Myasthenia Gravis in Dogs
Myasthenia gravis is a disease in which there is a malfunction in the transmission of signals between the nerves and muscles. Dogs with myasthenia gravis exhibit extreme weakness and excessive fatigue. Myasthenia gravis is a disease that interrupts the way nerves communicate with muscles. In order to understand this disease, Some breeds are predisposed to an inherited/congenital form of this disease, including Jack Russell Terriers, English Springer Spaniels, Smooth Fox Terriers, and Smooth-haired Miniature Dachshunds.
Acquired myasthenia gravis is, like other immune-mediated diseases, a complex condition requiring that multiple factors come into play, including environmental, infectious, and hormonal influences. The Newfoundland, Great Dane, Golden Retriever, Labrador Retriever, Akita, and Scottish Terrier are predisposed to acquired myasthenia gravis. Some dogs who acquire myasthenia gravis have a tumor mass in the chest cavity called a thymoma.
Puppies with congenital myasthenia gravis are typically diagnosed at 6-8 weeks of age. Acquired myasthenia gravis tends to be diagnosed in dogs 1-4 years of age, or in dogs at 9-13 years of age. Most cases are acquired, not inherited.
A neuron is a nerve cell. It has a head called a cell body at one end, a long strand called an axon, and a foot piece with small branching fingers called foot processes. Myasthenia gravis concerns neurons that have their cell bodies (their heads) in the spinal cord and their foot processes (their feet) in the voluntary muscles that we use to move our bodies (our “skeletal muscles”). The spinal cord sends a message to move a certain muscle. The neuron receives this message and carries it to that muscle.
A nerve is a group of axons. The white fibrous structures we call nerves are actually groups of axons bundled together similar to the way electrical wires in your home are bundled inside an insulating cord.
Neurotransmitters are chemicals. In order for a message to be transferred between neurons, a chemical is released from the foot processes of the first neuron and is taken up by one of the branches of the receiving neuron’s cell body. After the chemical message has been successfully transferred, an enzyme destroys the neurotransmitter molecule in order to prevent ongoing stimulation. The neurotransmitter we are concerned with in myasthenia gravis is called acetylcholine and the enzyme that degrades it is called acetylcholinesterase.
The neuromuscular junction is the area where nerves and muscle interface. There are three types of muscle: the heart muscle; smooth (or involuntary) muscle, (the kind that moves food through your intestine or constricts your pupils); and striated (or skeletal) muscle (the kind you use for voluntary activities like walking, typing, singing, and controlling facial expressions). It is the neuromuscular junction on the striated muscle that is stricken in myasthenia gravis.
There are several forms of myasthenia gravis:
Myasthenia gravis is the condition where the neuromuscular junctions are diseased. The acetylcholine message from the nerve trying to stimulate the muscle is quickly blunted because of damage to the nerve/muscle interface. An analogy would be like trying to receive a signal with a damaged radio or, worse, a radio with ongoing new damage. Some of the messages might get through or get through weakly. Treatment focuses on stopping the damage and strengthening the signal.
Congenital Myasthenia Gravis
In this condition, the patient is born without normal neuromuscular junctions to striated muscles. There is no effective treatment. Myasthenia gravis has been described as a recessive genetic disease in Jack Russell terriers, Springer Spaniels, and Smooth Fox Terriers. The miniature dachshund gets a congenital form that actually resolves with age.
What are the signs of myasthenia gravis?
Many dogs with acquired myasthenia gravis develop megaesophagus, which is a dilation of the esophagus that holds food rather than allowing food to pass into the stomach. These dogs then have their food return up through their mouths without the retching and abdominal muscle contraction associated with vomiting. They may exhibit difficulty swallowing, a decreased ability to blink, and/or acute collapse.
Other signs of myasthenia gravis include:
- voice changes
- exercise-related weakness and/or collapse
- progressive weakness
- inability to close the eyes, even when sleeping
- excessive drooling
- difficulty breathing
- cramping with mild exercise
Elevated blood levels of antibodies against acetylcholine receptors provide an important diagnostic confirmation and monitoring test.
Diagnosis
If your pet suffers from muscle weakness, your veterinarian will take a thorough history and perform a complete physical exam on him.
