What is laryngeal paralysis?
Laryngeal paralysis occurs when the vocal folds are unable to abduct (open) in response to exercise or respiratory demands. The dorsal cricoayrtenoid muscle is the only laryngeal muscle involved in vocal fold abduction; the recurrent laryngeal nerve innervates this muscle.
Laryngeal paralysis is a common sub-clinical condition in many elderly dogs, and a small number of cats. Signs are often attributed to ‘old-age’ and are frequently overlooked by the owner. Rapid escalation into a respiratory crisis can occur, particularly as summer approaches and ambient temperatures rise. Concomitant systemic disease can also contribute to exposing sub-clinical laryngeal disease. Prompt and effective management of most laryngeal diseases can be associated with excellent resolution of clinical signs, and restoration of an acceptable quality of life.
What causes laryngeal paralysis?
There are several possible causes of laryngeal paralysis including congenital disease (in Bouvier de Flanders, Bull Terriers and Dalmatians especially), traumatic and neoplastic infiltration (e.g. from a mediastinal mass). However, idiopathic laryngeal paralysis is probably the most common laryngeal condition that will be encountered. Idiopathic laryngeal paralysis is usually seen in the middle-aged and older (median 9.5 years) large breed dog. However, it is important to remember that laryngeal paralysis can also occur in the smaller breed dog and even the cat. It is likely that differences in lifestyle and respiratory dynamics limit the clinical expression of the disease in these smaller animals.
How common in laryngeal paralysis?
Laryngeal paralysis is relatively common in the older dog, but does not always cause clinical problems. Frequently, the pathology affecting the larynx develops slowly. As a consequence, dogs with laryngeal disease may display relatively minor clinical signs, the significance of which may be overlooked by their owners. However, it is also possible for animals to present as an acute emergency with severe cyanosis and respiratory distress induced by a period of excitement, exercise or hot weather. Successful management of these conditions requires immediate and effective resuscitation, followed by prompt alleviation of the obstructive process.
What are the signs of laryngeal paralysis?
Laryngeal paralysis may cause the following clinical signs: exercise intolerance, inspiratory stridor, coughing and gagging, change or loss of voice. Many of the clinical signs of laryngeal disease that may be apparent (e.g. a soft cough, exercise intolerance), are often attributed to ‘old age’ or ‘heart disease’, particularly as they will develop insidiously. However, there are two audible features that are very characteristic of laryngeal disease – a soft ‘ineffectual’ cough, and inspiratory stridor. The clinician who is tuned into these noises is unlikely to overlook a diagnosis of laryngeal disease.
Many dogs with laryngeal paralysis are asymptomatic at rest. However, rapid decompensation of the respiratory status can occur if the dog becomes excited, is exercised more intensively than usual, or is unable to find a cool area on a hot day. When this happens, the dog can suddenly develop respiratory distress, with rapid escalation into a life-threatening crisis if appropriate action is not taken immediately.
How is laryngeal paralysis diagnosed?
Clinical recognition of laryngeal disease is usually very straightforward for the clinician who is experienced with the disease. The acoustic ‘footprint’ of laryngeal disease is very characteristic, and the sensitivity of physical examination alone has been shown to be more than 90% for clinicians experienced with the disease. Features of examination that are useful include direct auscultation of the larynx to aid localisation of the stridor to the larynx. In less severe cases, listening to the larynx before and after a short period of exercise may accentuate the stridor that is present.
Definitive diagnosis of laryngeal disease requires visualisation of the laryngeal structure, and correlation of movement of the vocal folds during respiration. This can sometimes be performed in the conscious dog with an ultrasound probe placed directly on the cricothyroid membrane, or more commonly, by direct visualisation of the larynx under a light plane of anaesthesia (laryngoscopy). Because of the high sensitivity of physical examination alone, laryngoscopy is usually unnecessary as an isolated procedure particularly if the dog is severely affected, and is usually performed prior to definitive surgical repair.
When laryngoscopy is performed, care must be taken to only assess laryngeal function with the animal under a light plane of anaesthetic. Dogs with clinically significant disease usually have complete paralysis of both vocal folds (Grade IV); but some may still retain some very mild muscle tone and movement on the right side (Grade III).
The other important aspect in the clinical diagnosis of laryngeal disease is the recognition of other concomitant or complicating disease factors. For examples, population studies have shown that sub-clinical laryngeal paralysis is remarkably common in the dog. Transition from sub-clinical to clinical disease may occur if the animal’s ability to compensate for the underlying laryngeal pathology is affected by secondary disease (e.g. aspiration pneumonia) or another unrelated disease (e.g. hypothyroidism, hyperadrenocorticism, obesity). Most dogs with laryngeal paralysis are elderly, and may have features of other endocrine or systemic disease on examination or blood work. The challenge for the clinician is to recognise the significance of these other disease processes on the presenting condition. In most cases, definitive surgical management of the laryngeal disease is the treatment of choice. In others, control of the underlying systemic disease will allow alleviation of the laryngeal compromise and thus delay the need for definitive surgical management of the laryngeal disease.
Routine diagnostic work-up of the patient with laryngeal disease should therefore include a full neurological examination, complete blood count, biochemistry profile and urinalysis, and thoracic radiography. Pursuit of any abnormalities detected in this diagnostic investigation will depend on the severity of the laryngeal disease, and the considered role of the abnormality on the presenting condition.
How is laryngeal paralysis treated?
Definitive surgical management of laryngeal paralysis is directed at permanently securing the vocal fold(s) in an abducted (open) position. Surgeons differ in their preference of suture location and placement, but clinical function appears to be similar regardless of technique. Most surgeons only secure a single vocal fold (usually the left). It has been stated that “the aim of surgery in the dog is … directed towards restoring an acceptable quality of life rather than achieving athletic function” and for the elderly dog, surgical stabilisation of just one vocal fold is generally considered to provide adequate airway function.
Laryngoplasty should only be performed by a surgeon who is experienced with the technique. The consequences of a failed procedure can be devastating for the patient, and there are limited salvage options available. In experienced hands, however, good success rates and reported, with few unexpected complications.
Are there any complications associated with surgery?
The most significant complication to develop following laryngeal paralysis surgery is aspiration pneumonia. This is more likely to occur in those animals with megaoesophagus or dysphagia prior to surgery, and every precaution should be taken to recognise these clinical features prior to surgery so that the owner may be warned of the greater prospect of complications. Careful feeding in the immediate post-operative period should be performed until normal swallowing ability is assured.
Aspiration pneumonia is more a consequence of the disease rather than being directly attributable to the surgery. This complication occurs due to progression of the loss of nerve function to the larynx, causing a reduction in the sensitivity of the gag reflex. Some disruption to the muscle tone of the oesophagus is also recognised in some affected dogs. The risk for a dog to develop aspiration pneumonia is highest in the 6 months after surgery, and often occurs when the dog is recumbent (e.g. sleeping).
Aspiration pneumonia can usually be successfully treated, but is a serious illness for an elderly dog to deal with. Prompt and aggressive treatment with antibiotics, fluid support and nursing care is important.
Recurrence of signs
A small proportion of dogs (<10%) may have a relapse of clinical signs in the weeks or months following surgery. In these animals, this may be because the suture holding the vocal fold open has failed prematurely, causing the airway to become restricted again. The cartilage in the older larynx can be very fragile and brittle, and the suture may fail to hold securely. The risk of this complication is highest in the first week or so after surgery. Although it is frustrating when this complication occurs, it is usually possible to rectify the problem by resuturing the original side, or operating on the opposite vocal fold.