Why an ‘Interstitial Pattern’ Isn’t Just Interstitial: CT Exposes Hidden Airway Disease in Dogs

The thoracic radiographic unstructured interstitial pattern underestimates and may fail to identify canine respiratory disease compared to computed tomography

R. Baumgardner; A. Vientos-Plotts; I. Masseau; C. Reinero

Background

In dogs, respiratory disease presenting with a thoracic radiographic unstructured interstitial pattern (UnIP) is diagnostically challenging due to nonspecific clinical signs and broad differential diagnoses. Although UnIP is traditionally associated with pulmonary interstitial disease, radiographic pattern terminology does not reliably reflect underlying anatomic localization. Computed tomography (CT) provides superior lesion detection and anatomic resolution, but data are limited on how CT findings compare with radiography in dogs presenting with a sole UnIP. The study aimed to determine whether dogs with a radiographic UnIP have CT-detectable disease beyond the pulmonary interstitium and whether CT-supported final diagnoses extend beyond those typically implied by a UnIP.

Methods

This retrospective study included 36 dogs with respiratory clinical signs, a sole UnIP on three-view thoracic radiographs, and thoracic CT performed within 14 days without change in clinical status. Thoracic CT scans were acquired using ventilator-assisted inspiratory and expiratory breath-holds and evaluated for four major CT pulmonary patterns and 14 subpatterns. Airway collapse was graded from the trachea to subsegmental bronchi. Final diagnoses were determined through comprehensive integration of clinical findings, imaging, endoscopy, cytology, microbiology, and histopathology when available, and classified as definitive or suspect.

Results

Thoracic CT identified disease extending beyond the pulmonary interstitium in all dogs with a radiographic UnIP. Thirty-five of 36 dogs had multiple CT patterns or subpatterns, most commonly ground-glass opacity, consolidation, peribronchovascular thickening, and subpleural interstitial thickening. Dynamic segmental or subsegmental airway collapse consistent with bronchomalacia was identified in 58% of dogs and was not detected on radiographs. Most dogs (92%) had more than one final diagnosis, with a median of five diagnoses per dog. Final diagnoses included large airway, small airway, parenchymal, and mixed airway–parenchymal disorders, with bronchomalacia being the most frequent definitive diagnosis.

Limitations

The study population was drawn from a tertiary referral center, which may limit generalizability. Its retrospective design resulted in variability in diagnostic workup across cases. Static radiographs were not standardized for respiratory phase, and obesity was not controlled for. Although CT minimized atelectasis through ventilator-assisted breath-holds, some recumbency-related artifacts may have persisted.

Conclusions

A thoracic radiographic UnIP frequently underestimates the extent and anatomic distribution of respiratory disease in dogs. CT reveals a wide range of large airway, small airway, and mixed airway–parenchymal disorders that are not apparent on radiographs, including dynamic bronchomalacia. The findings demonstrate that UnIP should not be assumed to represent isolated interstitial disease and support consideration of thoracic CT or expanded differentials when evaluating dogs with this radiographic pattern.

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