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Myasthenia Gravis in Dogs

Myasthenia gravis is a disorder of signal transmission between the nerves and muscles (known as neuromuscular transmission), characterized by muscular weakness and excessive fatigue. The disorder is congenital (present at birth) and familial (runs in families or lines). Jack Russell terriers, English springer spaniels, smooth fox terriers; smooth-haired miniature dachshunds have an autosomal recessive mode of inheritance for the disease.

It can also be acquired (not inherited, but present later in life/after birth), but as with other autoimmune diseases, it requires the appropriate genetic background for the disease to occur. Multiple factors are involved, including environmental, infectious, and hormonal influences. The familial forms of acquired myasthenia gravis occur in the Newfoundland and Great Dane breeds.

Acquired forms affect several dog breeds: golden retrievers, German shepherd dogs, Labrador retrievers, dachshunds, Scottish terriers, and Akitas.

The congenital form becomes apparent at 6-8 weeks of age. The acquired form has a bimodal age of onset. Either at 1-4 years of age, or 9-13 years of age. There may be a slight susceptibility for females in the young age group, but none in the old age group.

Symptoms and Types

The acquired form may have several clinical presentations, ranging from localized involvement of the esophagus' muscles, the muscles of the throat, the muscles adjacent to the eye, and acute generalized collapse.

Any dog with acquired enlargement of the esophagus, loss of normal reflexes, or a mass in the front central area of the chest should be evaluated for myasthenia gravis. Regurgitation is common, but it is important to first differentiate it from vomiting.

Physical findings

• Voice change
• Exercise-related weakness
• Progressive weakness
• Fatigue or cramping with mild exercise
• Acute collapse
• Loss of muscle mass usually not found
• Sleeps with eyes open
• May look normal when at rest
• Excessive drooling, repeated attempts at swallowing
• Difficulty breathing with aspiration pneumonia

Subtle nervous system findings

• Decreased or absent blink reflex
• May note a poor or absent gag reflex
• Spinal reflexes are usually normal but may fatigue

Risk Factors

• Appropriate genetic background.
• Tumor or cancer - particularly thymus tumor
• Vaccination can exacerbate active myasthenia gravis
• Intact (non-neutered) female

Causes

• Congenital (present at birth)
• Immune-mediated disease
• Secondary to cancer

Diagnosis

There are other disorders of neuromuscular transmission, such as tick paralysis, that may have the same symptoms, so your veterinarian will want to rule them out before coming to a conclusion about the diagnosis. To do that, he will need a careful history, thorough physical and neurologic examinations, and specialized laboratory testing.

A complete blood profile will be conducted, including a chemical blood profile, a complete blood count, and a urinalysis. Your veterinarian may also check for such things as thyroid functioning. Diagnostic imaging will include chest X-rays to look for an enlarged esophagus and aspiration pneumonia, and an ultrasound-guided exploration of the chest, to look for a mass. If a mass is found, a biopsy will need to be performed to confirm whether the growth is cancerous.