In routine management of muscle disorders three recognizable MRI patterns have been described: edema,
fat component and mass lesion.
1) EDEMA
Characterized by an increase in free water,
muscle edema is the most common MRI pattern and is well depicted in fluid-sensitive sequences,
either T2-weighted images with chemically selective fat suppression or STIR sequences.
Given the many potential causes of muscle edema,
it is helpful to subdivide them considering three different groups depending on its distribution: diffuse,
focal,
or multifocal.
- Diffuse edema: Multiple muscles and muscle groups are involved
bilaterally,
typically with the entire muscle affected.
For instance,
in
inflammatory disorders (polymyositis or dermatomyositis),
inclusion
body myositis,
viral myositis or medication-induced myopathy.
- Focal edema: The abnormality is confined to one muscle or muscle
group or to a contiguous area involving multiple adjacent muscles.
We can see it in traumatic injuries,
pyomyositis,
diabetic muscle
infarction,
compartment syndrome,
denervation or radiation.
- When evaluating osteomyelitis changes,
especially in children,
one should take special care on looking at soft tissues involvement (Fig. 1, Fig. 2).
- Patients who have undergone radiation therapy may develop edema within various tissues in the irradiated field,
including skeletal muscle,
which characteristic sharp margins over the radiation field (Fig. 3, Fig. 4).
- Compartment syndrome occurs when an elevated pressure within a confined myofascial compartment (produced by trauma,
burns,
heavy exercise,
extrinsic pressure,
or intramuscular hemorrhage) leads to venous occlusion,
muscle and nerve ischemia,
arterial occlusion,
and finally tissue necrosis.
Muscles in the gluteal region are distributed in three distinct compartments (gluteus maximus,
gluteus medius and minimus,
and tensor fascia lata) that are covered with non-distensible fascia and aponeuroses that confine them to a limited space,
predisposing to a potential compartment syndrome.
Fig. 5: Scheme shows the three different compartments of gluteal area: the Tensor Fascia Lata (orange), The Gluteus Medius and Minimus (yellow) and The Gluteus Maximus (red).
In gluteal region,
this syndrome mainly results from
atraumatic conditions such as prolonged immobilization or
incorrect surgical position,
and drug abuse and alcoholic
intoxication have been related to possible predisposing factors.
Postoperative patients have several factors to consider:
- Prolonged immobilisation: The most important one. Often
from lying on hard uneven ground in an unaccustomed
position
- Special surgical positions (prone or lateral decubitus or dorsal
lithotomy positions)
- Overweight
- Prolonged operative time
- Epidural anaesthesia: It's believed that an impaired sensation
and motor blockade due to analgesia may prevent the normal,
spontaneous changes in posture that redistribute the load on
the gluteal area
We describre a case of this syndrome in gluteal muscles which
is very uncommon and only discussed from our knowledge in a
few published case reports.
MR imaging findings depict swelling
and edema within the affected compartment, highlighting
in our case the presence in both gluteal maximus of
hypoperfused- ischemic areas,
acquiring a morphology that
resembles "butterfly wings" (Fig. 6, Fig. 7, Fig. 8).
- Potential mechanisms of denervation are numerous,
including spinal cord injury,
poliomyelitis,
peripheral nerve injury or compression,
and neuritis.
Peripheral nerves,
particularly in the upper extremities,
are vulnerable to compression by mass lesions and entrapment in narrow anatomic spaces.
- Ischiofemoral impingement is a syndrome defined by narrowing of the space between ischial tuberosity and lesser trochanter,
that courses with hip pain and leads to abnormalities of the quadratus femoris muscle,
ranging from deformity and edema to tears and atrophy (Fig. 9, Fig. 10).
- Multifocal edema: several areas of muscle edema are remote from
each other and often have a patchy or nodular appearance.
For
example pyomyositis,
sarcoid myopathy,
denervation,
diabetic
myopathy and neuropathy or metastases could depict this pattern.
- Diabetes neuropathy.
The most common form of diabetic neuropathy is the distal symmetric form with predominantly sensory and autonomic manifestations.
