Myasthenia Gravis

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Introduction

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).

The underlying cause of the immune response against the acetylcholine receptors is not known but an association with thymomas and other neoplasia has been documented. Additionally immune mediated endocrinopathies such as hypothyroidism and hypoadrenocorticism have also been associated with the disease.

Acquired myasthenia Gravis is the most common form of the disease and cases present with generalized muscle weakness which is accentuated by mild exercise. The focal form of myasthenia gravis affects only the extraocular muscles, the oesophagus and the muscles innervated by cranial nerves V, VII and IX. Additionally 90% of animals diagnosed with myasthenia gravis have megaoesophagus at presentation.

Signalment

Acquired Myastehenia Gravis:

Akitas, German Shorthaired Pointers, Chihuahuas and some Terrier breeds are predisposed to the acquired Myasthenia Gravis, 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.

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

Congenital Myasthenia Gravis:

English Springer Spaniels, Fox Terriers and Jack Russell Terriers are genetically predisposed to congenital Myasthenia Gravis, it has also been documented in Siamese and Domestic Short Haired cats.

Clinical Signs

Three presentations of acquired Myasthenia Gravis have been reported. Focal disease and Generalized disease which can be acute or chronic in presentation.

Animals affected with the focal form present with laryngeal, pharyngeal, 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.

An association between thymomas and myasthenia has been made in both dogs and cats.

Congenital Myasthenia Gravis presents with similar signs to the acquired form except that megaoesophagus is only rarely associated.

Laboratory Tests

Acquired Myasthenia Gravis:

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

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

Additionally an ELISA is now available which identifies circulating antibody to the acetylcholine receptor.

Congenital Myasthenia Gravis Autoantibodies are not present in this form of the disease as animals are born without the receptors and consequently diagnosis is based on the response to anticholinesterase treatment.

Radiography

Lateral thoracic radiographs will often reveal megaoesophagus in dogs and or a thymoma or other neoplastic mass in dogs and cats.

Treatment

Anticholinesterase agents are the most common treament for Myasthenia Gravis. They inhibit the cholinesterase enzyme breaking down acetylcholine resulting in increased levels and duration of the neurotransmitter in the synaptic cleft. Anticholinesterase agents include pyridostigmine bromide and neostigmine.

Animals should be fed from a height and also have their head held in an elevated position for 5 minutes after feeding to avoid aspiration of food and the development of aspiration pneumonia.

Any accompanying megaoesophagus must also be treated appropriately (antibiotics and oxygen). Certain drugs including aminoglycosides and antiarrhythmic drugs can interfere with neuromuscular transmission and should be avoided in myasthenia gravis patients.

The use of immunosuppressants in Myasthenia Gravis is documented but controversial, particularly in patients suffering from aspiration pneumonia.

Prognosis

Animals suffering from acquired Myasthenia Gravis may be affected over a varied time span from months to years and animals can go into remission. The prognosis also depends on the underlying cause of disease which may be neoplastic. Animals with the congenital form have a poor prognosis they do not go into remission and often deteriorate.


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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




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