Myasthenia Gravis in Dogs
Myasthenia Gravis is a deficiency of the neurotransmitter, acetylcholine, that results in a neuromuscular junction dysfunction. Think of it like talking to toddlers: they do not have enough words to communicate effectively. In this instance, there is not enough acetylcholine sending vital information from the nerves to the muscles. This results in muscle weakness of the limbs, face, and/or the esophagus.
Symptoms
The symptoms – which are progressive – can vary from dog to dog.
Megaesophagus – relaxing of the esophageal muscle so that food cannot be pushed by normal muscle contraction down into the stomach. Dogs can develop aspiration pneumonia from inhaling regurgitated food.
Pharynx laxity – changes occur to the bark
Limb muscle weakness and stiffness – use of the muscles exhaust the limited amount of acetylcholine available. Affected dogs will usually exhibit shortened strides and stiffening limbs during exercise, but also muscle weakness after exercise. As the disease progresses, the periods of exercise exertion shorten while rest time lengthens. This can lead to paralysis.
Facial weakness – inability to blink; drooping lip; and facial paralysis
Drooping tail
Trouble controlling urine stream or squatting
Lethargy
Forms of Myasthenia Gravis
Two forms of Myasthenia Gravis have been identified: congenital (hereditary) and acquired.
Congenital Myasthenia Gravis
Congenital Myasthenia Gravis requires that both parents carry an autosomal recessive trait that creates a deficit in the number of the acetylcholine receptors. It is a very unfortunate condition:
Genetic testing does not exist yet to screen out the recessive trait
Diagnosis is best done with muscle biopsy of the afflicted
The onset of generalized muscle weakness starts around 6 – 8 weeks of age
It is progressive
Treatment is available but usually pups do not respond
Death often occurs before puberty
Jack Russell Terriers, Springer Spaniels, Smooth Fox Terriers and Dachshunds have all been identified as carriers of the gene. Interestingly, it has been noted that sometimes Dachshunds can spontaneously recover. Spontaneous recovery or not, these dogs should not be bred and the parents should never be used for breeding with any dog.
Acquired Myasthenia Gravis
Acquired Myasthenia Gravis is an immune-mediated (autoimmune) neuropathy (disease of the nerves). The body produces autoantibodies that attack and destroy the acetylcholine receptors specifically. Like most immune-mediated diseases, we do not know what outside forces (food, infections, chemicals, vaccines, environment, etc.) trigger the body to produce these antibodies that attack itself, but believe there is also a genetic component. In the instance of Myasthenia Gravis, it might be an abnormal thymus gland but researchers are not certain yet.
A dog’s thymus gland functions similarly to that in humans. It is a vital organ in the development of the immune system since it builds up the T-lymphocytes (T-cells) to help fight infection and other immune challenges. Once this developmental process is complete, the thymus stops active production and stays dormant. In humans and dogs with myasthenia gravis, it has been observed that the thymus gland remains large and is abnormal (forms a thymoma).
Researchers believe the thymus gland may give incorrect instructions to developing immune cells, ultimately resulting in autoimmunity and the production of the acetylcholine receptor antibodies, thereby setting the stage for the attack on neuromuscular transmission.
We can also assume that there is a genetic predisposition because we find it more often in Golden Retrievers, Labrador Retrievers, Akitas, Miniature Dachshunds, Scottish Terriers, German Shorthaired Pointers, Shetland Sheepdogs, and Collies. However, it must be remembered that any dog of any breed can develop Myasthenia Gravis.
For some reason, we often see peaks of Acquired Myasthenia Gravis, between the ages of 2 – 4 and then again from 9 – 13 years of age. Unfortunately, pet parents may think it is just the signs of aging of their senior dog, and not seek out proper diagnosis, management and treatment.
With this form of Myasthenia Gravis, we break it down further into three groups:
Mild or Focal – only one body part is afflicted and it is usually the esophagus
Moderate Generalized – limb weakness with or without megaesophagus
Severe Generalized or Acute Fulminating – rapidly progressive and usually fatal
Diagnosis
As stated previously, the best way to diagnose Congenital Myasthenia Gravis is with a muscle biopsy that is sent to Dr. Diane Shelton at the University of California San Diego’s Comparative Neuromuscular Laboratory.
