Korean Journal of
Veterinary Service

Tracheal collapse (TC) is a progressive, degenerative respiratory disease caused by the weakening and flattening of the cartilage rings (Maggiore, 2014). This disease is prevalent in small and toy breed dogs such as Pomeranians, miniature and toy poodles, Yorkshire terriers, Chihuahuas, and pugs, and it generally occurs with age (Moritz et al, 2004; Payne et al, 2006). The typical symptoms of TC include a goose-honking sound, difficulty breathing, and exercise intolerance.

The diagnosis of TC is made using radiography taken during both inspiration and expiration, as well as fluoroscopy (Macready et al, 2007). Additionally, tracheobronchoscopy and computed tomography (CT) can be useful adjunct imaging modalities (Payne et al, 2006). The recommended measures to alleviate TC symptoms include weight loss and exercise restriction. Medical management options may involve the use of bronchodilators, cough suppressants, tranquilizers, and corticosteroids (Tappin, 2016). Endoluminal tracheal stenting should be considered if patients do not respond to medical treatment or are in an emergency state of respiratory distress (Moritz et al, 2004; Weisse, 2015).

Lung lobe torsion (LLT) is a rare but potentially life-threatening condition in dogs, characterized by the axial rotation of a lung lobe and its pedicle (Neath et al, 2000). The rotation leads to impaired venous blood circulation, progressive lobar congestion, and subsequent alveolar edema, hemorrhage, and necrosis. While primary LLT occurs more frequently in dogs, small breed dogs are commonly affected by secondary LLT, which is associated with thoracic trauma, neoplasia, diaphragmatic hernia, and post-operative complications (Park et al, 2018; Rossanese et al, 2020). The present case report presents a Pomeranian dog that developed LLT four months after undergoing tracheal stenting for TC.

A 7-year-old castrated male Pomeranian weighing 2.7 kg presented with clinical signs of goose honking, severe panting, and dyspnea for two days and was referred to the Smart Animal Hospital. The dog had a history of TC diagnosed several years ago, and weight loss was recommended. No cardiac murmur was detected on auscultation, and thoracic radiographs showed a narrowed tracheal diameter at the thoracic inlet, while the cardiac silhouette and lung field appeared normal (Fig. 1A). The complete blood count and serum biochemistry parameters revealed no significant findings, except for an elevated concentration of C-reactive protein (172.8 mg/L, reference range 0∼35). The antitussive effect of intravenous (IV) butorphanol at a dose of 0.055∼0.11 mg/kg was temporary. Despite oxygen therapy and administration of medications such as aminophylline, dexamethasone, and codeine, the severity of respiratory distress did not improve. Thus, the owners consented to endoluminal tracheal stent placement.

Figure 1. (A) Thoracic radiographs showing a narrowed tracheal diameter at the thoracic inlet (arrow), with the vertebral heart score measuring at 10.4. (B, C) The maximal trachea diameters for stent selection measured on computed tomography images during positive-pressure ventilation at 20 cm H₂O.

Based on the maximal tracheal diameter measured on radiographic and CT images, a metal stent with a diameter of 8 mm and length of 80 mm (FAUNA STENT, Tracheal stent, MI tech, South Korea) was selected, taking into consideration the potential issue of stent shortening (Fig. 1B, 1C). Atropine 0.044 mg/kg was subcutaneously administered for premedication. Anesthesia was induced using IV propofol at a dose of 6 mg/kg and maintained using an air-oxygen mixture with isoflurane. The stent was implanted intratracheally under the guidance of fluoroscopy. The dog had an uneventful recovery, and there was a significant improvement in breathing pattern immediately after the procedure. The patient was discharged after three days of hospitalization, with the symptom of infrequent coughing. Medical treatment continued as before stent placement, and the use of prednisolone (0.5 mg/kg twice a day) was gradually tapered off until it was eventually discontinued. During the two-month follow-up period, the patient exhibited coughing only when excited or at night. As a result, all oral medications were discontinued, and the inhaled salmeterol/fluticasone propionate (Seretide 125 Evohaler, GlaxoSmithKline, Brentford, UK) was used only when necessary, one actuation twice a day.

