A dog's vagus nerve impacts laryngeal paralysis in various ways and is now considered to be play an important role in the generalized neurological condition now known as geriatric onset laryngeal paralysis and polyneuropathy, commonly abbreviated as GOLPP. In order to better understand what happens with this condition, it therefore helps to take a close look into a dog's vagus nerve and its anatomy. By learning more about this nerve, it is possible to better understand how it impacts the dog's overall health and what happens when things go wrong.
Dog Vagus Nerve Anatomy
A dog's vagus nerve is part of the cranial nerves, which are special nerves that emerge directly from the brain, rather than emerging from the spinal cord (spinal nerves).
The cranial nerves are important and are known for carrying out important tasks such as relaying information from the sense organs to the brain, controlling muscles and connecting to special glands or important organs such as the heart and lungs.
Dogs are known for having 12 cranial nerves and they are often described by their Roman numbers. The vagus nerve is referred to as CN X, which stands for cranial nerve number 10.
The term "vagus" derives from the Latin and refers to wandering, hence the word vagabond. And this nerve sure knows how to wander!
The vagus nerve is the longest cranial nerve. It is believed to extend from the dog's head (more specifically, a part of the brain known as myelencephalon), all the way down to the abdomen.
Along this route, this nerve innervates pretty much everything (esophagus, heart, stomach etc.) up to the a portion of the dog's transverse colon.
It is therefore not surprising the important role this nerve plays in maintaining the dog's heart rate, triggering the wave-like muscular contractions, by which food is moved through the gastrointestinal tract, evoking vomiting and providing dogs with the ability to bark.
Did you know? The vagus nerve is actually two large nerves, with one emerging from the right side of the medulla, and the other from the left.
A Dog's Vagus Nerve Impacts Laryngeal Paralysis
The larynx is a hollow, muscular organ that allows air to reach the lungs. It's equipped with a sphincter that closes the airway during swallowing to prevent aspiration of food and saliva into the lungs.
The larynx is also known as the voice box, allowing humans to speak and dogs to whine, bark, bay and howl.
The dog's larynx is, overall, a complex structure that is composed by four muscles: the posterior cricoarythenoid muscle, the lateral cricoarytenoid muscle, the cricothyroid muscle and the thyroarytenoid muscle.
The posterior cricoarythenoid muscle, the lateral cricoarytenoid muscle and the thyroarytenoid muscle are innervated by the recurrent laryngeal nerve, while the cricothyroid is instead innervated by the cranial laryngeal nerve.
Both the recurrent laryngeal nerve and the cranial laryngeal nerve originate from the vagus nerve. It is thanks to these nerves that humans and dogs are gifted with a gag reflex. The purpose of this reflex is to keep food or any debris from entering the airway (out of the throat and lungs) which could potentially cause causing choking or aspiration pneumonia.
When there is interruption of nerve transmission from the laryngeal nerves or the vagus nerve, the arytenoid cartilages (a pair of cartilage pieces that act as “doors” to the trachea that open and close) fail to receive messages from the nerves to contract upon inhalation and relax during exhalation. This leads to noisy, problematic breathing.
On top of this, during swallowing, the risks for food aspiration increase as the cartilages fail to close the opening to the trachea, leading to dogs often choking during eating or drinking.
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Laryngeal paralysis is a condition caused by the disruption of nerve impulse transmission to the muscles that control the arytenoid cartilages of the dog's larynx. The underlying causes for this condition can be several.
The acquired form of laryngeal paralysis in dogs affects mostly senior dogs and may sometimes stem from a primary disease such as hypothyroidism, cancer, Addison’s Disease, and immune-mediated diseases, however, many times an underlying cause cannot be found, in which case, it's referred to as idiopathic.
Some dogs may develop laryngeal paralysis as a result of injury (dog bite to the throat, complication of a surgical procedure) or organophosphate toxicity, and some dog breeds may be affected by a hereditary form.
Other Bodily Functions
The vagus nerve, being the longest nerve of the autonomic nervous system, and one of the most important nerves in the body, branches off to several muscles other than the ones controlling the the arytenoid cartilages of the dog's larynx.
Further down, the vagus nerve wraps around the dog's digestive tract, including the esophagus, stomach and the large and small intestines. At the level of the esophagus, the vagus nerve along with the cranial nerve IX, allows swallowing and regulates motility of the esophagus.
The gastrointestinal tract is known to be densely innervated by the vagus nerve. As mentioned, this nerve is responsible for triggering the wavelike muscle contractions which allow food to move through the dog's gastrointestinal tract.
Interestingly, in humans, the vagus is known to react to stress, triggering that dry throat before a public speech and the high heart rate. It also can also lead to excess stomach acid, explains Jean Abitbol, in the book "Odyssey of the Voice."
As seen, the vagus nerve does a whole lot!
"The nerves responsible for laryngeal movement arise from the vagus nerve, one of the body’s major nerves. Clinical investigations have shown that other nerves are also affected, leading to clinical signs, such poor swallowing function, slowly progressing hind-end weakness, and loss of muscle mass."~Michigan State University
A Form of Polyneuropathy
Lately, it has been discovered that laryngeal paralysis may be just a component of a more generalized type of polyneuropathy (condition affecting multiple nerves affecting both sides of the body).
It was Dr. Bryden J. Stanley, a board-certified surgeon at Michigan State University College of Veterinary Medicine, to first suggest that laryngeal paralysis in dogs was the first sign of a slowly progressing, generalized paralysis known as geriatric onset laryngeal paralysis polyneuropathy (GOLPP).
Dr. Stanley's research demonstrated that 31 percent of dogs with laryngeal paralysis had some sort of neurological dysfunction affecting the hind legs causing muscle wasting, weakness, unsteadiness and gait problems, and that 75 percent developed some level of esophageal dysfunction. Muscle wasting of the spinal muscles and muscles on top of the head were seen too.
GOLLP commonly affects large dog breeds such as Labrador retrievers, Rottweilers, Afghan hounds, Irish setters, golden retrievers, Saint Bernards and standard poodles. When affecting the hind legs, owners may report a wobbly gait, nails on the back feet scuffing against the pavement and difficulty rising from lying down. When affecting the esophagus dog owners report regurgitation, reflux and trouble swallowing.
Unfortunately, while tieback surgery in dogs can open up the airway and make it easier for dogs to breathe, there is no treatment for the neurological deterioration affecting the rest of the dog's body.
“ I think of the condition as a “trifecta”– three regions are affected and show us clinical signs, 1) larynx, 2) rear limbs, and 3) esophagus. "~Dr. Lara Marie Rasmussen
Small Animal Neurological Emergencies, By Simon Platt, Laurent Garosi