This is a rare disease caused by a deficiency of acetylcholine receptors, normally present at the junction of nerve endings and muscle cells. When an animal decides to move a muscle, the nerve endings release acetylcholine, which is a neurotransmitter.
A reduction in the number or function of these acetylcholine receptors produces generalized muscle weakness, made worse by exercise. Weakness is most apparent in the hind quarters. Dogs with myasthenia gravis have difficulty getting up and exhibit a swaying or staggering gait.
There is a focal form of myasthenia gravis that affects only the muscles involved in swallowing. The dog is unable to swallow solid food and develops enlarged, dilated megaesophagus. Aspiration pneumonia often follows.
A congenital form of myasthenia gravis is inherited as an autosomal recessive trait. It occurs in Jack Russell Terriers, Springer Spaniels, and Smooth Fox Terriers.
An acquired form of myasthenia gravis occurs in all breeds, but is seen most often in golden retrievers, German Shepherds, Labrador Retrievers, Dachshunds, and Scottish Terriers, often occurring at 1 - 4 years of age or 9 - 13 years of age.
Acquired myasthenia gravis is an immune-mediated disease in which auto-antibodies are directed at and destroy the acetylcholine receptors.
Hypothyroidism can occur at the same time as autoimmune myasthenia gravis. Occasionally,myasthenia gravis is related to a tumor of the thymus gland, but this is rare.
The diagnosis of myasthenia gravis is based on neurological examination. One test involves injecting a drug called edrophonium chloride. This drug blocks the enzyme that breaks down acetylcholine, resulting in higher concentrations of this neurotransmitter at the receptor sites. The test is positive if muscle strength improves after the injection. A serologic test for diagnosing autoimmune myasthenia gravis is available.
Drugs are available that increase the concentration of acetylcholine at receptor sites, thereby reversing muscle weakness. These drugs can be given as a syrup or by injection. The dose varies according to the dog's activity and stress levels. Close veterinary monitoring is required. Drugs to counter immune responses may also be helpful. Thymus glans tumors are removed surgically.
With appropriate treatment, the outlook for complete recovery and the return of normal swallowing is good.
There is no prevention for this condition.
Please contact your veterinarian with questions regarding this condition.
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