Radiographic abnormalities in cats with feline bronchial disease and intra- and interobserver variability in radiographic interpretation: 40 cases (1999–2006)

Julie Gadbois Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Marc-André d'Anjou Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Marilyn Dunn Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Kate Alexander Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Guy Beauregard Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Jérôme D'Astous Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Myriam De Carufel Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Luc Breton Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Guy Beauchamp Companion Animal Research Group, Département de Sciences Cliniques, Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, QC J2S 7C6, Canada.

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Abstract

Objective—To determine prevalence of various radiographic signs in cats with feline bronchial disease (FBD) and evaluate intra- and interobserver variability in radiographic interpretation for examiners with variable degrees of experience in radiographic interpretation.

Design—Retrospective case series.

Animals—40 cats with FBD and 40 control cats without thoracic disease.

Procedures—Radiographic abnormalities in cats with FBD were scored by consensus of 2 radiologists. Radiographs of control cats and cats with FBD were examined twice by 5 other individuals, and diagnostic accuracy and observer agreement were assessed.

Results—In cats with FBD, the most common radiographic signs were bronchial (n = 37) and unstructured interstitial (30) lung patterns, lung hyperinflation (31) and hyperlucency (21), aerophagia (19), and lung soft tissue opacities (11). Ratios of lung inflation on ventrodorsal views were significantly higher in cats with FBD. For the 5 examiners, sensitivity ranged from 71% to 89% and specificity ranged from 43% to 74%. Intraobserver agreement was good (N = 0.47 to 0.60), but the agreement between examiners was only poor to good (N = 0.22 to 0.70). For most examiners, significant associations were found between examiner diagnosis (correct vs incorrect), level of examiner certainty, and bronchial pattern severity.

Conclusions and Clinical Relevance—Findings suggested that several radiographic abnormalities can commonly be seen in cats with FBD but highlighted the limitations of thoracic radiography. Examiner diagnosis and level of confidence were significantly associated with severity of a bronchial pattern.

Abstract

Objective—To determine prevalence of various radiographic signs in cats with feline bronchial disease (FBD) and evaluate intra- and interobserver variability in radiographic interpretation for examiners with variable degrees of experience in radiographic interpretation.

Design—Retrospective case series.

Animals—40 cats with FBD and 40 control cats without thoracic disease.

Procedures—Radiographic abnormalities in cats with FBD were scored by consensus of 2 radiologists. Radiographs of control cats and cats with FBD were examined twice by 5 other individuals, and diagnostic accuracy and observer agreement were assessed.

Results—In cats with FBD, the most common radiographic signs were bronchial (n = 37) and unstructured interstitial (30) lung patterns, lung hyperinflation (31) and hyperlucency (21), aerophagia (19), and lung soft tissue opacities (11). Ratios of lung inflation on ventrodorsal views were significantly higher in cats with FBD. For the 5 examiners, sensitivity ranged from 71% to 89% and specificity ranged from 43% to 74%. Intraobserver agreement was good (N = 0.47 to 0.60), but the agreement between examiners was only poor to good (N = 0.22 to 0.70). For most examiners, significant associations were found between examiner diagnosis (correct vs incorrect), level of examiner certainty, and bronchial pattern severity.

Conclusions and Clinical Relevance—Findings suggested that several radiographic abnormalities can commonly be seen in cats with FBD but highlighted the limitations of thoracic radiography. Examiner diagnosis and level of confidence were significantly associated with severity of a bronchial pattern.

Feline bronchial disease, also known as allergic bronchitis or asthma, is an important cause of respiratory distress in cats, affecting an estimated 1% of all cats in the United States.1 Although the exact cause remains obscure, the condition is believed to involve a genetic predisposition to a type I hypersensitivity reaction to inhaled allergens2,3 that results in airway inflammation, airway smooth muscle contraction, and excessive mucus secretion.3–5

Cats with FBD are typically examined because of recurrent episodes of coughing,2–4 but there is no particular clinical sign or laboratory test result that is pathognomonic for this condition.5,6 The diagnosis is usually made on the basis of a combination of history; clinical signs; results of physical examination, thoracic radiography, bronchoscopy, pulmonary function testing, and clinicopathologic testing (eg, bronchoalveolar lavage, hematology, and fecal examination); and response to treatment. Most often, FBD is diagnosed by excluding other diseases that may be associated with similar clinical signs (eg, pneumonia, congestive heart failure, parasitism, neoplasia, pneumothorax, and lung contusion).