MRI findings include,
beyond the intrinsic nerve features (fascicle enlargement and high signal on T2),
muscle denervation changes such as edema pattern in the acute and subacute stages and fatty replacement with or without atrophy in subacute and chronic stages of disease (Fig. 11,
Fig. 12,
Fig. 13).
- Diabetic myopathy is one of the long-term complications of diabetes.
It’s believed to be secondary to a microvascular pathologic process,
with inflammation and ischemia,
of the affected muscles.
According to the literature,
the large muscles of the thigh are more commonly affected,
although calf muscles could also be affected (Fig. 14). Bilateral involvement and multiple discontinuous sites of muscle involvement,
which may be seen in diabetic myopathy,
may further help to distinguish it from necrotizing fasciitis. In some cases,
it can progress to a severe ischemia and infarction (Fig. 15).
- Metastases: Muscular metastases are relatively uncommon,
and they are usually seen in disseminated neoplasms,
although they could be the first manifestation of the disease.
The most frequent tumors that mestatasize to muscle are carcinoma,
leukemia and lymphoma.
Although any skeletal muscle can be involved,
paravertebral,
psoas,
abdominal rectus and deltoid muscles are the common sites for metastasic deposits.
They can manifest as focal lesions with characteristic surrounding and sometimes extensive edema (Fig. 16, Fig. 17).
2) FAT COMPONENT
Fat in muscular disorders can be visualized in two different patterns,
as a diffuse fatty infiltration within muscles or focal,
nodular and discrete component.
- Diffuse fatty pattern occurs in:
- Chronic denervation, usually accompanied by muscle atrophy,
represents irreversible muscle injury as we depict in a case of muscle atrophy in a girl due to myelomeningocele intervened in the neonatal period (Fig. 18)
- Myopathies - Hereditary including dystrophies,
myotonies,
congenital and metabolic disorders.
- Acquired,
that can be subclassified in inflammatory myopathies, toxic myopathies (Corticosteroids,
especially when used in high doses for long periods) and myopathies associated with systemic conditions,
including Diabetic Myopathy (Fig. 14).
- Focal and nodular fatty pattern:
- Muscular Tumors: Lipoma,
hemangiomas,
myxoid liposarcoma,
etc. Some tumors may contain fat,
and we can distinct them from fatty infiltration because do not respect the shape of the muscle and frequently involve adjacent tissues.
Hemangiomas,
as we depict,
also may contain well-circumscribed serpiginous blood-filled spaces with fluid-fluid levels (Fig. 19).
- Parasitic infections,
such as Hydatid disease.
It is as a helminthic infection caused by the tapeworm Echinococcus granulosus,
contracted in humans by ingestion of eggs contained in the feces of the dog.
Once the embryos are released,
traverse the intestinal mucosa and are disseminated systemically via venous and lymphatic channels targeting to several organs,
the most common ones the liver and lungs,
and less common the brain,
peritoneal cavity,
spleen,
heart,
and bones. Skeletal muscles are not usually affected,
accounting for 1±5%,
including subcutaneous tissue,
basically due to the presence of lactic acid which hinders their development.
The most common locations are the neck,
the trunk and the root of the extremities.
MRI findings show multiple and multiloculated cysts with mixed high signal on T2-weighted images (depending on the amount of proteinacous cellular debris). Cysts typically contain multiple vesicles (daughter cysts) attached to the wall secondary to endogeneous proliferation of the germinal layer, acquiring a "cyst within cyst" appearance. An extremely rare but significant pearl in the case we show is the presence of small,
nodular fat component within the lesion (Fig. 20,
Fig. 21). In our knowledge,
this special feature has only been described in very few reports,
and although the cause is still unknown,
it could be related to the presence of high amount of fat in the germinal layer as is seen in pig and sheep hydatid cysts.
3) MASS:
In this pattern,
a localized masslike region with morphology and signal intensity different than those of normal muscle is found with all sequences.
This pattern may be seen in:
- Infectious conditions: abscesses (Fig. 2),
parastic infections (Fig. 20)
- Traumatic injuries: Hematoma (Fig. 22),
myositis ossificans
- Inflammatory conditions,
like sarcoidosis
It’s also quite frequent appreciate slight edema in adjacent skeletal muscle,
which may reflect muscle edema,
tumor invasion,
or both.