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Anatomical study of the brachial plexus using surface ultrasound.

SUMMARY

The aim of this study was to define the anatomy relevant to brachial plexus regional anaesthesia and to identify the extent of variation between individuals. Surface ultrasound examination of the brachial plexus was performed on twenty volunteers. In the axilla there was considerable individual variation in the location of the median, radial and ulnar nerves in relation to the axillary artery. There was often more than one venous structure in this region, which was easily compressed by surface palpation. In the supraclavicular region, neural elements were located inferiorly to the subclavian artery in two volunteers. In one volunteer, a vein was identified between nerve trunks in the interscalene region. These findings indicate that the anatomical variation is considerable, even within the relatively small sample studied. For this reason, use of surface ultrasound may lead to increased success of brachial plexus regional anaesthesia and a decreased risk of intravascular injection.

Key Words: brachial plexus, regional anaesthesia, ultrasound, nerve block, interscalene, axillary

Ultrasound has been used in conjunction with upper limb anaesthesia as early as 1989. Ting and colleagues (1) used a 3.5 MHz transducer to determine the spread of local anaesthetic agent during axillary brachial plexus anaesthesia. This transducer had limited image resolution which made the identification of nerves difficult to distinguish from muscle. Since then, the development of high frequency transducers (10 to 15 MHz) has enabled discrete visualization of nerve bundles, enabling them to be separated from muscle and blood vessels. In the short axis view, nerves have been described as having a hyperechogenic circular or oval rim on cross-section imaging (2), as shown in Figure 1. The interior of the nerve however, appears hypoechoic and has a granular or speckled appearance. Imaging in the long axis shows nerves as strongly reflective parallel linear tubular structures. Appearance can vary and depends on the ultrasound frequency used, the angle of ultrasound beam, and the size of the nerve.

For regional anaesthesia of the upper limb, ultrasound guidance enables needle placement to be identified in real time and positioned next to target nerves (3). Identification of optimal routes of needle insertion, together with direct visualization of needle advancement may prevent vascular puncture or neural contact with needle tips. In comparison to the blind needle insertion required using surface landmark techniques (4), ultrasound guidance may improve accuracy of needle placement, and potentially reduce the incidence of neuropraxia arising from trauma to nerves (2,3). Other complications such as intravascular injection, haematoma from vascular puncture, pneumothorax, and subarachnoid injection may also be reduced (3,5).

[FIGURE 1 OMITTED]

Ultrasound is a non-invasive technique which can show the relationship of nerves to surrounding structures in the living subject without morbidity. It is valuable as a teaching aid because it illustrates individual anatomy in the living subject and continually reinforces the anatomical relationships used when performing nerve blocks.

The aims of this study were to investigate the anatomical relationship of nerves of the brachial plexus using surface ultrasound, to identify variation between individuals and to apply these findings to regional anaesthesia of the upper limb.

METHODS

The study was approved by the Human Research Ethics Committee of the University of Melbourne. Following informed written consent, surface ultrasound examination of the brachial plexus was performed in twenty volunteers.

Subjects were positioned on an examination couch in the supine position. Anatomical surface landmarks of the neck were identified and marked using a surgical marking pen. Measurements of surface landmark positions were taken as shown in Figure 2. The interscalene groove was palpated at the level of the C6 vertebra and an 'ideal' point for needle insertion for the interscalene approach was marked on the skin.

[FIGURE 2 OMITTED]

Ultrasound coupling gel was applied to the skin surface and the ultrasound probe placed in contact with the skin using gentle pressure. A 15MHz transducer (15-6L probe) coupled to a Sonos 5500 machine (Phillips Medical Systems, Andova, U.S.A.) was used.

Sonographic examination began by placing the probe lateral to the larynx and identifying the thyroid gland, internal jugular vein, carotid artery and vagus nerve before moving further laterally to the posterior triangle of the neck. Neural elements were identified adjacent to the scalene muscles, and the probe was moved downwards in a para-sagittal plane towards the clavicle. A series of seven ultrasound short-axis views at 2 cm intervals were taken, starting at the level of the C6 vertebra, and extending to the supraclavicular and infraclavicular regions (Figure 3).

