What is myasthenia gravis?
Myasthenia gravis (MG) has two forms: 1) congenital and 2) acquired. Acquired myasthenia gravis is more common and results from the development of antibodies against the nicotinic acetylcholine receptors on the muscle membrane.
What is the clinical presentation?
Cats present commonly with the generalized form which includes a wide variety of clinical signs and progressions. This may include weakness (the "floppy cat"), cervical ventroflexion, or pharyngeal weakness. Signs may be slowly or rapidly progression and even result in waxing-waning clinical signs.
What causes the acquired form of myasthenia gravis?
Most cats have an idiopathic MG however up to 30-50% of cats will develop paraneoplastic MG for which thymoma are commonly implicated. Spontaneous remission of idiopathic MG within 6-8 months is common in dogs, but was previously thought to be uncommon in cats. A 2019 article found remission within 6 months in ALL 8 cats evaluated. (Mignan T, et al. JVIM Nov 2019) Remission even occurred in several cats that didn't have any form of immunosuppression or acetylcholinesterase inhibitors prescribed (see treatment below).
How should I diagnose myasthenia gravis?
1. A thorough neurologic exam for appropriate lesion localization (yay!), and
2. An acetylcholine receptor (Ach-R) antibody titer through a reputable lab. Here in the states, I recommend Dr. Shelton's lab (http://vetneuromuscular.ucsd.edu/). This titer can be repeated to document biochemical remission along with the neurologic exam to document clinical remission.
What is the ideal treatment?
Oh, the million dollar question! Based on experience I'd say cats respond less favorably to acetylcholinesterase inhibitors (think pyridostigmine or edrophonium) and therefore immunosuppressive steroids have been my go-to treatment. Having seen the data on spontaneous remission, I might consider no treatment in a minimally affected cat.
What is the long-term prognosis?
Idiopathic myasthenia gravis carries a good prognosis in cats. Should pharyngeal weakness become a clinical problem, aspiration pneumonia may result in increased morbidity or mortality. The 2019 study by Mignan et al reported a 100% survival at 6 months, without signs of relapse up to 4 years after treatment. Cats with paraneoplastic myasthenia gravis have a poorer short and long-term prognosis.
Key Points:
1. Cervical ventroflexion, or a "floppy" cat on examination should prompt an Ach-R antibody titer for myasthenia gravis diagnosis. (Maybe even a neurology consult!)
2. Treatment could be immunosuppressive steroids OR no treatment at all if clinically mild
3. Prognosis is good if a thymoma or other neoplastic process is not identified.
Thanks for reading! Happy 4th of July to US folks reading and Happy July 1st to Canadian folks! The rest of ya - hope you're having a good week!
Also, I'm looking for a location that can hold up to 20 people for a possible CE event in February 2025 in the middle of the state (Portage/Dells/Stevens Point). Do you have a hospital conference room I could rent or do you have a reocmmenation for a conference center that you thought did a good job in that area? Please reach out if you do!