The larynx is the structure at the upper end of the windpipe or trachea that we refer to as the "voice box". The larynx is made of cartilage and muscle. The major functions of the larynx are to produce sound for vocalization and to allow air to pass into the trachea while preventing food or water from doing the same.
The larynx is composed of the thyroid, Cricoid and Arytenoid cartilages. The muscles of the larynx move the cartilages to open or close the opening to the trachea. All but one of the muscles helps to close the larynx and prevent aspiration. The one muscle that opens the larynx is the Cricoarytenoideus dorsalis. There is a single nerve that innervates this muscle known as the recurrent laryngeal nerve. It is responsible for stimulating the muscle to open the larynx to allow air to move into the trachea. See Figure 1.
Laryngeal paralysis is the disease process where the larynx is not able to open as well and to let air into the trachea. The most common cause is due to loss of the nerve supply to the Cricoarytenoideus dorsalis muscle. This can be due to trauma, infection, inflammation, a multiple nerve disease (polyneuropathy) or unknown cause (idiopathic). There has in the past been a strong correlation of this disease being linked with the endocrinopathy hypothyroidism. It is really debatable if there is a cause and effect relationship. Never the less, animals should be tested for hypothyroidism because it can affect their overall health. See Figure 2.
The most common animal affected by the disease is older (>8yrs) retriever type dogs. However other smaller dogs and cats have been seen with this problem. Most owners report that their pet has begun to slow down and doesn't want to exercise as much. The dog doesn't want to stay out in the heat and when it does the pet doesn't tolerate it well. There is an incredibly loud roaring noise noted on breathing in (inhalation). There might have been a change in the tone of the dog's bark. It may have more "hoarseness" to the bark. A lot of the episodes are sub clinical until the first part of the summer when the weather gets hot enough for them to have an acute episode that is extreme.
Diagnosis is achieved with consideration of clinical symptoms, history and laryngeal exam. The laryngeal exam is performed usually under heavy sedation or light anesthesia. The larynx is visualized and observed during several inspiratory and expiratory cycles. The larynx should open (widen) during inspiration and close (become slightly more narrow) during expiration. If this does not occur then laryngeal paralysis is a likely diagnosis.
Treatment of the condition usually consists of initial stabilization. Sedation and an oxygen cage are very helpful. Sometimes, to allow the patient to breath, a temporary tracheostomy is placed to bypass the larynx. Usually this is a temporary measure and the tracheostomy can be taken down within a very short time. Surgery is the treatment of choice in dogs that are clinically having difficulties living with the disease. The preferred method at our hospital is the Arytenoid Cricoid tie-back or Arytenoid lateralization. The basis of the surgery is that we will use two sutures to open up one side of the larynx to allow more air to get past the larynx and into the trachea. The sutures are basically placed at the same attachments of the Cricoarytenoideus dorsalis muscle. In this manner it mimics the same function that the muscle had before it did not work any longer. The difference is that the suture is permanent and it causes the larynx to stay open slightly more on that side forever. There are a few notable complications that can occur, such as, aspiration of food, water or vomit down the trachea. This has not been too much of a problem, but it can occur. Another complication is dependent on if the cartilage is weak and brittle and the sutures can pull through them. If this occurs we usually will attempt an intra-oral technique to rectify the problem. It is rare for the first two procedures not to work but if they fail, a permanent tracheostomy can be placed. Most dogs can carry out a completely normal life with any of the above procedures.
