The importance of X-rays in cats with dyspnoea

Introduction

Lower respiratory tract diseases (LRTD) are a common cause of dyspnoea in cats. They include both bronchial disorders (conditions affecting the airways distal to the trachea) and interstitial lung diseases (mainly pneumonia and neoplasms, but also pulmonary oedema and fibrosis). Patients with severe disease usually have dyspnoea or tachypnoea, whereas coughing or wheezing are more common in milder cases.¹

The aim of taking X-rays in cats with dyspnoea is to determine whether the cause of the respiratory problem is bronchial disease, interstitial disease, upper respiratory tract disease, heart disease, pleural effusion or intrathoracic mass.¹

Lower respiratory tract diseases in cats

Idiopathic inflammatory bronchopulmonary diseases, such as asthma and chronic bronchitis are the most common LRTD in cats, but they are not always to differentiate clinically.^1,2

Asthma in cats has been defined by the presence of reversible bronchoconstriction, which manifests clinically as wheezing or respiratory distress. Chronic bronchitis is usually characterised by cough as a consequence of airway remodelling in which there is fixed airflow obstruction.The inflammation of the airways in asthma is mainly eosinophilic, while neutrophilic inflammation predominates in chronic bronchitis.^1–3 Despite these differences, a recent study concluded that the clinical presentation and radiographic findings of the two diseases often overlap.³

Other potential causes of LRTD include thoracic trauma, smoke inhalation, aspiration pneumonia, idiopathic pulmonary fibrosis, neoplasms and infections (bacterial, viral, parasitic or fungal).^1,2 

Interpreting X-rays for cats with LRTD

Patients with severe dyspnoea should always be stabilised before taking X-rays. If an X-ray is considered essential for a cat with acute dyspnoea, the patient should be positioned to minimise stress, usually by placing the cat in sternal decubitus and obtaining a dorsoventral view.

A chest X-ray study should include three good-quality projections: left and right laterolateral and dorsoventral or ventrodorsal.^1–4 Generally, images should be obtained at maximum inspiration, but X-rays in expiration are preferred in certain situations. Examples include to detect bullae, chest hyperinflation or small pneumothoraces or to confirm intrathoracic tracheal collapse.⁴

Recognising the radiological pattern displayed in X-rays can reduce the list of differential diagnoses. However, other tests are often required for an accurate diagnosis, including a cytological study. In this regard, the radiographic appearance can help determine the best way to obtain samples, for example, when a bronchoalveolar lavage is indicated or when aspiration cytology is required.¹

It is important to be aware that many cats can present a mixed pattern. In these cases, the clinician must decide the steps to take based on the predominant pattern.¹ On the other hand, one disease can also produce more than one radiological pattern.⁵

In general veterinary practice, the attending vet typically assesses the radiological findings; in case of doubt, however, a radiologist can be consulted for a more accurate interpretation of the X-rays.

Recent advances have seen artificial intelligence return promising results when used to identify different types of lesions in chest X-rays.⁶

Bronchial pattern

The bronchial pattern is caused by the accumulation of fluid or cells in the bronchial walls and the perivascular and peribronchial connective tissue. This increases the bronchial walls’ radiopacity, so they appear as two parallel lines in longitudinal views which are often said to resemble railway tracks. In transverse sections, by contrast, the bronchus appears as a circle of radiodense tissue with a radiolucent centre, known as the doughnut sign. This pattern is typically observed in cats with chronic bronchitis, eosinophilic pulmonary infiltration, dirofilariasis, allergic pulmonary disease and feline asthma.^1,4

Vascular pattern

The vascular pattern is due to changes in the prominence of pulmonary vessels (arteries and/or veins). The vessels appear as narrowing radiodense linear structures running parallel to the airway. The vascular pattern is usually seen in cats with cardiovascular disease.¹

Alveolar pattern

The alveolar pattern is a sign of a collapse of the pulmonary alveolus or fluid-filled pulmonary acini. This produces an overall increase in pulmonary opacity, so the bronchial structures in the affected areas appear as radiolucent lines called air bronchograms. Other characteristic features of the alveolar pattern are the lobar and silhouette signs. The alveolar pattern is suggestive of aspiration pneumonia, bronchopneumonia, pulmonary oedema (cardiogenic and noncardiogenic), pulmonary haemorrhage, neoplasms and smoke inhalation.^1,4

Interstitial pattern

The interstitial pattern is caused by the accumulation of fluid or cells in the pulmonary interstitium rather than the airway. This produces haziness across the pulmonary fields that attenuates the vascular lines. This pattern can be structured or unstructured. The structured (miliary or nodular) interstitial pattern is usually seen in cats with lymphoma, carcinoma or pneumonia. The unstructured pattern is observed in cats with lymphoma, fibrosis, fungal infections, oedema, bleeding and infectious diseases, among others.^1–4

To find out more about the emergency management of cats with dyspnoea, watch the webinar on the Management of Cats in Shock by Lluis Bosch, a specialist in emergency and intensive care.

Conclusions

X-rays in cats with LRTD is an important diagnostic aid in determining the origin of the clinical picture. Nevertheless, additional diagnostic methods may be required to establish an accurate diagnosis. In all events, recognising the radiological pattern and knowledge of the associated diseases can narrow the list of potential differential diagnoses.  

References

1. Baral RM. (2012) Lower respiratory tract diseases. In Little SE (ed). The Cat: Clinical Medicine and management. Elsevier Saunders: 861–891.

2. Rozansky E. (2016). In Little SE (ed). Feline Lower Airway Disease. August’s Consultations in Feline Internal Medicine, 7th ed. Elsevier: 447–451.

3. Lee EA, Johnson LR, Johnson EG, et al. (2020). Clinical features and radiographic findings in cats with eosinophilic, neutrophilic, and mixed airway inflammation (2011–2018). J Vet Intern Med;34: 1291–1299. 

4. Bradley K. (2013). Radiology of the thorax. In Holloway A, McConnell JF (eds). BSAVA Manual of Canine and Feline Radiography and Radiology. British Small Animal Veterinary Association: 109–175.

5. Guglielmini C, Diana A. (2015). Thoracic radiography in the cat: Identification of cardiomegaly and congestive heart failure. J Vet Cardiol;17 Suppl 1:S87–101.

6. Boissady E, de La Comble A, Zhu X, et al. (2020). Artificial intelligence evaluating primary thoracic lesions has an overall lower error rate compared to veterinarians or veterinarians in conjunction with the artificial intelligence. Vet Radiol Ultrasound; 61: 619–627.