Radiography is an important component of the diagnostic investigation in cats suspected to have FBD. Several radiographic signs compatible with FBD have been described, including a bronchial pattern, which is reportedly the most common radiographic finding.1,2,5,6 Other radiographic features reported in cats with FBD include unstructured interstitial and alveolar patterns, lung hyperinflation and hyperlucency, and lobar atelectasis.2,7 Structured pulmonary soft tissue opacities attributed to the presence of eosinophilic granulomas and mucus plugs have also been described sporadically.8 However, studies evaluating the relative prevalence of all radiographic signs in a large population of cats with FBD are lacking.2–4,7 Furthermore, the intra- and interobserver variability in the radiographic assessment of this condition is unknown. Because many practitioners rely on thoracic radiography in the assessment of cats suspected to have FBD, it is critical to determine variability in radiographic interpretation, as it may greatly influence the reliability of this test.

The purpose of the study reported here was to determine the prevalence of various radiographic abnormalities in cats with FBD. We also wanted to evaluate intra- and interobserver variability in radiographic interpretation and to determine whether variability in radiographic interpretation was associated with experience of the individual examining the radiographs.

Materials and Methods

Case selection—Records of cats examined at the University of Montreal Veterinary Teaching Hospital between 1999 and 2006 were reviewed to identify cats with FBD. Cats were eligible for inclusion in the study if they had a history of acute or recurrent bouts of coughing, wheezing, or dyspnea without concurrent evidence of upper respiratory tract disease and a final diagnosis of asthma had been made by the attending clinician on the basis of clinical signs, results of bronchoalveolar lavage, response to treatment, or some combination of these. In addition, cats were included only if high-quality lateral and ventrodorsal radiographic views of the thorax obtained at the time of initial examination were available for review and if follow-up data, including thoracic radiographs, excluded other conditions, such as neoplasia and congestive heart failure, that could have caused similar clinical and radiographic signs.

Control selection—For each case cat included in the study, a single control cat was also included. Control cats were selected after a search through our radiologic database. Cats were eligible for inclusion in the study as control cats if high-quality lateral and ventrodorsal radiographic views of the thorax obtained at the time of initial examination had been interpreted by a radiologist as normal, there was no record of respiratory tract abnormalities, and the final diagnosis did not involve any thoracic diseases. Control cats were matched with case cats on the basis of age (ie, within 1 year) and body weight (ie, within 2 kg).

Prevalence of radiographic abnormalities—Thoracic radiographs of the case cats were evaluated by a board-certified veterinary radiologist (MAD) and a trained veterinary radiologist (GB), who assigned consensus scores for various radiographic abnormalities; the radiologists were aware of the clinical diagnosis at the time they assigned scores. A bronchial pattern was identified if bronchial walls were thickened; bronchial pattern was scored as absent (0), mild (1), moderate (2), or severe (3) and was described as ill-defined or well-defined. Interstitial and alveolar patterns were described as focal or diffuse and as uniform or heterogeneous. Other radiographic abnormalities were scored as present or absent, and the location was recorded; abnormalities that were examined included lobar atelectasis, structured pulmonary soft tissue opacities, ill-defined lung hyperlucencies, well-defined bullae, aerophagia, vascular tortuosity or dilatation, cardiomegaly, and mediastinal shift. Bronchiectasis was recorded as present when ≥ 1 bronchus appeared excessively wide, nontapering, or irregularly shaped. Lung hyperinflation was identified on the ventrodorsal radiographic view when the thoracic wall appeared excessively convex and on the lateral radiographic view when the distance between the diaphragm and the cardiac silhouette was increased and the diaphragm was flattened. Bronchial wall mineralization was recorded as present when bronchial walls were associated with an increased opacity that could not be explained by increased wall thickening alone.

Four ratios of lung inflation were calculated for each cat. Ratios calculated on the basis of measurements obtained from the ventrodorsal view included the width of the lung field at the level of T7 divided by the width of T7 and the maximal width of the lung field divided by the width of T7 (Figure 1). Ratios calculated on the basis of measurement obtained from the lateral view included the maximal length of the lung field divided by the length of T7 and the length of the lung field from the most cranial margin to the diaphragm at the level of the caudal vena cava divided by the length of T7.

