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CANINE
MYASTHENIA
GRAVIS
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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 myasthenia gravis inherited?
The mode of inheritance is autosomal recessive in congenital MG, and
unknown in acquired MG.
What breeds are affected by myasthenia gravis?
Congenital MG: Jack Russell terrier, springer spaniel,
smooth-haired fox terrier.
Acquired MG: mostly seen in medium to large breeds,
particularly the German shepherd, and golden and Labrador retriever.
For many breeds and many disorders, the studies to determine the mode
of inheritance or the frequency in the breed have not been carried out,
or are inconclusive. We have listed breeds for which there is a
consensus
among those investigating in this field and among veterinary
practitioners,
that the condition is significant in this breed.
What does myasthenia gravis 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 myasthenia gravis 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.
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.
How is myasthenia gravis 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 .
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.
Copyright
© 1998 Canine Inherited Disorders Database. All rights reserved.
Revised: October 30, 2001.
This database
is a joint initiative of the Sir James Dunn Animal Welfare Centre at
the Atlantic Veterinary College, University of Prince Edward Island,
and the Canadian Veterinary Medical Association.
reprinted
with kind permission from:-
Alice
Crook, DVM,Coordinator, Sir James Dunn Animal Welfare Centre, Atlantic
Veterinary College,University of Prince Edward Island, 550 University
Ave.Charlottetown, PEI C1A 4P3
The above information is simply
informational. It's intent is not to replace the advice of a
veterinarian nor to assist you in making a diagnosis of your pet.
Please consult with your own veterinarian for confirmation of any
diagnosis. Your pets life may depend on it.
