Understanding Myasthenia Gravis

My dog was just diagnosed with myasthenia gravis. What can you tell me about this disease?

Myasthenia gravis (MG) literally means grave (gravis) muscle (my-) weakness (asthenia). It is a common cause of generalized weakness in dogs. MG is a disease that interrupts the way nerves communicate with muscles.

To understand this disease, it helps to have some understanding of how things work under normal circumstances.

Each muscle in the body is controlled by its own nerve, but this nerve does not connect directly to the muscle. At the junction between the nerve and the muscle there is a small gap. Signals travel along the nerve as an electrical current. When the electrical nerve impulse reaches the end of the nerve, the signal must be conveyed across the gap to the muscle.

A chemical messenger called acetylcholine bridges this gap. The messenger is released from the end of the nerve, flows across the gap and fixes itself to a specific acetylcholine receptor on the muscle. The acetylcholine attaches to the receptor like a key fitting into a lock and triggers a signal that causes the muscle to contract.

In myasthenia gravis there is abnormal transmission of the message between the nerves and the muscles. If the muscles are unable to contract properly, they become weak. Muscle weakness can affect the limbs so that these dogs are unable to stand or exercise normally. It can also affect other muscles in the body. In severe cases, the muscles involved with breathing can be affected.

The esophagus, which is the muscular tube that carries the food from the mouth to the stomach, is often weak in dogs with MG. Affected dogs have difficulty swallowing and frequently regurgitate their food. This condition is called megaesophagus. The inability to swallow can lead to accidental inhalation of food and water, which can cause a serious lung infection known as aspiration pneumonia.

Are there different kinds of myasthenia gravis?

Two kinds of MG are recognized in veterinary medicine. The congenital form happens when dogs are born with a reduced number of acetylcholine receptors. This form carries a very poor prognosis. It is inherited as an autosomal recessive trait and is seen in very young dogs.

The second form of MG is the acquired form. This form is considered a less ominous disease in which the immune system attacks its own acetylcholine receptors, thereby interfering with normal nerve communication. The mode of inheritance for the acquired form is unknown. It is most common in middle-aged to older dogs.

What are the signs of myasthenia gravis?

The symptoms of MG are muscle weaknesses that affect the eyes, facial expression, throat/esophagus and legs. Owners may notice muscle fatigue brought on quickly by exercise. Other signs include vomiting, regurgitation, laryngeal paralysis or difficulty swallowing. The symptoms may appear suddenly over days to weeks. The characteristic exercise-induced weakness may not be seen at all, and sometimes regurgitation caused by a dilated esophagus is the only sign.

Because swallowing difficulties and regurgitation occur with MG, these dogs are at high risk for developing pneumonia resulting from the accidental inhalation of food, saliva or vomit. This complication is called aspiration pneumonia and substantially worsens the prognosis.

Unlike humans with myasthenia gravis, some dogs will cure spontaneously. The prognosis is good if they survive the initial month when the weakness is most severe and the risk of aspiration pneumonia is the greatest. Owners should be prepared to treat the dog long term, although treatment periods of only a few months are not unusual.

Myasthenia gravis is more common than one would think, so any dog with general muscle weakness, difficulty swallowing or megaesophagus (dilated esophagus) should be tested for this disease.

How would my veterinarian diagnose myasthenia gravis in my dog?

A blood test is available to test for antibodies against acetylcholine receptors. It is called an AChR test. This blood test is able to detect 98 percent of dogs with MG. The disadvantage of this test is that it is run at only one laboratory, located at the University of California, San Diego, and the results may take a couple of weeks.

There is a window of time during which a dog that has an early case of MG will not have a high enough antibody level to be considered positive. For dogs that test negative, a second test should be performed after a couple of months before considering ruling out MG as a diagnosis.

Another diagnostic test is the tensilon test, which involves giving an injection of edrophonium chloride (Tensilon) intravenously to a patient suspected of having MG. Edrophonium chloride is a short-acting anti-cholinesterase. This allows acetylcholine to accumulate in the neuromuscular junction, strengthening the message from the nerve to the muscle.

The tensilon test allows for a tentative diagnosis to be made quickly rather than waiting for the AChR test results. Dogs with MG that can barely walk will rise up and walk strongly after being given a tensilon injection. The issue with this test is that edrophonium chloride is currently not being manufactured, so this form of testing has limited availability.

Radiographs of the chest should be taken to look for megaesophagus and aspiration pneumonia.

Is there treatment for myasthenia gravis?

Pyridostigmine (Mestinon) is the typical medication used to prolong the action of acetylcholine. By inactivating the enzyme acetylcholinesterase, the receptors that have not been destroyed by the immune system can bind acetylcholine longer. This drug is typically given orally two to three times daily and may cause nausea, cramping and diarrhea. Pyridostigmine can be thought of as a longer-acting version of edrophonium chloride.

Corticosteroids, such as prednisone and similar drugs, can suppress the production of the antibodies that are destroying the neuromuscular junctions. Other stronger immune-suppressive drugs, such as azathioprine, cyclosporine or mycophenalate, are used if there are reasons that corticosteroids cannot be used due to diabetes, high blood pressure or the presence of infection such as aspiration pneumonia. Any other existing conditions related to MG, such as low thyroid levels, should also be addressed.

What is the prognosis for dogs with myasthenia gravis?

Myasthenia gravis is a serious disease, and a dog’s overall prognosis depends on the underlying cause or associated complications. Early diagnosis, diligent treatment and management can lead to favorable outcomes. Some cases will even go into remission after several months. Other affected dogs may require lifelong treatment of special feeding and medication.

Severe or recurrent aspiration pneumonia, and difficulties breathing or swallowing, will lead to less favorable outcomes.