Figure 1—
Figure 1—

Illustration of methods for calculating ratios of lung inflation in cats. On the ventrodorsal view (A), ratios consisted of width of the lung field at the level of T7 (a) divided by the width of T7 and maximal width of the lung field (b) divided by the width of T7. On the lateral view (B), ratios consisted of maximal length of the lung field (c) divided by the length of T7 and length of the lung field from the most cranial margin to the diaphragm at the level of the caudal vena cava (d) divided by the length of T7.

Citation: Journal of the American Veterinary Medical Association 234, 3; 10.2460/javma.234.3.367

Thoracic radiographs of the control cats were evaluated by the same 2 radiologists for evidence of radiographic abnormalities. In addition, ratios of lung inflation were calculated.

Thoracic radiographs had been obtained by means of standard film radiography or computerized radiography with an edge-sharpening algorithm routinely used for thoracic studies. For purposes of the present study, images obtained by means of computerized radiography were printed on film and viewed without the possibility of magnification or adjustment of viewing parameters.

Intra- and interobserver agreement and diagnostic accuracy—Thoracic radiographs from case and control cats were subsequently examined in random order by 5 individuals (an individual completing an internship in small animal medicine and surgery [JD], a veterinarian in general practice [MDeC], an individual board-certified in veterinary internal medicine [MD], an individual recently board-certified in veterinary radiology [KA], and a trained radiologist with 30 years of experience teaching veterinary radiology [LB]) not otherwise involved in the study. The 2 radiologists were not the same as the 2 radiologists who examined radiographs from case cats to determine prevalence of radiographic abnormalities. Examiners were unaware of the proportion of cats in each group, but were aware of the objective of the study. An evaluation period of 2 min/case was suggested to best simulate the clinical situation. After a delay of 1 week, radiographs were interpreted a second time in a different order by the same examiners.

For each interpretation, each examiner was asked to state whether radiographs were normal or compatible with a diagnosis of FBD and to assign a grade indicating how confident they were in their diagnosis (0 = uncertain; 1 = certain; and 2 = highly certain).

Statistical analysis—The Student t test was used to compare the age and weight between case and control cats and confirm the similarity of these populations. The Student t test was also used to compare lung inflation ratios between case and control cats. For evaluation of intra- and interobserver agreement, the N statistic was calculated, and level of agreement was classified as excellent (N between 0.93 and 1.0), very good (N between 0.81 and 0.92), good (N between 0.61 and 0.80), fair (N between 0.41 and 0.60), slight (N between 0.21 and 0.40), poor (N between 0.01 and 0.20), or none (N < 0.01).9 Sensitivity, specificity, and accuracy were calculated for the first and second examinations of each of the 5 examiners, with the clinical diagnosis considered the gold standard. For results of the initial examination of radiographs by each of the 5 examiners, the F2 test was used to determine whether the diagnosis made by the examiner (correct vs incorrect) was significantly associated with severity of a bronchial pattern (absent or mild vs moderate or severe), as assigned by the initial 2 radiologists, and whether examiner diagnosis (correct vs incorrect) was significantly associated with degree of examiner certainty (uncertain, certain, or highly certain). The Cochran-Mantel-Haenszel test was used to determine whether degree of examiner certainty (uncertain, certain, or highly certain) was significantly associated with severity of a bronchial pattern (absent or mild vs moderate or severe). Logistic regression was used to examine the association between presence of bronchial wall mineralization and age in control cats. All analyses were performed with standard software.a For all analyses, a value of P < 0.05 was considered significant.

Results

Signalment—Forty case and 40 control cats were included in the study. Age of cats with FBD (mean ± SD, 6.1 ± 4.1 years) was not significantly (P < 0.88) different from age of control cats (6.2 ± 4.1 years); body weight of cats with FBD (5.0 ± 1.7 kg [11.0 ± 3.7 lb]) was not significantly (P < 0.37) different from body weight of control cats (4.7 ± 1.4 kg [10.3 ± 3.1 lb]). Cats with FBD included 30 domestic shorthair cats, 3 Siamese, 3 Abyssinians, 1 Himalayan, 1 Maine Coon, 1 Persian, and 1 Russian Blue. Control cats included 31 domestic shorthair cats, 3 Siamese, 3 Himalayans, 1 Abyssinian, 1 Persian, and 1 Bombay. Cats with FBD consisted of 15 spayed females, 20 castrated males, 1 sexually intact female, and 4 sexually intact males. Control cats consisted of 17 spayed females, 20 castrated males, and 3 sexually intact males.

