Myasthenia gravis (MG) is an autoimmune disease characterized by autoantibodies against muscle acetylcholine receptors. Triggers for this disease are not yet known but are likely similar to those of MMM.

Corticosteroid usage can lower the acetylcholine receptor antibody (AChR-Ab) titer and result in a false negative result. Negative titers are also possible early in the course of the disease and retesting in 10-14 days would be suggested before ruling out a diagnosis of MG. A positive AChR-Ab test is real and diagnostic. Sensitivity in canine generalized MG is 98%. In focal MG sensitivity is 70-80%. The AChR-Ab test is accurate for the diagnosis of MG in both dogs and cats.

Monitoring. Data from treating cases of canine MG with pyridostigmine alone versus pyridostigmine + immunosuppressive drugs show the following (Shelton and Lindstrom 2001):

47/53 cases of generalized and focal MG treated with only anticholinesterase drugs went into spontaneous remission in an average time of 6 months (range: 1 to 18 months; AChR-Ab titers negative with the dogs off all medications and clinically normal). When present, megaesophagus often resolves, regurgitation ceases, and there is radiographic resolution. Based on this study, it is suggested to initially try Mestinon alone. If clinical signs are not greatly improved in 1-2 days, then addition of low dose corticosteroids or another immunosuppressive agent would be suggested. Avoidance of immunosuppressive dosages of corticosteroids is recommended since it can exacerbate the weakness in some myasthenic patients.

6/47 cases did not go into remission, AChR-Ab titers remained elevated and dogs required steroid treatment. All of these dogs developed neoplasia months to years following the diagnosis of MG. Four dogs developed thymoma, in one case the mass was found 2 years after the diagnosis of MG. If MG is associated with thymoma and there is complete surgical removal, AChR-Ab titers should decrease to within the normal range over 1-2 months. If the titer stays up or increases, there was in-complete surgical removal. Remission cannot be achieved and AChR-Ab titers will not return to normal in these cases.

Megaesophagus. In addition to MG, megaesophagus can be associated with Addison's disease, polymyositis (Evans 2004), esophageal disease, peripheral and central nervous system disease, and hypothyroidism. If these other diseases have been ruled out and the AChR-Ab is within the reference range, a low dose of Mestinon syrup (1 mg/kg BID) can be tried to see if regurgitation decreases. Dilute syrup half and half in water as it can be irritating to the stomach if given straight. If there is no response or if there is worsening of clinical signs, stop the medication. A diagnosis of idiopathic megaesophagus is likely.

Hypothyroidism. About 20% of dogs with MG are also hypothyroid.