In the supraclavicular fossa the superior, middle and inferior primary trunks divide into their anterior and posterior branches. The neural elements often appear as a cluster of nodules adjacent to the subclavian artery. Colour flow Doppler can be used to help distinguish neural elements from arterial and venous branches, in particular, suprascapular and transverse cervical branches. The subclavian vein lies anteriorly, separated from the artery by anterior scalenus as it inserts into the first rib at the tubercle of Lisfranc. The omohyoid muscle lies superficially and the strong reflective signal of the first rib is noted inferiorly.

[FIGURE 3 OMITTED]

Beneath the clavicle, the proximal infraclavicular neurovascular bundle comprising axillary vessels and plexus is identified in relation to the pleura. The probe was placed below the midpoint of the clavicle overlying pectoralis major. Colour flow Doppler was used to identify the axillary artery and also its thoracoacromial branch in this area. The most medial structure in the neurovascular bundle is the axillary vein formed by the confluence of the brachial veins. The axillary artery is positioned laterally with the plexus cords arranged around it. The medial cord often lies between the two vessels. Laterally, pectoralis minor inserts onto the coracoid process, and the neurovascular bundle can be found lying posterior to the muscle and inferior and medial to the coracoid process. The pectoralis major appears thick, and overlies pectoralis minor, with a strongly reflective hyperechogenic perimysium separating the two muscles.

The axilla was then examined with arm abducted and forearm flexed. Before sonography was performed, the bicipital groove between biceps and triceps muscles was identified and marked. The axillary artery continues into the axilla and passes along this sulcus. The superior wall of the sulcus is formed by both biceps and coracobrachialis. Starting at the lateral edge of pectoralis major, transverse images of the axillary neurovascular bundle were obtained at three levels as the probe was moved distally (Figure 3).

With all axillary images, the biceps muscle was oriented onto the left side of the image and skin surface made parallel to the top of the screen. The location of the vascular structures was confirmed using colour flow Doppler imaging. Ultrasound images were then stored onto magneto-optical disc for off-line analysis.

For each ultrasound image, the following measurements were taken:

1. depth of the neural sheath (scan 1-3).

2. depth of the neural elements (scan 1-3).

3. relationship between neural elements and vascular structures.

4. For the axillary images, the positions of the median, ulnar and radial nerves were analysed using a quadrant template arranged around the axillary artery (6). This template was divided into eight equal sections and centred with the middle of the axillary artery.

Examples of images for the interscalene, supraclavicular, and axillary regions, with the superimposed template are shown in Figure 4.

[FIGURE 4 OMITTED]

Statistical Analysis

Data are presented as mean [+ or -] standard deviation, and calculated using SPSS version 10.0.5 statistical software.

RESULTS

The sample population (17 males and 3 females) had a mean age of 23[+ or -]9 years, and body mass index of 23[+ or -]3.

Interscalene Region

Surface anatomical landmark data are shown in Table 1. The size and depth data for structures visualized with ultrasound are shown in Table 2. At the level of C6, the brachial plexus lies within the interscalene space beneath the external jugular vein and the sternocleidomastoid muscle. A chain of three discrete hypoechoic nodules (upper, middle and lower trunks) were located between the anterior and medial scalene muscles. Ultrasound showed this fatty tissue space to be a narrow crevice of 4.0[+ or -]1.1 mm width and depth of 11[+ or -]2.5 mm to the third plexus trunk. The trunks of the plexus had an average diameter of 4.0[+ or -]1.1 mm. The subcutaneous groove is formed by the two scalene muscle body humps producing a characteristic double hump or 'seagull sign' on sonogram (see Figure 1). This is relatively superficial being only 8.0[+ or -]2.6 mm below the skin surface. The upper trunk was on average only 10[+ or -]3.8 mm below the skin surface. In some subjects, more than three neural elements were identified, indicating component nerve roots were identified instead of single trunks. In one subject, a vein was identified between the scalenus anterior and scalenus medius muscles amongst the trunks (see Figure 5). The vein was differentiated from nerve trunks by being fully compressible with light pressure. Deep and medial to the trunks, the acoustic shadow of the transverse process of C6 may be seen. With small movement of the probe up and down the neck, the vertebral artery and vein may also be identified anterior to this bony process. Nerve roots lie posterior to the vertebral vessels. The plexus sheath cannot be identified as a distinctive structure, but its position is inferred from identification of aponeurosis investing adjacent muscles.