Prevalence of radiographic abnormalities—Radiographic abnormalities were observed in all 40 cats with FBD. A bronchial pattern was identified in 37 (93%) cats and was classified as mild in 13 (33%), moderate in 20 (50%), and severe in 4 (10%; Figure 2). The bronchial pattern was classified as ill-defined in 26 cats and as well-defined in 11. An unstructured interstitial pattern was seen in 30 (75%) cats; it was described as focal in 2 cats, diffuse in 28, uniform in 16, and heterogeneous in 14. Bronchial wall mineralization was identified in 25 (63%) cats. Nodular, tubular, or amorphous pulmonary soft tissue opacities were observed in 11 (28%) cats, and 8 of the 11 had a moderate or severe bronchial pattern (Figure 3). A lobar alveolar pattern was present in 2 (5%) cats, 1 of which had atelectasis of the right cranial lung lobe. Although distinct bullae were not seen, ill-defined lung hyperlucencies with variable distribution were observed in 21 (53%) cats (Figure 4). The thoracic wall appeared excessively convex on the ventrodorsal radiographic view in 26 (65%) cats, and excessive caudal displacement and flattening of the diaphragm were observed on the lateral radiographic view in 31 (78%) cats; both of these findings were considered suggestive of hyperinflation. Aerophagia (n = 19 [48%] cats), bronchiectasis (7 [18%]), and a mediastinal shift (4 [10%]) were also detected. Vascular tortuosity or dilatation was observed in 7 (18%) cats, and mild cardiomegaly was observed in 7 (18%); 2 cats had a combination of these signs. Echocardiography was performed in 6 of the 7 cats with mild cardiomegaly; 2 cats did not have any echocardiographic abnormalities, 2 had mild hypertrophic cardiomyopathy, 1 had an atrial septal defect, and 1 had signs of mild pulmonary hypertension. The mild echocardiographic abnormalities in these cats and lack of left atrial dilatation in conjunction with results of follow-up examinations indicated that cats did not have heart failure at the time FBD was diagnosed.

Figure 2—
Figure 2—

Lateral (A) and ventrodorsal (B) radiographic views of the thorax in a cat with FBD. Notice the moderate, diffuse, ill-defined bronchial pattern. The right cranial lung lobe is collapsed (*), resulting in shifting of the cardiac silhouette to that side; an alveolar pattern is present, causing a lobar sign at the caudal margin of the affected lobe (black arrows). There is caudal displacement and flattening of the diaphragm (white arrowheads) on the lateral view and convexity of the thorax on the ventrodorsal view, indicating hyperinflation. Ill-defined, amorphous soft tissue opacities are present in the caudal left hemithorax on the ventrodorsal view (white arrows).

Citation: Journal of the American Veterinary Medical Association 234, 3; 10.2460/javma.234.3.367

Figure 3—
Figure 3—

Lateral radiographic view of the thorax in a cat with FBD. Notice the numerous nodular soft tissue opacities in the caudal lung field that were presumed to represent organized, inflammatory lesions (granulomas).

Citation: Journal of the American Veterinary Medical Association 234, 3; 10.2460/javma.234.3.367

Figure 4—
Figure 4—

Ventrodorsal radiographic view of the thorax in a cat with FBD. The cardiac silhouette is mildly enlarged and displaced to the left. Additionally, some of the pulmonary arteries (between arrowheads) are enlarged. A multifocal pulmonary unstructured interstitial pattern is present in the caudal lobes (arrows) as well as an area of lung hyperlucency (*).

Citation: Journal of the American Veterinary Medical Association 234, 3; 10.2460/javma.234.3.367

Bronchial wall mineralization was identified in 21 of the 40 (53%) control cats (Figure 5). The presence of bronchial wall mineralization was not significantly (P = 0.61) associated with age in control cats. Width of the lung field at the level of T7 divided by the width of T7 and maximal width of the lung field divided by the width of T7 were significantly (P < 0.001) higher in cats with FBD (mean ± SD, 8.0 ± 1.5 and 11.3 ± 1.6, respectively) than in control cats (6.6 ± 0.9 and 8.8 ± 1.2, respectively). However, ratios of lung inflation measured on lateral radiographic views did not differ significantly (P ≥ 0.15) between groups.

