Information in this issue is summarized from the recent Antech Medicine Consultants Conference call with Dr. Diane Shelton of the University of California at San Diego (UCSD) and from their web site: www.medicine.ucsd.edu/vet_neuromuscular.

The clinical presentation of myasthenia gravis (MG) is highly variable. In dogs, focal MG (e.g. megaesophagus, dysphagia) comprises > 50% of cases. Peracute collapse with generalized weakness resembling polyradiculoneuritis can also occur. The classical episodic weakness with exercise intolerance is uncommon in dogs. Hypothyroidism is present in ~ 20% of canine MG cases. In cats, focal MG is uncommon as most cats present with generalized weakness. Dysphagia, megaesophagus, and decreased palpebral reflex can occur in cats. Thymoma related to MG is present in ~~ 3% of canine cases, but occurs in ~ 25% of feline cases, especially in Abyssinian and Somali cats.

Diagnostic confirmation of MG uses 3 approaches. The most commonly used and reliable test is for acetyl choline receptor antibody (ACRA) titer. This test is 98% sensitive for generalized MG and ~ 75% sensitive for focal MG. Immunosuppressive doses of corticosteroids given for > 7-10 days will lower ACRA titers. Hemolysis and lipemia do not appear to affect the test.

The "Tensilon" (edrophonium response) test can also be used, and is specific for MG if the response is dramatic. Weak responses can be seen in patients with MG, polymyositis, or peripheral neuropathy. False negative responses can occur. The dose of edrophonium is 0.1-0.2 mg/kg IV. Pre-treatment with atropine is usually unnecessary.

Repetitive nerve stimulation can also be used to diagnose MG but requires general anesthesia. A decremental muscle action potential in response to repetitive stimulation is seen with MG.

The clinical presentation of masticatory muscle myositis (MMM) in dogs reflects focal inflammatory myopathy of the muscles of mastication, while limb muscles are spared. Signs range from acute swelling of the temporalis and masseter muscles, restricted jaw movement, jaw pain, and exophthalmus, to muscule atrophy. The classical clinical sign of acute MMM is inability to open the jaws even under anesthesia. If the jaw can be opened, then temporomandibular joint disease (TMJ) is more likely. In chronic MMM, masticatory muscles are atrophied and there may be no pain or jaw restriction. CPK concentration is often normal or only slightly elevated because of the small amount of involved muscle mass and the chronic presentation.

Diagnosis of MMM is confirmed by the 2M-antibody test and a muscle biopsy. Antibodies against type 2M masticatory muscle fibers are detected in an immunohistochemical assay. The test is highly sensitive in acute stages of MMM, but can become negative in end-stage disease when little muscle antigen remains to stimulate an immune response. Breeds such as the Samoyed, Doberman Pinscher, and Rottweiler are especially affected with end-stage disease. Lowered antibody titers can result if the dog has received immunosuppressive doses of corticosteroids for > 7-10 days. Hemolysis and lipemia do not appear to affect the test.

Muscle biopsy is also recommended to determine the degree of myofiber destruction or fibrosis, for these affect prognosis. Open biopsy of the temporalis muscle is recommended, as punch biopsy is generally inadequate. Incise through the platysma muscle and thick fascia of the temporalis muscle to ensure biopsy of the correct tissue. Remove a 0.5cm 3 piece of muscle, moisten it by dipping in normal saline, and place in a red top tube and transport on ice directly to the UCSD lab, as it must be received within 24 hours. A formalin-fixed smaller (1/4 size) piece of muscle should also be submitted to UCSD at the same time, and is evaluated at no extra charge.

Early diagnosis and therapy with immunosuppressive doses of corticosteroids before muscle atrophy and fibrosis is advanced usually yields a good prognosis.