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unremarkable; however, decreased diameter of the air-filled oro- and nasopharynx may be detected. In brachycephalic syndrome, elongation of the soft palate, tracheal hypoplasia and increased soft tissues of the larynx can be identified radiographically. The normal
ratio of tracheal diameter to thoracic inlet diameter is .07-.21 in bulldogs and averages 0.16 in non-bulldog brachycephalic breeds. Non-brachycephalic breeds have a ratio of 0.2. Note: the trachea will appear narrower prior to one year of age in many breeds, especially brachycephalic ones.
In cats the pharyngeal area should be scrutinized for soft tissue masses associated with nasopharyngeal polyps.
The rigidity of the trachea decreases with age in
small breed dogs and has an affect on the diameter during inspiration and expiration, which can be shown radiographically. This makes the diagnosis of collapsing trachea in small breed dogs sometimes difficult. The difference between normal and pathologic is not always clear. Therefore, dynamic views of the trachea can
be very helpful for a more definitive diagnosis. This is best performed with fluoroscopy so that the change in diameter of the cervical and thoracic trachea can be observed during deep inspiration and expiration as
well as when the dog coughs. Unfortunately, this is only available at referral centers. Static radiographs, however, are a minimum data base and should be performed if the survey radiographs are negative. Both in inspiration and expiration have to be compared to detect many types of tracheal collapse. Typicallly, the extrathoracic trachea will collapse during inspiration, the intrathoracic trachea and stem bronchi during expiration. Collapse may also occur focally at the thoracic inlet. The significance of a soft tissue opaque shadowing of the dorsal tracheal border is subject to discussions. One possible explanation for the appearance is a redundant tracheal membrane, meaning the protrusion into the tracheal lumen.
Small breed dogs with large livers and a cranially displaced diaphragm can have direct compression of
the stem bronchi which can have a similar affect as
an enlarged left atrium in dogs with left heart failure. Reduction in the diameter of the trachea may also occur due to compression by mediastinal masses, enlarged tracheobronchial lymph nodes or an enlarged left atrium. Collapse of the stem bronchi is commonly the cause of coughing in small breed dogs with mitral insufficiency due to endocardiosis and enlargement of the left
atrium. Tracheal collapse and compression should
be differentiated from stenosis secondary to space- occupying lesions. Granulomas (foreign body, parasites) are rare. Neoplasia is also very uncommon. Tumors such as osteochondroma, osteosarcoma and chondrosarcoma appear as soft-tissue opaque lesions that contrast with the air-filled tracheal lumen. Inhaled foreign bodies such as small pebbles are not uncommon, especially in cats.
Foreign bodies are usually readily visible due to contrast with the air-filled lumen. Stenosis can occur due to prior foreign body or aspiration of gastric juices. They present radiographically as a focal narrowing of the tracheal lumen and can be present anywhere along the length of the trachea.
Tip: for squirming small breed dogs where you are
trying to get inspiratory and expiratory views in lateral recumbency and respect radiation safety at the same time, make a doggie burrito: swaddle the tiny dog in a towel with its legs pulled forward and it won’t struggle on its side.Lower Airway Diseases
Peribronchial infiltrates and edema, narrowing of the bronchial lumen due to either thickening of the bronchial wall or build up of secretions as well as enlargement of the bronchi are common consequences of dogs and cats with lower airway disease.
Radiographic findings of lower airway disease are
rings with a relatively small air-filled lumen represent transverse sections of the affected bronchi along with
an increased number of linear structures throughout the lung. The lung will appear to have a diffusely increased opacity due to the presence of thickened bronchi, bronchial secretions or peribronchial infiltrates. The difficulty lies in differentiating disease from age related changes of the bronchial tree, which can appear similar. Mineralized bronchial walls due to age appear thin and finely mineral opaque and sharply delineated. Thickening of the bronchial walls leading to “doughnuts” and “tramlines” is a sign of chronic bronchial inflammation. Primary differential diagnoses are chronic bronchitis, eosinophilic infiltrates or parasitic infections. Thickening of the peribronchial tissues (bronchial cuffing) due
to edema or inflammation can mimic bronchial wall thickening but belong to another list of differentials (bronchopneumonia, cardiogenic edema in large breed dogs, or allergic reactions).
Chronic bronchitis is an exclusion diagnosis. Thickened bronchial walls and their increased visibility is a reliable sign of chronic bronchitis in dogs. In severe cases the bronchi can be completely opacified by mucus and can be confounded with vessels or even small nodules. In cases of acute bronchitis the thoracic study may be inconspicuous or resemble chronic cases.
Bronchiectasis is much less common and appears as widened, irregularly shaped bronchial branches with
a thickened wall. This represents end-stage bronchial disease, usually following chronic bronchitis. Soft
tissue opacities may also be present in the periphery
of the lung due to secretion build up and appears as
a peripheral alveolar pattern, either in one or multiple lobes. Bronchiectasis may only be evident in one or two lung lobes or can be generalized.
Acute bronchitis may appear unremarkable
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