Myasthenia Gravis

Description

Myasthenia gravis is a disease of the neuromuscular junction that occurs due to a defect or absence of nicotinic acetylcholine receptors on the post-synaptic membrane (in the congenital form) or due to the presence of antibodies that bind to the receptors and prevent them from functioning normally (in the acquired form). Acquired myasthenia Gravis is the most common form of the disease. The classical focal form of myasthenia gravis affects only the extraocular muscles, the cranial oesophagus and the muscles innervated by cranial nerves V, VII and IX. 80% of animals diagnosed with myasthenia gravis have megaoesophagus at presentation.

Signalment

Akitas, German shorthaired pointers, Chihuahuas and some terrier breeds are predisposed to the disease, however Labradors and Golden retrievers are frequently diagnosed with the disease. Dogs 2-3 years old and also older than 9 years old are more commonly affected by the disease.

Abyssinian and Somali are the most commonly affected cat breed and can be affected at any age.

Clinical Signs

Three types of disease have been reported in the dog.

• Focal
• Generalized disease which can be acute or chronic in presentation.

Animals affected with the focal form present with laryngeal, pharangeal, facial and oesophageal dysfunction.

Animals affected by the generalised form present in non-ambulatory tetraparesis often with dyspnoea. Of these dogs 90% will have concurrent megaoesophagus some of whom will have a history of regurgitation or aspiration pneumonia.

Megaoesophagus does not occur as frequently in cats however thymomas are commonly reported as a cause of the disease.

Laboratory Tests

Identifying serum antibodies for acetylcholine receptors is the test most commonly used to diagnose the disease.

Additionally the tensilon (edrophonium chloride)test can be undertaken where edrophonium chloride is given intravenously and an improvement is noted in affected patients which is demonstrated by increased muscle strength.

Also an ElISA is now available which identifies circulating antibody to the acetylcholine receptor.

Radiography

Lateral thoracic radiographs will often reveal megaoesophagus in dogs and a thymoma in cats.

Treatment

Anticholinesterase agents inhibit the cholinesterase enzyme breaking down acetycholine resulting in increases level and duration of the neurotransmitter in the synaptic cleft. Animals should be fed from a height to avoid aspiration of food. Any accompanying megaoesophagus must be treated appropriately.

References

Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition) W.B. Saunders Company

• Animals develop antibodies to nicotinic acetylcholine receptors.
• Results in generalised muscle weakness and or megaoesophagus.

Pathogenesis: Type II hypersensitivity