36 Change in voice and swallowing
A difficulty or inability to swallow is called dysphagia and it presents as: gagging, retching, choking, extending or flexing the neck during swallowing, repeated attempts to swallow, coughing when eating or drinking, nasal discharge, and pseudo-ptyalism.
Food, voluntarily pushed to the pharynx by the masticatory muscles (CN V), the palatine muscles (CN V), and caudal tongue movement (CN XII), stimulates the reflex which is coordinated by the swallowing centre within the medulla oblongata.
Laryngeal paralysis can present with the following signs: an altered volume, pitch or character of voice (dysphonia), lack of voice (aphonia), inspiratory dyspnoea, sonorous respiration, high-pitched wheezing sound (stridor), gagging or coughing when eating or exercise intolerance with or without collapse.
Voice is generated by air passing through the larynx. Alteration of this process changes voice quality. An inability to create and expel a thoracic reservoir of air results in hypophonia (quiet voice) or aphonia. Weakness or paralysis of the respiratory muscles from UMN lesions (e.g. C1–5 spinal cord disease or tetanus), or LMN origin (e.g. myasthenia gravis or polyradiculoneuritis) is generally accompanied by non-ambulatory tetraparesis or tetraplegia.
Airflow through the larynx is impeded by paralysis of the dorsal cricoarytenoid muscle. This muscle, innervated by a branch of the vagus nerve (CN X), functions as chief abductor of the paired vocal folds and arytenoid cartilages which together create the glottis, the cranial opening of the larynx. Narrowing of the airflow creates stridor, dysphonia, and most importantly, inspiratory dyspnoea. Obstruction of the upper airway by nasopharyngeal polyps or a long soft palate creates a turbulent airflow and loud, stertorous respiration (e.g. snoring or snorting) but rarely results in dyspnoea.
Inability to close the glottis risks aspiration of pharyngeal contents (saliva, food and water). The chief laryngeal adductor is the thyroarytenoid muscle, also innervated by the vagus. Coughing and gagging while eating and drinking is a reflex protective of the airways. Sensation to laryngeal mucosa is supplied by the internal branch of the cranial laryngeal nerve, a division of CN X.
Concomitant signs of aspiration pneumonia may be present: fever, tachypnoea, cough or anorexia. Severely dyspnoeic animals choose to breathe rather than eat as to do both simultaneously would be impossible.
Increased muscle tone in myotonia congenita produces stridorous breathing and a high-pitched bark along with the generalized appendicular muscle stiffness. Dyspnoea and cyanosis have been reported during the increased muscular effort of whelping, or a sudden change of posture. Laryngospasm is a complication of tetanus.