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Brachial Plexus Sonography: A Technique for Assessing the Root Level

¹ Cattedra di Radiologia "R," DICMI, Università di Genova, Largo Rosanna Benzi, 8, I-16132 Genova, Italy.
² Division de Radiodiagnostic, Hôpital Cantonal Huniversitaire, Rue Micheli du Crest, Geneve, Switzerland.
³ Department of Radiology, The Tel Aviv Ichilov, Sourasky Medical Center, Tel Aviv, Israel.

Received January 9, 2002; accepted after revision February 20, 2002.

 
Address correspondence to C. Martinoli.

Abstract

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OBJECTIVE. Our study was intended to establish a technique to assess the level of the roots of the brachial plexus using high-resolution sonography.

MATERIALS AND METHODS. The skeleton of a cervical spine was examined in vitro to determine whether the vertebrae may be identified individually on sonography by means of the evaluation of their transverse processes. Then 20 healthy subjects and five patients who had undergone CT of the cervical spine were evaluated sonographically, and we attempted to identify the level of individual roots of the brachial plexus using the transverse processes as landmarks. To establish the reliability of this method, a blinded review of sonograms of the paravertebral area obtained at various levels was performed independently by three examiners.

RESULTS. In vitro, sonography was reliable in depicting the level of the C7 vertebra because of the absence of the anterior tubercle from its transverse processes. In healthy subjects, this feature allowed us to establish the level of the roots outside the spine. In our series, the C4-C7 roots were visible sonographically in all cases, whereas the C8 and T1 levels were seen, respectively, in only 16 of 20 and eight of 20 cases. All examiners correctly identified the C7 level in the blinded review of sonograms.

CONCLUSION. High-resolution sonography can reveal the level of the roots of the brachial plexus on the basis of the different morphology of the transverse processes of the vertebrae. Our study has implications for confirming the exact level of pathologic roots before surgery.

Introduction

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High-resolution sonography is capable of imaging brachial plexus nerves [1,2,3,4]. Although this technique has some limitations when compared with MR imaging, including the inability to track the roots inside the foramina or behind the clavicle, sonography provides excellent nerve depiction and may be helpful in guiding percutaneous anesthesia maneuvers to the brachial plexus [1]. This modality is also useful in evaluating patients with brachial plexopathies (Graif M et al., presented at the Radiological Society of North America meeting, November 2000; Bodner G et al., presented at the Radiological Society of North America meeting, November 2001).

In brachial plexopathies, the accurate mapping of the lesion is as important as a specific diagnosis, especially in the setting of acute trauma, when prognosis and treatment strategy depend on accurate preoperative location of the level of injury [5, 6]. To our knowledge, no prior studies have been attempted using sonography to classify the roots of the brachial plexus according to their level of origin. In undertaking this study, we theorized that the different morphology of the transverse processes of the vertebrae might prove helpful. Accordingly, our aim was to develop a sonographic technique based on evaluation of such bony landmarks to identify the roots individually as they exit the neural foramina.

Materials and Methods

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The first part of our study was designed to determine whether the cervical vertebrae could be identified individually on sonography by the evaluation of their transverse processes. A bony specimen of human spine extending from the atlas to the T2 vertebra, obtained from the anatomic museum of our university, was examined in vitro using a broadband 5-12-MHz linear array sonographic transducer (HDI 5000; Advanced Technology Laboratories, Bothell, WA). The vertebrae were kept in anatomic position by metallic wires. The spine model was placed in a dish with its left lateral aspect up. Then it was submerged in a jelly preparation of low to intermediate echogenicity that provided a tissue-mimicking environment to allow sonographic exploration of the bony surfaces without interposition of air.

Axial sonograms were obtained throughout the spine in an attempt to image the transverse processes of the vertebrae. The anterior tubercle of the transverse process, which is selectively absent in the C7 vertebra, was considered the key landmark to distinguish this vertebra from the others (Fig. 1). With this landmark, an examiner who was unaware of the orientation of the spine within the phantom was asked to insert a 22-gauge needle under sonographic guidance to label C7. The needle was infixed deeply into the bone; then the jelly was removed by flushing the specimen with water, and the position of the infixed needle was evaluated. This procedure was repeated by alternating three examiners, whereas a different author was always involved in the preparation of the model.

