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Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian. I am so sorry to hear that your fellow Lenny has been through a tremendous amount of testing without getting any answers and he is not improving with steroid use. How frustrating and sad for all of you.
Myasthenia gravis would certianly fit with this picture so it made sense to test for this. Sometimes we do get false negatives when testing for antibody levels. Was a drug called edrophonium chloride (“Tensilon response test”) ever tried to see if he improved?
Another possible cause for weakness includes hypothyroidism. We have seen some cases lately where the only symptoms seem to be neurologic, including muscle weakness. If he has not had a full thyroid profile that should be done. T-4 testing alone is not suffucient as that hormone level fluctuates wildly, enough so that we can get reading that looks normal when it really is not.
If he doesn't have Myasthenia Gravis or hypothyroidism, and hasn't responded to high doses of steroids, then unfortunately we need to consider that he may have a tumor affecting the nerve roots that control his front legs. The only good way to diagnose those is to have an MRI done at a specialty clinic. Treatment would be surgery to try and remove the tumor and probably follow-up chemotherapy based upon the type of tumor present.
Best of luck with your fellow, please let me know if you have any further questions.