Introduction

When cats present for dyspnea, the manner is often acute and the threat of exacerbation with even gentle, basic handling is of high concern. The goal of diagnosis is, ultimately, to be as rapid as possible without further reducing the stability of the cat in the process. In many cases, in addition to a tempered physical exam that limits unnecessary handling, radiography is a component of evaluating dyspneic cats. Additionally, the likelihood of obtaining a more complete assessment and the ability to share images through telemedicine is inherently more rewarding with radiography.

Top Causes of Dyspnea

The common causes of dyspnea in cats fall into one of four categories: cardiac, respiratory, neoplastic, or traumatic/other.

Table 1. Summary of diagnoses in cats enrolled for dyspnea

Given the most likely diagnoses in cats presenting for dyspnea, the ultimate goal of diagnostic testing is to determine a cardiac versus non-cardiac cause of dyspnea. According to each of three studies where cats were enrolled due to presentation of dyspnea, cardiac disease and failure is the most statistically likely cause of the dyspneic presentation.^1-3

Determining the Cause of Dyspnea

Determining a cardiac cause of dyspnea is also confounded by the inability to detect a murmur in a significant portion of cats presenting with cardiac failure.² Also, even in the presence of an audible murmur, the definitive diagnosis of cardiac disease cannot be assumed.⁴ Ultimately, the decision tree of treatment for dyspnea leads to the choice of whether to treat with diuretics or with airway therapeutics.

Cardiac

The most common acquired cardiac disease in cats falls under the category of cardiomyopathy, where hypertrophic cardiomyopathy (HCM) is the most common diagnosis. Pulmonary edema due to left-sided congestive heart failure has no clear, reliable pattern of distribution and can mimic some non-cardiac lesion distributions. Also, in cats, left-sided failure can also cause pleural effusion. Both cardiogenic edema and pleural effusion can reduce visibility of cardiac margins. The classic radiographic appearance of the most common cardiomyopathies in cats is associated with a shape change with or without measurable enlargement. On the lateral view, a concavity can be identified on the caudal cardiac margin that makes the heart appear kidney shaped. On the ventrodorsal view, the cardiac silhouette will be largest in width near the heart base, rather than mid silhouette.

Respiratory

The most common respiratory diseases in cats are two specific types of lower airway disease: chronic bronchitis and asthma. These airway diseases are separate entities that can have some overlap in clinical presentation and radiographic appearance.^6,7 The most common radiographic pattern detected in cats with lower airway disease is a bronchial pattern (about 75% of diagnosed cases).⁸ Twenty-three percent of cats with lower airway disease have normal thoracic radiographs.⁸ The distinction between chronic bronchitis and asthma relies on detection of any indicator of expiratory failure. Cats with asthma have intermittent expiratory respiratory distress due to bronchoconstriction which would not be a feature of chronic bronchitis.⁷ The only radiographically distinguishable feature that can indicate bronchial constriction (or, more likely, bronchiolar constriction) associated with asthma is the presence of hyperinflation associated with air trapping. A barrel-shaped chest, flattened diaphragm with or without apparent increased lung lucency might be identified. As air trapping occurs due to smooth muscle hypertrophy and edema, and constriction of small airways (bronchiolar level) that are radiographically too small to detect, the incidence of a bronchial pattern in cats with air trapping may not be present unless larger airways are also affected.⁹

Other respiratory causes of dyspnea include pneumonia, congenital diseases, neoplasia, and trauma. Alveolar and interstitial pulmonary patterns that are ventral in position are most typically identified in cases of infectious pneumonia in cats. Additionally, a “para-pneumonic” spread of infection to the pleural space is considered the most common cause of pyothorax in cats.¹¹ Non-cardiac and non-neoplastic pleural effusions (e.g., pyothorax and FIP-associated pleural effusion) can be considered in the respiratory category of diseases causing dyspnea in cats.^2,3 Chylothorax is most often idiopathic, but cardiac disease must be considered and ruled out.¹²

Neoplasia

The most common neoplastic causes of dyspnea are due to primary pulmonary neoplasia, pleural effusion due to pulmonary or cranial mediastinal masses, or mass effect originating within the cranial mediastinum with little to no pleural effusion. The most common primary pulmonary neoplasm is pulmonary adenocarcinoma, which can have pulmonary metastases in about 50% of cases.¹³ About 30% of cats having primary pulmonary neoplasia have concurrent pleural effusion, reducing ability to assess lungs and the heart.¹³

Traumatic, Etc.

Trauma may or may not have a matching clinical history. Rib or other skeletal fractures are sometimes present as clues. Rib fractures must be interpreted with caution because serial or singular rib fractures can occur secondary to chronic airway disease, as well.¹⁴ Pulmonary contusions or pleural effusion could be responsible for a dyspneic presentation.

Making the Most of the Radiograph

Cats presenting for dyspnea of cardiac or noncardiac/pulmonary causes are at risk of having a complicated lung pattern or pleural fluid that reduces visibility of the heart, mediastinum, and other soft tissue structures. Regardless of pulmonary or pleural status, the tracheal position is typically easy to assess. An indication of cardiomegaly can be made based on positional elevation of the trachea, even when entire cardiac silhouette is not visible. If enough of the cardiac silhouette is visible, an objective measurement of cardiac size could be obtained via a vertebral heart score (VHS) to determine likelihood of the heart as being the cause of dyspnea. In one study of 67 cats presenting for acute respiratory distress, a VHS cutoff of 9.3 or greater was “highly specific” for heart disease as being the cause of the respiratory distress.⁴ Seven cats (10%) in this study had pleural effusion to the point where a VHS could not be performed. Additionally, a VHS of 8 or less is specific for heart disease not being the cause of dyspnea. In determining whether dyspnea is cardiac or not, an assessment of cardiac size has to be made—where some have advocated, due to the prevalence of congestive heart failure as a cause of acute dyspnea in cats, treatment with diuretics in cases where complex pulmonary patterns are found, keeping in mind that pulmonary disease remains a possibility.⁴

References

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14.  Adams C, Streeter EM, King R, Rozanski E. Cause and clinical characteristics of rib fractures in cats: 33 cases (2000–2009). J Vet Emerg Crit Care (San Antonio). 2010;20(4):436–440.