Topographic Sonoanatomy of Infraclavicular Brachial Plexus: Variability and Correlation with Anthropometry.

OBJECTIVE: The aim of the study was to assess the effectiveness of ultrasound in analyzing topographic relationship of nerve cords with axillary artery at lateral infraclavicular level, their variations, and the distance from the skin and to correlate findings with anthropometric parameters.
MATERIALS AND METHODS: Two hundred patients aged 18-75 years were enrolled for the study after informed written consent. A 7-12 MHz linear ultrasonic transducer was used for scanning of the brachial plexus at lateral infraclavicular fossa. The results of the cord positions were expressed on a 12-section pie chart, and the number of arteries and veins was reported. Measurements included the vertical distance from the upper part of the artery to the skin, diagonal distance to the apical corner of the ultrasound image, and distances from center of cords to the center of artery. Age, sex, weight, height, body mass index, and biceps girth were recorded. Data were expressed as mean with standard deviation or frequency and percentage for categorical variables, and statistical analysis was done using correlation analysis and two-sample t-test.
RESULTS: The most frequent positions of the cords were observed in 2-4 sections (92%) for the medial cord, 6-7 sections (92%) for the posterior cord, 10-11 sections (89%) for the lateral cord, and 4-5 sections (95%) for the axillary vein. More than one axillary vein was seen in 12.5% and two axillary arteries were seen in 1.5% of cases. Cord visibility and distance between artery and apical corner of the ultrasound image correlated with anthropometric parameters.
CONCLUSIONS: A topographical study of brachial plexus at lateral infraclavicular fossa showed anatomical variations and abnormal vascular formations. This sonoanatomic knowledge can be helpful in improving safety and success of nerve blocks.

INTRODUCTION

Infraclavicular brachial plexus block (ICBPB) is a very useful technique for surgeries of hand, wrist, elbow, and distal arm. Advantage of ICBPB is that there is less likely hood of tourniquet pain, avoidance of neurovascular structures of neck, and ease of securing a continuous brachial plexus catheter.[1]

In the infraclavicular region, the cords of the brachial plexus are in close approximation to the second part of the axillary artery. During ICBPB, major vessels and pleura can be injured as they lie in close approximation to the plexus.[23]

Many anatomical variants of infraclavicular region vessels have been described in the literature such as a bifida axillary artery[4] and abnormality of the superficial and deep axillary artery.[56] This anatomical variation could cause an accidental puncture of vessel or a difficulty to find response in the blind nerve stimulation technique. The knowledge of cross-sectional view of anatomy is more useful for identification of neurovascular structures in contrast to usual longitudinal view. There is a paucity of literature on topographic pattern of brachial plexus in the infraclavicular fossa that is solely based on ultrasound imaging. For these reasons, we performed this study for determination of variations in brachial plexus at infraclavicular fossa and difficulties in sonographic view based on physical anthropometric characteristics.

The purpose of this study was to assess ultrasonic topography of locations of the nerve cords relative to axillary artery, their variations, and distance from the skin in the infraclavicular fossa and to correlate findings with anthropometric parameters.

MATERIALS AND METHODS

Following approval of the institutional ethics committee, this observational study took place on 200 patients at a single center. Patients in the age group of 18–75 years were included in the study. Purpose and methods of this study were explained to the patients, thereafter written consent was taken. Exclusion criteria were refusal to participate in the study at any part of the procedure, inability to communicate with doctor, difficulty in positioning, and previous injury to arm. The examination was performed by one of the two anesthesiologists who had experience of performing more than 50 ultrasound-guided infraclavicular blocks. The side of the body to be scanned was determined by randomization with coin toss.

