Dermatomyositis versus Polymyositis
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Features |
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Incidence |
About
1/3 of all adult inflammatory myopathy and almost all of the childhood
cases. |
Less
common than dermatophysis and it is questionable whether it occurs in
children. |
|
Age |
First
peak: 9 years old. Second
peak: 40-70 years old. |
40-60
years old. |
|
Sex |
M:F:
1:2-3. |
Equal. |
|
Clinical |
Subacute
onset with skin rash, symmetrical proximal weakness, and often with limb
muscle pain. |
Chronic,
insidious and progressive. Proximal weakness, dysphagia may be present.
Myalgias and arthralgias are less common than in dermatomyositis. |
|
Serum CK |
Usually
but not always elevated. |
Usually
but not always elevated; can be 100 times of normal. |
|
Skin rash |
Usually
appear with weakness. |
No. |
|
Association with
malignancy |
About
20% of adult cases are associated with internal malignancy. Childhood
cases are not. |
No. |
|
Response to
treatment |
Yes. |
Yes. |
|
Antibodies |
A
wide variety is present. |
A
wide variety is present. |
|
Perifascular
atrophy |
Yes. |
No. |
|
Fiber necrosis |
More
commonly seen in adults than in children and often in form of single fiber
necrosis at the edge of the fascicle. Groups
of necrotic fibers at the center of the fascicle suggesting infarction may
be present. |
Randomly
distributed. They may or may not be associated with the inflammatory
infiltrates. No grouping of necrotic fiber. |
|
Delay in
phagocytosis |
Yes.
Many featureless necrotic fibers are present. |
No. |
|
Regenerating
fibers |
Occur
in the same distribution as necrotic fibers. |
Present. |
|
Inflammatory
cell infiltration |
Usually
confined to septa, sometimes in
fascicles and ocaasionally in both locations. |
Perimysial and
endomysial lymphocytic infiltration within
fascicles and in non-necrotic fibers. |
|
Lymphocytic
profile |
Similar
number of B- and
T-cells around blood vessels. B-cells
are much less
common within fascicles. CD4
cells predominate. CD4/CD8 ratio
is highest at perivascular and lowest at endomysial infiltrates. |
Most
of the inflammatory cells are T-cells.
B-cells comprise only about one-eigth of the inflammatory cells. Most
of the perivascular cells are composed of B-cells. Most
of the endomysial infiltrates are composed of T-cells. Most
of the cells infiltrating necrotic fibers are macrophages. |
|
Lymphocytic
infiltration in non-necrotic fiber |
No. |
Yes. |
|
Class I MHC
antigen |
Detectable
in areas with damaged fibers, not expressed in normal fibers. |
Expressed
by all fibers in the biopsy regardless on whether they have lymphocytic
infiltrates. Fibers infiltrated by lymphocytes have stronger expression. |
|
Number of
capillaries |
Number
of capillaries is reduced because of continuous destructiojn of
capillaries. |
No
reduction. |
|
Complement |
Membrane
attack complex an be demonstrated in capillary walls, especially those of
more recent onset. |
No. |
|
EM |
Damaged
capillaries can be seen in practically most if not all cases. Tubuloreticular
structures in endothelial cells are highly suggestive but not diagnostic. |
Dark
bodies are seen in some patients, they are suggestive but not diagnostic. |
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