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Case Report |
1 Department of Radiology, University of Michigan Hospitals, 1500 E. Medical
Center Dr., Ann Arbor, MI 48109.
2 Department of Orthopaedic Surgery, University of Michigan Hospitals, Ann
Arbor, MI 48109.
Received September 13, 2002;
accepted after revision December 12, 2002.
Address correspondence to D. A. Jamadar.
Introduction
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MR imaging showed a mass along the course of the posterior interosseous nerve just proximal to its passage between the superficial and deep heads of the supinator muscle (Fig. 2A). The mass was isointense to the posterior interosseous nerve on T1-weighted and T2-weighted sequences.
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Sonography revealed a mass continuous with the posterior interosseous nerve immediately proximal to the supinator muscle (Figs. 2B and 2C). A transverse band was located just distal to the mass, crossing anteriorly to the nerve (Fig. 2D). Proximal to the mass, the nerve was thickened relative to the opposite side.
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At surgical exploration, the supinator muscle was divided at its proximal extent. A fibrous band was found compressing the nerve, crossing anteriorly and obliquely to the nerve (Fig. 2E). We found focal swelling of the nerve just proximal to the fibrous band, and the nerve was thickened proximal to the focal swelling. The nerve was traced along its course through the supinator between the superficial and the deep heads. There was no additional compression, and the nerve was of uniform diameter within the supinator muscle. The fibrous band was then surgically released.
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Two weeks after surgical decompression the patient had partial return of motor functions with return of function of the extensor digiti minimi.
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Posterior interosseous nerve syndrome occurs when there is compression of the posterior interosseous nerve sufficient to cause paresis or, if more severe, paralysis [2]. Many possible causes for posterior interosseous nerve syndrome are known. Compression of the posterior interosseous nerve by the arcade of Frohse, intermuscular septa, fibrous bands, muscle margins, vessels, rheumatoid arthritis, fractures, scar adhesions, ganglionic cysts, and other soft-tissued masses are well-known causes [2, 3]. The posterior interosseous nerve can also be stressed during repetitive supination and pronation, as has been reported in swimmers, Frisbee players, tennis players, violinists, and orchestra conductors [4]. Electromyographic studies show a nerve action potential with marked slowing of conduction and low amplitude across the lesion, which affects the severity and location of the deficit. Initial nonoperative treatment includes rest, activity modification, and splinting. If no improvement is seen with conservative therapy, patients are usually offered surgery to relieve any extrinsic compression [2].
Compression-induced neuronal swelling is well known. First described in animal models, nerve swelling can occur within 1 week after a constriction is applied. Axonal diameter equalizes a few days after the nerve is released [4]. The swelling of the neuron has been attributed to an obstruction of normal cytoplasmic axonal transport termed "axonal damming" [5], when the physiologic passage of cytoplasmic content along the axon is obstructed by a constriction, resulting in proximal swelling. Other authors have attributed this swelling of a nerve entirely to ischemia [5] and not to axonal damming. Cadaveric studies have also shown swelling of the nerve proximal to a compression [1, 3]. These histologic studies have associated an enlarged nerve with perineural [3] or epineural [1] fibrosis. The chronic compression and rapid recovery of motor function in our patient after surgery probably indicate that the swelling of the nerve was caused by long-term compression by the fibrous band.
Accurate identification of the posterior interosseous nerve on sonography relies on knowledge of anatomy and the sonographic appearance of normal peripheral nerves. On sonography, normal peripheral nerves show hypoechoic nerve fascicles and hyperechoic connective tissue [6]. In the transverse plane, a peripheral nerve has a speckled appearance and is relatively hyperechoic compared with surrounding muscle. In the longitudinal plane, a peripheral nerve exhibits a linear fascicular appearance. To identify the posterior interosseous nerve on sonography, we begin imaging the nerve proximally in the transverse plane, either at the radial groove of the posterior humerus or deeper at the brachioradialis muscle. More distal scanning shows the bifurcation of the radial nerve into superficial and deep components. The deep branch penetrates the supinator muscle and continues as the posterior interosseous nerve. Similar anatomic landmarks are used on MR imaging.
Sonography and MR imaging have been used to detect space-occupying lesions such as lipomas or ganglionic cysts [3] causing nerve entrapment. Similarly, sonography and MR imaging have been used to diagnose peripheral nerve abnormalities such as peripheral nerve sheath tumors [7]. We have found that sonography allows a detailed assessment of peripheral nerve continuity with a mass, which indicates an intrinsic nerve abnormality rather than an adjacent extrinsic mass. The differential diagnosis of peripheral nerve enlargement includes peripheral nerve sheath tumors, trauma, and nerve compression. We have shown the effectiveness of sonography in the detection of a fibrous band compressing the posterior interosseous nerve, resulting in a proximal swelling of the nerve. We believe that sonography, with its wide availability, high spatial resolution, lack of ionizing radiation, control of the imaging plane of orientation, and easy applicability, should be the first imaging modality in the investigation of possible posterior interosseous nerve syndrome.
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