There are many things that can cause muscle weakness in dogs; therefore, your veterinarian may recommend diagnostic tests such as:
- Chemistry tests to evaluate kidney, liver, and pancreatic function, as well as sugar levels
- Blood parasite screening to identify if your pet has been exposed to tick-borne or other infectious diseases
- Fecal tests to rule out intestinal parasites
- A complete blood count (CBC) to rule out blood-related conditions
- Electrolyte tests to ensure your pet isn’t dehydrated or suffering from an electrolyte imbalance
- Urine tests to screen for urinary tract infections and other diseases, and to evaluate the kidney’s ability to concentrate urine
- A thyroid test to determine if the thyroid gland is producing too little thyroid hormone
- X-rays of the chest, abdomen, and spine
- Myelogram contrast x-rays of the spine (dye is injected into the spinal column, then x-rayed)
- MRI or CT scan to gain a detailed picture of the brain and spine
- Specialized immunologic tests, including titers
- A Tensilon test, which checks muscle response
Is myasthenia gravis treatable?
Some dogs diagnosed with myasthenia gravis require treatment in the hospital until their medication dose is stabilized. These dogs are treated with a class of medication that inhibits a nervous system enzyme called acetylcholinesterase. Anti-acetylcholinesterase medications will be required for the rest of the dog’s life. Because of their compromised ability to swallow, some dogs will actually inhale food, liquid, or vomit, resulting in aspiration pneumonia. Aspiration pneumonia is extremely serious and often requires aggressive intensive care including oxygen therapy, antibiotics, IV fluid therapy, and supportive care. If the dog is unable to eat or drink without regurgitation, a feeding tube may be needed until the dog’s medication doses are stabilized.
Ancillary treatment of myasthenia gravis is as important as determining appropriate medication doses. In cases where there is a thymoma, it must be removed surgically. Food and water dishes should be elevated, and these dogs often do best with smaller, more frequent meals of a high-quality, high-calorie food. There is no single “best” nutritional formulation for dogs with myasthenia gravis. It is important to assess what works best for the individual dog.
Most dogs with myasthenia gravis will limit their own activity based on the severity of their muscle weakness.
Treatment of Myasthenia Gravis in Dogs
There are a few treatment options for myasthenia gravis, and your veterinarian will discuss each in detail with you once your dog has been diagnosed. Treatment methods include:
Support with Nutrition
You can adjust the feeding dishes so your dog doesn’t have to bend down so far to eat or drink. Once he eats, it is important to hold him with his head up for at least 20 minutes so he doesn’t become aspirated. Supporting him while he is eating is very important so he can continue to get proper nutrition with this condition. Some dogs with myasthenia gravis need to have a gastrostomy tube placed in order for them to continue to get the nutrition they need.
Fluids
It is important to prevent aspiration and the pneumonia which can follow in dogs with this disorder. If your dog already has aspiration pneumonia, treatment methods such as antibiotics, a nebulizer, and percussion therapy can be performed to expel the mucus from the lungs.
Respiratory System Support
In severe cases of dogs that have pneumonia, placement in intensive care on a ventilator may be what the veterinarian needs to do in order for your dog to recover.
Medications
There are medications that are used to help with the motility of the esophagus and surrounding muscles. Drugs such as metoclopramide, cisapride, and ranitidine can help the muscles of the esophagus and the surrounding system work more effectively.
Specific Therapies
Depending on your dog’s condition and the severity of the muscle weakness, targeted therapies may be performed in order to help your dog gain mobility. There are many different types of therapies, and the type used is dependent on your dog’s condition. Therapies to increase neuromuscular transmission, immunosuppressive therapies, and glucocorticoid therapies may be considered for your loved one.
What medications are recommended for dogs with myasthenia gravis?
Acetylcholine is a chemical that transmits messages between nerves and muscles at the neuromuscular junction. Because dogs with myasthenia gravis have an excess of the enzyme acetylcholinesterase, which breaks down acetylcholine, drugs that inhibit that enzyme prolong the action of acetylcholine. Pyridostigmine bromide (brand name Mestinon®) is the drug of choice.
Corticosteroids, azathioprine (brand names Imuran®, Azasan®), or mycophenolate may also be used for their ability to suppress the immune system.
How are dogs with myasthenia gravis monitored?
Improved muscle strength is an obvious barometer of response to therapy. In addition, chest radiographs (X-rays) are evaluated every 4-6 weeks for resolution of megaesophagus. Finally, acetylcholine receptor antibody levels are evaluated every 8-12 weeks, and should decrease into the normal range with remission.
What is the prognosis for dogs with myasthenia gravis?
For dogs who do not experience severe aspiration pneumonia or weakness of the throat and difficulty swallowing, the prognosis is good for complete recovery within 6-8 months. For dogs with a thymoma, the prognosis is guarded unless the mass is completely removed and control of clinical signs is achieved.
Although myasthenia gravis is treatable, most pets require months of special feeding and medication. Anti-acetylcholinesterase medication and immunosuppressive therapy will likely be required for the life of the dog. If the dog achieves remission, life quality is generally excellent.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
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