Definition

• Static or dynamic reduction in the luminal diameter of the large conducting airway with respiration.
• Can involve the cervical trachea, the intrathoracic trachea, or both segments.
• Airway collapse (bronchomalacia) refers to collapse of lobar bronchi and smaller airways, which can be seen in conjunction with tracheal collapse (tracheobronchomalacia) or alone.
• Compression of the trachea or bronchi due to hilar lymphadenopathy or external mass lesions-not considered part of this condition.

Pathophysiology

• Hypocellular tracheal cartilage in the cervical region identified historically in some small-breed dogs.
• Lack of chondroitin sulfate and/or decreased glycoproteins within the cartilage matrix results in a reduction in bound water and loss of rigidity in the cartilage.
• Causes of bronchomalacia not established-cartilage abnormalities could include a mechanism similar to cervical tracheal collapse, defects in chondrogenesis, nutritional deficiencies, or degenerative changes caused by chronic airway disease.
• Collapse-weak tracheal cartilage allows flattening of the normal ring structure; trachea collapses in a dorsoventral direction when pressure fluctuates within the airway lumen. During inspiration, intrapleural pressure becomes more negative leading to a drop in intra-airway pressure. Atmospheric pressure exceeds airway pressure in the cervical region and lack of cartilage support results in cervical collapse. During forced expiration, intrapleural pressure becomes positive and exceeds intrathoracic intra-airway pressure. When cartilaginous airway walls are weakened by bronchomalacia, intrathoracic airway collapse occurs, on expiration.
• Increased tension on the trachealis dorsalis muscle or neurogenic atrophy of the muscle causes stretching of the dorsal tracheal membrane with protrusion into the airway lumen.
• Coughing-mechanical trauma to the tracheal mucosa from collapse of the dorsal tracheal membrane exacerbates airway edema and inflammation.
• Upper airway obstruction worsens clinical signs, and chronic increases in respiratory effort could lead to secondary abnormalities in laryngeal structure and function.
• Small airway disease augments the trans-airway pressure gradient and potentiates collapse in the intrathoracic region.

Systems Affected

• Respiratory-chronic airway irritation.
• Cardiovascular-pulmonary hypertension.
• Nervous-can be involved when syncope develops from hypoxia or a vasovagal reflex associated with cough.

Genetics

Unknown, tracheal collapse common in small toy breed dogs.

Incidence/Prevalence

Common clinical entity

Geographic Distribution

Worldwide

Signalment

Signs

Causes

• Unknown etiology-congenital, nutritional, or familial defects of chondrogenesis suspected.
• Chronic small airway inflammation suggested to contribute to bronchomalacia but relationship not clearly established.

Risk Factors

• Obesity
• Airway infection or inflammation
• Upper airway obstruction
• Endotracheal intubation

Differential Diagnosis

• Infectious tracheobronchitis
• Tracheal or laryngeal obstruction or foreign body
• Chronic bronchitis
• Pneumonia-viral, bacterial, fungal, parasitic, eosinophilic
• Bronchiectasis

CBC/Biochemistry/Urinalysis

• CBC-can show an inflammatory leukogram secondary to chronic stress or concurrent infection.
• Increased liver enzymes common.

Other Laboratory Tests

Elevated bile acids-mechanism unclear.

Imaging

Diagnostic Procedures

Caution is warranted in anesthetizing and intubating dogs with tracheal collapse because endotracheal tube irritation can worsen clinical signs. Loss of respiratory drive from anesthestic drugs or excess excitement on recovery can precipitate a crisis.

Pathologic Findings

• Dorsal trachealis muscle-elongated.
• Cartilage rings-flattened.
• Tracheal mucosal inflammation in some cases.
• Hypocellularity of the cartilage with low glycoproteins and chondroitin sulfate-can be noted via histopathologic examination or electron microscopy.
• Can also see changes associated with chronic inflammatory airway disease.

Appropriate Health Care

• Outpatient-stable patients.
• Inpatient-oxygen therapy and sedation for severe respiratory distress. Sedation and cough suppression-butorphanol (50 µg/kg SC); addition of acepromazine (25 µg/kg SC) can enhance sedative effects and further reduce the cough reflex.

