Imaging Use in Focal Rhabdomyolysis of the Left Shoulder

Focal rhabdomyolysis and acute compression at the posterior shoulder leading to compressive brachial plexopathy is rare, with only 3 cases reported in the literature, all occurring with IV drug use.^1-3 This patient had compression of the brachial plexus posterior cord from rhabdomyolysis and prolonged immobilization. Intravenous drug abusers may delay medical care due to perceived illicit drug effects and frequently present to the ED confused, agitated, or obtunded. Acute extremity swelling, a palpable lump, and pain can be due to various etiologies, such as trauma, fluid collection, muscle tear, myopathy, venous thrombosis, neoplasm, or rhabdomyolysis.

Diagnosis of nontraumatic rhabdomyolysis depends on clinical history and biochemical tests, such as serum CK and urine myoglobin.^1,8 Creatine kinase is present in large quantities in the myocytes and is 100% sensitive as a marker for rhabdomyolysis.^1,8 Creatine kinase may increase acutely > 1,000 U/L, suggesting muscle lysis and necrosis as etiology for pain as opposed to other causes such as hematomas, abscesses, or venous thrombi.^1,9 Serum CK decreases rapidly at a rate of 39% per day, and it may normalize by the time a patient presents for medical care.^1,10-12 Imaging plays a significant complimentary role. During the patient’s second ED presentation, the CK was normal at 54 U/L, whereas ultrasound and MRI findings were suggestive of focal muscle abnormalities.

Although there are diverse etiologies of rhabdomyolysis, the ultimate consequences of rhabdomyolysis are muscle cell membrane injury, metabolism malfunction, and destruction of the myofibril, resulting in inflammatory changes, such as muscle edema, hemorrhage, and myonecrosis and disruption of muscle fibers.^1,2,8,9,13 This may cause an alteration in muscle size, shape, and echogenicity on sonography and abnormal signal intensity on MRI.¹³ The sensitivity of MRI in the detection of muscle involvement is higher than that of CT or ultrasound due to the high soft tissue contrast.^4,13,14 Specificity of all 3 modalities is low and not reported.

Although the sensitivity of ultrasound is lower than that of MRI, use of ultrasound in neuromuscular evaluation has been increasing recently due to technical refinements. Ultrasound can be effectively used as a first-line screening modality, especially in an emergency.⁵ Magnetic resonance imaging best assesses the distribution and extension of the affected muscles, especially when fasciotomy is considered for treatment, and initially reveals edema, inflammation, and findings of myonecrosis; muscle atrophy and fatty degeneration occur later.^4,13-15 Typical MRI findings include increased signal intensity on T2-weighted and STIR (short-tau inversion recovery) sequences and variable enhancement on T1 postcontrast images, as was seen in this case, which indicated edema, inflammation, and necrosis of the muscle tissue.

Shintani and colleagues described the reversibility of the MRI findings, showing that the high-intensity lesions seen on T2-weighted images resolved in parallel with the clinical course.^14,16 Lu and colleagues investigated 10 patients with rhabdomyolysis and found 2 distinct imaging types: Type 1 shows homogenous signal changes and enhancement in the affected muscles, and Type 2 shows rim enhancement on contrast-enhanced MRI, a “stipple sign” indicating areas of myonecrosis.¹⁷ Magnetic resonance imaging signal alterations in the musculature can be nonspecific and overlap with those of inflammatory myopathies such as polymyositis, connective tissue diseases with inflammatory myositis, muscle infection, muscle infarction such as diabetic myonecrosis, muscle contusion, drug-induced myotoxicity, corticosteroids use, and use of cholesterol-lowering agents.^18,19

Sonography is a useful screening modality for pain and swelling of the extremity, because it can detect a muscle tear, muscle sprain, and fluid collection, especially in emergent cases. There is scant literature about sonographic findings in rhabdomyolysis and compression nerve entrapment. The sonographic findings of rhabdomyolysis are local disorganization of the damaged muscle, decreased muscle echogenicity, and enlargement of the muscle, with preservation of the muscle boundaries.^5-7

Intramuscular hyperechoic areas are seen due to hypercontractility of injured muscle. In this case, noted findings included patchy, irregular, hypoechoic areas, enlargement of the muscles and tendons, and irregular hyperechoic areas without focal defects. These findings differentiated an abnormality from a muscle tear or rupture, as these often show a focal muscle gap and focal defect, signifying the ruptured muscle retracting.

A study by Su and colleagues used the large number of crush injuries after an earthquake in China.⁵ The characteristic sonographic findings were edema and thickened disrupted striated muscle, good overall muscle continuity, vague muscle texture, and enhanced cloudy or ground-glass-like echo. There was no blood flow signal in the hypoechoic areas.⁶ Ultrasound was deemed a cost-effective, easily available modality by the authors.

Conclusion

Nontraumatic, focal rhabdomyolysis is rare and should be detected and differentiated from other causes of swelling, lump, pain, or other muscle disorders to prevent late complications. Sonography is an important screening diagnostic modality. MRI is used for assessment of the extent and distribution of injury. Awareness and familiarity with imaging findings can play a significant role, along with clinical and laboratory findings in the diagnosis and management of rhabdomyolysis.