Jenny T. Bencardino, MDa*, Zehava Sadka Rosenberg, MDb aDepartment of Radiology, Huntington Hospital, North Shore Long Island Jewish Health System, 5 Twelvepence Court, Melville, NY 11747, USA
bDepartment of Radiology, Hospital for Joint Diseases, New York University Medical Center, 305 East 17th Street, New York, NY 10003, USA
Entrapment neuropathy secondary to nerve compression by mechanical or dynamic forces may be a cause of upper extremity pain and weakness in the athlete. Anatomically narrow passages predispose individual nerves to entrapment neuropathies. Dynamic changes within these narrow tunnels during repetitive athletic activity can produce further compression of a nerve with only minimal anatomic variation . Nerve compression may also be produced by space-occupying lesions such as tumors, cysts, inflammatory processes; or by post-traumatic conditions such as hematoma, myositis ossificans, and scar formation. Other causes for nerve compression are associated with hormonal alterations and systemic diseases, such as pregnancy, oral contraceptive ingestion, diabetes mellitus, and hypothyroidism.
Muscle weakness, with or without associated sensory loss, sharp burning pain, and paresthesias over a localized skin area, may be associated with entrapment neuropathies. In chronic instances, muscle atrophy and vegetative disturbances can occur. Although electromyographic and nerve conduction studies remain the preferred diagnostic method for entrapment neuropathies, MRI may play a role when the deep course of some nerves, such as the posterior interosseous nerve, may result in unreliable electromyographic analysis.
The treatment of entrapment neuropathies includes conservative measures such as immobilization, local heat, and anti-inflammatory medications. In refractory cases, percutaneous steroid injections and surgical release of the nerve may be required.
Other than MRI, most imaging techniques are insensitive for detecting entrapment neuropathies. Exostosis, osteophytes, fracture callus, and anatomic osseous variants, which may contribute to nerve compression, can occasionally be depicted using conventional radiography and CT. Direct visualization of the nerve, however, is best achieved with MRI.
*Corresponding author. E-mail address: [email protected] (J.T. Bencardino).
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In the setting of entrapment neuropathies, MRI can confirm the presence of nerve compression or entrapment, assess for space-occupying lesions, and exclude other lesions that can present in a similar clinical fashion to entrapment neuropathy (eg, rotator cuff tear). Normal peripheral nerves are depicted on MR images as low-to-intermediate signal intensity structures highlighted by fat. MRI neurography and high resolution studies often depict the honeycomb, fascicular pattern of the nerves. Spurious mild increased signal of the nerve is often seen on water-sensitive sequences.
MRI features of peripheral entrapment neuropathy can be subdivided into direct signs involving the nerve, and indirect signs related to muscle denerva-tion. Direct evidence of peripheral entrapment include changes in signal intensity, size, and position of the affected nerve. Focal changes in the nerve are particularly useful in distinguishing the mild increased nerve signal normally seen on fluid-sensitive images from true neuropathy. Nerve edema can result in focal effacement of the fascicular pattern of the larger nerves.
Denervation muscle injury follows a pattern of signal alteration characterized by normal T1 signal and interstitial T2 hyperintensity in the acute setting, and increased T1 signal with normal T2 signal consistent with muscle atrophy and fatty infiltration in chronic cases [2,3]. Often, increased signal on both T1 and fluid-sensitive images is noted, indicating an ongoing denervation process. The distribution pattern of denervated muscles can isolate the exact site of entrapment as well as depict variations in muscle innervation.
MRI is also useful in identifying the cause for entrapment. Osseous abnormalities such as bony spurs, fracture fragments, and callous can easily be identified. Mass occupying lesions such as tumors, aberrant muscles, ganglia, bursa, and scar tissue are also well-depicted with MRI.
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