Tracheal Collapse

Tracheal collapse is a progressive, degenerative condition of the tracheal rings and dorsal tracheal membrane, leading to dynamic airway obstruction. It is most commonly seen in small and toy breed dogs, characterized by a dorsoventral flattening of the tracheal lumen due to weakening of the tracheal cartilage rings and laxity of the dorsal tracheal membrane. The collapse can occur in the cervical or intrathoracic trachea, or both, and is often associated with mainstem bronchial collapse. The condition is classified into four grades (I-IV) based on the degree of luminal obstruction observed during fluoroscopy or bronchoscopy. Surgical intervention is indicated for severe cases (grade III or IV) that are refractory to medical management, with procedures including extraluminal prosthetic ring placement, intraluminal stenting, or in select cases, tracheal resection and anastomosis. The disease significantly impacts respiratory function, quality of life, and can be life-threatening if not appropriately managed.

The exact etiology of tracheal collapse is multifactorial. Primary factors include a congenital or developmental weakness of the tracheal cartilage, characterized by a reduction in chondroitin sulfate and glycoprotein content, leading to decreased cartilage rigidity. This is often exacerbated by chronic respiratory disease, obesity, and environmental irritants such as smoke or dust. The dorsal tracheal membrane becomes lax and redundant, allowing the dorsal edges of the cartilage rings to approximate during inspiration (cervical collapse) or expiration (intrathoracic collapse). In some cases, there is an association with other airway abnormalities such as laryngeal paralysis or bronchomalacia. Traumatic causes are rare but can include blunt trauma to the neck or iatrogenic injury from previous surgery. The biomechanical trigger is the negative pressure generated during inspiration, which causes the weakened cartilage rings to flatten dorsoventrally, and the positive pressure during expiration, which can cause collapse of the intrathoracic portion. Cellular mechanisms involve degeneration of the cartilage matrix, with loss of glycosaminoglycans and abnormal collagen composition, leading to reduced tensile strength.

Tracheal collapse is primarily a disease of small and toy breed dogs, with a high incidence in Yorkshire Terriers, Pomeranians, Chihuahuas, Poodles, and Maltese. It is less common in cats, and when present, is often associated with trauma or neoplasia. The condition typically affects middle-aged to older dogs, with a mean age of onset around 6-7 years, but can be seen in younger dogs with congenital predisposition. There is no strong sex predilection, though some studies suggest a slight male predominance. Obesity is a significant risk factor, as it increases intra-abdominal pressure and respiratory effort, exacerbating the collapse. The disease is often progressive, and affected dogs may have concurrent respiratory conditions such as chronic bronchitis or laryngeal paralysis. In working dogs, the condition is rare due to their larger size, but can occur secondary to trauma or chronic respiratory disease.

The pathophysiology of tracheal collapse involves a combination of structural and dynamic factors. The tracheal cartilage rings normally maintain a C-shape, providing rigidity and patency. In affected dogs, the cartilage rings become hypocellular and the matrix loses its normal composition, with a decrease in chondroitin sulfate and an increase in collagen cross-linking, leading to reduced flexibility and strength. The dorsal tracheal membrane, which is composed of smooth muscle and elastic tissue, becomes stretched and redundant, allowing the dorsal edges of the cartilage rings to approximate. During inspiration, the negative intrathoracic pressure is transmitted to the cervical trachea, causing the dorsal membrane to be drawn into the lumen and the cartilage rings to flatten dorsoventrally, resulting in collapse of the cervical trachea. Conversely, during expiration, the positive intrathoracic pressure can cause collapse of the intrathoracic trachea and mainstem bronchi. This dynamic obstruction leads to turbulent airflow, increased respiratory effort, and chronic coughing. Over time, the chronic inflammation and irritation can lead to mucosal edema, increased mucus production, and secondary bacterial infections, further compromising the airway. The systemic inflammatory response may contribute to the progression of the disease, and in severe cases, respiratory distress and cyanosis can occur.

