Coughing is a voluntary or reflex action in which deep inspiration is followed by forced expiration against a closed glottis. This requires strong contraction of the respiratory muscles and the larynx. The sudden opening of the glottis releases air in an explosive manner. Its passage through the vocal folds creates the characteristic sound. It may be followed by a terminal retch.
It is stimulated by irritation of the pharynx, larynx, trachea, extrapulmonary bronchi and rarely by other intrathoracic structures. The reflex is mediated by the medulla oblongata. Afferent input is supplied by the glossopharyngeal nerve (CN IX) innervating the pharynx and the vagus (CN X) which contains sensory input from the pharynx, larynx and thoracic structures. The motor arc is governed by the intercostal muscles (T1–12), the diaphragm (phrenic nerve, C5–7) and the larynx (CN X).
The Valsalva manoeuvre is forceful exhalation against the resistance of a closed mouth and nose, as occurs during vomiting, straining to defecate, and coughing with the subsequent release of resistance and exhalation. This gives rise to autonomically mediated cardiovascular reflexes with changes in heart rate and blood pressure and is the basis for ‘cough syncope’. Cough syncope may result from the increased intrathoracic pressure blocking venous return to the heart or strong vagal stimulation inducing vasodilation and bradycardia (vasovagal syncope).
Laryngeal paralysis is failure of abduction of the arytenoid cartilages by the dorsal cricoarytenoid muscles during the respiratory cycle. This causes a reduction in the diameter of the rima glottidis during the inspiratory phase, and clinical signs are most prominently exercise intolerance due to a reduced capacity to ventilate. Most dogs are classified as ‘idiopathic’ laryngeal paralysis and no underlying cause can be identified, but disease occurs as a result of loss of innervation of the dorsal cricoarytenoid muscle by the recurrent laryngeal nerves. The traditional view is that nerve dysfunction occurs as a result of the relative extreme length of these nerves. It has been more recently observed, however, that idiopathic laryngeal paralysis is part of a generalized neuropathy. Many other areas of the animal may therefore be affected, although the most prominent clinical sign is usually the laryngeal dysfunction. The underlying neuropathy in most cases is a disease that progresses over a period of weeks to years.
Idiopathic laryngeal paralysis typically occurs in large and giant breed dogs over the age of 9 to 10 years. It is commonly seen in Labrador retrievers, golden retrievers, collies and English and Irish setters. A congenital form of the disease has been described in the Bouvier des Flandres, Dalmatian, husky and bull terrier, with animals showing clinical signs at less than 1 year of age.
All animals presenting with laryngeal paralysis should be investigated for the presence of an underlying cause (see later in this chapter), and this should be especially thorough if the signalment is not typical. Other causes that should be ruled out are traumatic or surgical injury (usually to the neck), masses that interfere with the pathway of the recurrent laryngeal nerves (e.g. mediastinal or neck mass), endocrinopathies and infectious diseases. The prognosis for dogs following surgery that do not have ‘idiopathic’ laryngeal paralysis is likely to be worse and so it is imperative that the underlying cause is found out before surgery is embarked upon in these cases.
Dogs presenting with laryngeal paralysis usually have a gradual onset of respiratory stridor, cyanosis and decreasing exercise tolerance; there may be worsening of signs in hot weather as the dog will be attempting to shift more air through the larynx to thermoregulate. Syncope is sometimes seen with light as well as vigorous exercise and animals may present in severe respiratory distress. It is only with careful questioning that a more chronic history becomes apparent. A change in bark will have been apparent in some cases.
There may also be a history of coughing, sometimes with lethargy and pyrexia. There may be a low level of irritation due to small amounts of saliva and food entering the trachea, or if more severe aspiration occurs, aspiration pneumonia may result. This has previously been thought to occur because the vocal folds are not guarding the rima glottidis as effectively as normal; however it may also be due to reduced function of pharyngeal nerves and muscles caused by the generalized neuropathy. Owners may notice this as increased coughing during eating or drinking, or if the animal struggles or is slower to eat.
Megaoesophagus is sometimes reported with this condition, and so a history of regurgitation should be ruled out. If this is diagnosed, concurrent aspiration pneumonia should always be considered and the prognosis is more guarded.
The generalized neuropathy may also lead to pelvic limb weakness. This is usually mild and may be misdiagnosed as osteoarthritis as the animals are large breed and quite old. Owners sometimes report scuffing of the nails as the dog walks on hard surfaces, or stumbling on tight turns. Weakness on rising is very common. More severely affected dogs may be unable to rise and progression of this aspect of the disease is sometimes a cause for the animal to be euthanized.
General examination is usually unremarkable. The most prominent finding is noise on laryngeal auscultation, predominantly (but not usually exclusively) on the inspiratory part of the cycle. This will usually be significant enough to be detected on thoracic auscultation.
Heart sounds and other cardiac parameters such as pulse strength and quality and heart rate should all be normal; it is imperative that cardiac disease is eliminated as a cause for the clinical signs. Mucous membranes are often cyanotic because of respiratory compromise. Lung sounds may be harsher in the presence of aspiration pneumonia and referred laryngeal noise is common.
Muscle atrophy, especially of the pelvic limbs, is very common. A basic neurological examination will reveal pelvic limb weakness in most cases, along with conscious proprioceptive deficits. Pharyngeal sensitivity is reduced in some dogs, which will have a very poor gag reflex.