Barbara M. Kirby, BS, RN, DVM, MS, DACVS, DECVS, Australasian Registered Specialist in Small Animal Surgery
Section of Veterinary Surgery, School of Agriculture, Food Science and Veterinary Medicine, University College Dublin
Belfield, Dublin, Ireland
Laryngeal paralysis is common in dogs and horses, but rare in cats. It can be partial or complete, unilateral or bilateral, and congenital or acquired. Acquired laryngeal paralysis is often idiopathic, but can be iatrogenic, traumatic, neoplastic, or polyneuropathic.
Regardless of cause, complete laryngeal paralysis results in failure of the arytenoid cartilages and vocal folds to abduct during inspiration. This causes air flow resistance at the level of the rima glottidis during inspiration, resulting in turbulence leading to laryngeal oedema, which results in further decrease in size of the rima glottidis and increase in airflow resistance, resulting in a vicious cycle. In essence, this is an upper airway obstruction.
Presentation and Clinical Signs
The signalment and clinical presentation vary with the form of the disease. In the congenital form, English and Staffordshire bull terriers are most commonly affected in the UK, with onset of signs between 4 and 6 months of age. In contrast, the acquired idiopathic form of the disease affects middle-aged to elderly dogs. There are many predisposed breeds, including Saint Bernards, Labrador retrievers, Golden retrievers, Irish setters, other large and giant breeds, as well as occasional mixed bred dogs. Laryngeal paralysis is rare in cats, with no reported breed, sex, or age predilection.
Clinical signs may include voice change or hoarse bark, exercise intolerance, gagging, coughing, stridor, dyspnea, cyanosis, collapse, vomiting, absence of purring in cats, others. Clinical signs are often exacerbated by hot weather, exercise or excitement, or the stress associated with clinical examination.
Diagnostic work-up includes complete blood count, serum chemistry profile and thyroid function testing. Thoracic radiographs and lateral cervical radiographs are obtained to look for evidence of non-idiopathic primary causes of laryngeal paralysis, such as cervical or thoracic inlet masses. Thoracic radiographs are carefully evaluated for evidence of aspiration pneumonia. When present, aspiration pneumonia should be treated with appropriate antibiotics based on transtracheal wash or bronchoalveolar lavage sample for culture and sensitivity before surgery. In one retrospective study, 70% of dogs with laryngeal paralysis had abnormal thoracic radiographs including 20% with megaesophagus, 15% with bronchopneumonia, and 20% with narrowing or dilation of the extrathoracic trachea. It is also important to evaluate thoracic radiographs carefully for evidence of cardiogenic or noncardiogenic pulmonary edema and treat appropriately before surgery. These tests together with careful clinical examination will generally rule out the non-idiopathic causes of laryngeal paralysis. A full neurological examination is an important component of the evaluation of a suspected laryngeal paralysis case, particularly in cats.
Additional diagnostics warranted in selected cases of laryngeal paralysis include tracheoscopy or bronchoscopy, transtracheal wash or bronchoalveolar lavage, barium swallow under fluoroscopy, EMG's and/or nerve conduction studies, ACH antibodies and/or Tensilon response testing, or arterial blood gas measurements.
Definitive diagnosis is by direct laryngoscopy. A rigid laryngoscope with a long blade and good light source is required. A flexible stylet or long tongue depressor to elevate the soft palate is needed in some animals. The animal is positioned in ventral recumbency with the head suspended by the maxillae. A light plane of anaesthesia (I prefer IV propofol 2-6 mg/kg IV to effect after pre-medication with low-dose acepromazine and morphine) is required, just enough to look. Any anaesthetic agent can obliterate normal laryngeal function. The trick is to have the animal deep enough to prevent being bitten, but light enough that you are certain laryngeal movement is not absent because of drugs. It's useful to have another person watching the respiratory pattern and calling out 'in' and 'out' with inhalation and exhalation. Avoid touching the arytenoids or vocal folds during laryngoscopy as this may induce laryngospasm or cause asymmetrical movements of the larynx. Animals with laryngeal paralysis usually have a narrowed resting rima glottidis with erythema and edema of the mucosa overlying the arytenoid cartilages. They have absence of abduction of one or both sides (vocal folds plus arytenoid cartilages) during inspiration. They often have forced passive movement of the vocal folds during expiration, which mimics abduction, thus the importance of a second observer and 'in-out' calls. When in doubt, some examiners like to administer a dose of doxapram IV to stimulate exaggerated respiratory effort. In my opinion, this is not necessary.
Some confusion on the status of the patient's laryngeal function can be caused by caudal movement of the larynx by contraction of the accessory respiratory muscles, fluttering of the vocal folds during exhalation, movement of the vocal folds by contraction of the vocalis muscles, or vocal folds can be sucked in toward the midline during inspiration and appear to abduct as they passively return to their original position. Other conditions that need to be ruled-out during direct laryngoscopy include laryngeal collapse, other causes of laryngeal obstruction such as laryngeal neoplasia or laryngeal inflammatory disease, and brachycephalic upper airway syndrome anomalies including everted laryngeal saccules, elongated soft palate, etc.
Urgent treatment for severe respiratory distress includes a cool, non-stressful environment and sedation (morphine 0.25 to 0.5 mg/kg IV plus acepromazine 0.02 to 0.05 mg/kg IV). This often alleviates the crisis, but sometimes worsens it. If worse, general anaesthesia and endotracheal intubation plus specific surgery or temporary tracheostomy placement are required.
Arytenoid lateralisation ('tie-back') permanently fixes the arytenoid and vocal fold on one or both sides in abduction, enlarging the airway. Other surgical options include partial laryngectomy, castellated laryngofissure, and permanent tracheostomy.