucent and bronchial and vascular markings may be more visible in the periphery. The opposite is true for obesity where the lung has a hazy interstitial appearance due to the opacity of superimposed fat. Both situations make the lung challenging to assess and emphasis on the clinical signs is important for interpretation.
Air Space Disease
The alveoli are filled with air and this allows the vessels and bronchial structures to be sharply marginated and therefore well visible. These structures are extremely small in the periphery of the feline lung and this is the reason the periphery is so lucent. When an increased opacity is identified, the first duty is to determine if it is effacing the border of any soft tissue structure. The margins of the heart, pulmonary vessels, caudal vena cava and diaphragm should be scrutinized first. If any one or more of those margins are blurred, the pulmonary opacity is likely in the air space: it replaces air with soft tissue and the air no longer outlines soft tissue structures, making them less visible.
Air space disease may be lobar. If the right middle lobe is homogeneously soft tissue opaque and is the only lobe affected, then atelectasis due to lower airway disease is the likely cause. Aspiration pneumonia is rare in cats and would only be diagnosed if there is a clinical history of vomiting or regurgitation. If the clinical history fits with best with increased respiratory rate and wheezing, then atelectasis is the diagnosis.
Other radiographic features of air space disease are air bronchograms, consolidated lobes with lobar sign, and patchy opacities that silhouette borders of vessels, heart and diaphragmatic contours. Lobar consolidation is when the entire lobe is homogenously soft tissue opaque and not reduced in volume, with or without air bronchograms. It is usually due to pneumonia, neoplasia or contusion. Atelectasis is collapse of the lobe due to pleural space disease or bronchial obstruction. The lobe is opaque and there is a decreased volume and mediastinal shift of the heart to the affected lobe.
Common disease categories causing air space disease are pneumonia, edema, hemorrhage, atelectasis, infection, allergic inflammatory disease, and some neoplasia.
Multifocal, ill-defined, patchy soft tissue opacities that obscure the airspace and vessels in their surrounding are often due to infection or edema. Fungal pneumonia and cardiogenic edema are the most common of these, but neoplasia and contusions also have this appearance. Histoplasmosis can also have a patchy ill-defined mixed or airspace pattern as can cardiogenic edema. Other causes of infectious pneumonia are mycobacterial, cryptococcal, blastomycosis, aspergillosis, toxoplasmosis, Paragonimus and Aelurostrongylus.1 Lipid pneumonia is less common but consistently seen in cats. Radiographic abnormalities in Aelurostrongylus infection are dependent on severity and duration of infection. Early changes of bronchial thickening and small, poorly defined nodules progress to a generalized alveolar pattern in severe cases. After partial resolution of the alveolar pattern, an unstructured, patchy interstitial pattern develops.
Airway Disease
Lower airway disease can appear radiographically normal or have varying severities of airway pattern. Tracing the trachea to the carina and then tracing each main bronchus of each lobe should be performed. Two-thirds of the way out from the carina, the visualization of the bronchial walls and vessels should slowly disappear. If larger numbers of branching structures are visible, then an airway pattern is present. However, clinical signs are not always present. The clinical signs of airway disease may wax and wane, but chronic airway patterns are persistent on thoracic radiographs, regardless of clinical activity. This is where reader bias can sway the importance placed upon the presence of an airway pattern, or even lead the reader away from other abnormalities due to tunnel vision.2 Airway disease is usually due to allergic airway disease, asthma and heartworm infection. A recent study confirmed that the most common radiographic abnormality is a bronchial pattern, but an unstructured interstitial pattern can be present in many cats. More than half of the cats in that study had lung hyperinflation also.2 Bronchiectasis can be identified in a smaller number of cats. Right middle lobar atelectasis can be seen, as can small nodules throughout the lung and represent mucous plugs with granuloma formation.
Severe inflammatory lower airway disease can lead to hyperinflation with a flattened diaphragm. The bronchial pattern can be mixed with small nodules due to mucous plugging and exudates.
Interstitial Lung Disease
Primary Neoplasia
Primary pulmonary neoplasia is relatively uncommon in cats and generally has a poor prognosis. Radiographically, it is typically a solitary or multiple masses, or a disseminated lung pattern or lobar consolidation that looks like pneumonia. Adenocarcinoma may become cavitated. Bronchoalveolar cell carcinomas and squamous cell carcinoma are usually diffuse in the lung. Most pulmonary tumors are in the caudal lobes. Adenocarcinoma is reported as the predominant tumor type, but shares many features with less common tumor types.
Prevalence of suspected intrapulmonary metastasis was higher than in previous radiographic studies of cats with lung tumors.3
Metastatic Neoplasia
Metastatic neoplasia generally presents as multifocal, small, round, soft tissue pulmonary nodules. In cats, lung-digit syndrome is an unusual pattern of metastasis that is seen with various types of primary lung tumors, particularly bronchial and bronchioalveolar adenocarcinoma. Tumor metastases are found at atypical sites, notably the distal phalanges of the limbs; the weight-bearing digits are most frequently affected, and multiple-digit and multiple-limb involvement is common.4
Pulmonary Fibrosis
Pulmonary fibrosis is a progressive fatal interstitial lung disease that is often idiopathic, occurs in multiple species, and may be caused by a number of inciting factors. A recent study of nine cats showed that all patients had a broad range of radiographic characteristics that included bronchointerstitial pattern, alveolar pattern, pulmonary masses, pulmonary bullae, pleural effusion, and cardiomegaly.5 Cats in that study with echocardiographic studies had characteristics that included right ventricular dilation and hypertrophy and pulmonary arterial hypertension interpreted to be secondary to primary lung disease. Cats with pulmonary fibrosis have highly variable radiographic characteristics and these characteristics may mimic other diseases such as asthma, pneumonia, pulmonary edema, or neoplasia.5
Vascular Disease
Heartworm disease can cause enlarged pulmonary arteries. However, the pulmonary findings may also be rather unremarkable in infected cats. An airway pattern is often present in most cases as well. Cardiac abnormalities are not typical.
References
1. Dennler M, Bass DA, Gutierrez-Crespo B, Schnyder M, Guscetti F, Di Cesare A, Deplazes P, Kircher PR, Glaus TM. Thoracic computed tomography, angiographic computed tomography, and pathology findings in six cats experimentally infected with Aelurostrongylus abstrusus. Veterinary Radiology & Ultrasound. 2013;54:459–69.
2. Gadbois J, d’Anjou MA, Dunn M, Alexander K, Beauregard G, D’Astous J, De Carufel M, Breton L, Beauchamp G. Radiographic abnormalities in cats with feline bronchial disease and intra- and interobserver variability in radiographic interpretation: 40 cases (1999–2006). Journal of the American Veterinary Medical Association. 2009;234:367–75.
3. Aarsvold S, Reetz JA, Reichle JK, Jones ID, Lamb CR, Evola MG, Keyerleber MA, Marolf AJ. Computed tomographic findings in 57 cats with primary pulmonary neoplasia. Veterinary Radiology & Ultrasound. 2015;56:272–7.
4. Goldfinch N, Argyle DJ. Feline lung-digit syndrome: unusual metastatic patterns of primary lung tumours in cats. Journal of Feline Medicine and Surgery. 2012;14:202–8.
5. Evola MG, Edmondson EF, Reichle JK, Biller DS, Mitchell CW, Valdes-Martinez A. Radiographic and histopathologic characteristics of pulmonary fibrosis in nine cats. Veterinary Radiology & Ultrasound. 2014;55:133–40.