The Supraclavicular Region

In this study, neural elements were arranged into one of four major patterns with reference to the subclavian artery. Most neural elements were clustered cephalo-posterior to the subclavian artery (85%), whilst some were directly superior to the subclavian artery (5%). In 10% of patients the inferior trunk was located inferiorly to the subclavian artery, and in the remainder, nerves were arranged unclustered around the subclavian artery (5%).

Infraclavicular Region

Beneath the clavicle, plexus cords are distributed around the axillary artery and are positioned medially, laterally and posteriorly around the vessel. The pectoralis major appears thick, and overlies pectoralis minor, with a strongly reflective hyperechogenic perimysium often separating the two muscles. The plexus was relatively superficial, lying 3 to 5 cm beneath the skin, but became deeper as it progressed distally. In our subjects, the mean distance from the jugular notch where the plexus travelled beneath the clavicle was 99[+ or -]12 mm, which was more lateral than the midpoint of the clavicle (mean length 166[+ or -] 1.3 mm).

Axillary Region

Hypoechoic nodules were identified surrounding the axillary artery. These represented the median, radial and ulnar nerves which were distributed around the artery and shown schematically in Figure 6. Although the nerves were broadly clustered in the same anatomical quadrants, considerable variation in location was seen between individuals. The median nerve was positioned most often in the supero-anterior position, the ulnar nerve infero-posterior, and radial nerve posterior, though the distribution of spread was across a number of sectors. These positions altered slightly as the plexus travelled distally. The musculocutaneous nerve, positioned within the body of coracobrachialis, was not consistently seen. The position of the axillary vein was variable and often two easily compressible venous structures were identified.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

DISCUSSION

This study illustrates the ultrasound appearance of the brachial plexus in a small group of volunteers. Simultaneous measurement of surface landmarks indicates there is variation when locating the subcutaneous position of the brachial plexus. Such variations may lead to failure when performing brachial plexus anaesthesia. Secondly, variation in the position of veins as seen in the axilla, and in the case of an aberrant vein being seen in the interscalene space can lead to an increased risk of intravascular injection of local anaesthetic. Our finding of a vein located between the middle and lower trunks has not to our knowledge been previously described.

Importantly, use of surface palpation when performing nerve blocks not only distorts underlying anatomy but also compresses veins, which may mask intravascular placement. This highlights the need to test for intravascular needle placement by frequent aspiration when delivering agent in titrated doses, particularly without any manual pressure on the skin surface.

Light surface pressure also causes neural elements to 'slide' within the sheath (6). The clinical importance of this observation relates to axillary brachial plexus blockade, which is based on locating the axillary artery by palpation. Practitioners may palpate the axillary artery as well as slide the neural elements against the vessel to identify their position more accurately. Ultrasound showed that pressure exerted during palpation may distort the anatomical location of the nerves and relationship to vessels. This could cause the neural elements to be displaced sufficiently from their original location and be missed by the injection of the local anaesthetic.

The ideal surface landmark marked on the skin was often found to vary with ultrasound determination of the ideal surface landmark position. In some cases this lay beneath sternocleidomastoid muscle indicating that landmark positions are not always constant. Similarly, the midpoint of the clavicle has traditionally been used as a reference for infraclavicular puncture, and recently the mid-point of the distance between acromion and jugular notch has been advocated as a more accurate landmark for the vertical infraclavicular approach (7). However in a study by Greher and colleagues', the ultrasound determined landmark was inconsistent, illustrating further that variation can be significant between individuals.

