[pyrnet] Myositis

["Linda Weisser" <lmweisser@attbi.com>]

http://www.ivis.org/special_books/Braund/braund20a/chapter_frm.asp?LA=1#Mast
icatory_Myositis

Masticatory Myositis -
This inflammatory myopathy (synonym is eosinophilic myositis) is one of the
most common forms of myositis in dogs and is particularly common in adult,
larger-breed dogs [318-322]. Results of one retrospective study indicated
that most dogs were under 4 years of age with no gender or breed
predilection [321]. This disease is characterized by recurrent inflammation
of muscles, especially those of mastication (masseteric, temporalis, and
pterygoid muscles), sometimes in association with peripheral blood
eosinophilia. In most instances, the condition is restricted to muscles of
mastication. This is an autoimmune disease in which B-lymphocyte-mediated
antibodies are directed against type 2 M fibers in masticatory muscles. Type
2 M fibers are the dominant fiber type in masticatory muscles [323,324].
Biochemical studies have shown that masticatory muscles contain a unique
myosin isoform, unique myosin light chains, and unique myosin heavy chains
[325]. In one study of dogs with masticatory myositis, 86% of cases had
autoantibodies fixed to type 2 M fibers of the temporalis muscle [326].
Incubation of normal muscle with sera from affected dogs resulted in
labeling of 82% of type II M fibers. Immunocytochemical studies suggest that
transforming growth factor-beta (TGF-beta) and latent transforming growth
factor-beta binding protein (LTBP) may play a role in muscle tissue repair,
inflammation and fibrogenesis in masticatory myositis [327].
Lesions consist of myonecrosis, hemorrhage, edema and multifocal or diffuse
cellular infiltrates including macrophages, lymphocytes, plasma cells,
occasional neutrophils, and rarely, eosinophils. Skeletal muscle fiber
atrophy involving all fiber types may be pronounced, sometimes with foci of
small round fibers comprising entire muscle bundles [321]. Fiber hypertrophy
is usually not a feature. Perimysial and endomysial fibrosis is usually
marked. Regeneration of muscle fibers, characterized by vesicular nuclear
changes and fiber basophilia is frequently found.
Clinical signs are characterized by acute onset of painful, swollen,
masticatory muscles. The jaw is held partially open (pseudotrismus) and
passive manipulation is painful. Unilateral or bilateral exophthalmos may
also be present [319], which in some cases may cause optic nerve compression
or stretching resulting in blindness [321]. Dogs are often febrile, and
tonsils and mandibular lymph nodes may be swollen. The acute phase may last
2 to 3 weeks, with signs reaching a peak by 10 to 14 days. Serum CK levels
are elevated early in the disease and gamma globulin levels may be
increased.
Diagnosis is based on signalment, clinical, and muscle biopsy data, although
histological demonstration of antibodies against type 2M fibers is also a
sensitive index (the antibody titer may be reduced if corticosteroids have
been administered previously). Prognosis is usually favorable. In most
cases, the acute disease responds to corticosteroids, e.g., prednisone 1.0
to 2.0 mg/kg PO bid. The dose is reduced after remission of signs, and
gradually withdrawn using alternate day therapy. Note that the lowest
alternate-day dosage may be required for up to 6 months [317]. Repeated
clinical episodes are not uncommon, which usually result in muscle atrophy.
In one study, better clinical responses were noted in dogs receiving
prednisone early in the course of the disease, for at least one month, and
with the dosage tapered gradually from the initial immunosuppressive dosage
[321]. Other immunosuppressive drugs such as azathioprine (at 0.6 mg/kg PO
every one to three days) may also be used in conjunction with prednisone,
with a steroid-sparing effect, or alone, to maintain remission. There is no
apparent correlation between response to treatment and the extent/severity
of the muscle lesions. Note that manual manipulation of the jaw carries an
inherent risk of mandibular luxations/fractures [321]. In some severely
affected dogs, there may be a permanent inability to adequately open the
jaw, necessitating blending of the food for intake/ingestion [317].
Recently, a masticatory myositis has been reported in dogs with
leishmaniasis (Leishmania infantum) [328] (see infectious myositis).
I have observed masticatory myositis in several cats and an autoimmune
process is suspected. Note that many tissue samples received in our
laboratory from dogs with suspected masticatory myositis have evidence of
neurogenic atrophy with little or no sign of inflammation. These cases
probably represent idiopathic trigeminal neuritis.

Atrophic Masticatory Myopathy/myositis -
This is a chronic degenerative myopathy that is characterized by atrophy of
muscles of mastication which can occur in dogs of any breed [329,330]. It
has also been termed atrophic myositis and cranial myodegeneration. The
pathogenesis of this condition is uncertain. It may be a stage of
masticatory myositis or it might represent neurogenic atrophy secondary to
idiopathic trigeminal neuritis associated with severe axonal degeneration.
In some dogs with leishmaniasis, severe masticatory muscle atrophy may be
present [331] (see infectious myositis). Atrophic masticatory myopathy may
also be prominent in younger dogs with dermatomyositis. There is no
peripheral or local eosinophilia present. The atrophy is accompanied by a
state of trismus (lock-jaw) which may not be reduced, even under general
anesthesia, and which may interfere with eating (although this is not a
feature seen in dogs with leishmaniasis). Pathological studies reveal large
numbers of atrophic fibers and increased perimysial connective tissue. Focal
areas of lymphoplasmacytic infiltrates may be seen occasionally in
masticatory and other skeletal muscles.
Prognosis of this form may be guarded because of the severe trismus.
However, in most dogs jaw function returns to normal. Some animals appear to
respond to corticosteroids. Note that bilateral masticatory muscle atrophy
may be seen in some cats with nemaline myopathy.