Myasthenia gravis (MG) is a disease which interrupts the way nerves communicate with striated muscles. Striated muscles control voluntary movement, such as those used to walk or change facial expression.
• Some breeds are more susceptible, suggesting a genetic component
• Acquired MG is the result of autoimmune disease, or a non-cancerous tumor in the chest cavity
• Symptoms may develop quickly, and include weakness in the limbs, face, or esophagus resulting in difficulty moving, eating, and in severe cases, breathing
• MG often leads to life-threatening complications including megaesophagus and aspiration pneumonia
• Blood tests, biopsies, diagnostic imaging, and a tensilon test are used to diagnose MG
• Specific therapy includes medications along with surgical removal of relevant tumors when present
• All patients require supportive care such as the use of a Bailey chair for upright feeding
The impact of an MG diagnosis varies in dogs. In uncomplicated cases without thymoma (a benign lymphatic tumor near the heart) or aspiration pneumonia, the prognosis is good. Severe acute MG, MG due to thymoma, or accompanied by megaesophagus or pneumonia are all far more dangerous.
The symptoms of MG can be nonspecific, and comparable to symptoms of other conditions. Any dog exhibiting physical weakness and regurgitation requires emergency medical attention for proper diagnosis and treatment.
Note: regurgitation is not the same as vomiting. Vomiting includes use of the abdominal muscles to expel the stomach contents. Regurgitation is passive, with food dropping out of the mouth suddenly and without effort.
Megaesophagus is a common secondary condition resulting from MG. The muscles used to swallow food are weakened, and the esophagus enlarges. A dog with megaesophagus regurgitates food. Regurgitation is passive and is not accompanied by retching, gagging, or heaving. This is distinct from vomiting (which is an active process and requires noticeable effort).
Dogs with megaesophagus are at high risk of aspiration pneumonia. If food or liquid in the esophagus is inhaled into the lungs, it can lead to life-threatening infection.
A genetic component of congenital MG is suggested. Dogs with known cases of MG in their blood line may be at higher risk of having MG.
Myasthenia gravis interrupts the signals between the brain and striated muscle, leading to a reduced muscular response.
Congenital MG is a developmental condition, and is present from birth.
Acquired MG develops as a result of autoimmune disease, where the body’s immune system interrupts its own nervous signals. Acquired MG is also associated with thymoma, a benign tumor of lymphoid tissue near the heart.
Symptoms of MG include
• Weakness of limbs
• Muscle paralysis
In cases of congenital MG these symptoms develop within the first couple weeks of life. Weakness may manifest with exercise, and pass after rest, affecting the rear legs in particular.
Acquired MG can present itself more variably.
Focal MG only affects one area of the body, commonly the face and throat. This may lead to facial nerve paralysis and regurgitation.
Generalized MG affects most of the animal’s striated muscles. This manifests as difficulty eating and swallowing, a hunched posture, and weakness that becomes apparent with exercise but passes with rest.
Fulminating MG progresses rapidly, and is usually fatal. It can mimic diseases that cause paralysis, and usually affects all four limbs simultaneously.
Diagnostics to determine MG include:
• Physical examination
• Diagnostic imaging
Specific treatment for MG includes oral medication to boost signal from the brain to the muscles, and immunosuppressive therapy to suppress immune-mediated destruction of receptors.
Further treatment varies, specifically targeting underlying causes and complications.
• Thymoma requires surgical removal of the tumor.
• Megaesophagus requires special feeding management, such as with a bailey chair.
• Pneumonia may require hospitalization and additional antibiotics.
• Acute fulminating MG requires intensive care and oxygen support
• Unspayed female dogs benefit from spaying, as heat cycles and pregnancies can exacerbate MG.
Congenital MG is incurable and usually fatal by the time a puppy reaches nine months of age.In rare instances smooth-haired miniature dachshunds may recover naturally by six months of age and Jack Russell terriers may survive with the condition for years.
Without complications, the prognosis for immune-mediated acquired MG is generally positive, and recovery can happen within six months. Cases associated with megaesophagus, thymoma, and pneumonia have a far more guarded prognosis, as does fulminating MG.
Congenital MG can be avoided by breeding dogs without genetic susceptibility.
The condition is not contagious.
MG is a rare condition in dogs.
• Thymoma surgery
• Supportive care and adaptations
• Care for complications and secondary conditions
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