View larger version (111K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Photograph of cervical spine specimen shows typical appearance of transverse processes, which exhibit prominent anterior (asterisks) and posterior (stars) tubercles. Note absence of anterior tubercle at C7 level and flat lateral aspect of T1 without bony prominence.

The second part of our study involved 20 healthy subjects (10 female and 10 male; age range, 12-48 years; mean age, 35 years) and five consecutive patients (one woman and four men; age range, 24-43 years; mean age, 31 years) who had undergone helical CT of the C6 and C7 vertebral levels because of suspected degenerative disk disease. All 25 underwent sonographic examination of the neck using the same equipment as was used in the in vitro experiment with the aim of determining whether the different appearance of C7 could be regarded as a reliable reference for assessing the level of origin of the roots of the brachial plexus. The paravertebral area was examined with the patient seated and the head either in neutral position or turned to the opposite side.

During scanning, the thyroid served as the initial reference from which the probe moved laterally to the region of the transverse processes. Although axial and longitudinal scans were obtained at the lateral aspect of the vertebrae to image the bony structures abutting the roots, only the axial planes proved to be effective in depicting the relation between the roots and the tubercles. Color Doppler sonography was used initially to avoid confusion between roots and vertebral vessels but, as we gained confidence with the identification of nerves near the foraminal outlets, using color Doppler sonography rapidly became unnecessary. Then 10 sonograms showing the transverse processes in an axial plane were obtained at different levels in healthy subjects; only one image referred to the C7 vertebra. The three examiners who performed the in vitro experiment were asked to select the sonogram obtained at the C7 level. Each examiner was unaware of the results achieved by the others; final data were checked for accuracy by the author who had performed the sonography.

Results

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Sonographic examination obtained in vitro along the short axis of the spine was suitable to identify the transverse processes of the cervical vertebrae that projected laterally from the junctions of the pedicles and laminae on the basis of their typical shape. When the probe was swept on axial planes along the cervical spine, these processes appeared as hyperechoic bony prominences at the apex of which the anterior and posterior tubercles could be reliably distinguished (Fig. 2A). At the C7 level, absence of the anterior tubercle was recognized because of the different configuration of its transverse process, which exhibited a larger posterior tubercle ending in a smooth down-sloping contour anteriorly (Fig. 2B). Using only this feature, all examiners readily distinguished the C7 level from those of the upper vertebrae and were able to guide the needle correctly on it.

View larger version (168K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —Phantom containing spine shown in Figure 1. Axial in vitro 5-12—MHz sonogram of C6 vertebra shows two hyperechoic bony processes on its lateral aspect representing anterior (asterisk) and posterior (star) tubercles.

View larger version (166K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —Phantom containing spine shown in Figure 1. Axial in vitro 5-12—MHz sonogram of C7 vertebra shows only one process (star) referred to posterior tubercle.

The transverse processes could be recognized as confidently in vivo as they were in the in vitro experiment because of the strong reflectivity of their bony outlines (Fig. 3). To assess the level of individual roots, scanning first identified the C7 level and then moved either upward or downward on axial planes. The C7 root could be identified on the same plane used for the C7 vertebra as a hyperechoic structure bordered by the posterior tubercle only (Fig. 4A). When the transducer was shifted upward, the C6 vertebra could be recognized by the presence of prominent anterior and posterior tubercles; the C6 root appeared as a hypoechoic structure held between them (Fig. 4C). One-to-one comparison of sonograms and CT scans in the group of five patients confirmed these differential features (Figs. 4B and 4D).

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —25-year-old healthy man. Axial 5-12—MHz sonogram reveals relationship of C6 vertebra with soft-tissue structures of lateral neck including thyroid (T), sternocleidomastoid muscle (S), common carotid artery (CCA) and internal jugular vein (IJV). C6 vertebra shows hyperechoic contour, which delineates vertebral body (B), pedicles (P), and anterior (asterisk) and posterior (star) tubercles. C6 root (arrow) appears as hypoechoic image contained between tubercles.

