Myasthenia gravis (MG) is an autoimmune disease that causes muscle weakness.
The esophagus muscles are often affected, and when these muscles weaken, it creates a condition called megaesophagus that makes swallowing difficult. This, in turn, can lead to accidental inhalation of food and water, which can cause a serious lung infection known as aspiration pneumonia.
With an early diagnosis, adjustments in feeding and appropriate medications, some dogs may go into remission after several months, while others may require lifelong treatment.
MG is most commonly caused by an autoimmune disease that reduces muscles’ ability to process signals from the nervous system that would normally tell them how to move and function.
Motor neurons transmit this kind of information by sending a chemical messenger called acetylcholine (ACh). However, MG destroys the receptors that muscles use for receiving and activating ACh, making it harder for the muscles to work properly.
This immune-mediated response may be associated with hypothyroidism, cancer or a chest tumor called a thymoma. In some cases, there is no apparent known cause.
Acquired MG can happen in any breed, but may be more common in Akitas, German Shepherds, Labrador Retrievers, Golden Retrievers, Newfoundlands and Scottish Terriers.
Rarely, MG can be caused by a congenital condition, which means that these dogs are born with fewer ACh receptors. Congenital MG may be more common in Jack Russell Terriers, Springer Spaniels and Smooth Fox Terriers.
MG may cause generalized muscle weakness after activity, but this improves with rest. The disease may also cause focal muscle weakness, which only affects a few muscles around the dog’s face and neck.
MG may include any of the following clinical signs:
Early exercise fatigue or muscle weakness that improves with rest
Hindlimb weakness or limb stiffness
Inability to close eyelids
Physical exam and blood work: Your veterinarian will perform a thorough physical exam and recommend a general health screening that includes blood work and thyroid testing.
Antibody test: A definitive diagnosis requires a specific blood test that looks for antibodies to ACh receptors.
Tensilon test: A Tensilon test may be used if MG is suspected, but you are still waiting for antibody test results. This test involves providing an IV medication, and if the dog is MG-positive, then it will temporarily improve their muscle strength. However, the effects will wear off a few minutes after receiving the injection.
X-rays: Chest X-rays are used to look for megaesophagus, aspiration pneumonia or cancer.
Medications: The most common medication for treating MG is pyridostigmine bromide. This medication improves muscle function by prolonging the time ACh stays at the receptor. Some dogs may need an immunosuppressive medication, such as prednisone. Any concurrent conditions related to MG, such as hypothyroidism, must also be addressed.
Megaesophagus: Feedings need to be adjusted to prevent aspiration pneumonia for dogs with megaesophagus. Elevated food bowls, or a “Bailey Chair,” specially constructed to help the dog sit upright while eating, can both help avoid accidental inhalation. With this condition, smaller and more frequent feedings of a high-calorie food, placing the dog in an elevated position, and keeping your dog in that position for 20-30 minutes after each meal can help prevent aspiration pneumonia.
Aspiration pneumonia: Treatment for aspiration pneumonia requires antibiotics, but in severe cases it can require hospitalization and intensive care.
Surgery: MG associated with a thymoma (chest tumor) may require surgery to remove, if conditions look favorable.
MG is a serious disease, and a dog’s overall prognosis depends on the underlying cause or associated complications. Early diagnosis, diligent treatment and management can lead to favorable outcomes, with remission possible over several months. Other affected dogs may require lifelong treatment.
Severe or recurrent aspiration pneumonia, and difficulties breathing or swallowing, will lead to less favorable outcomes. The prognosis for thymoma-associated MG is also less favorable, but complete surgical removal of the tumor may improve the prognosis.