Myasthenia gravis is a disease of the nervous system that occurs in dogs and (rarely) cats. Myasthenia gravis in dogs can be present at birth (ie, congenital), but is usually acquired in adult dogs. The most commonly affected breeds include German Shepherds and retrievers. The most common cause of myasthenia gravis in dogs is an autoimmune condition in which antibodies in the blood attack receptors for a chemical called acetylcholine, which transmits nervous impulses to the muscles. Sometimes the autoimmune response is “triggered” by another cause such as vaccines, infection or even cancer. Searching for the problems will likely be recommended.
Dogs affected with myasthenia gravis typically show up with stiffness, shaking, and weakness after exercise. The signs usually go away with rest. The muscles in the head and throat may also be affected, leading to a slack appearance in the face and difficulty swallowing. Some dogs will spit up their food and may even aspirate some into the wind pipe, which can cause pneumonia.
Diagnosis is based on signs of illness and several clinical tests. A tensilon test involves injection of a drug that increases acetylcholine at the nerve/muscle junction. If a dog has myastenia gravis the weakness often resolves after the administration of tensilon. A positive response to this test is suggestive of myasthenia gravis, but a definite diagnosis of acquired disease involves checking for specific antibodies in the blood. Diagnosis of congenital cases requires a muscle biopsy. Other blood test may be recommended to look for endocrine disorders. Chest radiographs are generally performed to look for a ‘megaesophagus’ and pneumonia.
Treatment involves daily administration of drugs that replace the missing acetylcholine. Immunosuppressive drugs are sometime used as well to suppress the over active immune system.
The prognosis for dogs that have myasthenia gravis is variable. The most common life-threatening complication is aspiration pneumonia