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CANINE MASTICATORY
MUSCLE MYOSITIS
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Masticatory Muscle Myositis
Neuromuscular case of the month
What is masticatory
muscle myositis?
This disease affects the masticatory muscles - the large muscles
on the top and sides of the head which act to close the jaw. These muscles
have a unique muscle protein composition. Masticatory muscle myositis
is an immune-mediated disease in which the body’s immune system attacks
the unique muscle protein found in the masticatory muscles. This causes
the muscles to become painful and swollen initially, and the muscles
will atrophy as the disease progresses. Opening the mouth is particularly
painful.
How is masticatory muscle myositis inherited?
Unknown.
What breeds are affected by masticatory muscle myositis?
The German shepherd dog is most frequently affected, but this
disease is also reported in other large breed dogs, including the English
pointer and springer spaniel.
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 masticatory muscle myositis mean to your dog &
you?
Many dogs affected with masticatory muscle myositis will improve
with drug therapy.
How is masticatory muscle myositis diagnosed?
Your veterinarian will suspect this disease if your dog has painful,
swollen masticatory muscles, with pain on opening the jaw. The diagnosis
can be confirmed with a muscle biopsy to examine the cells with a microscope.
Special techniques may be used to test for specific antibodies.
How is masticatory muscle myositis treated?
Affected dogs are usually treated medically, with glucocorticoids
such as prednisone. Most dogs improve with drug therapy, although the
treatment may be long-term.
Breeding advice
Affected animals should not be bred.
FOR MORE
INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Resources
Kornegay JN. 1995. Disorders of the skeletal muscles. In EJ Ettinger
and EC Feldman (eds) Textbook of Veterinary Internal Medicine, p.727-736.
WB Saunders Co., Toronto. Ackerman L. 1999. The Genetic Connection: A
Guide to Health Problems in Purebred Dogs, p. 121. AAHA Press,Lakewood,
Colorado.
This database is funded jointly by
the Animal Welfare Unit at the Atlantic Veterinary College, University of
Prince Edward Island, and the Canadian Veterinary Medical Association.
Copyright © 1998 Canine Inherited
Disorders Database. All rights reserved.Revised: December 15, 2000.
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
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NEUROMUSCULAR
CASE OF THE MONTH - MAY 2004
Exophthalmos and
masticatory muscle swelling in a 7 year old M German Shepherd dog
Clinical History
A 7 year old M German Shepherd dog presented for an approximately
1 week history of bilateral exophthalmos that was worse on the right
side, swollen hemorrhaghic third eyelids, firm and swollen facial muscles,
and mild submandibular lymphadenopathy (A,B,C). Although pain was
reported on opening the jaw, the dog did not have any difficulty eating.
click
on image to enlarge
Physical Examination
Abnormal findings on physical examination were localized to the
muscles of mastication and the eyes. The masticatory muscles were firm
and swollen, and there was marked enlargement of submandibular lymph
nodes. Ophthalmologic examination revealed bilateral exophthalmos that
was worse on the right, protrusion and vascular engorgement of the nictitating
membrane bilaterally, focal corneal ulceration in the right eye, anisocoria,
and negative direct pupillary light response and absent menace reflex in
the right eye. The dog was blind in the right eye.
Diagnostic Testing
CBC – Unremarkable with the exception of decreased platelets
(30,000/ml; reference range 175-500,000)
Creatine kinase– Mildly elevated (298 IU/L; reference range 10-200)
Total protein – Elevated (95 g/L; reference range 52-82)
Globulin – Elevated (62 g/L; reference range 25-45)
Antinuclear antibody titer – Negative
2M antibody titer – Positive at 1:1000 (reference range <
1:100)
Chest radiographs – Normal
Diagnosis
Based on the clinical signs and the positive serum antibody titer
against masticatory muscle type 2M fibers, a diagnosis of masticatory
muscle myositis (MMM) was made. Since platelets were decreased on the
initial CBC, a decision was made not to perform a biopsy of the temporalis
muscle. A repeat CBC later showed the platelet count within the reference
range and it was concluded that initial count was laboratory error.
Treatment
Prednisone therapy was initiated at immunosuppressive dosages
and ophthalmic ointments were used for treatment of the corneal ulceration.
Outcome
On reevaluation 7 days later, all clinical signs with the exception
of the blindness in the right eye had resolved (D,E,F). A gradual
tapering dose of prednisone was planned for over the next 3 to 6 months.
Clinical signs were most likely a result of MMM due to swelling of the
temporalis and pterygoid muscles. In one study (Gilmour et al 1992),
ocular signs including conjunctivitis, exophthalmos and permanent blindness
with optic nerve atrophy occurred in 44% of the cases. Temporary blindness
was also described in a case report (Glauberg and Beaumont 1979). On reexamination
3 weeks following the initial presentation, the dog was still blind in
the right eye with no other clinical signs of MMM.
click
on image to enlarge
References
Gilmour MA, Morgan RV, Moore FM. Masticatory myopathy in the
dog: A retrospective study of 18 cases. J Am Anim Hosp Assoc 28:300-306,
1992.
Glauberg A, Beaumont PR. Sudden blindness as the presenting sign
of eosinophilic myositis: A case report. J Am Anim Hosp Assoc 15:609-611,
1979.
Contributed by Dr. Lina Bravo Alta
Vista Animal Hospital, Gloucester, Ontario, Canada
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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.