Fortunately, a blood test exists for the Acquired form. This blood test, AChR, checks for the autoantibodies that are destroying the acetylcholine receptors. Remember, the congenital form does not have the autoantibodies. The AChR specificity rate is 98%. If the veterinarian is fairly certain that your dog lands within this 2% zone, other tests are available such as the Tensilon Response Test to see if the muscles move after administering edrophonium chloride, and electrodiagnostic testing to see if electrodes stimulate the muscles to respond at various levels. He can also retest with the AChR test a couple of weeks later.
Before performing these secondary or follow-up tests, I would also consider thyroid disease as a possible cause of these symptoms. Thyroid dysfunction can cause neuropathies such as megaesophagus and muscle weakness. Indeed, hypothyroidism could be concurrent with Myasthenia Gravis. Thus far, we believe they are separate conditions and that one does not precipitate the other. Regardless, I recommend a complete thyroid panel that measures the hormones T4, FT4, T3 and FT3, and the thyroid autoantibody, TGAA, at either Hemopet’s Hemolife Diagnostic Laboratory, Michigan State University, or another veterinary reference diagnostic laboratory.
Treatment
Treatment outcomes for Myasthenia Gravis are variable. Fortunately, while many dogs have spontaneous remissions, the stated 87% rate of spontaneous remissions within the first four months is of concern. It could relate to a number of reasons such as incorrect original diagnosis, another cause (e.g. tick-borne paralysis, vaccines), and giving optimal treatment dosage and supportive care by the pet caregiver. Secondly, the diagnostic group involved – focal, moderate, or severe – also impacts the outcome.
Once diagnosis is confirmed, veterinarians can prescribe anticholinesterase agents. Basically, the body has a naturally occurring acetylcholinesterase enzyme that breaks down acetylcholine, which is needed to put the brakes on the neurotransmission. Think about it, if acetylcholinesterase did not exist, we would have muscle spasms or facial tics all the time. So, if we stop or slow the production of it, the acetylcholine has time to replenish and do its job.
Use of other drugs such as glucocorticoids (a class of corticosteroids) and other immunosuppressive drugs is controversial and inadvisable. They can enhance certain symptoms like muscle weakness or cause side effects like weight gain, instead of effectively treating the condition.
The most innovative procedure to date is therapeutic plasma exchange (TPE). Basically, the plasma containing the autoantibodies is removed and exchanged with fresh healthy canine donor plasma. However, it is not a cure for Myasthenia Gravis, expensive and usually requires going to a research veterinary institution.
Other supportive care really comes down to a daily ritual depending upon what part of the body is affected. For instance, megaesophagus requires fine-tuned care that includes slow eating, swallowing small chunks of food whole, lifting the food and water bowls to chest height, and possibly using a Bailey Chair to sit the dog in an upright position.
Myasthenia Gravis demands patience and commitment from the pet caregiver. Remember, it is a progressive condition that does not have a cure but we can try to slow the progression and side effects with the right supportive care and medications for a longer, quality life.
W. Jean Dodds, DVM
Hemopet / NutriScan
11561 Salinaz Avenue
Garden Grove, CA 92843
References
Kumar, Rajesh et al. “Therapeutic Plasma Exchange in the Treatment of Myasthenia Gravis.” Indian Journal of Critical Care Medicine: Peer-reviewed, Official Publication of Indian Society of Critical Care Medicine 19.1 (2015): 9–13. PMC. Web. 15 Jan. 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4296418/.
"Myasthenia Gravis." National Institutes of Health. U.S. Department of Health and Human Services, n.d. Web. 15 Jan. 2017. http://www.ninds.nih.gov/disorders/myasthenia_gravis/detail_myasthenia_gravis.htm.
"Myasthenia Gravis." VetMed. University of California Davis, n.d. Web. 15 Jan. 2017. http://www.vetmed.ucdavis.edu/vsr/Neurology/Disorders/Myasthenia%20Gravis.html#Pubs.
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA: Sinauer Associates; 2001. Acetylcholine. https://www.ncbi.nlm.nih.gov/books/NBK11143/.
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