Myasthenia gravis is a neuromuscular disease that causes weakness of skeletal muscles. Myasthenia gravis literally means “grave muscle weakness”. Myasthenia gravis is commonly seen in dog breeds like Jack Russell, Terrier, Springer spaniel, german shepherds, golden retrievers, Akitas, and labrador retrievers.
Generally, there are two forms of this disease,
1. Congenital (animal born with)
2. Acquired (animal develops during lifetime)
Myasthenia gravis (MG) is caused by the failure of transmission of acetylcholine at the neuromuscular synapse or junction. Every muscle of the body is controlled by its own nerve. At the junction of nerve and muscle (neuromuscular junction), there is a gap, also known as a synapse. Along with nerves, signals or messages travel as an electrical current. When the electrical signal reaches the end of the nerve it is transmitted in the form of a chemical messenger across the gap (synapse). Acetylcholine is also a chemical messenger that helps in this transmission after binding to specific receptors on the muscle.
In congenital from, animal borns with the reduced number of acetylcholine receptors. Due to the reduced number of receptors and insufficient innervation, muscles are unable to contract and they become weak. MG is inherited as an autosomal recessive trait and it is found in very young animals. The prognosis for congenital form is very poor.
In acquired form, the immune system of the body attacks the acetylcholine receptors. Due to faulty immune function, antibodies are produced against the acetylcholine receptors. These antibodies attack and destroy the receptors. When the number of receptors is reduced, acetylcholine cannot attach to muscle cells. Ultimately, muscle contraction and working is lost and muscle weakness results.
Signs & Symptoms
- May have respiratory symptoms
- difficult breathing
- Generalized weakness
- Difficult swallowing
- Excessive drooling
- Difficulty in walking
- Poor exercise tolerance
These patients regurgitate food in formed, cylinder-shaped boluses shortly after eating. Such patients also show severe weakness after only a few minutes of exercise.
Diagnosis is achieved through blood testing to find out the presence of acetylcholine receptor antibodies (anti-AChR antibody titre).
The “tensilon test” is also performed to diagnose the myasthenia gravis.. In this test, an acetylcholinesterase inhibitor (edrophonium) is injected intravenously. In affected animals, a dramatic effect is seen after injection, collapsed animals may get up and run about.
Treatment involves the use of a long-acting antidote that reduces the acetylcholine breakdown. Pyridostigmine hydrochloride is used as a drug of choice in canine cases. Depending upon the individual scenario, it may be necessary to use immun-o- suppresser drugs. Remissions are common in such patients, depending on the cause. Prognosis is usually good, unless severe pneumonia, difficulty eating, or underlying cancer is present.