Canine atrophic masticatory muscle myositis case report

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					                                                          ORIGINAL WORK

  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.

INTRODUCTION                                                            Submitted
                                                                                                 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 + 34.957.21.81.43 - Fax + 34.957.21.87.40 – E-mail:

      Canine atrophic masticatory muscle myositis: A case report

                                              Fig. 1:
                                              Frontal view
                                              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 [11]. 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                [1] ANDERSON (J.G.), HARVEY (C.E.). – Masticatory muscle myositis,
end-stage, showing destruction of type IIM fibres and marked                   J. Vet. Dent., 1993, 10, 68.
fibrosis, and little muscle antigen remains to stimulate an                [2] BLOMME (E.A.), PIEL (M.J.), FOUANT (M.M.) and coll. – What’s your
immune response. The IIM antibody assay does not provide                       diagnosis ? Bilateral head swelling in a male beagle. Masticatory
                                                                               muscle myositis (acute form), Lab. Anim., 2001, 30, 23-25.
information about the degree of myofibre destruction or
fibrosis, which are important in determining a prognosis [14].             [3] BLOT (S.), FUHRER (L.). – Les myopathies des carnivores
                                                                               domestiques Deuxième partie : étude spéciale, Prat. Méd. Chir.
Histological evaluation of muscle biopsy from affected                         Anim. Comp., 1995, 30, 27-43.
muscles reveals severe atrophy and degeneration of                         [4] BROGDON (J.D.). – Diagnosing and treating masticatory myositis,
masticatory muscle fibres, infiltration with eosinophilic                      Vet. Med., 1991, 86, 1164-1170.
granulocytes, perivascular plasmacytic infiltration,                       [5] DELVERDIER (M.), LAUGIER (S.), JEANJEAN (S.). – Myopathie des
                                                                               muscles masticateurs chez le chien, Prat. Med. Chir. Anim. Comp.,
proliferation of the fibrous interstitial tissue, necrosis and
                                                                               1990, 25, 137-142.
phagocytosis of type IIM myofibres [4,13,15]. Severe atrophy
                                                                           [6] KORNEGAY (J.N.). – Disorder of the skeletal muscles. In:
and degeneration of masticatory muscle fibres, infiltration of                 Ettinger S.J., Feldman (E.C.) (eds). – Textbook of Veterinary Internal
eosinophilic granulocytes, and proliferation of the fibrous                    Medicine. Philadelphia, W.B. Saunders 1995, pp 727-736.
interstitial tissue are the hallmarks of CAMMM. Since episodes             [7] LEWIS (R.M.). – Immune-mediated muscle disease, Vet. Clin. North.
are recurrent, the microscopic features will depend on the                     Am. Small Anim. Pract., 1994, 24, 703-710.
activity of the lesion. In the acute stage there will usually be           [8] McGAVIN (M.D.). – Muscle. In: Thomson’s Special veterinary
numerous eosinophils and less lymphocytes, neutrophils and                     Pathology Second Edition. W.W. Carlton & M.D. McGavin (eds)
plasma cells. In chronic stages or during quiescent phases,                    1995, Mosby Year Book Inc. St.Louis Missouri USA, pp 393- 420.
these proportions seem to be reversed so that the plasma cells             [9] MORIN (C.), CAUZINILLE.(L.). – La myosite des muscles
predominate, cellular content is greatly reduced, but clusters                 masticateurs : revue bibliographique à propos d’un cas clinique.
                                                                               Prat. Méd. Chir. Anim. Comp., 1999, 34, 603-607.
of plasma cells and lymphocytes with dark nuclei persist [6,8].
                                                                         [10] NELSON (R.W.), COUTO (C.G.), KING (C.). – Enfermedades del
When segmental degeneration occurs, the separated parts of                     músculo, In: Nelson R.W., Couto C.G., King C. (eds): Manual de
the myofibrillar mass and the empty segment of sarcolemmal                     Medicina interna de pequeños animales, Harcourt 1a ed. 2000,
sheath are very rapidly enclosed by macrophages and                            pp. 640 -646.
aggressive, invading cells, only some of which are                       [11] PODELL (M.). – Inflammatory myopathies, In: Neuromuscular
macrophages [6,10]. This vigourous invasion of fragmented                      diseases. Vet. Clin. North. Am. Small Anim. Pract., 2002, 32, 147-167.
fibres leads to some unusual cross sectional structures also             [12] SHELTON (G.D.), CARDINET (G.H IIIrd.), BANDMAN (E.). – Canine
found with the other inflammatory myopathies, in which the                     masticatory muscle disorders: A study of 29 cases, Muscle Nerve
centre of the fibre is hollowed out and filled with invading                   1987, 10, 753-66.
cells (presumably T81+ cells or killer cells), while the outer           [13] SHELTON (G.D.), Differential diagnosis of muscle diseases in
                                                                               companion animals, Prog.Vet. Neurol., 1991, 2, 27-33.
ring of the fibre consist of apparently normal myofibrils
[7,8,15]. Supporting structures in muscles appear to be                  [14] SHELTON (G.D.), CARDINET (G.H.) 3rd, BANDMAN (E.) and coll. –
                                                                               Fiber type-specific autoantibodies in a dog with eosinophilic
unaffected; nerve end plates, neuromuscular spindles, blood                    myositis. Muscle Nerve 1985, 8, 783-90.
vessels and tendons are unaltered even in chronic disease,               [15] TAYLOR (S.M.). – Selected disorders of muscle and the
apart from the expected modifications which accompany                          neuromuscular junction, Vet. Clin. North. Am. Small. Anim. Pract.,
atrophy [3,8].                                                                 2000 Jan, 30, 59-75.

Immunosuppressive dosages of corticosteroids are used
(prednisone 2 mg/kg/day twice a day) in all dogs diagnosed                 5
                                                                                5 5

with CAMMM, regardless of the degree of muscle atrophy or                  5
                                                                                5     5
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but recurrences can lead to chronic lesions with atrophy                       and preferably English (or German) to the FECAVA or IAMS
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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
                                                                                    All persons donating books enter a free draw, potentially
in the differential diagnosis of disorders in dogs with trismus                      winning free registration at a future FECAVA Congress
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a very useful technique which enables, as shown in this case,                                       in Ankara, Turkey.
the definitive diagnosis and prognosis of canine atrophic
masticatory muscle myositis to be made.

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