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Myasthenia gravis is a neuromuscular disease that presents as forms of muscle weakness due to interference with nerve-muscle communication.
Diagnosis involves observing the patient's response to a drug that improves this communication, and therefore improves the weakness. A blood test can confirm this diagnosis.
Treatment involves daily supplementation with a similar drug.
Prognosis for control of this disease, by achieving remission, is good, provided that precautions are taken to guard against secondary complications like aspiration pneumonia.
What is myasthenia gravis?
- Myasthenia gravis is a neuromuscular disease characterized primarily as muscle weakness. It may affect a few specific muscles, e.g. the esophagus, or the whole body.
- Muscle contraction is controlled by impulses (commands) from nerves.
- Normal transmission of these impulses occurs via release by the nerves of the neurotransmitter acetylcholine (ACh) that activates receptors on the muscles, resulting in contraction.
- With myasthenia gravis (MG), the receptors for ACh on the muscles are decreased in number, therefore not functioning properly, resulting in fatigue and muscular weakness that become worse with exercise and improve with rest.
- There are 2 forms of myasthenia gravis:
- Congenital: the dog is born with the condition, lacking the ACh receptors in its muscles. Clinical signs are evident at an early age.
- Acquired: seen in adult dogs, where there is destruction by the dog's own immune system of the receptors, making it an auto-immune disorder. The acquired form may present in one of three ways: megaesophagus, generalized weakness, or fulminant MG. A weak esophagus (megaesophagus) means that food is not moved into the stomach appropriately, and the dog is at risk of inhaling it instead, causing aspiration pneumonia.
- Clinical signs of mysathenia can be quite variable, depending on the form.
- Megaesophagus: signs include regurgitation, coughing, and pneumonia. The veterinarian may discover abnormal lung sounds on physical examination. Whenever megaesophagus is diagnosed, MG should be suspected. Conversely, whenever MG is diagnosed, chest X-rays should be done to evaluate the esophagus, in case megaesophagus is present.
- Generalized weakness: the owners may note weakness that is more pronounced after activity, the veterinarian may note an easily fatigued palpebral (blink) response leading to incomplete palpebral closure.
- Fulminant MG: the dog presents immobile, having collapsed acutely, with difficulty breathing and swallowing.
- Breeds likely to have congenital MG: Jack Russell terrier, springer spaniel, smooth-haired fox terrier.
- Breeds likely to have acquired MG: medium to large breeds, particularly the German Shepherd, and Golden and Labrador Retrievers.
- MG can be presumptively diagnosed by a rapid response to an injection of edrophonium hydrochloride (Tensilon). Immediate improvement in muscle strength of short duration is expected.
- Specific lab tests for this disease are available to confirm the diagnosis, e.g. anticholinesterase receptor antibody levels.
- In order to reduce the risk of aspiration pneumonia, dogs should be fed from elevated food bowls and encouraged to remain standing for 10 minutes or so after eating.
- Anticholinesterase medications are very helpful. Pyridostigmine bromide (Mestinon syrup) is the most commonly used medication. If it is not effective, injections of neostigmine (Prostigmin) will sometimes work.
- In severe cases, corticosteroids may be necessary. It is very important to make sure that there are no other illnesses and no aspiration pneumonia signs prior to using corticosteroids.
- A number of dogs with acquired MG will achieve remission from the disease. Checking the levels of antibodies to anticholinesterase receptors prior to treatment and at regular intervals during treatment is important to monitor effectiveness of treatment and impending recurrence throughout the lifespan of affected dogs.
- Dogs that acquire MG later in life usually have a fairly good prognosis with treatment.
- Dogs can be maintained well in remission for a long time with this disease. The more attention paid to prevention of aspiration pneumonia, the better the long term prognosis.
- Many medications interfere with the actions of the anticholinesterase drugs, so careful thought must be given to the choice of medications to treat other problems in affected dogs.
MVS Neurology Team
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