Four months after the stent placement, the patient presented again with a history of reverse sneezing accompanied by a productive cough, exercise intolerance, and loss of appetite that had been ongoing for a week and had gradually worsened. There were no specific events that seemed to be related to the onset of these clinical signs. Radiographs showed an increase in cranial lobar opacity and fissure lines in both lungs (Fig. 2A, 2B). No significant findings were observed on the complete blood count and coagulation parameters, while the serum biochemistry revealed an elevated concentration of C-reactive protein (115.1 mg/L, reference range 0∼35). The results suggested that pneumonia was one of the leading differential diagnoses. However, despite treatment with oxygen, IV fluids, and antibiotics including marbofloxacin (2 mg/kg IV) and oral doxycycline (5 mg/kg), the patient’s symptoms worsened.

Figure 2. Lateral (A) and ventrodorsal (B) thoracic radiographs taken at presentation with respiratory distress showing an increase in cranial lobar opacity (arrows) and fissure lines (arrow heads) in both lungs.

A CT scan was conducted to investigate possible problems related to the tracheal stent or pulmonary fibrosis. The left cranial lung lobe was consolidated and swollen, with an abrupt ending of the left cranial bronchus and a trapped air bubble (Fig. 3). Emergency lung lobectomy was performed. Pre-oxygenation for 5 minutes and premedication with midazolam (0.1 mg/kg) and glycopyrrolate (0.05 mg/kg) were intravenously administered. General anesthesia was induced with IV propofol 6 mg/kg and maintained with isoflurane in an air-oxygen mixture. The patient was positioned in right lateral recumbency, and surgical exploration of the thoracic cavity was performed via a left fifth intercostal thoracotomy. A Finochietto retractor was used to gain exposure of the left hemithorax (Fig. 4A). Necrosis of the cranial part of the left cranial lung lobe and heavy congestion of the caudal part of the lobe were identified (Fig. 4B). The pulmonary arteries, veins, and lobular bronchus of the left cranial lobe were individually ligated using polyglyconate (Maxon, Covidien, Mansfield, Massachusetts). The cranial and caudal parts of the affected lung lobe were transected and submitted for histopathology. (Fig. 4C∼4E). The thoracic cavity was lavaged with warmed sterile normal saline. The resected pulmonary vessels’ margin and the bronchial margin were evaluated to ensure no hemorrhage or air leakage. The pleural cavity was suctioned, and a thoracostomy tube (ID-VAC Evacuator, INSUNG MEDICAL, South Korea) was placed at the left ninth intercostal space. Incised muscles, subcutaneous tissue, and skin were routinely closed. After recovering from anesthesia, the patient unfortunately developed postoperative complications, including hypotension and bradycardia. Despite medical intervention, the patient’s condition deteriorated, and the patient died on the second day after the surgery. A post-mortem examination could not be conducted as the owner did not consent.

Figure 3. Computed tomography images. (A) Consolidation and enlargement of the left cranial lung lobe in the dorsal view. (B) Abrupt ending of the left cranial bronchus and (C) Trapped air bubble of the left cranial lung lobe in the transverse images.

Figure 4. (A∼C) Surgical images of the left cranial lung lobectomy. (D) The necrotic excised cranial part of the left cranial lung lobe. (E) Heavy congestion of the resected caudal part of the lobe.

Histopathology was performed on the excised lung lobes. The parenchyma of the left cranial lobe showed coagulative necrosis characterized by eosinophilic outlines of remnant tissue that were replaced by hemorrhage and fibrin (Fig. 5A). Additionally, multifocal vascular thrombi of both medium and large vessels were observed. The remaining intact tissue showed marked collapse, making it difficult to discern any alveolar space, and the bronchi and bronchioles were also necrotic. Histopathological examination of the caudal portion of the left cranial lung lobe revealed marked peribronchiolar edema mixed with free red blood cells and fibrin (Fig. 5B). The intra-alveolar spaces were partially collapsed and filled with hemorrhage, fibrin, and macrophages, and the alveolar septa were thickened due to edema. The findings appeared to be consistent with LLT, for which the underlying cause could not be determined.

Figure 5. Histopathology of cranial (A) and caudal part (B) of the left cranial lung lobe. Coagulative necrosis, vascular thrombi, peribronchiolar edema, and collapsed intra-alveolar spaces were observed.

The etiology of TC remains unclear and is likely multifactorial. The hyaline cartilage in the trachea is replaced by fibrocartilage and collagen fibers, which have reduced levels of glycosaminoglycans and glycoproteins (Dallman et al, 1988). As a result, the cartilage loses its rigidity and ability to maintain normal tracheal conformation during the respiratory cycle, leading to dorsoventral flattening of the trachea. Intraluminal stenting to support the trachea can serve as a viable treatment option when affected dogs experience severe respiratory distress and have not responded to aggressive medical management (Moritz et al, 2004; Weisse, 2015). Our patient underwent tracheal stent placement due to severe respiratory distress accompanied by cyanosis and exhibited stable breathing right after the operation.

