C anine atrophic masticatory muscle
myositis: A case report
E. Quiroz-Rothe (1), E. Aguilera-Tejero (2), J.-C. Estepa (2), I. Lopez-Villalba (2), J.L.L. Rivero (1) *.
S U M M A R Y
Canine atrophic masticatory muscle myositis is presumed to be an immunologically mediated myopathy of
unknown origin. Severe atrophy and degeneration of masticatory muscle fibres, infiltration of eosinophilic
granulocytes, and proliferation of connective tissue are the hallmarks of this pathology. This report
examines a case of atrophic masticatory muscle myositis in a dog, concluding that muscle biopsy analysis
is a useful technique for diagnosis and prognosis of this disease.
Clinical pathology results demonstrated a
normal number of eosinophils in peripheral
for publication 2001 blood (158/L). No changes in muscular
Canine masticatory muscle myositis (CMMM) is
based on cases seen enzymes were detected (creatine kinase CK=
an immunomediated canine myopathy,
July – September 1999 36.6 UI/L). A muscle biopsy was taken from
which selectively causes focal myositis and
the masseter muscle (under general anesthesia).
progressive destruction of type IIM myofibres Multiple biopsy specimens were collected
from masticatory muscles (masseter, temporalis, from the same area to reduce the chances of
pterygoideus and tensor veli palatini) which leads to trismus in missing a focal pathological change. Three small pieces
dogs [4,10]. This disease must be included in the differential (approximately 0.2 cm x 0.2 cm x 0.4 cm) were frozen by
diagnosis list of problems of the temporo-mandibular joint (e.g.: immersion in isopentane, precooled in liquid nitrogen, and
trigeminal neuritis or the early form of tetanus in dogs) [3,8,13]. stored at –70ºC until analysed. Serial sections were cut at 10
This clinical report shows a 1 year old German Shepherd µm and stained with hematoxylin and eosin (HE) and
presented at consultation for chronic masticatory problems. periodic acid Schiff (PAS).
Histological examination from muscle biopsy enabled the
Histological observation showed a large area of chronic
definitive diagnosis of this myopathy to be made.
proliferative fibrosis with loss of muscular tissue.
Characteristic inflammatory cells of chronic reaction and the
presence of blood vessels were also observed (Fig. 2). The
few areas occupied by muscular fibres showed chronic
CASE PRESENTATION atrophy, signs of degeneration and hollow spaces inside
muscular fibres. These spaces did not contain any specific
A one year old male German Shepherd was referred to the material (glycogen or lipids) (Fig. 3). Occasionally, cellular
Small Animal Clinic of the University of Cordoba, with a elements of small size that could correspond probably to
history of progressive weight loss and difficulty in masticating killer cells or cells T81+ were observed. The damaged areas
and closing his jaws. At presentation, the dog showed severe were surrounded by a ring of apparently normal contractile
bilateral atrophy of the masseter and temporal muscles (head material. There was generalized atrophy and fibrilar
contour with a skull-like appearance) with trismus and fragmentation. Eosinophils were not observed and no
complete loss of functional control from affected muscles (Fig. abnormalities of nerves, neuromuscular junctions, blood
1). No abnormalities were found in other skeletal muscles. vessels and tendinous structures were found.
(1) Muscle Biopathology Laboratory Department of Comparative and Pathological Anatomy, Faculty of Veterinary Sciences of Cordoba, Campus de Rabanales,
Edificio de Sanidad Animal. Ctra. Madrid-Cádiz, km 396. E-14014 Córdoba.
(2) Department of Animal Medicine and Surgery, Faculty of Veterinary Sciences of Cordoba, Campus de Rabanales, Edificio de Sanidad Animal. Ctra. Madrid-
Cádiz, km 396. E-14014 Córdoba.
* Corresponding author Dr J.L.L. Rivero, Phone + 34.922.214.171.124 - Fax + 34.9126.96.36.199 – E-mail: email@example.com
Canine atrophic masticatory muscle myositis: A case report
of a German
Shepherd dog Fig 3: H-E stain 400x. Presence of hollow spaces inside
with bilateral cross-sectionally muscle fibres. These spaces are
cranial surrounded by ring of apparently normal contractile
and masticatory material. Cellular elements inside hollows probably
muscle atrophy, correspond to killer cells or cells T81+.
