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Focal myositis: a clinicopathologic study of 115 cases of an intramuscular mass-like reactive process.
[focal myositis]
Focal
myositis
is
an
uncommon
inflammatory
pseudotumor
of
skeletal
muscle
that
can
be
confused
with
a
variety
of
neoplastic
and
inflammatory
diseases
.
It
is
often
misunderstood
because
it
presents
as
a
tumor
-like
mass
,
but
histologically
resembles
a
skeletal
muscle
myopathy
or
dystrophy
.
We
wanted
to
discuss
the
detailed
morphologic
and
immunophenotypic
features
of
the
largest
reported
group
of
focal
myositis
patients
.
Two
hundred
and
six
cases
coded
as
"
focal
myositis
"
were
culled
from
our
files
.
Only
115
cases
with
adequate
material
,
a
solitary
lesion
,
and
correct
diagnosis
were
included
.
A
variety
of
immunohistochemical
studies
were
performed
,
as
were
polymerase
chain
reaction
for
T
cell
receptor
gene
rearrangement
and
immunoglobulin
heavy
chain
rearrangement
.
Age
ranged
from
7
to
94
years
(
mean
41
,
median
36
y
)
.
Most
patients
were
otherwise
healthy
,
and
with
the
exception
of
10
cases
,
lacked
antecedent
trauma
.
Masses
that
ranged
in
size
from
1
.
0
to
20
.
0
cm
(
median
3
.
0
cm
,
mean
3
.
9
cm
)
were
reported
in
specific
muscles
of
the
lower
extremities
(
including
vastus
lateralis
,
adductor
muscle
,
and
groin
muscles
,
n
=
39
;
gastrocnemius
,
n
=
22
)
,
followed
by
the
trunk
,
neck
(
mentalis
,
n
=
8
;
sternocleidomastoid
muscle
,
n
=
8
)
,
and
upper
extremity
.
Histologically
,
these
were
solitary
intramuscular
processes
composed
of
variable
myopathic
(
93
%
)
and
focal
neurogenic
(
89
%
)
changes
,
fibrosis
,
and
inflammation
(
97
%
)
,
occasionally
accompanied
by
prominent
eosinophils
(
n
=
20
)
.
By
immunohistochemistry
,
most
cases
had
CD
163
-
positive
macrophages
that
were
negative
for
S
100
protein
and
CD
1
a
.
Lymphocytes
were
mostly
CD
3
,
CD
4
-
positive
lymphocytes
that
were
negative
for
cytotoxic
markers
,
TIA
-
1
and
granzyme-
B
.
Polymerase
chain
reaction
did
not
show
B
cell
or
T
cell
rearrangement
.
In
situ
studies
for
Epstein-
Barr
-encoded
receptor
were
negative
,
as
was
ALK
-
1
immunohistochemistry
.
Major
histocompatibility
complex
-
1
and
weak
IgG
4
were
focally
positive
in
skeletal
muscle
.
Cases
with
severe
inflammation
had
increased
numbers
of
CD
2
0
-
positive
B
cells
and
CD
123
-
positive
plasmacytic
dendritic
cells
.
S
100
was
strongest
in
skeletal
muscle
fibers
with
vacuolar
change
.
Clinical
diagnostic
considerations
ranged
from
benign
entities
such
as
rhabdomyoma
,
intramuscular
lipoma
,
fibromatosis
,
myositis
ossificans
,
proliferative
myositis
,
inflammatory
myofibroblastic
tumor
,
and
inflammatory
myopathy
to
malignant
entities
such
as
rhabdomyosarcoma
,
leiomyosarcoma
,
liposarcoma
,
and
lymphoma
.
Available
follow-up
revealed
spontaneous
regression
.
Focal
myositis
occurs
in
specific
muscle
groups
of
young
adults
of
both
sexes
without
significant
trauma
.
It
is
a
largely
unrecognized
entity
with
specific
histology
including
myopathic
,
focal
neurogenic
,
fibrosis
,
and
inflammatory
features
.
It
can
be
easily
mistaken
for
an
inflammatory
myopathy
,
dystrophy
,
alternate
reactive
,
or
even
neoplastic
process
.
Focal
myositis
seems
to
be
a
macrophage
and
T
-
cell-rich
lesion
that
changes
to
B
cell
and
dendritic
plasmacytoid
cells
when
markedly
inflamed
,
but
does
not
seem
to
have
a
known
viral
or
molecular
etiology
.
IgG
4
presence
may
be
linked
to
the
fibrosis
in
these
lesions
;
a
possible
transient
autoimmune
etiology
can
not
be
excluded
.
Careful
attention
to
reproducible
clinicopathologic
features
can
aid
diagnosis
and
spare
patients
from
excessive
surgery
or
adverse
therapy
.
Diseases
Validation
Diseases presenting
"myopathy"
symptom
coats disease
cushing syndrome
dystrophic epidermolysis bullosa
epidermolysis bullosa simplex
familial mediterranean fever
focal myositis
homocystinuria without methylmalonic aciduria
inclusion body myositis
junctional epidermolysis bullosa
lymphangioleiomyomatosis
megacystis-microcolon-intestinal hypoperistalsis syndrome
pyruvate dehydrogenase deficiency
This symptom has already been validated