Overview of Canine Myasthenia Gravis
Myasthenia gravis is a disorder characterized by muscular weakness that is aggravated by activity and relieved by rest. It is caused by an impairment of transmission of nerve impulses to the muscles, which results in muscle weakness. The disease can be an inherited congenital defect (rare) or can be acquired later in life (common in dogs, rare in cats).
The acquired form is due to a defect in the immune system whereby antibodies are made against the animal’s own muscle receptors, preventing signal transmission from occurring properly.
Both sexes are equally affected, and all breeds are susceptible, although a recent study shows Akitas, Scottish terriers, German shorthaired pointers and Chihuahuas to be at higher risk. Rottweilers, Doberman pinschers, Dalmatians and Parson Russell Terriers had lower relative risk. The acquired disorder rarely strikes animals less than 1 year of age. There seems to be two age peaks for the disease: one at 2 to 3 years of age, and another peak at 9 to 10 years of age.
The disease causes generalized weakness that often affects the esophagus, causing difficulty eating and swallowing. This can lead to regurgitation and aspiration of material into the lungs, and subsequent pneumonia. Many dogs succumb to this type of pneumonia.
Myasthenia gravis sometimes occurs secondary to a tumor of the thymus gland, or possibly some other type of tumor.
What to Watch For
Signs of myasthenia gravis in dogs may include:
Diagnosis of Myasthenia Gravis in Dogs
Treatment of Myasthenia Gravis in Dogs
Home Care and Prevention
Administer medication as prescribed and follow any special feeding instructions.
There are no significant preventative measures for myasthenia gravis.
In-depth Information on Myasthenia Gravis in Dogs
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.
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.
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.