View larger version (164K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —Lateral neck of 38-year-old man. Axial 5-12—MHz sonogram at C7 vertebral level shows C7 root (arrow) between vertebral artery (arrowhead) and posterior tubercle (star). Anterior tubercle is absent.

View larger version (164K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4C. —Lateral neck of 38-year-old man. Axial 5-12—MHz sonogram at C6 vertebral level reveals C6 root (arrow) and vertebral artery (arrowhead) between anterior (asterisk) and posterior (star) tubercles.

View larger version (139K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —Lateral neck of 38-year-old man. CT scan at C7 vertebral level corresponding to A confirms absence of anterior tubercle. Note C7 root (arrow) and vertebral artery (arrowhead) running anterior to posterior tubercle (star).

View larger version (150K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4D. —Lateral neck of 38-year-old man. CT scan at C6 vertebral level corresponding to C shows both anterior (asterisk) and posterior (star) tubercles. C6 root (arrow) is located between tubercles.

Sonographically, the transverse processes of C5 and C4 had basically the same shape as seen at C6; they could be identified as successive steps cranial to the C6 level by counting the number of transverse processes encountered while sweeping the transducer cranially from C7. The C4-C7 levels were recognized in all cases. More cranially, the C3 level was visualized in 16 of 20 cases and the C2 level, in 10 of 20 cases. As a rule, the higher the level, the closer the intervening space between the tubercles. When the transducer was moved downward from C7, the lateral aspect of the T1 vertebra appeared flat and showed no tubercle; at this level, the C8 root was identified near the foraminal outlet in 16 of 20 cases. More caudally, the T1 nerve was visible in only eight of 20 cases, because the location of the intervertebral foramen deep between the T1 and T2 vertebra created problems of access. The T1 root showed a curving course below the first rib and could be examined using an axial oblique plane of approximately 45°. The retrospective blinded review of the sonograms resulted in a correct identification of the C7 level by all examiners.

Discussion

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The brachial plexus is composed of the anterior divisions of the spinal nerve roots of C5 down to T1 with a minor contribution from C4 [7]. The roots combine to form the trunks, divisions, cords, and distal branches that supply motor and sensory innervation to the upper extremity. The anatomic progression of the nerves is complex, and the nonspecific symptoms often associated with a brachial plexopathy can make proper localization of the lesion troublesome because different sites of disease can produce similar signs [8].

MR imaging is currently the technique of choice for imaging the brachial plexus because of its multiplanar capabilities and excellent soft-tissue contrast [8, 9]. Recently, high-resolution sonography has proven to be effective in depicting normal brachial plexus anatomy at several levels, including the paravertebral, interscalenic, supraclavicular, and infraclavicular areas [1,2,3,4]. Using this technique, the nerves of the plexus can be reliably delineated as hypoechoic structures that are tubular on longitudinal scans and rounded on transverse scans. However, no definite criteria have been advocated for attributing a precise location to these nerves with respect to their order as they exit from the intervertebral foramina.

It is important to determine whether a lesion is preganglionic rather than postganglionic, to recognize whether it is infraclavicular rather than supraclavicular. Because the list of possible clinical syndromes in a patient with brachial plexopathy is different according to the pattern of the injured roots and trunks (e.g., upper: C5 and C6, associated or not with C7; lower: C8, T1; complete: C5-T1), it is important to attribute an abnormality to a given level of nerve involvement [5]. Electrodiagnostic studies can help to define the extent of brachial plexopathies, but they have several limitations and may be used only as supplement modalities when selecting patients for surgery [6]. Therefore, a confident determination of the extent of a lesion based on imaging findings may help clinicians to make treatment decisions.

