Neurogenic laryngeal paralysis may be caused by interruption of the recurrent laryngeal nerve or by motor neuron disease that involves the motor neurons of the recurrent laryngeal nerves in the nucleus ambiguus. Laryngeal paralysis caused by muscular disease affects both the abductor and adductor muscles and is usually but one component of polymyositis; dyspnea is then part of the muscular weakness syndrome. In myoneural junction disease, laryngeal dysfunction is due to weakness of both the abductor and the adductor muscles and is also part of a generalized disorder. Ankylotic laryngeal paralysis is found in older dogs and in the rare cases that have been diagnosed the cricoarytenoid articulations were the only joints affected. Ankylosis of the cricoarytenoid articulations also occurs in humans and arthritis of the cricoarytenoid articulations can occur as an uncommon symptom of rheumatoid arthritis. Although laryngeal paralysis does occur in cats, it is more common in dogs.

In cats, laryngeal paralysis is usually unilateral. The unilateral laryngeal dysfunction does not always cause dyspnea but is found by laryngoscopy, together with laryngitis or pharyngitis, and may therefore be associated with herpesvirus type 1 or calicivirus infection. Laryngoscopy should be repeated several times and under various levels of anesthesia before it can be concluded that one vocal fold is definitely immobile. One "lazy" vocal fold may be due to the level of anesthesia and there may be a spontaneous reappearance of normal abduction as the cat awakens. In most of the reported cases of unilateral laryngeal paralysis in cats, the cause of the disease was unknown. When the cause of unilateral laryngeal paralysis in a cat is not known, it is difficult to decide upon appropriate therapy. If the paralyzed vocal fold causes obstruction of the airway it can be lateralized, but since spontaneous improvement can occur, the examination should be repeated in three weeks before deciding to undertake surgery. Medical treatment is less invasive, but is not based on knowledge of the cause. Treatment with a glucocorticoid may be tried, but only for a very short period, because of the possibility of an underlying viral infection. When obstruction of the airway is not the main symptom, it is best to avoid treatment and to repeat the laryngoscopy after three weeks.

Bilateral laryngeal paralysis is characterized by severe dyspnea and a laryngeal stridor. The cat should be assured of an open airway, for which tracheotomy is often necessary. Spontaneous improvement may also occur in bilateral vocal fold paralysis when no specific cause of the disease is found. A short period of treatment with a glucocorticoid may be tried. If a tracheal tube is needed, a broad-spectrum antibiotic should be given to prevent bronchial infection. Laryngoscopy should be repeated after one week and if there is even slight spontaneous improvement, such as restoration of activity in one vocal fold, surgical lateralization should be postponed for another week. When there is no improvement, lateralization of one vocal fold is usually sufficient to relieve the severe dyspnea.

When one recurrent laryngeal nerve is transected in the dog, there is no loss of voice or hoarseness and there is no observable effect on endurance during normal exercise. The nonparalyzed vocal fold is observed to cross the midline during adduction and it closes the laryngeal opening when it touches the paralyzed vocal fold. Similarly, sufficient abduction of the nonparalyzed vocal fold compensates for the inactive vocal fold and provides an adequate laryngeal opening. Thus unilateral laryngeal paralysis caused by recurrent laryngeal nerve interruption is usually subclinical. This is in contrast to laryngeal paralysis caused by degenerative neurogenic diseases. There may be asymmetry in movement of the vocal folds but the innervation of the intrinsic laryngeal muscles is affected bilaterally, as EMG recordings will clearly reveal. Trauma to the recurrent laryngeal nerve can occur in a dog fight, usually together with trauma to the trachea, or as a result of scar tissue contraction following neck wounds. Unilateral laryngeal paralysis can also result from trauma to the recurrent laryngeal nerve caused by use of a choke chain.

Compression of the thyroid cartilage against the cricoid cartilage may damage the nerve, which lies between them.

Arytenoid cartilage lateralization is the most common treatment for bilateral laryngeal paralysis. The aim of this surgical procedure is to widen the laryngeal inlet, which is inadequate when there is bilateral paralysis. The first publication on its use in dogs was based on experience with the technique in young Bouviers with spontaneous laryngeal paralysis. The aim was to permanently widen the opening of the larynx with a minimum of laryngeal trauma, and improvement in respiratory ability was given priority over vocalization. The technique is recognized as being successful in both dogs and cats. Its advantage is that it can be performed without bilaterally rupturing the laryngeal mucosa and this minimizes webbing.

