Remember that ad on TV about asthma? It told us having asthma was like trying to breathe through a straw. Get a straw yourself and try it. I didn’t last long before I gave up!
Asthma is rare in dogs but laryngeal paralysis, which also affects breathing ability, is quite common with some estimates suggesting as many as 25% of older dogs are affected. Most dogs are only mildly affected. As the laryngeal paralysis progresses it becomes as bad as breathing through a straw, leading to collapse and even death from asphyxia.
How does the larynx work?
The larynx is a complicated structure at the entrance to the windpipe. Its job is to make sure that we can breathe freely, as well as eat and drink without getting food material or liquids down into our airways. That’s quite a challenge, as most of us have discovered at one time or other. The larynx also houses the vocal cords which can be tensed to produce a range of sounds. The functions of the larynx are controlled by nerves. These mainly operate in a reflex way, without conscious control. The key nerves are the recurrent laryngeal nerves. In many older dogs these nerves cease to function properly. Damage from a fight, other trauma, or pressure from acancerous mass is occasionally identified. Usually no cause is found, but affected dogs often show other signs of long nerve degeneration. Hypothyroidism, an under-functioning thyroid gland, has often been associated with laryngeal paralysis, but there is no cause and effect relationship. Laryngeal paralysis is not reversed with thyroid hormone supplementation unlike the other nerve paralysis syndromes sometimes seen with severe hypothyroidism.
The dogs most often affected are middle-aged or older, large breed dogs, and especially Labradors. Obesity markedly increases the severity of the symptoms. Males are affected three times more often than females.
One key function of the recurrent laryngeal nerves is to control a reflex that tenses the vocal cords, pulling them outwards every time the dog breathes. This opens the airway and decreases the resistance to the flow of air. In laryngeal paralysis the paralysed vocal cords hang slackly and are drawn into the airway by the negative pressure of inspiration, partly blocking the airway and making the dog work much harder to breathe. With a narrower air passage the air must move faster, making the slack vocal cords vibrate rapidly and causing the characteristic harsh rasping whistle (stridor) that allows an experienced vet to diagnose laryngeal paralysis on the spot.
A second key function of the recurrent laryngeal nerves is to sense the presence of food or liquid that may have gotten into the larynx and then prompt a cough reflex. This protects the sensitive lung tissue from contamination with foreign material. Dogs with laryngeal paralysis are at much greater risk of aspiration pneumonia – infected lung tissue from breathed-in food or liquid. Studies have shown between 10 – 15% of dogs presented with laryngeal paralysis already have aspiration pneumonia. This is a serious and potentially fatal complication.
What are the signs of laryngeal paralysis?
The signs of laryngeal paralysis typically develop gradually and can often be misinterpreted as ageing. Initially louder breathing sounds are heard, especially during inspiration. The dog will pant more and have decreased tolerance for exercise. Many clients comment that their dog’s bark is hoarser and at a lower pitch. Breathing becomes increasingly difficult and the dog’s tongue may become grey or blue even with minimal exercise. This is followed by collapse and, if nothing is done, death.
Affected dogs may have a throaty cough, followed by a dry, gagging retch, especially after drinking water. Some dogs have swallowing difficulties or regurgitation of food.
Obesity and warm, humid conditions make the situation much worse for clinically affected dogs. Overweight dogs overheat more easily. Once dogs are clinically affected it is easy for a vicious circle to develop. Dogs need to pant to cool down. If exercised in warm temperatures they pant more. When they have some degree of laryngeal paralysis they have to work much harder to breathe. That increased effort causes greater negative inspiratory pressures, drawing the slack vocal cords in to occlude the airway more fully. The vocal cords become inflamed and swollen due to the laryngeal stridor, causing even more breathing difficulty. This is a highly dangerous situation!
What can be done?
