Getting At the Root Cause of Laryngeal Paralysis in Dogs
Historically, laryngeal paralysis has been thought of as an idiopathic disease, meaning no cause had ever been identified. It appeared to spring up on its own, with a changed bark noted about half the time and two out of three dogs with the condition experiencing difficulty inhaling, gagging, throat-clearing, coughing, or choking sensations, most often associated with eating and drinking.
But researchers at Michigan State University’s College of Veterinary Medicine have found evidence that the condition is part of a larger syndrome — a general, gradual deterioration of nerves throughout the body called polyneuropathy. They made the discovery when trying to figure out which dogs with laryngeal paralysis were more apt to get aspiration pneumonia once they had surgery to treat the condition.
The answer: dogs whose esophageal function was compromised along with their laryngeal function.
The researchers followed dogs who had been treated for laryngeal paralysis and compared them to dogs who had never been diagnosed with the condition. Over a year’s time, the dogs with laryngeal paralysis were found much more likely to have problems swallowing than dogs without the problem. It was directly related to a breakdown in the nerve supply to the esophagus, just as it is a problem with nerves that causes laryngeal paralysis. “The worse the dogs’ esophageal function,” says lead veterinary researcher Bryden J. Stanley, BVMS, “the more likely they were to get aspiration pneumonia.”
It might seem that all the nerve degeneration in afflicted dogs occurs in the throat — the larynx, the esophagus, and so on. But the problem is bigger than that. “Just to be very thorough as we were following the dogs,” Dr. Stanley says, “we conducted neurologic exams on them throughout the year of the study, and what we found was completely unexpected. More than one in four of the dogs with laryngeal paralysis had neurologic signs throughout their bodies. They were weak in their hind legs. They also had what are known as proprioceptive deficits, meaning it was difficult for them to tell where their bodies were in space. They weren’t sure where their feet were, for instance, and would trip or stumble while trotting upstairs or making their way over bumpy ground.”
Six months out, 58 percent of the dogs with laryngeal paralysis had such signs. By the end of a year, 100 percent of them had some degree of neurologic deficit. Four couldn’t use their hind legs at all anymore. There was muscle wasting, too. “Nerves give tone to muscle,” Dr. Stanley explains. “Take away the tone and tautness nerves provide, and the muscles atrophy — a very common course of events in neuropathy.”
So why has laryngeal paralysis been the one piece of this multi-nerve degeneration that has gotten noticed for so many years, while the other aspects of the condition went unrecognized? It could simply be that because laryngeal paralysis affects breathing, it presents more of a life-and-death scenario than, say, weak hind legs, which many might simply think is the result of their dog’s old age. The neurologic degeneration is not even painful; affected dogs remain bright, alert, and happy, further throwing off the trail even particularly tuned-in owners — and veterinarians.
Bringing the proof to the next level
The Michigan State researchers are now taking a step further their discovery that laryngeal paralysis is but one aspect of a dog’s general neurologic decline in old age. They have received funding from the American Kennel Club to conduct another, more involved study with dogs whose owners have given permission. So, when they perform a surgery to mitigate the effects of laryngeal paralysis, they do small nerve and muscle biopsies in a dog’s hind legs while he is still under anesthesia. They also look at muscle activity throughout the body and nerve conduction in the front and hind legs, with electrodes. In addition, any dog in the study that dies of the disease undergoes a full post mortem with a look at more than 35 nerves and muscles from all over the body, including the brain and spinal cord.
“We’re just finishing up,” Dr. Stanley says. “There’s a lot of tissue to analyze,” as well as numbers to crunch, comparing affected dogs to dogs without laryngeal paralysis. But it does appear, she says, that nerve conduction slows significantly in both the front and hind legs of dogs with laryngeal paralysis. There appear to be changes in the spinal cord and brain as well.
Studies looking at genealogical sequencing are aiming to nail down exactly where in a dog’s DNA a gene mutation may cause the neurologic decline. “If we can come up with that,” Dr. Stanley says, “then maybe we can screen for breeding and eventually breed out the condition.”
In the meantime, the Michigan State researchers recommend that for esophageal dysfunction in a dog who has been diagnosed with laryngeal paralysis, it’s a good idea to feed that dog at a 30-degree incline, with the dog’s head facing upward and the dog sitting for 10 minutes after he eats. That will make him more comfortable as well as less prone to developing aspiration pneumonia. Additionally, most dogs end up on some medication to help swallowing and decrease reflux.