Intraluminal tracheal stenting offers several advantages, including minimal invasive placement, quick and effective relief of clinical signs, and better adaptability to various diameters (Weisse et al, 2019). However, there are risks associated with this procedure, such as tracheal irritation, respiratory infections, stent fracture, migration, shortening, and the formation of fibrotic scar tissue which are often related to stent sizing and stent placement technique (Chisnell and Pardo, 2015; Weisse, 2015). A retrospective study reported that minimal and acceptable mean stent shortening without substantial migration occurred in dogs that underwent stent placement for TC treatment, highlighting the importance of precise stent sizing and deployment technique (Raske et al, 2018). Stent sizing is generally performed using fluoroscopy or digital radiography. Ideally, a stent should be 2∼3 mm larger than the maximum diameter of trachea, ensuring good apposition of the stent and limiting the possibility of stent migration (Weisse, 2015). The maximal trachea diameter at the cervical, thoracic inlet, and intra-thoracic regions was measured during positive-pressure ventilation at 20 cm H₂O (Raske et al, 2018; Violette et al, 2019). The patient remained free of complications and experienced no issues for several months following tracheal stent placement.

In humans, lung torsion can be caused by trauma, thoracocentesis, thoracoscopic surgery, and various thoracic procedures including transesophageal operations, aorta repairs, and hernia corrections, although it may also occur spontaneously (Dai et al, 2016; Sahota and Anjum, 2023). Pediatric patients with lung torsion commonly have a history of intrathoracic procedures to correct congenital malformations, such as tracheoesophageal stricture, hiatal hernia, and patent ductus arteriosus (Sahota and Anjum, 2023). In dogs, LLT has been reported in association with thoracic trauma and hernia, with the majority of cases being idiopathic in nature (Benavides et al, 2019). When the patient in the present case study presented with lung torsion, no structural damage related to TC, as well as no observations of stent migration or fracture, were noted. Therefore, the condition was considered idiopathic.

The prognosis following lung lobectomy for idiopathic LLT is considered good to excellent, with an overall survival rate to discharge of 87∼95% (Benavides et al, 2019; Rossanese et al, 2020). There have been reports of pre- or post-operative deaths attributed to underlying tumors or trauma, and comorbid diseases including thrombosis, kidney disease, and myasthenia gravis (Murphy and Brisson, 2006; Benavides et al, 2019; Rossanese et al, 2020). Postoperative mortality or euthanasia was associated with complications such as pyothorax, chylothorax, aspiration pneumonia, hypoxemia, as well as ongoing issues with persistent pleural effusion and hemorrhage (Murphy and Brisson, 2006; Rossanese et al, 2020). Our patient was diagnosed with LLT through a CT examination 48 hours after being admitted to the veterinary hospital, and subsequently underwent lung lobectomy. Throughout this period, the patient consistently recorded low SpO2 levels despite oxygen supplementation. It is considered that the patient’s death was likely due to hypoxia and subsequent multiple organ damage. For 64% of dogs with LLT, the time from diagnosis to surgery was less than 24 hours after the initial presentation (Rossanese et al, 2020).

The differential diagnoses of a consolidated lung lobe on radiographs include pneumonia, edema, hemorrhage, atelectasis, and neoplasia. LLT is a relatively uncommon differential diagnosis. The vesicular pattern of the affected lung lobe, which is a specific finding of LLT, was observed in only about half of the thoracic radiographs (Benavides et al, 2019). Vesicular emphysema was seen only on CT images, not on radiographs, in the present case. CT is the most sensitive diagnostic tool for LLT because it visualizes the abnormalities of pulmonary and bronchial structure clearly, without superimposition of concurrent pleural effusion (Benavides et al, 2019).

LLT can be challenging to diagnose solely based on radiological findings, but it should be included in the differential diagnosis list when a vesicular pattern is observed in dogs with respiratory distress. In addition, to the best of the authors’ knowledge, this is the first case of LLT occurring in a dog with a stent placement for TC. Although a direct causality between TC or the stent and lung torsion was not established in this case, regular follow-up observations are necessary for both the tracheal stent and the thoracic cavity.

No potential conflict of interest relevant to this article was reported.