Overview
Myasthenia gravis is a disease that affects the way an animals’ nerves and muscles communicate. Normally, when an animal wants to move its muscles to walk, run, or even swallow, its nerves send signals to the muscles to tell them what to do. In animals with myasthenia gravis, these signals don’t get through properly because the muscle receptors aren’t working correctly or the immune system is mistakenly blocking the communication between the nerves and muscle receptors.
Causes
There are two forms of myasthenia gravis:
- Acquired Myasthenia Gravis occurs when the immune system mistakenly produces antibodies that block receptors on the muscles, disrupting the nerve signals needed for movement. This can be triggered by autoimmune disease, hormone imbalances (such as hypothyroidism or Cushing’s disease), or certain tumors like thymomas. The most common cause is an autoimmune reaction in which the body attacks its own muscle receptors.
- Congenital Myasthenia Gravis is a genetic condition in which animals are born with fewer or poorly functioning receptors on their muscles. This form is typically seen in young animals and is an inherited condition. This condition is incredibly rare.
Clinical Signs
Myasthenia Gravis causes generalized muscle weakness that worsens with even mild activity like walking or playing. They are often observed repeatedly “sitting down” after brief periods of activity. This weakness is not painful. This condition is best described as exercise intolerance—the dog’s strength improves with rest and often appears normal after a period of inactivity.
The most severe clinical sign of myasthenia gravis is megaesophagus, a condition where the esophagus becomes enlarged and loses its ability to properly move food. This can lead to difficulty swallowing, regurgitation, and an increased risk of stomach contents being inhaled into the lungs, which may cause repeated and potentially life-threatening aspiration pneumonia. Even if myasthenia gravis is treated appropriately, megaesophagus usually remains permanently.
Diagnosis
The most accurate way to diagnose acquired myasthenia gravis is a blood test that looks for antibodies against the muscle receptors, called an acetylcholine receptor antibody test. This test is considered the gold standard and usually takes about 1 to 2 weeks to get results. Although it’s very reliable, a small number of pets (less than 2%) may test negative even if they have the disease. This can be repeated every few months to see if the patient has gone into remission, which can happen infrequently.
In addition to this test, a full health evaluation is important. This includes blood work, hormonal testing, X-rays, and an abdominal ultrasound to check for tumors or hormone-related diseases that might be causing the antibodies to attack the muscle receptors.
Treatment
When an underlying condition such as a hormonal disorder or tumor is identified, it is essential to address and manage that primary disease appropriately.
Pyridostigmine is the treatment of choice for myasthenia gravis. This oral medication enhances stimulation of the remaining functional receptors and typically requires lifelong administration. It has a wide dosing range, and careful titration is necessary to reach the optimal therapeutic dose for each patient. Overmedication can lead to significant and potentially harmful side effects, so monitoring and adjustment are critical.
Given the often immune-mediated nature of the disease, prednisone or other immunosuppressive therapies may be considered as part of the treatment plan. However, this approach is somewhat controversial, as immunosuppression may worsen aspiration pneumonia if it is already present. The decision to initiate immunosuppressive therapy is typically made on a case-by-case basis, depending on what’s best for each individual patient.
For dogs with megaesophagus, the risk of aspiration pneumonia is significantly increased. To minimize this risk, all food and water must be administered while the dog is in an upright position, and they should remain upright for approximately 20 minutes after eating or drinking. The most effective method to achieve this is by using a Bailey Chair, a specially designed feeding seat for dogs with megaesophagus.
There is no treatment for congenital myasthenia gravis.
Prognosis
The prognosis of acquired myasthenia gravis depends on whether megaesophagus is present.
If megaesophagus is not present, prognosis is good. Animals with myasthenia gravis often do well with lifelong medication and have a good quality of life. In some cases, animals may even go into remission after a few months of treatment and no longer need medication.
However, if megaesophagus is present, prognosis if often poor. These dogs are at high risk for repeated episodes of aspiration pneumonia, which require frequent hospitalization and can be life-threatening.
For animals born with the congenital form of myasthenia gravis, the outlook is guarded. Most of these animals will gradually get worse over time.