Intrinsic predisposing factors include breed-specific conformational traits, such as a small tracheal diameter relative to body size, and a genetic predisposition to cartilage weakness. Age-related degeneration of the cartilage matrix is also a factor. Obesity is a major extrinsic factor, as it increases the work of breathing and intra-abdominal pressure, which can exacerbate the collapse. Environmental factors such as exposure to cigarette smoke, dust, and other airborne irritants can cause chronic bronchitis, leading to increased coughing and further weakening of the tracheal cartilage. Concurrent respiratory diseases, such as laryngeal paralysis or bronchomalacia, can also predispose to tracheal collapse. Prior surgeries, such as tracheostomy or cervical surgery, may cause iatrogenic damage to the tracheal rings. Excessive exercise or excitement can trigger acute episodes of respiratory distress, but is not a primary cause.

The most common clinical sign is a chronic, harsh, dry cough, often described as a 'goose-honk' cough, which may be exacerbated by excitement, exercise, eating, or drinking. The cough is often paroxysmal and may be followed by retching or gagging. In severe cases, dogs may exhibit respiratory distress, with increased respiratory effort, cyanosis, and syncope. Physical examination may reveal a cough easily elicited by palpation of the trachea, and in some cases, a palpable flattening of the tracheal rings. Auscultation of the trachea may reveal crackles or wheezes, and lung sounds may be increased or have referred upper airway sounds. In cases with intrathoracic collapse, expiratory wheezes may be heard. Dogs may also show signs of exercise intolerance, and in advanced cases, weight loss and poor body condition. The severity of clinical signs often correlates with the grade of collapse, with grade I and II causing mild signs, and grade III and IV causing severe respiratory distress.

Differential diagnoses for tracheal collapse include: 1) Chronic bronchitis: Characterized by a chronic cough, but radiographs may show bronchial thickening, and bronchoscopy reveals inflammation without dynamic collapse. 2) Laryngeal paralysis: Presents with inspiratory stridor and respiratory distress, but laryngeal examination reveals absent or paradoxical arytenoid movement. 3) Tracheal neoplasia: Can cause a cough and respiratory distress, but radiographs or CT may show a mass, and bronchoscopy with biopsy is diagnostic. 4) Foreign body aspiration: Acute onset of cough and respiratory distress, with a history of possible ingestion, and imaging may show a radiopaque foreign body or air trapping. 5) Bronchomalacia: Often concurrent with tracheal collapse, but bronchoscopy shows collapse of the mainstem bronchi. 6) Heart disease, such as mitral regurgitation: Can cause coughing due to cardiomegaly and airway compression, but echocardiography and thoracic radiographs can differentiate. 7) Allergic bronchitis or asthma: More common in cats, but can cause cough and wheezing, with eosinophilic inflammation on cytology. 8) Infectious tracheobronchitis (kennel cough): Acute onset, often with a history of exposure, and may be self-limiting or respond to antibiotics. 9) Tracheal stenosis: Can be congenital or acquired, and may be due to previous intubation or trauma, with imaging showing a fixed narrowing. 10) Ciliary dyskinesia: A rare congenital condition causing chronic respiratory infections and cough, with a history of recurrent pneumonia.

The diagnostic algorithm for tracheal collapse begins with a thorough history and physical examination, including gentle palpation of the trachea to elicit a cough. Baseline thoracic radiographs, including inspiratory and expiratory views, may show a narrowing of the tracheal lumen, but are not always diagnostic. The gold standard for diagnosis is fluoroscopy, which allows dynamic assessment of the tracheal lumen during inspiration and expiration, and can identify the site and severity of collapse. Bronchoscopy is also valuable, as it allows direct visualization of the tracheal lumen, assessment of the degree of collapse, and collection of airway samples for cytology and culture. Advanced imaging such as CT can provide detailed anatomical information, especially for surgical planning, but is not dynamic. In cases where surgery is considered, a complete respiratory workup, including arterial blood gas analysis and cardiac evaluation, is recommended. The diagnostic algorithm should proceed from non-invasive to invasive, and the choice of diagnostic tests depends on the severity of clinical signs and the need for surgical intervention.

