Myasthenia gravis

What is ... ?: 

 Normal transmission of impulses from nerves to muscles occurs via a substance in the body called acetylcholine (ACh), which serves as a neurotransmitter. With myasthenia gravis (MG), the receptors for ACh on the muscles are decreased in number, resulting in fatigue and muscular weakness that become worse with exercise and improve with rest.

There are 2 forms of myasthenia gravis. In the congenital form (ie the dog is born with the condition) there is a lack of receptors in the muscle. Clinical signs are evident at an early age. In the acquired form which is seen in adult dogs, there is destruction by the dog's own immune system of the receptors. Acquired MG is therefore, an auto-immune disorder.

How is ... inherited?: 

The mode of inheritance is autosomal recessive in congenital MG, and unknown in acquired MG.

What does ... mean to your dog & you?: 

 The classical picture of this disease is an exercise-related muscle weakness that improves following rest. With congenital MG, signs are generally noticed in pups by 6 to 9 weeks of age, when they are learning to walk. They may walk a few steps and then have to rest. When tired, the face may droop. Chewing and swallowing may be difficult and they may regurgitate food and develop a dilated esophagus (megaesophagus), particularly the smooth fox terrier.

The average age for the development of acquired MG is around 5 years. The signs are similar - muscle weakness that becomes worse with exercise, and improves after the dog rests. Regurgitation and megaesophagus are more common with the acquired form and may lead to aspiration pneumonia due to inhalation of regurgitated food particles.
Sometimes the weakness may be restricted to only one group of muscles, such as the esophagus, or it may be generalized.

How is ... diagnosed?: 

 Diagnosis is made based on the clinical signs and diagnostic tests. Helpful tests include nerve conduction studies, and the injection of an drug that causes a build-up of acetylcholine at the receptors. A rapid, temporary improvement in your dog's strength means that the problems are likely due to myasthenia gravis. In acquired MG, a special blood test can be used to detect antibodies against the acetylcholine receptors.

A chest x-ray is recommended as well, because megaesophagus is so common with this disorder.

How is ... treated?: 

 The basis of treatment for MG is a drug that slows the breakdown of acetylcholine at the muscle receptors (anticholinesterase), allowing more time for neurotransmission to occur effectively. Your veterinarian will work with you to adjust the levels of this drug, based on the duration of increased muscle strength, and also to determine  the slight daily adjustments that may be necessary due to changes in your dog's activity or stress levels. S/he will also discuss with you the signs of overdosage with an anticholinesterase, such as increased salivation and anxiety.

Many dogs will recover completely from acquired MG in a few weeks or months while others, after a period of successful treatment, will no longer respond to the anticholinesterase.
Aspiration pneumonia because of megaesophagus is the major complication with this condition. It is very important that this be diagnosed and treated early, and subsequently avoided as much as possible by appropriate management, such as providing food and water from an elevated position and having your dog remain standing for 5 to 10 minutes following feeding. Your veterinarian will discuss this with you.
Congenital MG: The same drugs are used, but unfortunately are less effective. Pups usually become progressively worse despite treatment, to the point of generalized weakness, loss of muscle mass, and inabilty to walk.
For the veterinarian: A trial-and -error approach is used in the medical management of MG. Dosage levels of long-acting anticholinesterase drugs  (eg. pyridostigmine bromide) are based on the severity of the signs and the size and activity level of the dog, and varied as necessary. Overdose can produce signs of muscarinic (urination, defecation, salivation, bradycardia), nicotinic (muscle fasciculations, tremors) or CNS (anxiety, anorexia, hyperactivity) stimulation.
Immunosuppressive doses of corticosteroids can be tried in refractory cases of acquired MG, but only in the absence of aspiration pneumonia.
Certain drugs (including aminoglycoside antibiotics, phenothiazines, and methoxyflurane) may worsen disorders of neuromuscular transmission and should be used with caution, if at all, in dogs with suspected MG .

For the veterinarian: 

 Acquired MG should be considered in all cases of adult onset megaesophagus or generalized weakness, as well as with the more classical clinical picture. Barium studies should be used judiciously due to the risk of barium aspiration pneumonia.

both forms of MG: There is an increase in muscle strength in response to the short-acting anticholinesterase drug, edrophonium chloride (Tensilon). Dogs improve within 30 seconds of IV injection, and the weakness returns in about 5 minutes. There is also a decremental compound muscle response to repetitive nerve stimulation.
acquired MG: Definitive diagnosis is by demonstration by radioimmunoassay (RIA) of serum  ACh-receptor antibodies (absent in congenital MG). Response to treatment and/or evidence of spontaneous remission can be monitored with periodic serum ACh receptor titers.

Breeding advice: 

 For the congenital form of myasthenia gravis, affected dogs, their parents (carriers of the trait), and normal-appearing siblings (suspect carriers) should not be bred.

In the acquired form, affected dogs should not be bred and, although the mode of inheritance is unknown, it is also prudent to avoid breeding parents and siblings.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources: 

 Shelton, G.D. 1992. Canine myasthenia gravis. In R.W. Kirk and J.D. Bonagura (eds.) Kirk's Current Veterinary Therapy XI Small Animal Practice. pp.1039-1042. W.B. Saunders Co., Toronto.

 Braund, K.G. 1995. Peripheral nerve disorders. In S.J. Ettinger and E.C. Feldman (eds.) Textbook of Veterinary Internal Medicine, pp. 701-726.  W.B. Saunders Co., Toronto.


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