MYASTHENIA GRAVIS

About the Diagnosis

Myasthenia gravis is a disease that decreases the function of muscle and causes premature muscle fatigue or even profound muscle weakness approaching paralysis. It occurs much more commonly in dogs than cats. The symptoms of myasthenia gravis include a weakness of all four legs (especially after exercise), with the classic appearance being of a dog that appears to tire very quickly even though the desire to continue is still there (total loss of strength). This is a weakness problem; there is no pain. Passive regurgitation of food, muscle tremors, excess salivation, voice changes, dilated pupils, and coughing may be other symptoms observed as part of myasthenia gravis. These symptoms usually progress gradually over days to months; however, in rare cases, symptoms have a very sudden onset. The dog or cat may not even be able to stand.

Cause: In most animals with myasthenia gravis, the disease is immune-mediated. This means that the body's immune system mistakenly attacks the body's normal muscle tissue. With myasthenia gravis, the attack is at the level of the acetylcholine receptors, which are microscopic regions in each muscle fiber that are critical to muscle function. It is a case of mistaken identity: normally, the immune system recognizes intruders in the body that should not be there, such as bacteria and viruses, and produces antibodies to fight off these invaders. However, in animals with myasthenia gravis, the acetylcholine receptors on muscles are mistaken for foreign invaders by the immune system. Antibodies to the receptors are made, and the receptors are destroyed. In these animals, symptoms can develop at any time but are often first noticed by owners when the animal is approximately 3 to 9 years old.

Rarely, puppies and kittens can be born with myasthenia gravis. In these animals, the immune system probably does not play a role in the disease, but rather they are born with defective muscle (at the microscopic level).
Myasthenia gravis can also be associated with or triggered by other diseases. If a dog or cat has one of these other diseases, they have an increased risk of developing myasthenia gravis. These diseases include some types of cancer, hypothyroidism, problems with the thymus (an organ in the neck and/or chest), and others.

Diagnosis: Myasthenia gravis is an uncommon disease. Therefore, if your veterinarian suspects that your dog or cat has myasthenia gravis, several tests may first be performed to eliminate the possibility of other diseases as the cause and to assess for complications. Chest x-rays can be done to determine if your dog's esophagus is affected (megaesophagus) or if tumors are present in the chest since both of these disorders occur in patients with myasthenia gravis. Rarely, certain heart problems can cause symptoms that are very similar to those of myasthenia gravis, and an electrocardiogram (ECG, EKG) can be performed to look for these abnormalities. A blood sample may be taken to look for indications of kidney, liver, or other problems.

The most reliable test for myasthenia gravis involves examining a blood sample for specific antimuscle antibodies made by the immune system. Your veterinarian will send the blood sample to a special laboratory. Finding these antibodies (positive result) in patients with symptoms described above establishes the diagnosis of myasthenia gravis.

Living with the Diagnosis

Dogs and cats with myasthenia gravis require special attention at home. Depending on the situation, you may need to give your pet with myasthenia gravis his/her medicine 2 or 3 times daily. The dosage and type of medicine may need to be changed several times before the best combination is found. This requires several visits to the veterinarian.
Dogs and cats diagnosed with megaesophagus as part of myasthenia gravis have a greater chance of regurgitating food and developing aspiration pneumonia, an inhalation of food, water, and esophageal secretions into the lungs that occurs disproportionately more often in patients with megaesophagus. Aspiration pneumonia can have very serious side effects and is the most common cause of death in dogs and cats with myasthenia gravis. Therefore you must carefully monitor your pet during and after meals. Food and water bowls should always be placed on an elevated surface, so that gravity can help with the passage of food and water down to the stomach (since the esophagus is hampered in its ability to do so). If possible, the pet should be held upright for 5 to 10 minutes after each meal for the same reason.

Some dogs and cats that develop the disease may go into remission after several months or years of treatment. Your veterinarian may recommend performing the test for antibodies (see above) periodically to monitor progress. However, animals born with myasthenia gravis do not go into remission. These pets do not generally respond favorably to treatment, and the symptoms progress, often making for a poor long-term outlook (poor prognosis). It is important to differentiate this condition from juvenile megaesophagus, a similar situation where only the swallowing process is affected and where there is an opportunity for complete resolution (cure). In juvenile megaesophagus, the myasthenia blood test is negative.

Treatment

For dogs and cats with generalized weakness, exercise intolerance, and without megaesophagus, medicine is available (pyridostigmine or Mestinon) that can be given by mouth. Many animals respond well to this treatment and have a marked improvement or even complete resolution of their symptoms over time. Occasionally, cortisone-like drugs (corticosteroids) are given. If regurgitation is a problem, your veterinarian will discuss the best way to administer food and medicine so that it is absorbed into the system.

If your pet aspirates regurgitated food into the lungs, vigorous treatment or, in the most serious cases, intensive care may be necessary to control pneumonia. These pets should be taken to the veterinarian or emergency clinic immediately if there is very labored breathing or respiratory distress. Antibiotics may be given to control bacterial infections, an intravenous (IV) catheter may be placed in a vein to give fluids for rehydration, and oxygen may be given to help such severely affected pets breathe more easily.

DOs

• Give medicine(s) exactly as directed.

• Keep food and water bowls on elevated platforms.

• Inform your veterinarian if your pet has ever been diagnosed with any medical condition or is currently taking medicine.

DON'Ts

• Do not allow your dog or cat to continue eating or drinking if coughing begins during a meal.

• Do not stop giving medicine until you talk to your veterinarian or the veterinarian at the emergency clinic. Some medicine(s) needs to be decreased gradually or serious side effects can result.

When to Call Your Veterinarian

• If you cannot keep a scheduled appointment.
• If the problems get worse, especially after giving medicine.
• If you are unable to give medicine as directed.
• If you see signs of not feeling well or of potential complications (see below).

Signs to Watch For

• Adverse reactions to medicine may include weakness, anxiety or nervousness, nausea, vomiting, diarrhea, salivation, difficulty breathing, seizures, and so forth.

• Signs of complications or recurrence include loss of appetite, recurrent coughing (often moist or raspy), ongoing lethargy or sluggishness, persistent muscle weakness, and relapse of (or failure to eradicate) original symptoms.

Routine Follow-Up

• Usually within 1 week after the initial diagnosis; then periodically as determined by response to treatment and presence or absence of complications. Animals with myasthenia gravis often have this condition for life, requiring periodic but lifelong medical follow-up.