Myasthenia gravis is a disorder of signal transmission between the nerves and muscles (neuromuscular transmission). It is caused by a breakdown of the transmission of impulses from the nerves to the muscles. This keeps the muscles from contracting, causing affected dogs to become weak—the number one symptom of this neuromuscular disease. It is a neuromuscular disease that presents as forms of muscle weakness due to interference with nerve-muscle communication.
The most common cause of myasthenia gravis in dogs is an autoimmune condition in which antibodies in the blood attack receptors for a chemical called acetylcholine, which transmits nervous impulses to the muscles.
This disorder can be congenital (present at birth) and familial (runs in families) or it can be acquired (develops later in life). Congenital form affects breeds such as Jack Russell terriers, English springer spaniels, smooth fox terriers; smooth-haired miniature dachshunds.
Acquired form of myasthenia gravis still requires the appropriate genetic background for the disease to occur. Predisposed breeds to acquired form are golden retrievers, German shepherd dogs, Labrador retrievers, dachshunds, Scottish terriers, and Akitas while the familial forms of acquired myasthenia gravis occur in the Newfoundland and Great Dane breeds.
Symptoms in congenital disorder typically become apparent at 6-8 weeks of age. Average age of onset of symptoms in acquired form is either at 1-4 years of age, or 9-13 years of age.
Symptoms of myasthenia gravis can vary greatly from dog to dog. Acquired form of this condition causes enlargement of the esophagus, loss of normal reflexes, or a mass in the front central area of the chest. Regurgitation is common.
Fatigue or cramping with mild exercise
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
Appropriate genetic background.
Tumor or cancer – particularly thymus tumor
Vaccination can exacerbate active myasthenia gravis
Intact (non-neutered) female