Our use of ultrasound to interrogate the brachial plexus compares well with studies performed by Yang and co-workeer (2), and Retzl and co-workers (6). Our study found that the upper, middle and lower nerve trunks remained together in the interscalene region. The spread of the neural elements at the two levels recorded were no different from one another. This implies that anaesthesia may be adequately performed at a range of levels within the interscalene groove, rather than the classical position at the level of the C6 vertebra.

When performing an interscalene block, the dermatomes of the lower trunk may be missed (ulnar nerve, medial cutaneous nerve of forearm and medial cutaneous nerve of arm). The lower trunk of the brachial plexus has been described as being located behind the subclavian artery and this anatomical configuration is postulated to be responsible for ulnar sparing(8). In 10% of our subjects, elements of the lower trunk were found inferior to the subclavian artery.

Our study shows similar results to those of Retzl and co-workers (6) with the distribution of peripheral nerves around the axillary artery in the axilla. Ultrasound was unable to identify the neurovascular sheath in the axillary region or identify the presence of septa or compartments within the sheath that could contribute to the uneven distribution of local anaesthetic within the sheath. Ultrasound-guided brachial plexus anaesthesia, though, can identify maldistribution of local anaesthetic during injection, because the sheath can be seen to swell and nerves become surrounded by agent(9). The needle can be redirected during the procedure to facilitate even spread within the sheath and specifically target individual nerves.

Our study has several important limitations. We performed an anatomical study, rather than an interventional study, and we are unable to draw conclusions as to whether the use of ultrasound will improve the efficacy and safety of brachial plexus anaesthesia. Other studies have found improved success rate or facilitation of brachial plexus anaesthesia using ultrasound, but most are observational studies rather than randomized trials' (4,10-12). Furthermore, our population was comprised mostly of young healthy volunteers with a low body mass index. This is due to the study being conducted at the university by a medical student researcher, with most of the volunteers known to him. This biased the sample to a young and healthy population. It is possible that anatomical variation, especially in relation to the depth from the skin to nerve bundles, may be different in an older or more obese patient population. The study was descriptive and twenty volunteers were selected to provide a reasonable range of anatomical variation. A larger study may have found even greater variation.

The brachial plexus is relatively superficial in the supraclavicular and axillary regions (depth 1 to 2 cm), and this requires the use of high resolution and high frequency systems such as a 10 to 15 MHz probe to obtain quality imaging. In the infraclavicular region or in obese patients, the plexus is situated deeper and a lower frequency probe enabling greater depth of penetration may be appropriate.

Ultrasound can be used to identify the anatomy relevant to brachial plexus regional anaesthesia, and identify variations that could either reduce the efficacy of the block or increase the risk of intravascular injection. It is a valuable teaching aid in applying regional techniques to living anatomy.

ACKNOWLEDGEMENTS

We thank Philips Medical Systems Australia for the loan of echocardiography equipment.

Accepted for publication on November 11, 2005.

REFERENCES

(1.) Ting PL, Sivagnanaratnam V Ultrasonographic study of the spread of local anaesthetic during axillary brachial plexus block. Br J Anaesth 1989; 63:326-329.

(2.) Yang WT, Chui PT, Metreweli C. Anatomy of the normal brachial plexus revealed by sonography and the role of sonographic guidance in anesthesia of the brachial plexus. AJR Am J Roentgenol 1998; 171:1631-1636.

(3.) Ootaki C Hayashi H, Amano M. Ultrasound-guided infraclavicular brachial plexus block: an alternative technique to anatomical landmark-guided approaches. Reg Anesth Pain Med 2000; 25:600-604.

(4.) Greher M, Retzl G, Niel P et al. Ultrasonographic assessment of topographic anatomy in volunteers suggests a modification of the infraclavicular vertical brachial plexus block. Br J Anaesth 2002; 88:632-636.

(5.) Kirchmair L, Entner T, Wissel J et al. A study of the paravertebral anatomy for ultrasound-guided posterior lumbar plexus block. Anesth Analg 2001; 93:477-481.

(6.) Retzl G, Kapral S, Greher M, Mauritz W Ultrasonographic findings of the axillary part of the brachial plexus. Anesth Analg 2001; 92:1271-1275.

