What is myasthenia gravis?
Myasthenia gravis is a disease in which there is a malfunction in the transmission of signals between the nerves and muscles. Dogs with myasthenia gravis exhibit extreme weakness and excessive fatigue. Some breeds are predisposed to an inherited/congenital form of this disease, including Jack Russell Terriers, English Springer Spaniels, Smooth Fox Terriers, and Smooth-haired Miniature Dachshunds.
Acquired myasthenia gravis is, like other immune-mediated diseases, a complex condition requiring that multiple factors come into play, including environmental, infectious, and hormonal influences. The Newfoundland, Great Dane, Golden Retriever, Labrador Retriever, Akita, and Scottish Terrier are predisposed to acquired myasthenia gravis. Some dogs who acquire myasthenia gravis have a tumor mass in the chest cavity called a thymoma.
"Most cases are acquired, not inherited."
Puppies with congenital myasthenia gravis are typically diagnosed at 6-8 weeks of age. Acquired myasthenia gravis tends to be diagnosed in dogs 1-4 years of age, or in dogs at 9-13 years of age. Most cases are acquired, not inherited.
What are the signs of myasthenia gravis?
Many dogs with acquired myasthenia gravis develop megaesophagus, which is a dilation of the esophagus that holds food rather than allowing food to pass into the stomach. These dogs then have their food return up through their mouths without the retching and abdominal muscle contraction associated with vomiting. They may exhibit difficulty swallowing, a decreased ability to blink, and/or acute collapse.
Other signs of myasthenia gravis include:
- voice changes
- exercise-related weakness and/or collapse
- progressive weakness
- inability to close the eyes, even when sleeping
- excessive drooling
- difficulty breathing
- cramping with mild exercise
Elevated blood levels of antibodies against acetylcholine receptors provide an important diagnostic confirmation and monitoring test.
Is myasthenia gravis treatable?
Some dogs diagnosed with myasthenia gravis require treatment in the hospital until their medication dose is stabilized. These dogs are treated with a class of medication that inhibits a nervous system enzyme called acetylcholinesterase. Anti-acetylcholinesterase medications will be required for the rest of the dog’s life. Because of their compromised ability to swallow, some dogs will actually inhale food, liquid, or vomit, resulting in aspiration pneumonia. Aspiration pneumonia is extremely serious and often requires aggressive intensive care including oxygen therapy, antibiotics, IV fluid therapy, and supportive care. If the dog is unable to eat or drink without regurgitation, a feeding tube may be needed until the dog’s medication doses are stabilized.
"Anti-acetylcholinesterase medications will be required for the rest of the dog’s life."
Ancillary treatment of myasthenia gravis is as important as determining appropriate medication doses. In cases where there is a thymoma, it must be removed surgically. Food and water dishes should be elevated, and these dogs often do best with smaller, more frequent meals of a high-quality, high-calorie food. There is no single “best” nutritional formulation for dogs with myasthenia gravis. It is important to assess what works best for the individual dog.
Most dogs with myasthenia gravis will limit their own activity based on the severity of their muscle weakness.
What medications are recommended for dogs with myasthenia gravis?
Acetylcholine is a chemical that transmits messages between nerves and muscles at the neuromuscular junction. Because dogs with myasthenia gravis have an excess of the enzyme acetylcholinesterase, which breaks down acetylcholine, drugs that inhibit that enzyme prolong the action of acetylcholine. Pyridostigmine bromide (brand name Mestinon®) is the drug of choice.
Corticosteroids, azathioprine (brand names Imuran®, Azasan®), or mycophenolate may also be used for their ability to suppress the immune system.
How are dogs with myasthenia gravis monitored?
Improved muscle strength is an obvious barometer of response to therapy. In addition, chest radiographs (X-rays) are evaluated every 4-6 weeks for resolution of megaesophagus. Finally, acetylcholine receptor antibody levels are evaluated every 8-12 weeks, and should decrease into the normal range with remission.
What is the prognosis for dogs with myasthenia gravis?
For dogs who do not experience severe aspiration pneumonia or weakness of the throat and difficulty swallowing, the prognosis is good for complete recovery within 6-8 months. For dogs with a thymoma, the prognosis is guarded unless the mass is completely removed and control of clinical signs is achieved.
Although myasthenia gravis is treatable, most pets require months of special feeding and medication. Anti-acetylcholinesterase medication and immunosuppressive therapy will likely be required for the life of the dog. If the dog achieves remission, life quality is generally excellent.