Myasthenia gravis is a disorder of the neuromuscular junction, where nerves intersect and interact with muscles. This interaction is done via the release of acetylcholine, a neurotransmitter, from the end of the nerve. Acetylcholine normally binds to receptors on the muscle cell and triggers the muscle cell to contract. In myasthenia gravis, this interaction is interrupted by the presence of antibodies which bind to the acetylcholine receptor. Antibodies are normally used to fight tissues foreign to the body, such as bacteria. In myasthenia gravis, the immune system mistakes the acetylcholine receptor for a foreign tissue and sends antibodies to attack it. This results in destruction of acetylcholine receptors, leaving only a few on muscle membranes. Without the normal constituent of receptors, the muscle cell cannot receive signals normally from its associated nerves. This results in muscle weakness, especially noticeable with exercise. Another common sign of myasthenia is regurgitation; this is because the esophagus is made of muscle, and without proper input it becomes floppy and unable to propel food into the stomach.
Dogs develop myasthenia gravis more commonly than cats, and nearly any age of pet can be affected. The severity and distribution of signs varies amongst dogs with myasthenia. While most develop the inability to exercise without tiring and regurgitation due to a dilated esophagus (megaesophagus), some dogs may have only megaesophagus, may have only facial weakness, or may have weakness so severe it is life-threatening due to impairing the ability of the dog to breathe.
The best test for myasthenia gravis is to test the patient’s blood for the presence of antibodies directed against the acetylcholine receptor, the so-called anti-acetylcholine receptor antibody test. As this test may potentially take a week or more for results to become available, an edrophonium-response test may be done at the initial appointment to give supporting evidence of the diagnosis while awaiting the antibody test results. Edrophonium is a drug which increases the amount of acetylcholine available to muscle cells. In dogs with myasthenia gravis, increasing the amount of available acetylcholine will typically result in a drastic improvement in the patient’s ability to exercise. While the results of this test are instantaneous, they are not 100% accurate, thus necessitating confirmation with the antibody test to ensure the proper diagnosis has been made.
In addition to the diagnostic testing above, all dogs suspected to have myasthenia gravis should have chest radiographs (x-rays) performed. Radiographs allow the diagnosis of megaesophagus, common in patients with myasthenia, and can also be evaluated for evidence of pneumonia. Many dogs with esophageal disease will develop pneumonia from stomach fluid being aspirated into the lungs after an episode of regurgitation. This is very common in dogs with myasthenia, being recorded in 85% of patients. While the direct effects of myasthenia are not commonly life-threatening, pneumonia can be very deadly and in fact accounts for 60% of all deaths amongst myasthenic dogs. Prompt, aggressive treatment is the best chance at a successful outcome in myasthenic patients with pneumonia. Owners of myasthenic dogs should be on the alert for signs of pneumonia, including cough, lethargy, and decreased appetite, and should see a veterinarian immediately if any of these signs arise.
Treatment for myasthenia gravis is with the drug pyridostigmine, a long-acting form of edrophonium. Changes in eating habits are also required for dogs with a megaesophagus, including feeding from an elevated position and elevating water dishes. A healthy and active immune system is required to fight off infections such as pneumonia; for this reason, drugs are not commonly given to suppress the immune system despite myasthenia being an autoimmune disease. Fortunately, many dogs with myasthenia gravis will stop making offending antibodies and will be able to discontinue pyridostigmine within 6 months of developing signs. To monitor for remission, anti-acetylcholine receptor antibody levels may be checked every 6-8 weeks after the initial diagnosis of myasthenia gravis. Once the level drops to zero, pyridostigmine may be discontinued. Relapses are rare.
If you or your veterinarian feels that your pet may have myasthenia gravis, or if you pet has unexplained weakness, you may schedule a consultation with our Neurologist by calling us at our Manchester or Newington location today.