Synopsis
This presentation will review clinical and MR
imaging features of inflammatory and ischemic myopathies.Target Audience
Radiologists, imaging scientists and clinical
providers interested in understanding the MR imaging appearance of myopathies
and features that may help distinguish among etiologies.
Outcome/Objectives
At the conclusion of the talk, the participant should be able to:
» Recognize the three main
patterns of muscle pathology observed by conventional MRI
» Describe MR imaging
differences observed with various myopathies
» Understand the potential
utility of diffusion weighted and
spectroscopic MR imaging in myopathy
Purpose
To review muscle anatomy and MR imaging findings
of inflammatory and ischemic myopathies for improved diagnosis and treatment.
Highlights
o
Three main pathologic muscle
MR imaging appearances:
·
Mass lesion
·
Fatty infiltration
·
Edema
o
Muscle edema is the primary
finding in inflammatory myopathy
o
Muscle distribution and/or anatomic
location of edema can suggest a diagnosis and direct biopsy
o
Late stage myopathies
progress to fatty infiltration
Muscle Anatomy and Myopathy Review
MR Imaging muscle pathology
Muscle pathology on MRI can be divided into
mass lesions, edema, and fatty atrophy [1]. Mass lesions can include sarcomas, injury
with hematoma, myositis ossificans, abscess, sarcoidosis, or parasitic
infection. Fatty infiltration is
typically the end stage of various muscle pathologies, including disuse,
myopathy, end-stage of tendon tears (particularly in the shoulder and hip),
chronic denervation, or potentially could represent a fusiform lipoma. Muscular edema is the other muscle pathology
commonly encountered with clinical MR imaging.
Muscular edema can be seen with muscle injury, denervation, drug-induced
myopathy, or can be an early finding of myopathies. The focus of this presentation is muscle edema
and atrophy as the muscle edema and atrophy patterns can be helpful in
distinguishing etiologies.
Muscle Edema on MR Imaging
Muscle edema on imaging can be from a number of
causes. Careful evaluation of the edema
location and additional clinical information can help determine the etiology of
muscle edema in most cases. For example,
traumatic and denervation edema tend to be unilateral whereas inflammatory or drug-induced
myopathies are often bilateral. Findings
in denervation and traumatic muscle injuries are the focus of other lectures
within the muscle disease section and will not be further discussed here. Muscle edema located within a confined area
corresponding to a radiation port can differentiate radiation myositis from
other muscle pathologies, especially when supported with a history of radiation
treatment. Sometimes the etiology of
muscle edema or atrophy is not apparent from the history or location of
pathology. The remainder of the syllabus
will focus on reviewing muscle anatomy and myopathies to assist in identifying
an etiology for muscle edema or atrophy.
Muscle
Anatomy Review as it relates to MRI:
Recalling the structure of muscle and relating it
to the location of edema by MR imaging can help narrow the differential diagnosis.
Remember that the muscle is composed of muscle fibers that contain myofibrils [2]. Each muscle fiber
containing numerous myofibrils is encompassed by connective tissue layer called
the endomysium (Figure 1). Muscle fibers
are grouped together to form muscle fascicles, which are surrounded by a
perimysium. Muscle fascicles are grouped
together to form each muscle, which is surrounded by the epimysium. The epimysium and perimysium can be
visualized on MR imaging, while the endomysium connective tissue is contained
within the center of the muscle (Figure 2).
MR Imaging Edema - Differentiating features
The characteristics of muscle involvement in
patients with muscle weakness can help in diagnosis and treatment. Muscle edema is typically present in active
myopathy and absent in patients without symptoms or those who have responded to
treatment [3]. Since muscle edema suggests active disease, these would be the optimal
areas for targeting biopsy. When
gadolinium is administered, areas of myonecrosis will not enhance and should be
avoided when biopsy is performed [4]. Both the muscles that have
edema and the characteristics of the edema may suggest one diagnosis over
others.
o
Subcutaneous vs. absence of
subcutaneous edema
o
Endomysial vs. Perimysial
edema
o
Unilateral vs. bilateral edema
o
Muscle distribution
o
Presence vs. absence of
muscle atrophy
o
Presence vs. absence of
myonecrosis
Idiopathic Inflammatory Myopathies[5, 6]
The idiopathic inflammatory myopathies include
dermatomyositis, polymyositis, and inclusion body myositis. Diagnosis is based on clinical history,
laboratory tests, including autoantibodies, electromyography (EMG), and biopsy. MR imaging, more than any other imaging
modality, has become important for diagnosis and treatment monitoring,
especially since the EMG is painful, time-consuming, and can cause local
myositis that may interfere with biopsy.
