Myasthenia Gravis in Dogs

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Myasthenia gravis is a neuromuscular disorder in dogs. It can be congenital (inherited) or acquired, but the congenital form is rare.

When a dog is affected by this condition, there is a problem with the transmission of impulses from the nerves to the muscles. That means that the signal from the brain to activate a muscle group isn't properly received. The result is muscle weakness and fatigue.

Causes of Canine Myasthenia Gravis

The congenital form of myasthenia gravis in dogs is the result of an inherited problem with the structure or function of specific nerve receptors (nicotinic AChRs or acetylcholine receptors). It occurs in the following breeds:

• Jack Russell terrier
• Springer spaniel
• Smooth fox terrier

The acquired form of canine myasthenia gravis is autoimmune in nature and involves the body destroying its own AChRs. It is commonly seen in the following breeds:

• Newfoundland
• Golden retriever
• Great Dane
• Akita
• Scottish terrier
• Labrador retriever
• German shepherds
• Dachshunds

The acquired form of myasthenia gravis in dogs can present four different ways:

• Focal, in which only one area of the body is affected. Often, this manifests as megaesophagus when the esophagus in the affected area.
• eneralized, in which all areas of the body are affected.
• Fulminating, in which the condition rapidly progresses and is often fatal.
• Paraneoplastic, in which there is a thymoma, or tumor of the thymus (located in the front part of the chest), involved in the disease process.

Signs of Myasthenia Gravis in Dogs

Signs of myasthenia gravis in dogs occur around 6-8 weeks of age in the congenital form and either between 1 and 4 years or 9 and 13 years in the acquired type. The following signs may be noticed by an owner or veterinarian:

• Voice changes
• Exercise-induced weakness/wobbliness
• Sudden collapse
• Vomiting
• Drooling and difficulty swallowing
• Fatigue or painful muscle cramps with mild exercise
• Trouble breathing *
• Regurgitation *

* Seen when secondary megaesophagus is present. Megaesophagus is a common occurrence in dogs with myasthenia gravis. Trouble breathing could indicate aspiration of ingested material because of the megaesophagus and is serious.

Diagnosis of Canine Myasthenia Gravis

When your dog is exhibiting signs consistent with myasthenia gravis, your veterinarian will perform a thorough exam and take a history of the condition from you. Other disease processes your vet will need to rule out (differential diagnoses), which can cause some similar signs, include:

• Tick paralysis
• Botulism
• Other neurologic and muscle conditions

General lab work is done to assess the patient's overall health and look for clues. A serum AChR antibody titer can be done to determine whether the dog's body has too many circulating antibodies which attack AChR receptors. A high value on this test is diagnostic for the acquired form of myasthenia gravis.

X-rays of the chest may show a suspicious mass if a thymoma if present, and they may also reveal an enlarged esophagus in the case of megaesophagus.

An ultrasound of the chest can also be done to explore the area for a thymoma further or to get an ultrasound-guided aspirate of a mass for diagnosis.

Treatment of Myasthenia Gravis in Dogs

If a dog has aspiration pneumonia secondary to megaesophagus caused by myasthenia gravis, hospitalization for antibiotics, oxygen, and fluid therapy may be required. A gastrostomy tube may also be necessary if the dog can't get enough food without regurgitating it. See more about treating megaesophagus here: "Megaesophagus in Dogs."

If a thymoma is present, surgery is required to remove it.

Anticholinesterase drugs can help the nerve receptors respond better to acetylcholine, resulting in better muscle strength.

The prognosis with medication is good unless the dog suffers from severe aspiration because of megaesophagus or has a thymoma that hasn't been completely surgically removed.

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