FEBRUARY 2010 - SPECIAL FEATURE

Most commonly asked questions about acquired myasthenia gravis

Acquired myasthenia gravis (MG) is a disorder of neuromuscular transmission resulting from autoantibody mediated destruction of the nicotinic acetylcholine receptors at the neuromuscular junction. Acquired MG is the most common neuromuscular disorder that can be diagnosed in dogs. It occurs less commonly in cats. There is a broad spectrum of clinical presentations ranging from focal MG associated with regurgitation due to esophageal dilatation or dysphagia, generalized weakness with or without an associated megaesophagus, MG associated with thymoma, or acute fulminating MG with rapid deterioration of clinical signs over a period of hours. The course of the disease can be variable and unpredictable. Unfortunately, the mortality rate in canine acquired MG is still unacceptably high with approximately 50% of the dogs diagnosed with acquired MG succumbing to aspiration pneumonia or respiratory paralysis. The importance of early recognition of regurgitation (and not vomiting) associated with esophageal dilation, an accurate diagnosis, and appropriate therapy cannot be stressed enough. The following is a list of frequently asked questions about this disorder.

Q: What is the diagnostic method of choice for acquired MG in dogs and cats? What is the sensitivity and specificity?
A:  An accurate diagnosis of acquired MG is important prior to beginning therapy. The immunoprecipitation radioimmunoassay for detection of circulating antibodies against the nicotinic acetylcholine receptor (AChR)  is the gold standard for the diagnosis of acquired MG in humans, dogs, and cats. The assay is specific, sensitive, and demonstrates the presence of antibodies specifically against AChRs. Over 100,000 dogs and cats have been tested by this method and to date, false positive results have not been demonstrated. Although 98% of dogs with generalized MG are seropositive, approximately 2% of the dogs with acquired MG are seronegative. This assay is available for dogs and cats through the Comparative Neuromuscular Laboratory at the University of California, San Diego. Previous corticosteroid therapy at immunosuppressive dosages for longer than 7-10 days will lower antibody levels so a pre-corticosteroid serum sample is recommended.

Q: What about the Tensilon test? Should I do it?
A: If  Tensilon is available, a dramatic increase in muscle strength following IV injection  (0.1-0.2 mg/kg) should give a presumptive diagnosis of acquired MG while waiting for the results of the AChR antibody titer. Treatment could be initiated based on the results of the dramatic positive test. Unfortunately not all dogs with acquired MG are responsive to Tensilon, and dogs with other neuromuscular diseases may show a subjective positive response.


Q:  What is the treatment of choice for acquired MG?
A:  This is not an easy question to answer as treatment needs to be tailored to the severity of the disease and adjusted according to the animal's response. If esophageal dilatation is present, elevation of food and water is of utmost importance. The consistency of the food (liquid, solid, gruel) may also need to be altered depending on which is handled best. If intractable regurgitation is present even with this altered feeding technique, a PEG tube should be placed for maintenance of fluid and nutritional support. Anticholinesterase drugs are the cornerstone of therapy for MG. Pyridostigmine bromide (Mestinon, 0.5-3.0 mg/kg BID-TID adjusted as needed) is recommended over neostigmine (Prostigmin) as muscarinic side effects are less. Pyridostigmine bromide is available in syrup, tablet, and timespan forms. If the syrup is used it should be diluted 1:1 in water as it can cause gastric irritation and vomiting if given straight. In dogs with milder forms of MG this may be all the therapy required. If muscle strength has not returned to normal with anticholinesterase therapy alone, addition of low dose daily (0.5 mg/kg/day) or alternate day corticosteroid therapy has been shown to be beneficial. Immunosuppressive dosages of corticosteroids may exacerbate muscle weakness early in treatment and worsen the clinical signs so avoidance of high dosages is encouraged. For the acute, fulminating form of MG, intensive care including respiratory support may be required. Therapies used in human medicine for the severe forms include IV immunoglobulin and plasmapheresis. Unfortunately, these are not generally available to the veterinary community. Many clinicians also use other immunosuppressants such as azathioprine, mycophenolate or cyclosporine, but efficacy in controlled studies has not been documented.


Q:  Does the dog need to be on medication for the rest of it's life?
A:  In most cases the answer is no. If the dog does not succumb to an aspiration pneumonia it should ultimately go into clinical remission. The average time for remission is 6-8 months with some dogs going into remission faster and others taking over 1 year. Periodic radiographic evaluation of esophageal dilatation and serial AChR antibody titers are suggested to monitor the course of the MG. The AChR antibody titer will go down into the normal range as the dog goes into clinical remission. This is in the absence of immunosuppression. It is important not to take the animal off medication too soon as clinical signs can return. If the AChR antibody titer remains elevated for over a year, then a search for a neoplasm (such as thymoma) should be initiated. A screen for any concurrent medical problems such as hypothyroidism should also be initiated and treated if identified.

From the Case of the Month Archives:
Aquired Myastenia Gravis in a 2 year old female spayed Golden Retriever mix.



Q: If the dog goes into a clinical remission will the MG come back?
A: If the clinical remission is a true remission (ie: not a drug induced remission where clinical signs may return following cessation of therapy) with the AChR antibody titer returning to the normal range, the chances are good that the remission is permanent. The author is aware of a few dogs that did come out of remission 1-2 years following the initial course of MG, but those cases are the exception and not the rule.

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