Dogs and Myasthenia Gravis

What is Myasthenia Gravis?

Portrait of dog, lying on sofa
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What in the world is Myasthenia Gravis? You may have heard this term (and thought it sounded like someone had a mouth full of marbles). However, there's a good chance you have no idea what myasthenia gravis actually is and how it can affect dogs.

What is Myasthenia Gravis?

Myasthenia Gravis is a neuromuscular disorder that can affect dogs, cats, and humans. It is caused by a deficiency of acetylcholine receptors.

The lack of adequate ACh-receptors causes a disruption in the signals between the nerves and muscles, leading to muscle weakness in various parts of the body.

How Dogs Get Myasthenia Gravis

Myasthenia Gravis can be congenital (present at birth) or acquired. Although neither form is very common in dogs, the congenital form is rarest.

Congenital myasthenia gravis generally becomes apparent in puppies between six to eight weeks of age. These dogs were not born with an appropriate amount of ACh-receptors. They typically show signs of exercise-induced weakness that can progress to paralysis and even death. Certain dog breeds are prone to myasthenia gravis, such as Springer Spaniels, Jack Russell Terriers, and Smooth Fox Terriers. Some  Dachshunds are born with a form of myasthenia gravis that actually resolves on its own. 

Acquired myasthenia gravis begins in adult dogs, especially around age two to four years.

This is an immune-mediated form of myasthenia gravis. The dog's antibodies destroy ACh-receptors, leading to a deficiency. Acquired myasthenia gravis can affect any dog. Certain dog breeds may be predisposed, such as the following breeds:

Without an adequate amount of ACh-receptors, there cannot be effective signal transmission between muscles and nerves. The muscles become weak and cannot perform important bodily functions.

Signs of Myasthenia Gravis in Dogs

In general, if your dog has myasthenia gravis, you may notice one or more of the following signs:

  • Exercise intolerance or weakness when exercising
  • Gradually worsening weakness
  • Sudden collapse or falling over
  • Sleeping with eyes open
  • Drooping of eyelids
  • Excessive drooling
  • Wretching/regurgitating
  • Change to the sound of bark and/or whine
  • Trouble swallowing (or excessive swallowing)
  • Difficulty breathing (may indicate aspiration pneumonia)
  • Coughing (may indicate aspiration pneumonia)

The muscle weakness caused by myasthenia gravis may be generalized (all over the body) or focal (only apparent in specific areas of the body). The most common focal areas affected are the muscles of the esophagus, pharynx, and face. In either case, signs range from mild to severe.

Generalized muscle weakness due to myasthenia gravis may appear in some dogs as exercise intolerance that improves with rest. Some dogs simply have trouble walking and tire easily. On the opposite extreme, some dogs develop sudden paralysis.

Focal muscle weakness commonly results in a condition called megaesophagus, perhaps the most common sign of acquired myasthenia gravis. This is actually a secondary condition that occurs due to myasthenia gravis. Megaesophagus is an enlargement of the esophagus (the tube that connects the throat to the stomach). The esophagus has muscles that move in a wave-like manner, sending food and liquid to the stomach. If a dog has megaesophagus, the esophagus loses muscle tone, becomes enlarges, and cannot function properly. Food and liquids may become trapped in the esophagus and/or get regurgitated (spit up) by the dog.

Megaesophagus can easily cause aspiration pneumonia. This occurs when food or liquid is inhaled into the lungs and an infection develops. The esophagus and the trachea (windpipe) are beside one another, so food or liquid can easily get into the trachea if the muscles in that region do not properly function.

Acquired myasthenia gravis may also cause some dogs to develop a type of tumor in the chest (called a thymoma).

How Vets Diagnose Myasthenia Gravis in Dogs

If you think your dog is showing signs of myasthenia gravis or any other illness, be sure to contact your veterinarian right away. 

Your vet will begin by discussing your dog's history with you, then performing a thorough physical examination. Additional diagnostics, such as lab work and radiographs (x-rays) may be recommended to look for various issues. It is very important to rule out other diseases, disorders, or injuries before making a definitive diagnosis. Your vet may recommend you bring your dog to a veterinary specialist (usually a veterinary neurologist) to help make a definitive diagnosis.

A specific blood test (AChR antibody test) can be done to check for antibodies against acetylcholine receptors. This test can effectively diagnose most dogs with myasthenia gravis.

If your dog's symptoms are easily noticed, then a special drug may be given to check for myasthenia gravis. This is often called a Tensilon test. The dog is given an intravenous injection of an acetylcholinesterase inhibitor called edrophonium. If the dog has myasthenia gravis, then the drug will cause a significant (though temporary) improvement in the muscle weakness. 

Myasthenia Gravis Treatment for Dogs

Unfortunately, there is no cure for myasthenia gravis. Most puppies with congenital myasthenia gravis will not survive. However, there are treatments for acquired myasthenia gravis that can help many dogs live happy lives. In fact, some dogs even experience spontaneous remission after being diagnosed.

The key to successfully treating your dog's myasthenia gravis is to effectively communicate with your vet and stay diligent in your dog's daily care. Work closely with your vet to get your dog on the best therapeutic plan. Stick to a steady routine and report changes in your dog's condition immediately. Medications should always be given exactly as prescribed by the veterinarian.

Never make treatment adjustments without consulting your vet. 

The following treatments may be used to treat your dog for myasthenia gravis:

Anticholinesterase agents (pyridostigmine or neostigmine) are prescribed to enhance neuromuscular signal transmission. These drugs can prolong the action of acetylcholine at the neuromuscular junction. Dogs with acquired myasthenia gravis will typically need to remain on this type of medication for life. For many dogs, this treatment is enough to manage their symptoms.

Immunosuppressive therapy may be considered if additional treatment is needed. Your vet may prescribe corticosteroids to suppress the immune system. Because acquired myasthenia gravis is immune-mediated, immunosuppressive medications may be effective. However, immunosuppressive therapy can increase the risk of infections, especially for dogs with megaesophagus who are already prone to developing aspiration pneumonia.

Therapeutic plasma exchange is a treatment sometimes used for humans with serious cases of myasthenia gravis. This is available for dogs in some regions but it may be cost-prohibitive. TPE involves removing of the "diseased" plasma and replacing it with plasma from a healthy donor. This therapy may be effective in dogs with very serious cases of myasthenia gravis.

Supportive care is a major part of treating dogs with myasthenia gravis.

  • Dogs with megaesophagus should be fed large "meatballs" of food while in an upright position. This type of feeding may allow food to get into the stomach more effectively and lessen the risk of aspiration pneumonia.
  • Fluid therapy may be required to avoid dehydration, particularly in dogs that regurgitate liquids. 
  • Antibiotics and breathing treatments (like nebulizers) may be necessary to treat aspiration pneumonia.
  • Medications to support the gastrointestinal system may also be helpful (metoclopramide, cisapride, cimetidine).

When your dog is first diagnosed with myasthenia gravis, hospitalization may be necessary to stabilize your dog, especially if secondary issues are a concern. The hospitalization will also help vets closely monitor your dog during the medication adjustment period.

Depending on how severe your dog's disease is, daily care may be time-intensive (especially if your dog has megaesophagus). Be sure to stay organized and pay close attention to detail. However, be patient with yourself and your dog. Ask for help from friends and family members if needed. Consider joining a community of fellow myasthenia gravis dog owners or megaesophagus dog owners.

No matter how closely you monitor your dog, it is always possible for problems to crop up. Your dog may need to be hospitalized periodically to treat aspiration pneumonia or other secondary problems. Ths is why it's so important to communicate with your vet about any change in your dog, regardless of how small the changes seem.