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. 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
• 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. 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 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. 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. Certain drugs including aminoglycosides and antiarrhythmic drugs can intefere with neuromuscular transmission and should be avoided in myasthenia gravis patients.

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. Animals with the congenital form do not go into remission and often deteriorate.

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