Myasthenia Gravis in Dogs

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:

• Muscle weakness that worsens with exercise and improves with rest
• Concurrent esophageal weakness, manifesting as regurgitation and possibly pneumonia (coughing, fever)

Diagnosis of Myasthenia Gravis in Dogs

• Anti-acetylcholine receptor antibody test
• Tensilon test
• Electromyography

Treatment of Myasthenia Gravis in Dogs

• Anticholinesterase drugs
• Drugs that suppress the immune system
• Plasmapheresis
• Thymectomy
• Elevated feedings or gastrostomy tube placement

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.

• A focal form, where the weakness affects specific muscle groups, such as the esophagus, throat and face.
• 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.

In-depth Information on Diagnosis

• Anti-acetylcholine receptor antibody test. A definitive diagnosis of myasthenia gravis is made by demonstrating an excessive level of antibodies against the ACh receptors in the bloodstream. This is an extremely sensitive test, detecting greater than 90 percent of all dogs with acquired myasthenia gravis. Dogs with the focal form of myasthenia gravis tend to have lower antibody levels, while dogs with the acute severe form tend to have higher levels.
• Tensilon test. Tensilon is the brand name of a drug called edrophonium chloride, a drug that allows the impairment of signal transmission to be overcome briefly. A dramatic, unequivocal, positive response to intravenous administration of Tensilon is suggestive of myasthenia gravis, and helps establish a clinical diagnosis while waiting for the anti-ACh receptor antibody test results.
• Electromyography. This procedure tests the ability of the muscle to be stimulated electrically. Although this test can be used as a good tool for diagnosing myasthenia gravis, it is rarely done because of the risks of anesthesia in dogs with esophageal dysfunction with or without pneumonia, as well as the need for specialized equipment.
• X-rays. X-rays will not diagnose the disorder, although greater than 80 percent of dogs with myasthenia have an enlarged esophagus that can be detected on x-rays. Aspiration pneumonia and thymomas, would also be detectable on x-rays.

In-depth Information on Treatment

In humans with myasthenia gravis, there are three major aspects of treatment: anticholinesterase drugs, drugs that modify the immune system and removal of the thymus gland. Usually, some combination of these therapies is employed to control the disease. In dogs, only anticholinesterase drugs have achieved widespread acceptance, although the other therapies have been tried with varying degrees of success.

• Anticholinesterase drugs. The cornerstone of therapy, these drugs inhibit the enzyme that breaks down ACh, so ACh levels increase and are able to somewhat break through the antibodies that are blocking the ACh receptors. Obvious improvement in muscle strength is apparent in just a few days after beginning treatment.
• Drugs that suppress the immune system. These drugs have greatly improved the outcome of the disorder in humans. It can be helpful in dogs, however, it has to be used carefully, because suppressing the immune system can make infectious diseases like aspiration pneumonia much worse.
• Plasmapheresis. This is a procedure whereby plasma is separated from whole blood, and then returning the blood cells to the dog along with plasma from a normal dog, or a plasma substitute. The clinical benefits of this procedure are due to the removal of the antibodies that bind the ACh receptors and inhibit nerve impulse transmission. This procedure requires special equipment and usually requires referral to a specialist.
• Thymectomy. Surgical removal of an abnormal thymus gland is associated with long term clinical improvement in humans with myasthenia gravis. Unfortunately, in the few reported cases of thymomas in dogs with myasthenia gravis, removal of the thymus has generally resulted in poor outcomes, unfortunately.
• Elevated feedings or gastrostomy tube placement. The majority of dogs with myasthenia gravis have a poorly contractile esophagus and need to be fed a gruel at an elevated height, followed by elevation of the head for 10 to 15 minutes after feeding to facilitate passage of food into the stomach. For dogs who continue to regurgitate despite elevated feedings, placement of a gastrostomy tube directly into the stomach through the body wall may be helpful. Food, medication, and water may be easily given to the patient through the gastrostomy tube.

Follow-up Care for Dogs with Myasthenia Gravis

Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve. Administer all prescribed medication as directed. Alert your veterinarian if you are experiencing problems treating your pet.

If aspiration pneumonia was present, return to your veterinarian for follow-up radiographs to see if the condition is resolving.

Feed a semi-solid or liquefied food from an elevated height, and keep the dog’s head elevated for 10 to 15 minutes after feeding, if esophageal dysfunction is part of the dog’s syndrome.

Return for regular check-ups to monitor the level of anti-ACh receptor antibody in the circulation. The level should be low if medical therapy has been successful. A check of the antibody level is recommended every 4 to 8 weeks.

The prognosis for recovery from myasthenia gravis is guarded. Most affected dogs die within 12 months from aspiration pneumonia. The pneumonia may come on suddenly and cause death, or may be a chronic recurring problem causing debilitation and eventual euthanasia. Occasionally, a mildly affected dog will respond well to medical therapy and may have their disease go into remission for many months.

To talk to other pet owners with dogs affected with this condition, go to http://groups.yahoo.com/group/megaesophagus.