Laboratory Findings
In all forms of polymyositis there may be
elevated serum levels of the enzymes present in
skeletal muscle, such as CK, aldolase, serum
glutamic oxaloacetic transaminase, lactic acid
dehydrogenase, and serum glutamic pyruvate
transaminase. The degree of elevation decreases
from the first to the last in this series of
enzymes, and the pattern is the reverse of that
seen in liver disease. The erythrocyte
sedimentation rate is elevated in about
two-thirds of cases. Tests for circulating
rheumatoid factor are positive in less than
one-half and for antinuclear antibodies in
about three-quarters of the cases. Most other
hematologic indexes are normal. Several
autoantibodies seem to be associated with
clinically distinct groups of patients.
Anti-Jo-1 antibodies are more common in
polymyositis, especially in patients with
interstitial lung disease, and anti-nRNP
antibodies are often associated with
polymyositis seen in lupus erythematosus. Other
antibodies seen in patients with
dermatomyositis and polymyositis in association
with connective tissue diseases include:
anti-Scl-70 (progressive systemic sclerosis) Myoglobin can be found in the
urine when muscle destruction is acute and
extensive; rarely, acute polymyositis causes
the full syndrome of rhabdomyolysis and
myoglobinuria. In about 40 percent of cases EMG
reveals a markedly increased insertional
activity (muscle irritability), together with
the typical myopathic triad of motor unit
action potentials, which are of low amplitude,
are polyphasic, and have an abnormally early
recruitment. In a further 40 percent of the
patients only myopathic changes are present.
The ECG is abnormal in about 5 to 10 percent of
the cases at presentation. Since the pathologic
process in myositis is patchy, greater
diagnostic yield is accomplished by obtaining a
biopsy from two clinically affected muscles and
by skip serial sectioning of all specimens.
Magnetic resonance imaging may serve to
identify sites of muscle involvement. Muscles
recently used for EMG or intramuscular
injection must be avoided as these procedures
can produce inflammatory changes and muscle
fiber damage, leading to false-positive
results. In about two-thirds of cases, the
biopsies will demonstrate the typical
pathologic changes of myositis, but despite
following the above recommendations, about 10
percent of cases have normal muscle biopsy.
Skeletal Muscle Pathology
The principal changes in muscle consist of
infiltrates of inflammatory cells (lymphocytes,
macrophages, plasma cells, and rare eosinophils
and neutrophils) and destruction of muscle
fibers with a phagocytic reaction. Perivascular
(usually perivenular) inflammatory cell
infiltration is the hallmark of polymyositis.
Interstitial inflammatory cell infiltration is
also a prominent feature of the disease, but
lesser degrees of it may be seen in other
conditions as a secondary reaction (e.g., in
facioscapulohumeral and Becker's muscular
dystrophy). Evidence of muscle fiber
degeneration and regeneration is almost
invariably present. Many of the residual muscle
fibers are small, with increased numbers of
sarcolemmal nuclei. Either the degeneration of
muscle fibers or the infiltration of
inflammatory cells may predominate in any given
biopsy specimen. Blood vessel changes and
perifascicular atrophy are more prominent in
childhood dermatomyositis than in adult
dermatomyositis and polymyositis. Capillary
loss due to endothelial cell necrosis occurs
particularly in the periphery of fascicles and
may explain the perifascicular atrophy. Other
features include reduplication of capillary
basement membrane and the presence of tubular
inclusions within endothelial cells. Type II
muscle fiber atrophy and muscle infarcts also
may be found. Vasculitis is also seen in
polymyositis or dermatomyositis associated with
connective tissue disorders.
anti-Sm (lupus erythematosus)
anti-Ro and
anti-La (Sjögren's syndrome and lupus
erythematosus)
anti-ENA (mixed connective
tissue disease)
