Title: Masticatory muscle myositis

{{Short description|Inflammatory jaw muscle disease in dogs}}
[[File:American Pit Bull Terrier (1).jpg|thumb|200px|Acute MMM in an [[American Pit Bull Terrier]]]]
[[File:Masticatory Muscle Myositis dog.jpg|thumb|200px|Chronic MMM in a [[Rottweiler]]]]
'''Masticatory muscle myositis''' (MMM) is an immune-mediated inflammatory disease in [[dog]]s affecting the [[muscle]]s of mastication (chewing).  It is also known as '''atrophic myositis''' or '''eosinophilic myositis'''.  MMM is the most common inflammatory [[myopathy]] in dogs.&lt;ref name=Neumann&gt;{{cite journal |vauthors=Neumann J, Bilzer T |title=Evidence for MHC I-restricted CD8+ T-cell-mediated immunopathology in canine masticatory muscle myositis and polymyositis |journal=Muscle Nerve |volume=33 |issue=2 |pages=215–24 |year=2006 |pmid=16270307 |doi=10.1002/mus.20456}}&lt;/ref&gt;  The disease mainly affects large breed dogs.&lt;ref name=Ettinger_1995&gt;{{cite book|author1=Ettinger, Stephen J. |author2=Feldman, Edward C. |title=Textbook of Veterinary Internal Medicine|edition=4th|publisher=W.B. Saunders Company|year=1995|isbn=0-7216-6795-3}}&lt;/ref&gt; [[German Shepherd Dog]]s and [[Weimaraners]] are most commonly affected.&lt;ref name=&quot;orbit&quot;&gt;{{cite book | last1=Gelatt | first1=Kirk N. | last2=Plummer | first2=Caryn E. | title=Essentials of Veterinary Ophthalmology | publisher=Wiley | date=2022-08-12 | isbn=978-1-119-80132-0 | doi=10.1002/9781119801368.ch5 | url=https://onlinelibrary.wiley.com/doi/book/10.1002/9781119801368|chapter=Canine Orbit: Disease and Surgery| pages=227–228}}&lt;/ref&gt; Symptoms of acute MMM include swelling of the jaw muscles, drooling, and pain on opening the mouth.  Ophthalmic signs may include [[nictitating membrane|third eyelid]] protrusion, red eyes, and [[exophthalmos]] (protruding eyeballs).&lt;ref&gt;{{cite web | last = Gelatt | first = Kirk | title = Treatment of Orbital Diseases in Small Animals | work = Proceedings of the 27th World Congress of the World Small Animal Veterinary Association | year = 2002 | url = http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2002&amp;PID=2641 | accessdate = 2007-02-10 }}&lt;/ref&gt; 
==Causes==
MMM is caused by the presence of [[2M fiber]]s in the muscles of the jaw, 2M fibers are unique to the masticatory muscles and are made up of [[myosin]].&lt;ref name=&quot;orbit&quot;/&gt; In affected animals, the [[immune system]] recognizes these proteins as foreign to the body and attacks them, resulting in [[inflammation]].&lt;ref name=Ettinger_1995/&gt;
==Symptoms==
The inflammation from MMM typically results in [[anorexia (symptom)|anorexia]], loss of body weight, [[pyrexia]], and torpidity. [[Optic neuritis]] and blindness can occur in some cases. Acute cases last for one to three weeks without treatment. Chronic or repeat cases can result in fibrosis, atrophy, and [[enophthalmos]]. The fibrosis and atrophy can result in an inability to open the mouth in severe chronic cases.&lt;ref name=&quot;orbit&quot;/&gt;
==Diagnosis==
Diagnostic results of acute myositis are [[eosinophilia]], heightened serum levels of [[creatine phosphokinase]], a positive 2M antibody test, and histopathology and biopsy of the temporalis or masseter muscles.&lt;ref name=&quot;orbit&quot;/&gt;

Biopsy reveals an orange colour to affected muscles and histopathology results are degeneration of the muscle fibre and neutrophil and eosinophil infiltration.&lt;ref name=&quot;orbit&quot;/&gt;
===Differential diagnoses===
*[[Abscess]]&lt;ref name=&quot;orbit&quot;/&gt;
*[[Orbital cellulitis]]&lt;ref name=&quot;orbit&quot;/&gt;
*[[Polymyositis]]&lt;ref name=&quot;orbit&quot;/&gt;
*[[Temporomandibular joint disease]]&lt;ref name=&quot;orbit&quot;/&gt;

==Treatment==
Treatment is usually with [[corticosteroid]]s such as [[prednisone]], often with decreasing doses for up to 4–6 months, and in the case of trismus, manual opening of the mouth under [[anesthesia]].&lt;ref name=Chrisman_2003&gt;{{cite book|author1=Chrisman, Cheryl |author2=Clemmons, Roger |author3=Mariani, Christopher |author4=Platt, Simon |title=Neurology for the Small Animal Practitioner|edition=1st|publisher=Teton New Media|year=2003|isbn=1-893441-82-2}}&lt;/ref&gt;  Feeding very soft or liquid food during this time is usually necessary. The ultimate degree of recovery of jaw function and muscle mass will depend upon the extent of damage to the muscle tissue. Recurrence of MMM may occur.&lt;ref&gt;{{cite journal |vauthors=Clooten J, Woods J, Smith-Maxie L |title=Myasthenia gravis and masticatory muscle myositis in a dog |journal=Can Vet J |volume=44 |issue=6 |pages=480–3 |year=2003 |pmid=12839242 |pmc=340171}}&lt;/ref&gt;
==References==
{{reflist}}

==External links==
*[http://vetneuromuscular.ucsd.edu/cases/2010/January.html Most Commonly Asked Questions about Masticatory Muscle Myositis]
*Masticatory muscle [[Myositis ossificans|myositis]] (MMM), [https://otterhound.dog/irish-otterhound-dog-breed-information Dogs]
*[http://cavalierhealth.net/masticatory_muscle_myositis.htm Masticatory Muscle Myositis in the Cavalier King Charles Spaniel]

[[Category:Dog diseases]]
[[Category:Chewing]]