Anatomically, the transverse processes project from the junctions of the vertebral pedicles and laminae and primarily act as attachments for muscles. Each process has a U shape: two prominent bony tubercles, anterior and posterior, form its walls, and a thin lamina forms its floor [7]. This lamina is pierced by the vertebral artery, which ascends through the foramina transversaria of C6 to C3. This configuration is repeated uniformly from C2 down to C6, but C7 appears different. Because the C7 vertebra represents a transition between the cervical and thoracic configurations, it has a larger transverse process in which the posterior tubercle is larger and more prominent and the anterior tubercle is absent or rudimentary. We have found no statistical studies in the literature that report the prevalence of the absent anterior tubercle at the C7 level, but this finding is constant and can be considered a reliable anatomic landmark for identification of C7 [10]. More caudally, the T1 vertebra differs from the cervical vertebrae because its transverse processes are directed more posteriorly to articulate with the head of the first rib. The rib overlaps the vertebra immediately inferior to the foramen, thus giving a flat and smooth appearance to its lateral aspect. As the nerve roots leave the spine, they cross the groove between the tubercles. Each root leaves the intervertebral foramen sliding on the transverse process of its corresponding vertebral level [7]. Because there are eight cervical nerves and only seven cervical vertebrae, the C8 root lies at the level of the T1 vertebra, and so forth.

Although sonography is generally not suitable for bone assessment, it can readily distinguish the profile of cortical bone as a regular hyperechoic line with posterior acoustic shadowing. This finding is related to the strong reflection of the ultrasound beam at the bone—soft-tissue interface, and it allows delineation of the contour of most superficial bones in the extremities. Furthermore, subtle bony abnormalities such as undisplaced, even radiographically occult, fractures can be diagnosed on the basis of detection of focal defects or interruptions of this cortical outline [11, 12]. In our study, the shape of the cervical vertebrae was easily delineated at sonography and allowed detection of some differences in the configuration of the transverse processes that could lead to a precise assessment of the level of the roots. We used the transverse process of C7 as the main landmark for identification of the roots on axial scans, because this level could be examined in all subjects without difficulty, and because its different shape could readily be compared with the adjacent process of C6 by simply moving the transducer slightly upward. The assessment of C7 was easy to perform, and all examiners involved in our study learned this method in a short time.

The T1 vertebra could be selected as a landmark of the C8 root because of the absence of both tubercles; however, we did not regard it for this purpose because this level was too caudad and deep, especially in subjects with thick, short necks, and therefore it was less easy to scan. The nerve roots have a monofascicular structure [13] and can reliably be visualized as homogeneously rounded or oval hypoechoic images—a different appearance from that of peripheral nerves in the extremities, which typically are seen as clusters of hypoechoic fascicles embedded in a hyperechoic background [14]. This latter appearance can be recognized downstream, at the level of trunks and proximal cords. The monofascicular appearance of the roots worked well for our purposes because it helped us to refer a definite nerve level to a single hypoechoic structure, rather than to a cluster of hypoechoic fascicles.

One advantage of sonography in brachial plexus imaging is that each component of the plexus can be followed continuously through the lateral neck by shifting the probe back and forth in an axial plane. The progression of the roots is anatomically constant down to the interscalenic area, where they make a first union to form the three trunks—upper (C5 and C6), middle (C7), and lower (C8 and T1) [7]. Therefore, it is conceivable that the capability of this technique to depict the root levels in the paravertebral area may also enable sonologists to make a more confident identification of the trunks by simply following the nerves from where the trunks arise. An individual identification of divisions and cords of the brachial plexus distal to the interscalenic area seems less feasible on sonography because these branches anastomose with each other in various combinations.

In conclusion, we believe that a method of assessing the level of the roots of the brachial plexus with high-resolution sonography could improve the accuracy of this modality and provide additional information in cases of brachial plexopathies. This information would be especially useful in outlining a treatment plan for patients in whom clinical and neurophysiologic features are inconclusive and for whom the primary imaging modalities, including CT and MR imaging, do not aid in lesion localization. Although we do not think that sonography will replace MR imaging as the primary diagnostic tool in this field, we believe its capability to depict the nerves of the plexus outside the spine and to allow identification of their proper level of origin will increase the use of sonography as a complementary technique. Further work in series of patients with brachial plexopathies is necessary to analyze the sensitivity and specificity of this method.

References

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