Figure 5—
Figure 5—

Lateral radiographic view of the thorax in a cat without evidence of thoracic disease. Some of the larger bronchial walls (arrows) appear excessively opaque, while remaining of normal thickness, suggesting incidental mineralization.

Citation: Journal of the American Veterinary Medical Association 234, 3; 10.2460/javma.234.3.367

All 3 cats with FBD that did not have a bronchial pattern at the time of diagnosis had other radiographic abnormalities, including a diffuse, uniform interstitial lung pattern (mild in 1 and moderate in 1), a focal alveolar pattern (1), lung hyperlucencies (1), aerophagia (2), and flattening of the diaphragm (1).

Diagnostic accuracy and intra- and interobserver agreement—When radiographs for the 40 case and 40 control cats were examined, the experienced radiologist had the highest sensitivity but also the lowest specificity (Table 1). For all 5 examiners, sensitivity, specificity, and accuracy did not differ significantly (0.054 ≤ P ≤ 0.690) between the first and second readings, except that for the experienced radiologist, specificity during the second reading was significantly (P = 0.009) lower than specificity during the first reading.

Table 1—

Sensitivity, specificity, and accuracy in the diagnosis of FBD for 5 individuals with various levels of experience in radiographic interpretation.

ExaminerSensitivity (%)Specificity (%)Accuracy (%)
FirstSecondFirstSecondFirstSecond
Intern708565736879
Practitioner756878707669
Internist757568637169
Inexperienced radiologist808360537068
Experienced radiologist85934838*6665

Each individual examined radiographs from 40 cats with FBD and 40 cats without thoracic disease on 2 separate occasions.

* Significantly (P = 0.009) different from value obtained during the first reading.

For all 5 examiners, there was good agreement between results of the first and second readings (intern, N = 0.47; practitioner, N = 0.55; internist, N = 0.60; inexperienced radiologist, N = 0.58; and experienced radiologist, N = 0.59). Interobserver agreement ranged from 0.22 to 0.70 (Table 2), suggesting poor to good agreement between examiners. For 4 of the 5 examiners, there was a significant association between examiner diagnosis (correct vs incorrect) and degree of examiner certainty (uncertain, certain, or highly certain; intern, P < 0.001; practitioner, P = 0.065; internist, P = 0.008; inexperienced radiologist, P = 0.006; and experienced radiologist, P = 0.003). For all 5 examiners, there was a significant association between examiner diagnosis at the first reading (correct vs incorrect) and severity of a bronchial pattern (absent or mild vs moderate or severe; Table 3). Degree of examiner certainty (uncertain, certain, or highly certain) at the first reading was significantly associated with severity of a bronchial pattern (absent or mild vs moderate or severe) for 4 of the 5 examiners (the only exception was the internist; Table 4).

Table 2—

Interobserver agreement (κ values) with regard to radiographic diagnosis of FBD among 5 individuals with various levels of experience in radiographic interpretation.

ExaminerExaminer
 PractitionerInternistInexperienced radiologistExperienced radiologist
Intern0.53/0.550.27/0.290.50/0.400.26/0.22
Practitioner0.70/0.450.38/0.280.26/0.24
Internist0.32/0.300.23/0.28
Inexperienced radiologist0.38/0.40

Each individual examined radiographs from 40 cats with FBD and 40 cats without thoracic disease on 2 separate occasions. Data represent κ value for first reading/κ value for second reading.

Table 3—

Association between bronchial pattern severity and diagnosis for 5 examiners with various levels of experience in radiographic interpretation examining radiographs from 40 cats with FBD.

ExaminerDiagnosisBronchial pattern severityP value
Absent or mild (n = 16)Moderate or severe (n = 24)
Intern< 0.001
Correct622
Incorrect102
Practitioner< 0.001
Correct723
Incorrect91
Internist< 0.001
Correct624
Incorrect100
Inexperienced radiologist< 0.001
Correct824
Incorrect80
Experienced radiologist0.03
Correct1123
Incorrect51

Data represent number of cats.