Nursing Care

Oxygen therapy and sedation with butorphanol and/or acepromazine for severely distressed patients.

Activity

• Severely limited until patient is stable.
• During management of disease-gentle exercise recommended to encourage weight loss.

Diet

• Many affected dogs improve after losing weight.
• Institute weight-loss program with restriction of caloric intake; use a gradual weight-loss program (1–2% weight loss per week).

Client Education

• Warn client that weight gain, overexcitement, and humid conditions can precipitate a crisis.
• Advise client to use a harness instead of a collar.
• Advise owners that tracheal collapse is irreversible and that treatment strategies are designed to lessen triggers of cough.
• For surgical candidates, advise owner of the likelihood of complications after surgery (e.g., persistent cough, respiratory distress, or laryngeal paralysis); some patients can require a permanent tracheostomy. For stent candidates, advise owners of need for extensive follow-up to avoid stent fracture, migration, or granulation tissue formation.

Surgical Considerations

• Treatment of upper airway obstructive disorders (elongated soft palate, everted laryngeal saccules)-can reduce tracheal signs.
• Placement of extraluminal C-shaped rings by a skilled surgeon in selected patients with cervical collapse will enhance quality of life and reduce clinical signs when adequate stabilization of the airway can be achieved and when bronchomalacia does not limit resolution of disease.
• Intraluminal stents are life-saving in selected cases with intrathoracic airway collapse that fail aggressive medical management.

Drug(s) Of Choice

• Narcotic cough suppressants (butorphanol at 0.5–1 mg/kg PO q4–8h or hydrocodone at 0.22 mg/kg PO q4–8h) used to break the cycle of cough; reduce dose rate to the least frequent administration that controls signs.
• Reduction of tracheal inflammation-prednisone (0.5 mg/kg PO q12h then 0.25 mg/kg q12h) for a total of 5–7 days may help. Inhaled steroids given via facemask and spacing chamber are preferred to avoid systemic effects of panting and weight gain.
• Sustained-release theophylline (10 mg/kg PO q12h) -thought to reduce the pressure gradient in small airways and decrease cough in dogs with intrathoracic airway collapse-bronchodilators have no effect on tracheal diameter.
• Bacterial infection uncommon but doxycycline (3–5 mg/kg PO q12h) is sometimes beneficial, perhaps through reduction in bacteria within the airway or reduction of inflammation.

Recautions

Avoid long-term steroid use because of the propensity for weight gain and diseases associated with immunosuppression.

Possible Interactions

Theophylline metabolism-increased by concurrent treatment with ketoconazole or phenobarbital, which results in inadequate plasma concentration; decreased by fluoroquinolones (e.g., enrofloxacin), erythromycin, cimetidine, steroids, -blockers, mexiletine, and thiabendazole, which results in toxic plasma concentration and gastrointestinal upset, nervousness, or tachycardia; adjust dosages when concurrent use is necessary.

Alternative Drug(s)

Over-the-counter cough suppressants-rarely reduce cough.

Patient Monitoring

• Body weight
• Exercise tolerance
• Pattern of respiration
• Incidence of cough

Prevention/Avoidance

• Avoid obesity in breeds commonly afflicted.
• Avoid heat and humidity.
• Use harness rather than leash.

Possible Complications

Intractable respiratory distress leading to respiratory failure or euthanasia.

Expected Course and Prognosis

• Combinations of medications along with weight control can reduce clinical signs, but patient will likely cough throughout life and can suffer recurrent exacerbations of disease.
• Surgery-benefits many dogs with cervical collapse.
• Stent placement-benefits some dogs, primarily those with intrathoracic collapse. Medications usually required post-procedure.
• Prognosis-based on bronchoscopic evidence of airway obstruction and development of complications.

Associated Conditions

• Chronic bronchitis.
• Laryngeal paralysis.
• Pulmonary hypertension.
• Breeds of dogs that develop tracheal collapse also commonly have mitral insufficiency.

See Also

• Bronchitis, Chronic
• Canine Infectious Respiratory Disease

Client Education Handout Available Online