Laboratory findings in tracheal collapse are often non-specific. Hematology may show a stress leukogram or eosinophilia if there is concurrent allergic disease. Biochemistry may be normal, but in obese dogs, elevated liver enzymes or hyperlipidemia may be present. Arterial blood gas analysis may reveal hypoxemia and hypercapnia in severe cases, indicating respiratory compromise. Inflammatory biomarkers such as C-reactive protein (CRP) may be elevated in cases with secondary bacterial infection. Bronchoalveolar lavage (BAL) fluid analysis may show neutrophilic inflammation, with or without bacteria, and cytology can help rule out other causes of cough. Coagulation panel is not routinely indicated unless surgery is planned, but a baseline PT/aPTT is recommended to assess surgical risk.

Radiography: Standard lateral thoracic radiographs may show a narrowing of the tracheal lumen, but this is often not dynamic. Inspiratory and expiratory views can help demonstrate the collapse, with cervical collapse more evident on inspiration and intrathoracic collapse on expiration. The tracheal diameter can be compared to the thoracic inlet, but this is not always reliable. Fluoroscopy: This is the gold standard for diagnosis, as it allows real-time visualization of the tracheal lumen during breathing. It can accurately grade the collapse and identify the site (cervical, intrathoracic, or both). Bronchoscopy: This provides direct visualization of the tracheal lumen and allows assessment of the degree of collapse, as well as evaluation of the mainstem bronchi. It also allows collection of airway samples. CT: High-resolution CT can provide detailed anatomical information, including the shape of the tracheal rings and the degree of collapse, but it is not dynamic. It is useful for surgical planning, especially for stent placement. MRI: Not commonly used for tracheal collapse, but may be useful to evaluate for concurrent mediastinal masses. Angiography/Fluoroscopy: Not typically used, but can be helpful in cases with suspected vascular ring anomalies.

Cytology of bronchoalveolar lavage fluid may show neutrophilic inflammation, with or without bacteria, and can help rule out other causes of cough. Histopathology of tracheal cartilage from affected dogs shows degeneration of the cartilage matrix, with a decrease in chondroitin sulfate and an increase in collagen cross-linking. The cartilage rings may be thinner than normal, and the dorsal tracheal membrane may show myxomatous degeneration. In cases with chronic inflammation, there may be mucosal metaplasia and fibrosis. Histopathology is not routinely performed for diagnosis, but may be obtained during surgical procedures such as tracheal resection.

Medical management is the first line of treatment for mild to moderate cases, and includes weight loss, cough suppressants (e.g., hydrocodone 0.22 mg/kg PO q6-8h), bronchodilators (e.g., theophylline 10-20 mg/kg PO q12h), corticosteroids (e.g., prednisone 0.5-1 mg/kg PO q12h, tapering), and antibiotics for secondary bacterial infections. In severe cases, surgical intervention is indicated. Surgical options include: 1) Extraluminal prosthetic ring placement: This involves placing polypropylene or silicone rings around the trachea to support the cartilage. The rings are sized to fit the tracheal diameter and are sutured in place with non-absorbable sutures. This is most effective for cervical collapse. 2) Intraluminal stenting: This involves placing a self-expanding metallic stent into the tracheal lumen under fluoroscopic guidance. This is minimally invasive and can be used for both cervical and intrathoracic collapse. Complications include stent migration, fracture, and granulation tissue formation. 3) Tracheal resection and anastomosis: This is reserved for focal lesions, such as a single collapsed ring, and involves removing the affected segment and anastomosing the healthy ends. This is rarely performed due to the diffuse nature of the disease. Preoperative stabilization is crucial, including oxygen therapy, sedation, and corticosteroids to reduce inflammation. Postoperative care includes pain management, cough suppression, and strict rest. The choice of surgical technique depends on the location and severity of the collapse, as well as the surgeon's preference and expertise.

