Respiratory disasters and how we managed them-Part 2 (Proceedings)

Upper airway obstruction in cats

Upper airway obstruction is a rare syndrome in cats, but can be very severe and can cause life-threatening clinical signs. The clinical signs are similar to those reported in dogs. Care must be taken to ensure that a detailed evaluation of the patient is performed, as many of the causes of upper airway obstruction in this species cannot be easily distinguished from each other without anesthesia for laryngoscopy/pharyngoscopy and biopsy.

Nasopharyngeal polyps

• Inflammatory lesions that develop in the middle ear and extend into the pharynx through the auditory tube

• Common in young cats

• Usually accompanied by otitis media

• Nasal discharge and sneezing often accompany dyspnea

• Dysphagia and weight loss may occur

• Can be easily surgically removed but the ear disease must also be addressed

Congenital lesions

• Choanal atresia rarely occurs in some pure breed cats

• Upper airway obstruction and exercise intolerance predominate

• Diagnosis is by caudal rhinoscopy

Viral upper respiratory tract disease

• Occasional cats with severe viral upper respiratory tract disease present with signs of upper airway obstruction

• Fever and oral ulceration are common, nasal discharge may not occur

• Any age cat can be affected

• Concurrent viral pneumonia may be present

Laryngeal paralysis

• Rare in cats, middle aged to older cats most commonly affected

• The idiopathic acquired form is most common

• Gradual onset of clinical signs over weeks to months, progressing to dyspnea

• Change in voice is common

• Diagnosis requires anesthesia and laryngoscopy, can be treated with a laryngeal tie-back

Laryngeal or tracheal neoplasia

• Often older cats are most severely affected

• Signs may be gradual and prolonged in onset, especially in the case of neoplasms such as squamous cell carcinoma, or may be quite acute, as in cats with laryngeal lymphosarcoma

• Weight loss is common

Trauma

• Trauma can result in rupture of the trachea in cats; the two ends of the trachea may be completely separated from one another or may be held together by a thin membrane of peritracheal adventitial tissue.

• Motor vehicle accidents and endotracheal intubation are the most common causes of tracheal trauma in cats.

• Swelling or hematomas in the neck or in the pharynx can easily cause airway obstruction in cats; similar lesions would usually not cause any problem in dogs.

Inhaled foreign bodies

• Inhaled foreign bodies such as blades of grass can lodge in the larynx, resulting in profound respiratory distress, presumably because they can cause laryngospasm in this species.

• Inhaled foreign bodies can also lodge in the trachea, often at the carina. These may be visible on thoracic radiographs. Surgery to remove tracheal foreign bodies is profoundly risky and it can be very difficult to achieve a successful outcome.

Inflammatory laryngeal disease

• Can occur at any age, even in young cats.

• Histopathology may reveal acute or chronic neutrophilic or lymphocytic inflammation, or can be granulomatous.

• The history can be chronic or acute, and previous viral upper respiratory tract infection or intubation may be a feature.

• Laryngeal inflammation can be accompanied by fever and severe dyspnea.

• Laryngeal inflammation can be so severe that these cats often require tracheostomy.

• The laryngeal swelling can be so profound that it mimics a neoplasm.

• Biopsy is necessary to distinguish laryngeal inflammation from neoplasia.

• Bacterial infection of the larynx can occur (consider Pasteurella spp as a likely suspect) and culture of the laryngeal tissue or an endotracheal lavage should be considered.

• Once the crisis has been managed and the diagnosis confirmed, medical management with corticosteroids and antibiotics can be successful, but corticosteroids may be required on a long-term basis.

Collapsing trachea

Tracheal collapse is a progressive degenerative disorder of the tracheal cartilage seen in predominantly small breed dogs. Although numerous breeds can be affected, it is very common in miniature poodles, Yorkshire terriers and Pomeranians. The disorder is associated with abnormalities of the cartilage of the tracheal rings, the cartilage is softer than normal because it is composed of deficient glycosaminoglycans and contains decreased numbers of chondrocytes. The normal ring-shaped cartilages become C-shaped and flattened. The dorsal tracheal membrane becomes stretched and floppy, and to varying degrees falls down and obstructs the tracheal lumen. The physical changes in the airway can be accompanied by variable inflammation of the tracheal mucosa, resulting in edema and increased mucous accumulation, which further exacerbates the clinical signs.

Tracheal collapse is categorized by its location and severity. Specifically, collapse can be extrathoracic, at the thoracic inlet, intrathoracic, or mainstem bronchus. The extent of collapse is also graded by the severity of airway obstruction: grade 1 collapse obstructs 25% of the airway, grade 2 obstructs 50% of the lumen, grade 3 obstructs 75% of the lumen, and grade 4 completely obstructs the tracheal lumen.

The history of patients with tracheal collapse usually includes coughing, which is often characterized by a loud, harsh, goose-honk quality. The coughing is often paroxysmal and precipitated by excitement or exercise. Dogs may have stable disease or may experience gradual progression of the airway disease over time. Acute exacerbations of clinical signs may be precipitated by excitement or by concurrent problems such as pneumonia.

