to begin, and to prevent worsening of the most difficult aspect of the disease - megaesophagus.  Although megaesophagus caused by myasthenia CAN BE reversible, too often it is not.  Perhaps early diagnosis and treatment would prevent complete esophageal dilatation, esophagitis, and aspiration pneumonia.  In early stages of the disease, or in some cases of focal myasthenia, the esophagus is not completely flaccid and dilated - it may be weak and have poor function, but may remain normal in appearance on survey radiographs.

The Tensilon (edrophonium) test  allows a more rapid diagnosis, and can be performed in the veterinarian's office, if proper precautions are taken.  Edrophonium is a cholinesterase inhibitor - it blocks the action of acetylcholinesterase, which causes more Ach to remain in the NMJ after a nerve discharges.  Thus, when any of the small number of receptors has recovered after the initial depolarization, if there is persistent Ach in the region of the receptors, it will bind and will increase the chance that the muscle membrane will reach threshold.  Edrophonium is a short-acting cholinesterase - it will usually reverse the muscle weakness in a myasthenic animal for only a period of a minute or two.   Since both nicotinic and muscarinic receptors will be affected, and since the abnormality in a myasthenic is only in the nicotinic receptors, pre-treatment with an anti-muscarinic drug (e.g., atropine) will prevent dangerous muscarinic hyperactivity (salivation, urination, defecation, lacrimation and BRADYCARDIA). 

In both generalized and in focal myasthenia, observation of  esophageal activity can be beneficial both for diagnostic and for therapeutic purposes.  Fluoroscopy provides an excellent means to observe the esophagus dynamically, so that a diagnosis of  poor esophageal function can be made, even if megaesophagus is not present or is not identified on survey thoracic radiographs.  In addition, the Tensilon test can be performed on an animal that has focal myasthenia, and the esophagus can be observed fluoroscopically during swallowing, before and after giving the drug. Unfortunately, the Tensilon test is sometimes equivocal or negative (the animal does not regain strength convincingly) in an animal with myasthenia gravis, and occasionally an animal with another kind of neuromuscular junctionopathy or another type of weakness will show some increase in strength after receiving Tensilon.

Electrodiagnostic testing can sometimes confirm a diagnosis of myasthenia.  Action potentials are recorded from a muscle membrane during repetitive motor nerve stimulation.  In normal animals, the amplitude of the action potential in the muscle will be consistent, and in the myasthenic animal, the response is decremental.  In the myasthenic, when a nerve is stimulated at 3-5 Hertz, each action potential recorded from the muscle is of lower magnitude than the one before it, as the number of available receptors (those that have recovered and are ready to depolarize again) declines.  Animals under

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