Myasthenia Gravis in Dogs

Myasthenia gravis is a complex disease that can cause your dog to become extremely weak and tired. The disease results in poor communication between the dog's muscles and nerves, making them debilitated and unable to perform bodily functions. Myasthenia gravis is sometimes inherited and other times acquired, but requires medication to control the disease.

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 on the surface of the muscle cells. The lack of adequate ACh-receptors disrupts the signals between the nerves and muscles, leading to muscle weakness in various parts of the body.​ Without the necessary amount of ACh-receptors, there cannot be effective signal transmission between muscles and nerves.

Symptoms of 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. The muscle weakness caused by myasthenia gravis may be generalized (all over the body) or focal (only appearing in specific areas of the body). The most common focal areas affected are In either case, signs range from mild to severe. Dogs with myasthenia gravis may display several signs other than weakness, including the following:

Exercise Intolerance

Generalized muscle weakness due to myasthenia gravis may appear in some dogs as exercise intolerance or weakness when exercising that improves with rest. Some dogs simply become gradually weaker, have trouble walking, and will tire easily.

Sudden Collapse or Falling Over/Paralysis

On the opposite extreme, some dogs can develop sudden paralysis due to myasthenia gravis.

Sleeping With Eyes Open

One of the most debilitating functions that a dog will lose with this disease is its ability to blink. This may result in your pet sleeping with its eyes involuntarily open.

Drooping of Eyelids

Myasthenia gravis affects the muscles of the face, including the upper eyelids. Your dog may begin to have drooping eyelids, in addition to the inability to blink, as a result.

Excessive Drooling

Your dog will begin to lose control of its lips and mouth. Your dog may experience excessive drooling, especially at the corners of its mouth.

Trouble Swallowing

A dog's difficulty in swallowing can cause it to inhale food, liquids, and its own vomit. Or the dog will begin to excessively swallow in an attempt to try to control the loss of function.

Regurgitating

The disease will affect the functioning of your dog's esophagus, causing a secondary condition called megaesophagus (enlargement of the esophagus). The esophagus, which is the tube that connects the throat to the stomach, will lose its ability to move food into the stomach. A dog with acquired myasthenia gravis will eat food, but it will become trapped in the esophagus or spit back up into its mouth without the retching or vomiting action because the contractions do not work. Megaesophagus can easily lead to aspiration pneumonia when food or liquid is inhaled into the lungs and an infection develops.

Difficulty Breathing/Coughing

Problems with swallowing can result in difficulty breathing, coughing, and can lead to the development of aspiration pneumonia.

Changes in Bark or Whine

The condition weakens a dog's laryngeal muscles. As a result, its voice, including its bark and whine, will begin to change.

Tumor in Chest

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

Causes of 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 adequate 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 spaniel
• Jack Russell terrier
• Smooth fox terrier
• Dachshund (some dogs of this breed are born with a form of myasthenia gravis that actually resolves on its own) 

Acquired myasthenia gravis begins in adult dogs, typically 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, but certain dog breeds may be predisposed, including:

• Akita
• Dachshund
• German shepherd
• Golden retriever
• Great Dane
• Labrador retriever
• Newfoundland
  

Diagnosing Myasthenia Gravis in Dogs

Your vet will discuss your dog's history with you and then perform a thorough physical examination. Additional diagnostics, such as lab work and radiographs (x-rays) may be recommended to look for underlying 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. 

Treatment

Work closely with your vet to get your dog on the best therapeutic plan. Medications should always be given exactly as prescribed by the veterinarian. Never make treatment adjustments without consulting your vet. Various treatments may be used to treat dogs with myasthenia gravis, including the following:

• Anticholinesterase agents: Pyridostigmine or neostigmine anticholinesterase agents are prescribed to enhance neuromuscular signal transmission. These drugs can prolong the action of acetylcholine at the neuromuscular junction. Dogs with acquired myasthenia gravis usually need to remain on this type of medication for life which may be enough to effectively manage their symptoms.
• Immunosuppressive therapy: This type of therapy may be considered if additional treatment is needed. Because acquired myasthenia gravis is immune-mediated, immunosuppressive medications may be effective. Your vet may prescribe corticosteroids to suppress the immune system. 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: This treatment is 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 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.
• Hospital care: Following the diagnosis of myasthenia gravis, hospitalization may be necessary to stabilize your dog, especially if secondary issues are a concern. The hospitalization will also help your veterinarian closely monitor your dog during the medication adjustment period.

Supportive care is a major part of treating dogs with myasthenia gravis, which includes the following therapies:

• 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. 
• Medications to support the gastrointestinal system may also be helpful (metoclopramide, cisapride, cimetidine).
• Antibiotics and breathing treatments (like nebulizers) may be necessary to treat aspiration pneumonia.
• In serious cases of megaesophagus, a stomach tube may need to be surgically placed to deliver food directly to the stomach.

Prognosis for Dogs With Myasthenia Gravis

There is no cure for myasthenia gravis. Many puppies born with inherited myasthenia gravis will not survive. However, there are treatments for acquired myasthenia gravis that can help many dogs live happy lives. Some dogs with acquired myasthenia gravis even experience spontaneous remission after being diagnosed.

If your dog does not experience remission, you will need to care for it carefully. Depending on how severe your dog's disease is, daily care may be time-intensive (especially if your dog has megaesophagus). Try to take the following steps:

• Pay close attention to details of your dog's behavior and communicate with your vet about any change in your dog, regardless of how small.
• Be patient with yourself and your dog.
• Ask for help from friends and family members if needed.
• Join a community of fellow myasthenia gravis or megaesophagus dog owners for extra support.
• Keep a log of daily medication and treatments so no steps in your dog's care are overlooked.

No matter how closely you monitor your dog, it is always possible for problems to occur. Your dog may need to be hospitalized periodically to treat aspiration pneumonia or other secondary problems.

How to Prevent Myasthenia Gravis

Unfortunately, there are no preventative measures for this neuromuscular disease in dogs.