Myasthenia gravis

What is myasthenia gravis?

Myasthenia gravis literally means "severe muscle weakness". The disease occurs when the nerve endings do not release enough acetylcholine, a chemical substance that triggers muscle contraction. In addition, antibodies produced by the immune system can block or destroy the acetylcholine receptors on the muscle cells. This disrupts the communication between the nerves and the muscles and the muscles cannot function normally.

There are two types of myasthenia gravis in dogs: congenital and acquired.

Congenital myasthenia gravis

Congenital myasthenia gravis is a rare hereditary disease that is more common in some breeds, such as Dachshunds, Jack Russell Terriers, Samoyeds, Smooth Fox Terriers and Springer Spaniels. Symptoms usually appear at six to eight weeks of age and include generalized muscle weakness, exercise intolerance, collapse and respiratory distress. The condition can worsen over time and lead to quadriplegia and death. There is no cure for congenital myasthenia gravis, but some dogs may recover spontaneously or survive with symptomatic treatment.

Acquired myasthenia gravis

Acquired myasthenia gravis is an immune-mediated disease in which the immune system mistakenly attacks and breaks down the acetylcholine receptors. The cause of this reaction is not exactly known, but there may be a genetic predisposition or an association with certain cancers such as thymomas (tumors of the thymus). Symptoms may appear between one and four years of age or nine to 13 years of age, but are rarely seen in dogs at any age in between. Predisposed breeds include Newfoundlands, Akita, German Shepherds, Chihuahuas and some terrier breeds. Symptoms can vary depending on the muscles affected and include

• Focal muscle weakness: mainly affects the pharyngeal, esophageal and facial muscles. This leads to problems with swallowing, speaking, drooling and gagging. A common sign is megaesophagus, a widening of the esophagus that can lead to regurgitation and aspiration pneumonia.
• Generalized muscle weakness: affects multiple muscle groups throughout the body. This leads to exercise intolerance, weakness or cramps in the limbs, collapse and shortness of breath. The severity of symptoms can vary from mild to severe and may improve or worsen with rest.
• Acute fulminant myasthenia gravis: a severe form of the disease that leads to rapidly progressive quadriplegia (paralysis of all four limbs), ventroflexion of the neck (neck bent forward), bladder distension (distended bladder) and respiratory failure.

How is myasthenia gravis diagnosed?

The diagnosis of myasthenia gravis is based on a combination of medical history, clinical examination, blood tests and special tests to assess neuromuscular function. These tests include:

• Tensilon test: this is a quick and simple test in which the dog is injected with a small dose of a drug called edrophonium chloride (Tensilon), which enhances the effects of acetylcholine. If the dog is suffering from myasthenia gravis, its muscle weakness should improve within minutes. However, the test is not always reliable and can give false positive or false negative results.
• Antibody test: This is a blood test that measures the presence of antibodies against acetylcholine receptors. A high antibody titer is a strong indication of acquired myasthenia gravis, but a normal or low titer does not rule out the disease. The test is not useful for congenital myasthenia gravis as these dogs do not produce antibodies.
• Electromyography (EMG): This is an invasive test in which a needle is inserted into a muscle to measure electrical activity. In dogs with myasthenia gravis, EMG shows an abnormal decrease in muscle response to repeated nerve stimulation. The test requires sedation or anesthesia and may be painful or uncomfortable.
• Chest X-ray: This is an imaging procedure used to assess the condition of the lungs and heart. In dogs with myasthenia gravis, a chest x-ray may show signs of megaesophagus, aspiration pneumonia or thymoma.

How is myasthenia gravis treated?

The treatment of myasthenia gravis depends on the type, severity and complications of the disease. The general goals of treatment are

• Improve neuromuscular function: this is achieved by administering drugs that increase the action of acetylcholine or reduce the production of antibodies. These drugs include pyridostigmine bromide (Mestinon), immunosuppressants such as prednisone or azathioprine and immunoglobulins or plasmapheresis (blood purification).
• Management of megaesophagus: This requires a special feeding technique where the dog is fed in an upright position to reduce the risk of regurgitation. In addition, the dog should remain in this position for a few minutes after eating to use gravity to get the food into the stomach. The type and amount of food should also be adjusted to allow easier passage. Some dogs require surgical placement of a feeding tube for nutrition.
• Treatment of aspiration pneumonia: This requires aggressive antibiotic therapy, fluid therapy and oxygen therapy to combat the infection and inflammation of the lungs. In severe cases, mechanical ventilation may be required.
• Removal of thymoma: This requires surgical resection of the tumor from the chest cavity. Removal of the thymoma may improve or even cure the symptoms of myasthenia gravis.

The prognosis for dogs with myasthenia gravis depends on several factors, such as the type of disease, the response to treatment, the presence of complications and the dog's general health. Some dogs may recover completely or achieve long-term remission, while others may remain chronically affected or die from life-threatening complications.

How can myasthenia gravis be prevented?

There are no specific measures to prevent myasthenia gravis in dogs.