Table 4—

Association between bronchial pattern severity and level of certainty in the diagnosis for 5 examiners with various levels of experience in radiographic interpretation examining radiographs from 40 cats with FBD.

ExaminerLevel of certaintyBronchial pattern severityP value
Absent or mild (n = 16)Moderate or severe (n = 24) 
Intern0.02
Uncertain73
Certain78
Highly certain213
Practitioner0.006
Uncertain104
Certain47
Highly certain213
Internist0.18
Uncertain21
Certain87
Highly certain616
Inexperienced radiologist< 0.001
Uncertain10
Certain101
Highly certain523
Experienced radiologist< 0.001
Uncertain10
Certain91
Highly certain623

Data represent number of cats.

Discussion

Results of the present study suggested that a bronchial pattern was the most common radiographic abnormality in cats with FBD, followed by an unstructured interstitial pattern and signs of lung hyperinflation. When 5 examiners with various levels of experience were asked to examine radiographs to determine whether cats had FBD, intraobserver agreement was good, but interobserver agreement was more variable, ranging from poor to good. Whether the examiner made a correct diagnosis was significantly associated with degree of examiner certainty and with severity of a bronchial pattern. Moreover, a more severe bronchial pattern was associated with a higher degree of examiner certainty for all but 1 examiner.

A bronchial pattern on thoracic radiographs of cats with FBD is typically attributed to bronchial wall thickening secondary to inflammatory infiltrates and local edema and has been described as an excessive number of ring-like and linear soft tissue opacities.1,5,6,8 As expected, a bronchial pattern was the most common radiographic abnormality in cats with FBD in the present study. However, the prevalence of this pattern among cats in the present study (93%) was higher than prevalences reported previously (59%2 and 90%3). This difference may be explained, at least in part, by differences in disease severity or chronicity between study populations. Importantly, the prevalence of this abnormality may be lower in cats examined in general practice than in cats examined following referral to a teaching hospital.

In 26 of the 37 cats in the present study with a bronchial pattern, bronchial margins were classified as ill-defined. Bronchial margins may be ill-defined on radiographs if the inflammatory process extends into the peribronchial area and secretions accumulate in the lumen and better defined if bronchial walls are thickened and there is less active inflammation.10,11

Previous studies12,13 have shown that many clinically normal dogs can have subclinical, age-related pulmonary radiographic changes, including bronchial mineralization and an unstructured interstitial pattern. In contrast, to our knowledge, bronchial wall mineralization in clinically normal cats has not been reported previously. In the present study, 2 radiologists identified bronchial wall mineralization when bronchial walls appeared excessively opaque, regardless of their thickness. However, whether the increased visibility of these walls was truly the result of a mineral content is uncertain. The use of computerized radiography with an edge-sharpening algorithm that enhanced the visibility of bronchial walls may have artifactually created the impression of mineralization. The increased wall visibility may also have resulted in the appearance of ring-like and linear opacities that could easily be confused with a true bronchial pattern. Although bronchial wall mineralization was observed in many of the cats with FBD in the present study (25/40 [63%]), it was also common in control cats without thoracic disease (21/40 [53%]). Furthermore, we did not detect a significant association between bronchial wall mineralization and age in the control cats.

An unstructured interstitial pattern was observed in 30 (75%) cats with FBD in the present study. Again, this was higher than prevalences reported in previous studies (24%,2 32%,3 and 46%7). This pattern may result from an extension of the inflammatory process beyond the bronchial borders and into the interstitial tissues.11

Lung hyperinflation was the third most common radiographic abnormality in the present study, with an excessively convex appearance to the thoracic wall on the ventrodorsal view in 26 (65%) cats and excessive caudal displacement and flattening of the diaphragm on the lateral view in 31 (78%) cats. Lung hyperinflation is mainly a result of air trapping in small airways caused by smooth muscle hypertrophy, bronchospasm, airway edema, and cellular infiltrates.14 It can eventually lead to emphysema.15 A mediastinal shift was identified in 4 (10%) cats without atelectasis and could have possibly been due to asymmetric lung hyperinflation.