(7.) Kilka HG, Geiger P, Mehrkens HH. [Infraclavicular vertical brachial plexus blockade. A new method for anesthesia of the upper extremity. An anatomical and clinical study.] Anaesthesist 1995; 44:339-344.

(8.) Lanz E, Theiss D, Jankovic D. The extent of blockade following various techniques of brachial plexus block. Anesth Analg 1983; 62:55-58.

(9.) Kapral S, Krafft P, Eibenberger K et al. Ultrasound-guided supraclavicular approach for regional anesthesia of the brachial plexus. Anesth Analg 1994; 78:507-513.

(10.) Chan VW, Perlas A, Rawson R, Odukoya O. Ultrasound-guided supraclavicular brachial plexus block. Anesth Analg 2003; 97:1514-1517.

(11.) Marhofer P, Schrogendorfer K, Koinig H et al. Ultrasonographic guidance improves sensory block and onset time of three-in-one blocks. Anesth Analg 1997; 85:854-857.

(12.) Marhofer P, Sitzwohl C Greher M, Kapral S. Ultrasound guidance for infraclavicular brachial plexus anaesthesia in children. Anaesthesia 2004; 59:642-646.

C. F. ROYSE *, S. SHA ([dagger]), P. F. SOEDING ([double dagger]), A.G. ROYSE ([section])

Cardiovascular Therapeutics Unit, Departments of Pharmacology and Anaesthesia and Pain Management, Royal Melbourne Hospital and University of Melbourne, Victoria, Australia

* M.B., B.S., F.A.N.Z.C.A., Associate Professor, Cardiovascular Therapeutics Unit, Department of Pharmacology, University of Melbourne, and Consultant Anaesthetist, Department of Anaesthesia and Pain Management, The Royal Melbourne Hospital.

([dagger]) B.Med.Sci., Advanced Medical Science student, Department of Pharmacology, University of Melbourne.

([double dagger]) M.B., B.S., F.A.N.Z.C.A., Senior Fellow, Cardiovascular Therapeutics Unit, Department of Pharmacology, University of Melbourne, and Consultant Anaesthetist, Department of Anaesthesia and Pain Management, The Royal Melbourne Hospital.

([section]) M.B., B.S., ER.A.C.S., Associate Professor, Cardiovascular Therapeutics Unit, Department of Pharmacology, University of Melbourne.

Address for reprints: A/Prof. Colin Royse, Cardiovascular Therapeutics Unit, Department of Pharmacology, Level 8, Medical Sciences Building, University of Melbourne, Victoria, Australia 3010.
TABLE 1
Surface anatomical landmark measurements for the interscalene
region

 Standard
Measurement Mean (mm) deviation (mm)

Length of clavicle (a) 166 13
Distance between jugular notch
 and ISG (b) 99 12
Distance between insertion point
 to clavicle (c) 58 9
Distance between insertion point
 to SCM (d) 13 8
Length of SCM (e) 167 17
Height of cricoid cartilage (f) 62 10

ISG = interscalene groove, SCM = sternocleidomastoid.

TABLE 2
Measurements obtained from the ultrasound images

 Standard
Measurement Mean (mm) deviation (mm)

Depth of neural sheath on scan 1 8 5.5
Depth of neural sheath on scan 2 8 2.6
Depth of neural sheath on scan 3 10 4.6
Depth of upper trunk on scan 1 10 3.8
Depth of upper trunk on scan 2 9 2.8
Depth of upper trunk on scan 3 11 5.0
Width of trunks scan 1 4 0.5
Width of trunks scan 2 4 1.1
Spread of trunks scan 1 11 2.5
Spread of trunks scan 2 12 2.3
Width of axillary artery scan 5 5 0.6
Width of axillary artery scan 6 5 0.5
Width of axillary artery scan 7 5 0.5

Spread of trunks = distance between top of upper trunk, and bottom
of lower trunk.
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Author:Royse, C.F.; Sha, S.; Soeding, P.F.; Royse, A.G.
Publication:Anaesthesia and Intensive Care
Date:Apr 1, 2006
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