If your dog shows any of the above signs, take him to your vet for a thorough check up. The initial signs – panting more and decreased exercise tolerance – have a wide range of causes including heart and lung problems and arthritis. The laryngeal stridor is almost diagnostic on its own, though more mildly affected dogs may need to be examined during or immediately after exercise. The diagnosis is confirmed by viewing the movement of the larynx and the vocal cords while the dog is under a very light general anaesthetic. It is then clear that the slack vocal cords are being sucked in to obstruct the airway instead of being tensed and drawn outwards to open the airway at each inward breath. The vocal cords are often inflamed and swollen.
Chest X-rays are recommended to check for aspiration pneumonia and for megaoesophagus, a condition where nerve control of the gullet is lost. This unusual condition is seen much more frequently in dogs with laryngeal paralysis and markedly worsens the prognosis. Blood tests are normally done to identify any other underlying disease problems.
Conservative management can be used very successfully in dogs that are mildly affected by laryngeal paralysis, especially if they are overweight.
Dietary management to achieve a lean bodyweight rapidly – aim for a 2% weight loss per week – makes a dramatic difference to a dog’s ability to cope with laryngeal paralysis.
Exercise in the cool of the morning is usually well tolerated, but vigorous exercise must be avoided.
Situations that can lead to overheating must be avoided, especially leaving the dog in a car in a sunny situation, even with windows partly open.
These dogs need regular reassessment every three to six months. Other health issues must be treated promptly.
More severely affected dogs are best treated by surgery. Several techniques are described but the most successful and commonly used are called arytenoid lateralisation or ‘tieback’ procedures. These involve approaching the larynx from the outside, disconnecting the cartilage to which one end of the vocal cord attaches, and re-attaching this cartilage further back in the larynx so that the vocal cord is drawn outwards and backwards.
This can be done on just one side (unilateral tieback) or both sides (bilateral tieback). This is tricky surgery with a steep learning curve, and so is best done by an experienced surgeon or referral to a surgical specialist. Complications, when they occur, can be fatal. However it is generally successful, with most dogs in a very large survey living more than 1000 days after their operation, and most eventually dying of unrelated causes.
It is important to note that surgery doesn’t restore nerve function. The airway is artificially held open so that the dogs can breathe well, but it can’t close fully to prevent aspiration of food or liquid. Nor can the dogs sense the presence of foreign material in the larynx. It is common for these dogs to cough or gag a bit, especially after drinking water. They are at increased risk of getting aspiration pneumonia. Swimming could be dangerous for these dogs.
A New Zealand veterinarian, Dr Hilary Burbidge, did considerable research into laryngeal paralysis. She advocated bilateral tieback because it creates a substantially bigger airway and so permits better activity after surgery. Some studies have shown that bilateral tieback has a much greater risk of aspiration pneumonia post-operatively. Most surgeons in the US and in UK favour operating on just one side, supporting that decision by the good improvement in post-operative exercise tolerance and a decreased risk of subsequent aspiration pneumonia.
I have personally favoured the bilateral approach. I’ve seen good to excellent return to exercise, and have seen very few complications with aspiration pneumonia. The surgery is often done as an emergency procedure when an affected dog is brought in collapsed, cyanotic (a blue/grey tongue) and sometimes unconscious. They recover quickly when a tube is placed in their airway and they breathe oxygen. However they won’t tolerate an endotracheal tube once they recover consciousness and so must be anaesthetised. They can be difficult to manage without cutting a hole in the windpipe (tracheotomy) to bypass the swollen, paralysed larynx. This creates another set of problems.
Proceeding straight to surgery once the patient is stabilised means that when the dog wakes after surgery it can usually breathe freely. It is a dramatic change!
Hereditary laryngeal paralysis
Laryngeal paralysis does occasionally occur in immature dogs as a hereditary condition. Sometimes it is associated with multiple other nerve problems. Bouviers des Flandres, Dalmatians, Rottweilers and Siberian Huskies are breeds that have been affected.
Laryngeal paralysis in cats
The condition is much less common in cats but may be under-diagnosed because cats self-regulate their exercise so well. Loss of meow or purr, increased laryngeal breathing sounds, and coughing or gagging after eating are the key signs to watch for. Laryngeal tieback works well in cats too. We have diagnosed and operated on just two at Halifax.
Hans Andersen BVSc MANZCVS