To more objectively assess lung hyperinflation, we calculated 4 ratios that compared lung field size to the size of a fixed, central landmark (T7). Although both ratios of lung inflation measured on the lateral radiographic view did not differ significantly between cats with FBD and control cats, both ratios measured on the ventrodorsal radiographic view were significantly higher in cats with FBD, indicating that cats with FBD had a wider lung field than did the control cats. Whether these ratios can be useful clinically is still unclear. Additionally, we were unable to determine what impact, if any, the stage of respiration at the time radiographs were obtained (eg, full inspiration vs expiration) might have had on these ratios.

Air trapping and pulmonary emphysema15–17 can be manifested radiographically as ill-defined lung hyperlucencies or well-defined air-filled bullae.4,15,16,18 Although bullae were not observed in any of the cats with FBD in the present study, ill-defined focal or diffuse hyperlucencies were observed in 21 (53%).

Bronchiectasis is defined as congenital or acquired pathologic destruction of the elastic and muscular components of the bronchial walls causing chronic bronchial dilatation and distortion.17 These morphologic wall changes can be categorized as cylindric, tubular, saccular, cystic, or varicose15,17 and may be focal or diffuse. In cats, bronchiectasis is a rare consequence of chronic bronchitis or bronchiolitis, obstructive neoplasia, and bronchopneumonia.19 The prevalence of bronchiectasis among cats with FBD in the present study (7/40 [18%]) appeared high, compared with a previous report19 of 12 cats with histologically confirmed bronchiectasis identified over a period of 12 years. However, because histologic examination is infrequently performed on cats with FBD, the prevalence of this anomaly may be underestimated. Other studies4,7 in cats with bronchitis and other bronchopulmonary diseases have not reported radiographic evidence of bronchiectasis; however, it is unclear whether this sign was specifically evaluated. The use of computerized radiography with edge enhancement may have allowed better visibility of bronchial walls in the present study. Unfortunately, the accuracy of our results could not be validated histologically.

Lobar atelectasis may be seen in cats with FBD2 and may be associated with an ipsilateral mediastinal shift.1,16 The right middle lung lobe is most susceptible to collapse, which may be attributable to the fact that the orientation and smaller size of its main bronchus could predispose it to mucus accumulation, resulting in obstruction.11,16 Right middle lung lobe atelectasis was previously reported in 11% of asthmatic cats with a bronchial pattern,7 and collapse of the caudal portion of the left cranial lobe has been reported less commonly.2,3,7,11 Interestingly, lobar atelectasis was seen in only one of the cats with FBD in the present study and affected the right cranial lobe.

Nodular, tubular, or amorphous pulmonary soft tissue opacities seen in 11 (28%) cats with FBD in the present study were most likely a result of granuloma formation or mucus plugs filling the airways.14,16 Some of these opacities were poorly defined and may have been a result of unstructured interstitial or alveolar inflammatory infiltrates. Such opacities could easily be confused for primary or metastatic neoplasms,16,20 both of which can cause bronchointerstitial patterns because of neoplastic infiltration along airways.21 Interpretation can therefore be challenging in cats with this combination of radiographic signs.

Cardiomegaly and vascular tortuosity or dilatation were each observed in 7 (18%) cats with FBD in the present study, but only 2 cats had a combination of these signs. Cardiac enlargement has been reported previously in cats with FBD and characterized as mild right-sided cardiomegaly (cor pulmonale)3,6,7 caused by pulmonary hypertension secondary to chronic lung disease. As there were no signs of cardiovascular disease on physical examination in any of these cats, echocardiography was not routinely performed. However, in all cats, there were sufficient follow-up data to reasonably rule out concurrent heart failure as contributing to the radiographic abnormalities. Serologic testing for heartworm disease was not performed because of the low prevalence of heartworm disease among cats in Quebec at that time.

Respiratory distress is often associated with aerophagia.7 Gas filling the esophagus or stomach was a common finding among cats with FBD in the present study (19/40 [48%]), and prevalence was higher than prevalence reported in a previous study7 (20%).

In addition to assessing the prevalence of various radiographic abnormalities in cats with FBD in the present study, we were also interested in assessing observer variability in radiographic diagnosis, which reflects how useful radiography would be in the diagnosis of this condition. Sensitivity, specificity, and accuracy of radiography for diagnosis of FBD varied considerably among the 5 examiners included in the study, with the experienced radiologist having the highest sensitivity but the lowest specificity. Surprisingly, the least experienced examiner had the highest accuracy, and accuracy progressively decreased with experience, although significant differences among examiners were not identified. The fact that diagnostic accuracy of thoracic radiography was only moderate (65% to 78.8%) may be explained by several elements. First, 16 (40%) cats with FBD had no or only a mild bronchial pattern, and bronchial wall mineralization was present in more than half of the control cats and the cats with FBD. Because the presence or absence of a bronchial pattern appeared to be an important decision criterion for all 5 examiners, cats with minimal bronchial changes or mineralization were more likely to be miscategorized.

