August 2025
In this VETgirl online veterinary continuing education blog, Dr. Missy Carpentier, DACVIM (Neurology) details myasthenia gravis (MG) in dogs and cats. As a both a congenital, albeit rare, and, more commonly, acquired disease, MG can cause debilitating weakness in patients. In this blog, we focus on diagnosis, treatment and prognosis of acquired MG in dogs and cats.

Myasthenia Gravis

By Missy Carpentier, DVM, DACVIM (Neurology), Minnesota Veterinary Neurology, Columbus, MN

Myasthenia Gravis (MG) can be both congenital and, more commonly, acquired with the dominating clinical sign being weakness. Dogs with the congenital form of the disease present at less than a couple months of age and have abnormal development of the acetylcholine receptors. The congenital form is rare. We are going to focus on the acquired form of the disease in this article, but if you have a juvenile patient who you suspect has MG, remember that there is a congenital form and they will not test positive on the acetylcholine receptor-antibody (ACHR-Ab) test, because they do not have autoantibodies against the receptor.

Acquired MG

Acquired MG is a disease in which autoantibodies are directed against acetylcholine (Ach) receptors on the postsynaptic membrane on the neuromuscular junction.

Canine Presentations

There are three classic forms for acquired MG in canines and they include:

  • Focal MG: Predominant pathology is megaesophagus and therefore the classic clinical sign is regurgitation. This form is seen in approximately 36-43% of cases.
  • Generalized MG: Clinical signs include regurgitation secondary to the megaesophagus and exercise induced weakness. This form is seen in approximately 57-64% of cases.
  • Fulminating MG: Clinical signs include profound weakness to the degree of non-ambulatory tetraparesis/plegia. This form is seen in approximately <5% of cases.(1)

Feline Presentations

In cats, the most common clinical presentation is generalized weakness, reported as the main sign in 54.3% of cases.(2) They can also present with megaesophagus or dysphagia, in addition to generalized weakness. Cats are much less likely to present with focal MG with clinical signs localized to the esophagus or pharynx only. The feline esophagus is a combination of skeletal and smooth muscle versus the canine esophagus that is entirely skeletal muscle.

 

Unlike in dogs, in which only 3.4% present with a mediastinal mass, 25.7-52% of cats will present with a mediastinal mass that is most often a thymoma.(2-4) MG presents with a greater incidence in Abyssinian and Somali cats.(2, 4) An adverse drug reaction to methimazole is another cause of acquired MG in cats.

Diagnosis

A presumptive diagnosis of MG is made off the clinical signs. Previously we had a fun test, the tensilon test, to strongly support the diagnosis, however, edrophonium is no longer available so this is no longer performed. To confirm the diagnosis, acetylcholine receptor-antibody (ACHR-Ab) tests can be performed in both dogs and cats. Ninety-eight percent of dogs that have acquired MG will demonstrate antibodies, however, 2% will be seronegative.(5) There is no correlation between a higher ACHR-Ab level and prognosis in dogs, however, in cats, there is a significant correlation between a higher ACHR-Ab titer and a decreased survival time.

 

It is very important once MG has been diagnosed that a thorough work up is performed to rule out an underlying cause, such as a mediastinal mass mentioned above or other possible neoplastic processes. This is often performed with radiographs of the chest and an abdominal ultrasound, or if available, a whole-body CT.

Treatment and Prognosis

Treatment consists of oral immunosuppressive medications, pyridostigmine, and intense supportive care. Common immunosuppressive medications that are used include prednisone, mycophenolate, cyclosporine, azathioprine, or leflunomide. Normally treatment will be initiated using one immunosuppressive medication and a second immunosuppressive medication will be added if necessary.

Pyridostigmine is an anticholinesterase medication that is used to allow the ACH to be present longer at the synapse to hopefully allow it to bind to an available receptor. Possible side effects of pyridostigmine are related to a cholinergic crisis or toxicity. You can use the mnemonic “SLUDGE” to warn your owners about what side effects to watch for, which include: salivation, lacrimation, urination, defecation/diarrhea, gastrointestinal signs, and emesis. These signs can also be consistent with MG, so once pyridostigmine is started it is important to evaluate these patients and get a thorough history if the owners are noticing these clinical signs to try and decipher if the clinical signs are secondary to the pyridostigmine or progressive MG (but sometimes this can be difficult).

 

Most commonly, cats will be treated with prednisolone and pyridostigmine. It is important to know that even in cats that have a mediastinal mass that is surgically removed, almost all cases of cats with MG-associated thymoma require postsurgical medical management to control the signs of MG. This is different than in dogs, where most cases will go into remission when the neoplastic mass is removed. It is important to know the significant risks that are associated with these surgeries.

Most recently, therapeutic plasma exchange (TPE) has become more readily available for patients with MG and is a wonderful treatment option. The use of TPE in dogs with MG appears to be well tolerated, is safe, and can be considered as a first line therapy in addition to the medications listed above, or it can be considered in patients that are not responding to medical management alone.

Overall, one of the most important aspects of treatment for these patients is supportive care. This can be drastically different in each patient based on their level of weakness, but it is aimed at providing appropriate nursing care, nutritional support, and physical therapy to try and keep them as comfortable and safe as possible. One of the highest complications of MG is aspiration pneumonia due to the megaesophagus. Diligent feeding practices of these animals with a Bailey-chair or something similar is of utmost importance. Any signs of aspiration pneumonia in these patients should be addressed promptly.  I have also found that some of my patients tend to have regurgitation when they are sleeping at night. Some owners have found it helpful to have their pets sleep with an inflatable cone (donut cone) around their neck to keep their neck in slight extension.

Dog with megaesophagus in a Bailey chair. Photo courtesy of Kimberly Perkovick, LVT.

On average, approximately 88% of dogs will go into spontaneous remission approximately 6 – 18 months after diagnosis and will no longer require medication if the cause for MG is not neoplasia.(6)

Want to hear more…tune in to hear a podcast with Dr. Carpentier on myasthenia gravis in dogs and cats HERE!

Of course, you might want to hear more about the long-term outcome of  acquired MG in cats specifically…check out this podcast where we review the findings of Mignan et al in a study published in the Journal of Veterinary Internal Medicine, entitled Long-term outcome of cats with acquired myasthenia gravis without evidence of a cranial mediastinal mass.

References:

    1. Khorzad R, Whelan M, Sisson A, et al. Myasthenia gravis in dogs with an emphasis on treatment and critical care management. J Vet Emerg Crit Care. 2011;21(3):193-208.
    2. Shelton GD, Ho M, Kass PH. Risk factors for acquired myasthenia gravis in cats: 105 cases (1986–1998). JAVMA. 2000;216(1):55-7.
    3. Shelton GD, Schule A, Kass PH. Risk factors for acquired myasthenia gravis in dogs: 1,154 cases (1991–1995). JAMVA. 1997;211(11):1428-31.
    4. Hague DW, Humphries HD, Mitchell MA, et al. Risk factors and outcomes in cats with acquired myasthenia gravis (2001–2012). J Vet Internal Med. 2015;29(5):1307-12.
    5. Shelton GD. Myasthenia gravis and disorders of neuromuscular transmission. Veterinary Clinics: Small Anim Pract. 2002;32(1):189-206.
    6. Shelton GD, Lindstrom JM. Spontaneous remission in canine myasthenia gravis: implications for assessing human MG therapies. Neurology. 2001;57(11):2139-41.

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