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Myasthenia Gravis

By: Dr. Arnold Plotnick

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Diagnosis In-depth

  • 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.

    Therapy In-depth

    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.

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