In the present study, all 5 examiners were fairly consistent in identifying cats as having or not having FBD when radiographs were reexamined (ie, good intraobserver agreement). However, the agreement between examiners was more variable and generally lower. Interestingly, the examiners with the most experience in radiographic interpretation (ie, the 2 radiologists and the internist) had the highest values for intraobserver agreement (ie, N values of 0.58, 0.59, and 0.60) but had low values for interobserver agreement. This was consistent with results of previous studies that evaluated the accuracy of radiography and observer variability in the diagnosis of bronchitis in dogs13 and inflammatory airway disease in horses.22,23 In the study13 involving dogs with bronchitis, it was concluded that thoracic radiography was a low-yield diagnostic method, mainly because of its low sensitivity for bronchial lesions.

Importantly, in the present study, we found a significant association between examiner diagnosis (correct vs incorrect) and severity of a bronchial pattern for all 5 examiners. Thus, regardless of level of experience for the individual interpreting the radiographs, it appeared that detection of a bronchial pattern was a reliable indicator of FBD if the bronchial pattern was moderate or severe. Moreover, for all but 1 examiner, a more severe bronchial pattern was associated with a higher degree of examiner certainty in the diagnosis. However, because cats in the present study had been referred to a veterinary teaching hospital, it was likely that they had more chronic disease and, possibly, more severe bronchial patterns than did the typical patient seen in general practice. Thus, diagnostic accuracy may be lower when thoracic radiography is used in general practice.

There were several limitations to the present study. Most importantly, there is no definitive way to diagnose FBD, and the diagnosis is often made on the basis of clinical, laboratory, and imaging findings. Thus, our criteria for case selection may have introduced some degree of bias because radiography had been used as a criterion when making the diagnosis. An effort was made to only select cases that fit our strict inclusion criteria. However, given the retrospective nature of the study, it is possible that some cats identified as having FBD may have been misdiagnosed. Second, because all of the cats with FBD were treated after the diagnosis was made, histologic confirmation of the radiographic signs identified in the present study was not possible. Although findings such as pulmonary soft tissue nodules could have been related to a separate process, such as neoplasia or granulomatous disease, response to treatment and results of follow-up radiography made these processes highly unlikely. Third, the 2 radiologists determining prevalence of radiographic abnormalities were aware of the clinical diagnosis, which may have caused a bias toward identifying signs that have previously been reported as being associated with FBD. To limit this bias, the 2 radiologists were asked to work together to arrive at consensus scores for all findings. Fourth, as mentioned before, our study population was composed of cats evaluated at a veterinary teaching hospital, and it is likely that these cats were generally more severely affected than cats encountered in routine veterinary practice, in which disease severity and radiographic signs may be more subtle. Finally, part of the selection criteria for our control group was that thoracic radiographs had to have been interpreted as normal. Therefore, it was not possible to accurately compare the prevalence of various radiographic signs in cats with FBD with prevalence in clinically normal cats. Indeed, many of the abnormalities identified in cats with FBD may also be seen in cats without any clinical signs of respiratory tract disease. Furthermore, some of these abnormalities may be seen in cats with other conditions, such as primary or metastatic pulmonary neoplasia and lung worms. Hence, sensitivity and specificity of the radiographic abnormalities identified in the present study when used to diagnose FBD remain to be determined.

In conclusion, several radiographic abnormalities can be observed in cats with FBD. However, our results highlighted the limitations of thoracic radiography in the diagnosis of this condition. Furthermore, our results demonstrated that examiner diagnosis (correct vs incorrect) and level of certainty in the diagnosis were both associated with severity of a bronchial pattern, regardless of the level of experience of the individual examining the radiographs. Therefore, a diagnosis of FBD must rely on clinical and laboratory findings as well as results of thoracic radiography.

ABBREVIATIONS

FBD

Feline bronchial disease

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