This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
Causes appear to be multiple. Trauma, such as excessive pulling on the
leash or other pressure/force to the neck have been implicated. Older
Labrador Retrievers account for a large proportion of cases.
Overactive thyroid levels can contribute to the problem.
While the symptoms can vary somewhat, you will notice coughing while
exhaling, particularly after exercise. The coughing sometimes sounds
very odd. As the condition progresses, the dog may have problems
breathing and panting.
Secondary complications such as bronchitis, etc. may occur.
As there are various possible causes for shortness of breath and/or
coughing, you must have your veterinarian look at the dog. The vet may
put your dog under in order to examine the laryngeal muscles, xray to
check the condition of the dog's lungs. Anesthesia is required to be
able to observe the laryngeal muscles at work.
The standard treatment for dogs with LP is to surgically "tie back"
one of the laryngeal muscles. This allows adequate air flow without
giving pathogens, etc. completely unimpeded access to the dog's lungs.
Typically, a specialist may be called in to do the surgery, it is not
normally done in most veterinary clinics.