What is myasthenia gravis?
Myasthenia gravis (MG) is a neuromuscular disease caused by congenital or acquired causes. Acquired MG is caused by an attack against the acetylcholine receptor on the muscle or the muscle specific kinase (MUSK) protein. Acquired MG will be the focus of this TidBit today.
How does myasthenia gravis present clinically?
Focal, generalized and fulminant forms have been described. Focal MG results in weakness of 1 or more muscle groups (ex: esophagus or pharynx) and does not cause limb weakness. Generalized MG results in limb weakness, with or without (usually with!) megaesophagus and pharyngeal weakness and the fulminant form causes acute severe flaccid paresis or paralysis often requiring hospitalization and intravenous intervention.
How is myasthenia gravis diagnosed?
Acquired MG is diagnosed by a combination of clinical signs, positive acetylcholine receptor antibody titer and/or positive response to edrophonium challenge. A positive acetylcholine receptor antibody titer is considered the gold standard diagnostic tool. Seronegative MG is reported in about 2% of cases and likely represents dogs with immune attack against the MUSK protein rather than the acetylcholine receptor. Edrophonium is a short acting acetylcholinesterase inhibitor that can be used in the preliminary stages of diagnosis. Animals are administered a small dose of edrophonium and monitored for clinical signs of improved strength. This test is not advisable for use in obtaining a diagnosis of focal MG. It is important to note that dogs with other neuromuscular diseases may have a positive edrophonium challenge therefore it should not be used as a sole diagnostic tool.
What is the treatment for myasthenia gravis in dogs and cats?
Acquired MG is most often treated with an oral, long acting acetylcholinesterase (pyridostigmine). Signs of overdose are similar to signs of acquired MG (SLUD plus weakness) therefore it is recommended to start at a lower dose and slowly increase the dose over 1-2 weeks until clinical signs resolve. Immunosuppression has been used for pets with focal or generalized MG however the risk of secondary pneumonia is higher with immunosuppression and the clinical improvement does not appear to be better than with pyridostigmine administration therefore it is rarely recommended.
Acquired MG will often result in remission within 8 months of the initial diagnosis. If remission is not achieved, follow-up testing for concurrent diseases that could trigger MG is recommended. This includes a thyroid panel (for thyroiditis), chest radiographs and abdominal ultrasound for a cancer screening.
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