"fox like head
Both forms are probably different phases during a single
disease process [1,13,15]. The pathological data showing the
simultaneous presence of chronic inflammation, support the
theory that CEMMM is an early stage of CAMMM. Canine
masticatory muscle myositis can occur in any breed of dogs,
German Shepherds, Retrievers, Dobermann Pinschers, Shar Pei,
Beagles and other large breeds are the most commonly
affected [2,5,9]. There is no apparent gender predilection but
young and middle aged dogs are primarly affected, with an
average age of 3 years at the onset of clinical signs [4,7,12].
Clinical signs of CMMM range from acute swelling of the
temporalis and masseter muscles, trismus, recurrent attacks of
jaw pain, weakness, exophthalmos to muscle atrophy with or
without pain and restricted jaw movement [1,10]. Muscular
Fig. 2: H-E stain 200x. Muscle fibres cross-sectionally showing atrophy gradually becomes very obvious, and the head appears
chronic atrophy, a degenerative appearance aspect, to have a fine fox-like or a skull-like contour [3,12]. Differential
extensive areas of chronic proliferative fibrosis with loss of diagnosis of CAMMM should include traumatic and neoplasic
muscular tissue and characteristic inflammatory cells of disorders affecting the teeth, eyes, mouth, and temporo-
chronic reaction. mandibular joints (e.g.: polymyositis, trigeminal neuropathy,
hypothyroidism Cushing’s syndrome, or the early form of
Based on clinical signs and muscular biopsy observations a tetanus in dogs) [8,9,13]. There is no apparent correlation
diagnosis of atrophic masticatory muscle myositis was made. between clinical signs, muscle enzyme concentrations and the
The dog was treated with oral prednisolone (2 mg/kg twice a degree of muscle necrosis on biopsy. Serum creatine kinase
(CK) is only moderately elevated in some dogs with acute
day, total daily dose 4 mg/kg). Treatment resulted in a rapid
canine masticatory muscle myositis and is often normal in the
improvement of clinical signs; nevertheless a guarded
chronic presentation [1,6,15]. A moderate leukocytosis is
prognosis was given, because muscular biopsy showed an
present (only during the acute phase), with 10 to 40% of the
extensive area of chronic proliferative fibrosis, with loss of circulating white cells being eosinophils [2,5,7].
muscular tissue and progressive atrophy of craniofacial
muscles, with total loss of functional control. Surgical biopsy of the temporalis or masseter muscle and
serum assay for circulating autoantibodies against masticatory
muscle type 2M fibres are the recommended tests in CAMMM
diagnosis . Antibodies against type IIM fibres are detected
DISCUSSION by the immunoreagent (SPA-HRPO). Type IIM fibres are
present only in the group of muscles supplied by the
AND CONCLUSION mandibular branch of the trigeminal nerve (e.g: masseter,
temporalis, pterygoideus muscles), providing the specificity
Two forms of canine masticatory muscle myositis CMMM are necessary to separate this disorder from polymyositis, which
recognized: an acute stage known as eosinophilic myositis affects the temporalis and limb muscles. The test is highly
(CEMMM) and a chronic stage or atrophic myositis (CAMMM). sensitive in the acute stages of canine masticatory muscle
QUIROZ-ROTHE, AGUILERA-TEJERO, ESTEPA AND COLL. EJCAP - Vol. 12 - Issue 2 - October 2002
myositis, but can become a false negative if the dog has been
on immunosuppressive dosages of corticosteroids for longer
than 7-10 days, or when masticatory muscle myositisis is at  ANDERSON (J.G.), HARVEY (C.E.). – Masticatory muscle myositis,
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these proportions seem to be reversed so that the plasma cells  MORIN (C.), CAUZINILLE.(L.). – La myosite des muscles
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ring of the fibre consist of apparently normal myofibrils
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Immunosuppressive dosages of corticosteroids are used
(prednisone 2 mg/kg/day twice a day) in all dogs diagnosed 5
with CAMMM, regardless of the degree of muscle atrophy or 5
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and tapered to the lowest possible alternate day dosage for at
least 6 months [8,15]. Remission often follows steroid therapy, Bring unwanted text books (under 10 years of age
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appropriate therapy leads to a good prognosis for CAMMM.
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To sum up, masticatory muscle myositis needs to be included
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the definitive diagnosis and prognosis of canine atrophic
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