QUARTERLY BEAT / JULY 2025
AIRWAY DISEASE Inflammatory airway diseases are also frequently seen in dyspneic cats. Common causes of airway disease in the cat include asthma, heartworm disease, chronic bronchitis, and other infectious disease caused by parasitism, fungal etiologies, and bacteria. All are indistinguishable on radiographs with the end result being airway inflammation and thickening. While airway disease can still be present in the face of normal radiographs, the vast majority of cats exhibit a bronchial pulmonary pattern. 6 “Bronchial” should not be confused with the term “bronchiolar,” the latter of which refers to small airways. Thickened bronchi may be sharply marginated or ill-defined. Many cats also exhibit an unstructured interstitial pulmonary pattern, which may indicate extension of inflammation into the surrounding interstitium, concurrent infection, and/or pulmonary fibrosis. More recently, extension of airway disease from the larger bronchi to the smaller bronchioles (< 2 mm in diameter) has been described and classified 7 with yet to be determined clinical significance regarding treatment. Additionally, primary bronchiolar disease without larger airway involvement may also occur. Radiographs are unable to evaluate for bronchiolar disease. While there are limited reports on the use of CT in diagnosing lower airway involvement, bronchiolar disease is best confirmed with histopathology. Sequalae associated with feline asthma are somewhat different than those seen in dogs with chronic bronchitis. As the disease progresses, airway narrowing secondary to spasms, inflammation, or fibrosis can lead to air trapping that causes hyperinflation of the affected lobe(s). The increased lung volume causes the lung fields to appear bigger and more lucent. In more severe cases, the diaphragm will displace caudally and appear flattened on lateral projections or tented on the ventrodorsal projection. The author has witnessed on more than one occasion severe pulmonary hyperinflation confused for pneumothorax due to the decreased visualization of pulmonary markings. Occasionally, mucus plugs are sometimes seen in the lumen of large bronchi as ill-defined or sharply marginated soft tissue opaque structures. When bronchioles fill with mucus, pus, or fluid, multi-focal nodules with or without linear branching may be seen, a pattern termed “tree-in-bud.” Whether present in large or smaller airways, mucus plugs may be mistaken for pulmonary nodules or fungal/ parasitic granulomas. Another complication of lower airway disease more commonly seen in cats is lobar collapse secondary to airway obstruction from mucus or exudative material. Although any lung lobe can be affected, the right middle is most commonly reported. Atelectasis will cause a severe alveolar pulmonary pattern in the affected lobe, characterized by increased opacity, air bronchograms, border effacement of surrounding soft tissue structures, and a lobar sign. In certain cases, the collapsed lobe may contract and appear missing or difficult to visualize. Reinflation often occurs with treatment of the underlying disease. Although less common, secondary airway infection can occur in asthmatic cats as it does in dogs with chronic bronchitis. Bronchiectasis is less common but reported. Finally, a small percentage of cats may fracture the mid-portion of their most caudal five ribs secondary to severe dyspnea or coughing. PLEURAL EFFUSION Due to its myriad of causes, pleural effusion is a common cause of dyspnea in young, middle-aged, and older cats. Congestive heart failure (discussed above) and neoplasia (discussed below) are the most common causes of pleural effusion in the cat. Other causes include pyothorax from penetrating
trauma like bite wounds, feline infectious peritonitis (FIP), other infectious etiologies, idiopathic chylothorax, and hemorrhage. Pleural effusion can cause dyspnea alone via hypoventilation or along with concurrent pulmonary disease which compounds hypoxemia. Radiographic signs of pleural effusion include widened, opaque soft tissue interlobar fissures, retraction of the lung lobes, rounding of the lung margins and costophrenic sulci, and effacement of the margins of other soft tissue structures such as the cardiac silhouette and diaphragm. Most causes of pleural effusion present bilateral due to the incomplete mediastinum in cats. However, unilateral pleural effusion may occur with exudative etiologies that plug mediastinum fenestrations, such as pyothorax and FIP. Small volume pleural effusion is easily confirmed via thoracic POCUS. When possible, fluid should always be sampled to help prioritize differentials. DIFFUSE NEOPLASIA Diffuse neoplasia is an uncommon cause of dyspnea in cats but is very tricky as it can mimic other more common diseases. For both primary and metastatic neoplasia, respiratory signs primarily occur when the disease process is diffuse and/or causing concurrent pleural effusion. Carcinomas are the most common primary lung tumor and typed according to their location as bronchial, alveolar, or bronchoalveolar. While carcinomas most often form a single solitary mass that sometimes contains cavitations, they also can present as multifocal nodules, complete lung lobe consolidation, or a diffuse bronchial pulmonary pattern. The latter two radiographic presentations are deceptive as they can easily be confused with lung lobe torsion/abscessation or inflammatory airway disease, respectively. Feline pulmonary carcinomas are highly metastatic and may cause concurrent pleural effusion and lymphadenopathy. Lymphoma can occur in the lungs as either multicentric disease or primary pulmonary disease. Consistent with the saying “lymphoma does what lymphoma wants,” it can manifest as any pulmonary pattern: a single solitary mass, multifocal nodules, bronchial, alveolar, unstructured interstitial, or a combination thereof. Most deceptive are unstructured interstitial/alveolar or bronchial patterns as they can mimic pulmonary edema/infectious etiologies or inflammatory airway disease, respectively. Lymphoma also commonly causes pleural effusion, lymphadenopathy, and/or mediastinal masses. Hepatosplenomegaly may also be seen in the viewable cranial abdomen.
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