With the dog under anesthesia in dorsal recumbency and with an endotracheal tube in place, the procedure begins with tracheostomy. The endotracheal tube passing through the laryngeal lumen is then removed and a sterile endotracheal tube is inserted through the tracheostoma, cuffed, and connected to the anesthetic equipment. A paramedian incision is made 1 cm off the midline over the larynx on the side of the intended lateralization. The subcutis is separated over the sternohyoid muscle. The caudodorsal edge of the thyroid cartilage can then be palpated through the thyropharyngeal muscle. Using a finger to lift the dorsal edge of the thyroid cartilage, an incision of 1 to 1.5 cm is made through the thyropharyngeal muscle over the caudodorsal edge of the thyroid cartilage. Using closed blunt-tipped scissors, the cricothyroid articulation is located and disarticulated without rupturing the joint cartilages. The caudodorsal edge of the thyroid cartilage is then lifted in order to view the intrinsic laryngeal muscles. The atrophy of the laryngeal muscles exposes the cricoarytenoid articulation as a cartilaginous elevation. If the muscles are not atrophied, the articulation can be palpated under the junction of the dorsal cricoarytenoid muscle and the thyroarytenoid muscle. The dorsal cricoarytenoid muscle is severed from its attachment to the arytenoid cartilage and the cricoarytenoid articulation is separated in the joint. The arytenoid cartilage is partly separated from the cricoid cartilage ventrally and dorsally, by cutting part of the mucosal connection (often indurated in older dogs) between the cricoid and arytenoid cartilages. The arytenoid is then separated from the contralateral arytenoid by severing the connective tissue and sesamoid cartilage joining them on the dorsal midline. The effect of this separation and the intended lateralization is inspected orally with the laryngoscope while the arytenoid cartilage is pulled into the intended position under visual guidance. The effect is considered satisfactory if the detached arytenoid cartilage can be lateralized without the need for traction on the contralateral arytenoid cartilage and if there is satisfactory widening of the laryngeal opening. Atraumatic twisted stainless steel suture material is placed through the caudodorsal edge of the thyroid cartilage and then through the muscular process of the arytenoid cartilage. A second stainless steel suture is placed through the caudodorsal edge of the thyroid cartilage just cranial to the previous suture and through the muscular process slightly dorsal to the previous suture. This extra suture prevents pivoting of the arytenoid cartilage after fixation. In cats and small dogs the second suture is omitted because of the smaller size of the muscular process of the arytenoid cartilage. The incision in the thyropharyngeal muscle is sutured and the wound is closed routinely. When the dog breathes spontaneously the endotracheal tube is removed from the tracheotomy wound and a tracheal cannula is put in place for five days.

Complications of laryngeal surgery often have severe consequences. The airway may again become obstructed or failure of the mucosal sensory function may result in aspiration. There are various reasons for failure of the lateralization of the arytenoid cartilage. The laryngeal opening can become obstructed if the sutures tear through the cartilage before the arytenoid becomes consolidated in its new position. To help prevent this, exercise, excitement, and barking should be strictly limited for four weeks. Secondly, a harness should be used in place of a collar--for the remainder of the dog's life--beginning from the moment of awakening after surgery. One fierce tug on a collar at that time could disrupt the fixation of the arytenoid. After surgery the larynx will be found by palpation to be hard and more voluminous and there may be recurrent laryngitis because of the unnatural laryngeal opening. If disruption of the arytenoid fixation occurs within less than a month after surgery, reattachment us usually possible. Otherwise, it is easier to lateralize the arytenoid cartilage on the opposite side.

Another reason for failure of lateralization is a lack of sturdiness of the cartilage, such as in very young dogs (before 5 to 6 months) and in dogs with a hypoplastic larynx. As a result of its softness, the larynx collapses after surgery and further surgery usually fails to create a sufficient laryngeal opening. A choice must then be made between a permanent tracheostoma and euthanasia.

After arytenoid cartilage lateralization the glottis is permanently abducted on one side. In most cases the contralateral side of the larynx is also paralyzed or partly paralyzed and the glottis cannot close properly. When the sensory nerves (the cranial laryngeal nerves) are intact, the normal swallowing action prevents the leakage of fluid and food into the larynx and trachea. When the internal branch of the cranial laryngeal nerve is crushed or transected during surgery, even unilaterally, the swallowing action may be impaired and aspiration may occur. In neurogenic degenerative diseases and in polyneuropathy, the sensory nerves may be involved in the disease and with the laryngeal inlet permanently open, aspiration may occur. Evaluation of the swallowing action should be included in the clinical evaluation before arytenoid lateralization.

References

1.  Venker-van Haagen AJ. The larynx. In: Ear, Nose, Throat, and Tracheobronchial Diseases in Dogs and Cats. Hannover: Schlütersche Verlagsgesellschaft, 2005: 121-161.