Masticatory muscle myositis
Encyclopedia
Masticatory muscle myositis (MMM) is an 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. The disease mainly affects large breed dogs. German Shepherd Dog
s and Cavalier King Charles Spaniel
s may be predisposed. There is a similar disease of the eye muscles found in Golden Retriever
s. Symptoms of acute MMM include swelling of the jaw muscles, drooling, and pain on opening the mouth. Ophthalmic signs may include third eyelid
protrusion, red eyes, and exophthalmos
(protruding eyeballs). In chronic MMM there is atrophy
of the jaw muscles, and scarring of the masticatory muscles due to fibrosis
may result in inability to open the mouth (trismus
). The affected muscles include the temporalis
, masseter
, and pterygoid
muscles. The disease is usually bilateral.
MMM is caused by the presence of 2M fibers in the muscles of the jaw. 2M fibers are not found elsewhere in the body, but they are close in structure to protein
s found on the surface of bacteria
. The immune system
recognizes these proteins as foreign to the body and attacks them, resulting in inflammation
. Diagnosis of MMM is through either biopsy
of the temporalis or masseter muscles or the 2M antibody assay, in which blood serum of the possible MMM-dog is reacted with temporalis tissue of a normal dog, or both. False negatives by the 2M antibody assay may be obtained if MMM is end-stage with destruction of type 2M fibers and marked fibrosis. 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
. 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. Misdiagnosis of MMM as a retroorbital abscess
based on physical examination and finding of trismus leads to inappropriate treatment with antibiotics, which will not impede the progress of MMM.
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s affecting the muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s of mastication (chewing). It is also known as atrophic myositis or eosinophilic myositis. MMM is the most common inflammatory myopathy
Myopathy
In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...
in dogs. The disease mainly affects large breed dogs. German Shepherd Dog
German Shepherd Dog
The German Shepherd Dog , also known as an Alsatian or just the German Shepherd, is a breed of large-sized dog that originated in Germany. The German Shepherd is a relatively new breed of dog, with its origin dating to 1899. As part of the Herding Group, the German Shepherd is a working dog...
s and Cavalier King Charles Spaniel
Cavalier King Charles Spaniel
The Cavalier King Charles Spaniel is a small breed of Spaniel-type dog, and is classed as a toy dog by most kennel clubs. It is one of the most popular breeds in the United Kingdom. Since 2000, it has been growing in popularity in the United States. It is a smaller breed of spaniel, and Cavalier...
s may be predisposed. There is a similar disease of the eye muscles found in Golden Retriever
Golden Retriever
The Golden Retriever is a medium-sized breed of dog. They were historically developed as gundogs to retrieve shot waterfowl such as ducks and upland game birds during hunting and shooting parties. As such, they were bred to have a soft mouth to retrieve game undamaged and have an instinctive love...
s. Symptoms of acute MMM include swelling of the jaw muscles, drooling, and pain on opening the mouth. Ophthalmic signs may include third eyelid
Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...
protrusion, red eyes, and exophthalmos
Exophthalmos
Exophthalmos is a bulging of the eye anteriorly out of the orbit. Exophthalmos can be either bilateral or unilateral . Measurement of the degree of exophthalmos is performed using an exophthalmometer...
(protruding eyeballs). In chronic MMM there is atrophy
Atrophy
Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations , poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, disuse or lack of exercise or disease intrinsic to the tissue itself...
of the jaw muscles, and scarring of the masticatory muscles due to fibrosis
Fibrosis
Fibrosis is the formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive process. This is as opposed to formation of fibrous tissue as a normal constituent of an organ or tissue...
may result in inability to open the mouth (trismus
Trismus
-Common causes:*Pericoronitis is the most common cause of trismus.*Inflammation of muscles of mastication. It is a frequent sequel to surgical removal of mandibular third molars . The condition is usually resolved on its own in 10–14 days, during which time eating and oral hygiene are compromised...
). The affected muscles include the temporalis
Temporalis muscle
The temporal muscle is one of the muscles of mastication.-Structure:It arises from the temporal fossa and the deep part of temporal fascia...
, masseter
Masseter muscle
In human anatomy, the masseter is one of the muscles of mastication.In the animal kingdom, it is particularly powerful in herbivores to facilitate chewing of plant matter.-Origin and insertion of the two heads:...
, and pterygoid
Pterygoid
Pterygoid can refer to:* Pterygoid processes of the sphenoid bone** The Lateral pterygoid plate by it* a muscle such as Lateral pterygoid muscle or Medial pterygoid muscle* a branch of the Mandibular nerve...
muscles. The disease is usually bilateral.
MMM is caused by the presence of 2M fibers in the muscles of the jaw. 2M fibers are not found elsewhere in the body, but they are close in structure to protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s found on the surface of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. The immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
recognizes these proteins as foreign to the body and attacks them, resulting in inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
. Diagnosis of MMM is through either biopsy
Biopsy
A biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...
of the temporalis or masseter muscles or the 2M antibody assay, in which blood serum of the possible MMM-dog is reacted with temporalis tissue of a normal dog, or both. False negatives by the 2M antibody assay may be obtained if MMM is end-stage with destruction of type 2M fibers and marked fibrosis. Treatment is usually with corticosteroid
Corticosteroid
Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...
s such as prednisone
Prednisone
Prednisone is a synthetic corticosteroid drug that is particularly effective as an immunosuppressant drug. It is used to treat certain inflammatory diseases and some types of cancer, but has significant adverse effects...
, often with decreasing doses for up to 4–6 months, and in the case of trismus, manual opening of the mouth under anesthesia
Anesthesia
Anesthesia, or anaesthesia , traditionally meant the condition of having sensation blocked or temporarily taken away...
. 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. Misdiagnosis of MMM as a retroorbital abscess
Abscess
An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...
based on physical examination and finding of trismus leads to inappropriate treatment with antibiotics, which will not impede the progress of MMM.