Myasthenia gravis is a neuromuscular disorder that affects dogs and (rarely) cats and is associated with failure of neuromuscular transmission. Acquired myasthenia gravis, an immune-mediated disorder in which antibodies target the nicotinic acetylcholine receptor (AChR) of skeletal muscle, is the most common neuromuscular disorder diagnosed in dogs and results in muscular weakness and excessive fatigability. Congenital myasthenia gravis is an inherited disorder that affects young dogs and is characterized by general muscle weakness; however, patients with congenital myasthenia gravis have presynaptic, synaptic, or postsynaptic defects as opposed to autoantibodies against the neuromuscular junction of skeletal muscle. Congenital myasthenia gravis has been reported in Jack Russell terriers, springer spaniels, smooth fox terriers, gammel Dansk honeshund breeds, and miniature short-haired dachshunds.
Dogs with acquired myasthenia gravis typically have episodic, generalized muscle weakness that is most obvious in appendicular muscles. This weakness is typically exacerbated by activity, and although it may be relieved with rest, this should not be a definitive criterion for diagnosis, as some dogs have profound weakness that does not improve with rest. Weakness may be confined to the pelvic limbs, mimicking an orthopedic or spinal cord disorder. Thorough neurologic and orthopedic examinations can differentiate these disorders. Weakness in patients with neuromuscular disease is typically equal in both the left and right pelvic limbs, but a disparity may exist between thoracic limbs and pelvic limbs. Spinal reflexes and proprioception are typically normal. The weight of the patient should be supported when assessing proprioception, as a very weak dog may not have the strength to correct the limb during testing, resulting in erroneous classification of proprioceptive deficits.
Acquired myasthenia gravis can be characterized as focal, generalized, or acute fulminating. Focal myasthenia gravis is characterized by megaesophagus alone. Generalized myasthenia gravis is characterized by obvious appendicular weakness (with or without megaesophagus), as well as facial, pharyngeal, or laryngeal weakness. Acute fulminating myasthenia gravis is characterized by rapid onset and progression of muscular weakness and is often rapidly fatal due to respiratory distress.
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Dr. Bailey discusses this condition from top to bottom on this episode of the Clinician's Brief podcast.