The physical examination is often unremarkable, except that there is an easily elicitable cough on tracheal palpation, and the trachea may feel softer than normal, with an easily palpated tracheal membrane. Physical examination changes relating to concurrent diseases such as hyperadrenocorticism and heart disease are common, probably reflecting the patient population rather than a specific relationship with tracheal collapse.

Diagnosis

Tracheal collapse can be highly suspected based on clinical suspicion and signs. Confirmation of the diagnosis can be made by numerous imaging techniques that allow visualization of the trachea. Plain radiographs of the neck and thorax can be helpful but are usually not definitive. Attempts can be made to radiograph the trachea during inspiration and expiration to document phasic changes in tracheal diameter. Similarly, the trachea can be radiographed during hyperextension and flexion of the neck, to document collapse during neck movement. Some collapse of the trachea can be normal, however, during neck hyperextension. Fluoroscopy is more useful than plain radiography, because it allows more specific documentation of the location and dynamic quality of tracheal collapse, particularly if the images are obtained while the patient is actively coughing. None of the plain radiographic and fluoroscopy studies distinguish whether the obstruction of the airway is occurring because of mucosal thickening or excess mucous accumulation, rather than because of anatomic changes in the cartilage or membrane. Similarly, they do not allow assessment of the presence of concurrent diseases such as laryngeal paralysis or chronic bronchitis.

Obtaining plain radiographs is a vital part of the initial evaluation of these patients to ensure that concurrent diseases such as left atrial enlargement or neoplasia are ruled out. Fluoroscopy is very important because it allows characterization of the collapse during coughing in an awake patient. The final and most definitive method of diagnosis is by performing tracheobronchosopy. Although anesthesia is somewhat risky in these patients, definitive diagnosis of the problem is a priority prior to performing invasive therapeutic procedures such as tracheal stent or extraluminal ring placement.

Tracheobronchoscopy initially allows confirmation that laryngeal function is normal, ruling out concurrent laryngeal paralysis. Then tracheobronchoscopy allows imaging of the lumen of the trachea and grading of the severity and location of tracheal collapse. Finally, imaging the bronchi allows the clinician to determine the extent to which mainstem bronchus collapse or chronic bronchitis might be contributing to the clinical signs. This is an important consideration prior to surgical intervention. Placement of a stent may be quite unsuccessful for control of the clinical signs if coughing is primarily caused by chronic bronchitis rather than tracheal collapse. Finally, tracheobronchosopy allows the clinician to obtain samples from the airway that can be submitted for cytologic evaluation and also for bacterial culture.

Medical management

Anti-tussives: Anti-tussive agents are one of the cornerstones of therapy; they are especially important when the cough is non-productive, and are often of considerable benefit when longterm coughing is interfering with the patient's ability to exercise and even to sleep. In such cases, the continued airway irritation caused by coughing can lead to more coughing, and thus can perpetuate a vicious cycle, which can be temporarily broken by anti-tussive agents. The primary drugs effective as anti-tussives are centrally acting opiate derivatives, which act on the cough center of the brain to depress its response to cough stimuli. Hydrocodone bitartrate (1.25-5 mg PO up to QID) is effective and widely used. It is a DEA Schedule CIII drug. Its main side effect is mild sedation, which can, however, prove to be helpful in some patients. Other prescription drugs, such as butorphanol tartrate (0.05-0.1 mg/kg PO, BID-QID) are also effective, with less central nervous system depression. Non-prescription drugs such as dextromethorphan (1-2 mg/kg PO, TID-QID) are available in various human proprietary cough mixtures, and play a useful role in symptomatic treatment of chronic cough.

Bronchodilators:Two classes of bronchodilators are widely used: methylxanthine derivatives and beta 2 agonists. Methylxanthine derivatives such as aminophylline (4-5.5 mg/kg PO TID) are well absorbed from the gastrointestinal tract. They are phosphodiesterase inhibitors that cause bronchodilation by decreasing the intracellular breakdown of cAMP. Recent studies have shown that they also act at the level of the diaphragm to increase its contractility and to render it less susceptible to fatigue. Thus, these agents may also prove useful in cases of chronic respiratory tract disease for reasons other than bronchodilation. The beta 2 agonists such as terbutaline sulfate (1.25-5 mg BID-TID) and albuterol (50 mg/kg PO BID-TID) activate adenylate cyclase and therefore increase intracellular camp, thus inducing bronchodilation.

Corticosteroids: Corticosteroids play an important role in therapy, but considering their negative side-effects, their use should be undertaken with caution. Anti-inflammatory doses of prednisone (0.5 mg/kg SID) can be beneficial. This dose can be effective in decreasing the inflammatory response, leading to reduction of secretions, and decreases in mucosal edema, airway thickening and bronchospasm. This results in clinical improvement in many patients, with decreased coughing and better exercise tolerance.

Surgical management

Surgical management of collapsing trachea involves either surgical placement of extraluminal tracheal rings, or endotracheal placement of intraluminal stents. Surgical management is usually reserved for only the minority of patients who are failing to respond to medical management.