Additionally, imaging may be helpful for cases where the muscle biopsy
is normal, which is reported in up to 10-15% of biopsies [5].
MR Imaging
o
STIR imaging helps identify
site – 97% sensitive to disease – sometimes before clinical manifestation of
muscle pathology
o
Gadolinum does not add to
diagnosis, but may help identify myonecrosis due to other diseases
o
Disease activity correlates
better with imaging than creatine kinase levels
o
May detect unsuspected
subcutaneous or fascial involvement
o
May be more cost-effective
than repeat biopsy for following treatment
Clinical and MR imaging in specific myopathies
All the
idiopathic myopathies present clinically with progressive muscle weakness and
increased STIR signal within the affected muscles, however some distinguishing
features can help differentiate among the myopathies. Ultimately, muscle biopsy is needed to make
the diagnosis and MR imaging can help ensure a diagnostic biopsy.
Inclusion Body Myositis[7-12]
Inclusion
body myositis is a sporatic age-related disease of skeletal muscle that results
in slowly progressive muscle weakness.
It typically affects individuals over the age of 50 with a male
predominance and has a predilection for the flexor digitorum profundus and
quadriceps muscles. Its prevalence
varies from 1-15 per million and delayed diagnosis is the norm. It is not associated with autoantibodies and
usually does not respond to immunosuppressive therapies.
o
More insidious onset of
weakness than dermatomyositis and polymyositis
o
Affects distal as well as
proximal muscles
o
No rash or Autoimmune
features (i.e. Raynaud’s)
o
Biopsy – rimmed vacuolar
inclusions in muscle cells, although not always present
MR Imaging
o
Symmetric involvement of
thighs
o
Quadriceps involvement with relative
sparing of rectus femoris muscles
o
Calf involvement – medial
gastrocnemius
o
Forearm involvement –
particularly flexor digitorum profundus
o
Frequently see fatty atrophy
of involved muscles
Dermatomyositis and Polymyositis [3, 5,
13-19]
Dermatomyositis
and polymyositis are autoinflammatory myopathies that result in subacute
development of symmetric proximal muscle weakness. Both tend to have interstitial lung disease
and skin involvement. There is a
predilection for women and age can vary, but has a peak between 30 and 50 years
of age. Both have an increased risk of
malignancy with dermatomyositis at higher risk.
Clinical and histological manifestations
Dermatomyositis
specific:
o
Pathognomonic heliotrope rash
– eyelids
o
Gottron’s sign – rash over
the extensor surfaces of the hand
o
Perifascicular muscle atrophy
Polymyositis
specific
o
Endomysial inflammation
MR Imaging
Dermatomyositis
o
Perimysial edema favors
perifascial inflammation
Polymyositis:
o
Endomysial inflammation by
histology favors intramuscular edema
Diffusion Weighted MRI (DWI) and MR Spectroscopy (MRS) [20, 21]
Few studies have looked at DWI and MRS in
patients with myopathies, but those that have show differences between normal
patients and myositis patients. These
techniques may prove useful for serial monitoring of response to treatment.
Ischemic Myopathy [22]
Myonecrosis typically presents as a focal,
tender mass. It can be associated with diabetes or alcoholism, but also may be
idiopathic. It usually presents on MR
imaging as mild swelling of the muscle, often with perifascial inflammation. Post-gadolinium imaging demonstrates a
peripherally enhancing area of muscle with no central enhancement or possibly small
linear areas of enhancement. Treatment
is generally conservative. Recognition
of the imaging features is necessary to avoid unnecessary biopsy or
overtreatment.
Mimics
o
Infectious Myositis/Necrotizing
Fasciitis
o
Compartment Syndrome
o
Radiation Therapy
o
Rhabdomyolysis
o
Amyloid Myopathy
Acknowledgements
I would like to thank Dr. M.K. Jesse from the University of Colorado for help with the preparation of the materials for this lecture and the ISMRM program committee for inviting me to give this presentation.References
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