Dr. Arnold Plotnick
Acquired myasthenia gravis is an immune-mediated disorder. That is, the disease occurs because of an abnormal response by the immune system. Normally, nerve endings release a substance called acetylcholine (ACh), which binds to a specific receptor on the targeted muscle. This allows the muscle to contract. In myasthenia gravis, the immune system produces antibodies against the receptors for ACh. These antibodies bind the receptors, blocking the normal binding of ACh. Because ACh cannot bind the receptor, the signal is now interrupted, and muscle contraction cannot occur. A focal form, where the weakness affects specific muscle groups, such as the esophagus, throat and face.
The typical clinical presentation is muscle weakness of the front and rear legs that worsens with exercise and improves with rest. Most dogs have a weak esophagus as well, and regurgitation is a common occurrence as a result. Many of these dogs aspirate some of the regurgitated food and develop aspiration pneumonia.
Although exercise-related muscular weakness is the most common presenting form of the disease, there are basically three main clinical forms.
A generalized form in which there is mild to moderate weakness in the legs and in most cases (87 percent) some esophageal dysfunction.
Acute severe myasthenia in which there is rapid onset of severe muscle weakness, collapse, and esophageal problems.
The generalized form is the most common. Dogs with this form of myasthenia gravis are weak. They are reluctant to exercise. Exercise exacerbates their weakness, and they take progressively shorter and shorter steps, and then may refuse to walk or they collapse. The rear legs are often more affected than the front legs. The majority of affected dogs have weakness of their esophagus, and have a history of regurgitation. Some may be ill from aspiration pneumonia. Some dogs have muscle weakness involving their face, and repeated touching of the corner of the eye demonstrates a progressively weaker blink reflex until an inability to blink becomes quickly apparent.
The focal form in which the legs are unaffected has been well documented. In these cases, esophageal problems are the main clinical feature. Regurgitation and aspiration pneumonia are the common clinical signs. Weakness of the throat muscles and facial muscles, with an easily fatigable blink reflex is often seen.
The acute form in which rapid loss of muscle strength is the primary sign has been described in a few dogs. These dogs have severe esophagus problems characterized by frequent regurgitation of large volumes of fluid. Respiratory failure, presumably caused by aspiration pneumonia and loss of strength in the muscles responsible for breathing, is common. Affected animals may be so weak as to be unable to lift their head.
Exactly why dogs with myasthenia gravis develop antibodies against their ACh receptors is unknown. In humans, approximately 75 percent of myasthenia patients have an abnormality involving the thymus gland, and thymus abnormalities are also seen in some dogs with myasthenia gravis. It has been suggested that the abnormality in the thymus gland induces the immune system to make antibodies against it, and these antibodies happen to be capable of binding the ACh receptors on the muscles, impairing the ability of nerve signals to get through.