The prognosis for tracheal collapse is variable and depends on the severity of the disease, the presence of concurrent conditions, and the response to treatment. Medical management can control clinical signs in many cases, but the disease is often progressive. Surgical intervention can significantly improve quality of life in severe cases, with reported success rates of 75-90% for extraluminal ring placement and 80-95% for intraluminal stenting. However, complications such as stent fracture, migration, and granulation tissue formation can occur, and may require additional intervention. Negative prognostic indicators include severe intrathoracic collapse, concurrent bronchomalacia, and the presence of chronic bronchitis. Long-term survival is possible with appropriate management, but the disease can be life-threatening if not treated.

Postoperative follow-up is essential to monitor for complications and assess response to treatment. For extraluminal ring placement, radiographs should be taken immediately postoperatively and at 4, 8, and 12 weeks to assess ring position and tracheal patency. For intraluminal stenting, fluoroscopy or radiography should be performed at 1, 3, 6, and 12 months to check for stent migration or fracture. Activity should be restricted for 4-6 weeks postoperatively, with a gradual return to normal activity. Cough suppressants and anti-inflammatory medications may be continued as needed. Long-term monitoring includes regular physical examinations and owner education on weight management and avoidance of respiratory irritants. In cases with concurrent bronchomalacia, additional treatment may be necessary.

Clinical pearls: 1) Always perform fluoroscopy to accurately diagnose and grade tracheal collapse, as radiographs may be normal. 2) When placing extraluminal rings, ensure the rings are not too tight, as this can cause tracheal necrosis. 3) For intraluminal stenting, choose a stent that is 10-15% larger than the normal tracheal diameter to prevent migration. 4) In cases with concurrent laryngeal paralysis, consider surgical correction of the larynx before addressing the tracheal collapse. Pitfalls: 1) Avoid surgical intervention in dogs with mild clinical signs, as medical management is often effective. 2) Do not place extraluminal rings on the intrathoracic trachea, as this is technically difficult and can lead to complications. 3) Be aware that intraluminal stents can fracture, especially in active dogs, and may require removal or replacement. 4) Do not forget to address obesity, as weight loss is critical for long-term success.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery. Postoperative antibiotics: Amoxicillin-clavulanate 13.75-22 mg/kg PO q12h for 7-10 days. Analgesics: For postoperative pain, use opioids such as hydromorphone 0.05-0.1 mg/kg IV q4-6h or buprenorphine 0.01-0.02 mg/kg IV q6-8h. NSAIDs such as carprofen 2.2 mg/kg PO q12h or meloxicam 0.1 mg/kg PO q24h can be used for 3-5 days, but caution in patients with renal or hepatic disease. Local anesthetic blocks: For cervical surgery, a cervical paravertebral block with bupivacaine 1-2 mg/kg can provide analgesia. Cough suppressants: Hydrocodone 0.22 mg/kg PO q6-8h or butorphanol 0.05-0.1 mg/kg PO q6-12h. Bronchodilators: Theophylline 10-20 mg/kg PO q12h or terbutaline 0.01 mg/kg SC q6-8h. Corticosteroids: Prednisone 0.5-1 mg/kg PO q12h, tapering over 2-4 weeks. Sedatives: For preoperative stabilization, acepromazine 0.01-0.02 mg/kg IV or butorphanol 0.2-0.4 mg/kg IV. All dosages are based on Plumb's Veterinary Drug Handbook.

Landmark studies on tracheal collapse include: 1) A retrospective study by Buback et al. (1996) evaluating extraluminal ring placement in 50 dogs, reporting good to excellent outcomes in 80% of cases. 2) A study by Sura et al. (2008) comparing intraluminal stenting to extraluminal rings, showing similar success rates but a higher complication rate with stents. 3) A consensus statement from the American College of Veterinary Surgeons (ACVS) on the management of tracheal collapse, recommending medical management as first-line therapy and surgical intervention for severe cases. 4) A study by Johnson and Fales (2001) on the use of fluoroscopy for diagnosis, emphasizing its importance in grading the collapse. 5) A meta-analysis by Tappin et al. (2016) on the outcomes of intraluminal stenting, reporting a 90% improvement in clinical signs but a 20% complication rate. These studies support the current treatment recommendations and highlight the need for careful patient selection and owner counseling.