Ultrasound-guided ICBPB is usually performed in lateral infraclavicular fossa with coracoid process as landmark. Axillary artery is easily identified here below pectoralis minor muscle where it is surrounded by three cords of the brachial plexus. The volunteers were placed in supine position with head turned away from the side of the scan. The ipsilateral arm was abducted to 90° and the elbow was flexed. Ultrasound imaging was done by Sonosite ultrasound instrument (Sonosite® Bothell, WA, USA). A 38 mm and 7–12 MHz linear ultrasonic transducer was used for the purpose. Coracoid process was identified and transducer was positioned in the parasagittal plane. Scanning was started just medial to the coracoid process and inferior to the clavicle. The transducer was placed in the parasagittal orientation and was then moved in cephalocaudal direction until axillary artery was identified deep to the pectoralis minor muscle. Axillary artery, vein, cords, and pleura were identified by adjusting the probe position. Scan with best identification of cords was recorded having artery in the center. Artery was distinguished from the veins by their lack of easy compressibility, more defined circular structure and pulsatility using real time ultrasonography. Cords were identified as singly dotted hypoechoic/hyperechoic structures or with distinctive honeycomb appearance. The cord positions around the axillary artery were recorded on a 12-section pie chart (numbered from 1 to 12; starting at 12 o' clock). The probe was positioned so that the axillary artery could be viewed at the center and the 3 o' clock direction may be placed in the medial direction and 9 o' clock direction in the lateral direction with respect to the axillary artery [Figure 1]. The section in which each cord is located was mapped on the pie chart on a transparent film. When a cord or vessel extended to two or more divisions, the nerve position was recorded on the division in which the greater portion of the nerve or vessel was included. Two experienced anesthesiologists independently assessed the number of identified cords and their relation with the axillary artery. Patients were enrolled in the study only when the analysis of the two anesthesiologists matched. The distribution of lateral, posterior, and medial cord around the artery was determined according to pie chart. The vertical distance from the upper part of the artery to the skin and diagonal distance to the apical corner of the ultrasound image was determined. Distances from center of cords to the center of artery were also determined. The presence of other venous or arterial structures was noted and mapped. After the compilation of results, the patients were divided in different groups on the basis of number of visible cords and distance of artery from skin and comparisons were done.

Figure 1

Schematic representation of cross-section ultrasound scan image with probe at infraclavicular fossa. The cord positions are shown as a 12-section pie chart radiating out from the central axis of axillary artery

Age, sex, weight, height, body mass index (BMI), and biceps girth of all patients were noted. The data were expressed as mean with standard deviation or frequency and percentage for categorical variables. Different parameters were compared using the linear Pearson correlation coefficient. Two-sample t-test was applied to observe the difference between two groups. P < 0.05 was taken as statistically significant. Analysis of the statistical data obtained from the study was carried out by statistical programming software Statistical Package for the Social Sciences-SPSS Statistics version 23.0.0 (SPSS Inc., Chicago, Illinois, USA).

RESULTS

We included 200 patients within an observation period of 6 months. The demographic data of these patients are shown in Table 1.

Table 1

Demographic profile of participants (n=200)

The average distance between the upper part of the artery and cutaneous surface and between upper part of the artery and corner of the ultrasound image was 2.29 ± 0.62 and 2.98 ± 0.52 cm, respectively.

The number of distinguishable cords was 3 in 129 patients, 2 in 47 patients, 1 in 20 patients, and 0 in 4 patients (64.5%, 23.5%, 10% and 2%, respectively). Lateral cord was visible in 90.5% of patients, medial in 87%, and posterior in 71%.

Anatomical variations in axillary vein and axillary artery were also seen [Figure 2]. Two axillary veins were visible in 9% of patients, three in 3.5%, and none in 1.5%. In rest of patients, single axillary vein was visible. Two axillary arteries were visible in three patients.

Figure 2

Ultrasound scan image showing axillary vascular variations at infraclavicular fossa. AA: Axillary artery, AV: Axillary vein

The distribution of cord positioning is shown in the 12 sections radiating from the axillary artery on the pie chart in Figure 3. The most common position (92%) of the medial cord was found in sections 2–4. The posterior cord was visible in sections 6 and 7 in 92% of cases. The lateral cord was visible in sections 10 and 11 in 89% of cases. Axillary vein was found mostly in sections 4 and 5 in 95% of cases [Figure 3].

Figure 3

Schematic representation of common arrangement of three cords and axillary vein around axillary artery. AA: Axillary artery, AV: Axillary vein, L: Lateral cord, M: Medial cord, P: Posterior cord

The mean distance from center of the axillary artery to medial cord, posterior cord, lateral cord, and axillary vein was 0.67 ± 0.14, 0.62 ± 0.10, 0.72 ± 0.16, and 1.38 ± 0.28 cm, respectively.

Anthropometric parameters (weight, BMI, and biceps girth) were having moderate positive significant correlation with number of visible cords while height weakly correlated with number of visible cords [Table 2].

Table 2

Anthropometric parameters in respect to the number of visible cords

Distance from artery to the apical corner of the ultrasound image signifies roughly to the minimum distance needed by the needle for the procedure. Weight (r = 0.818, P = 0.001), BMI (r = 0.815, P = 0.001), and biceps girth (r = 0.790, P = 0.001) strongly correlated with artery apical corner distance. The mean number of visible cords in patients having artery apical corner distance ≥ 4 cm and <4 cm was 1.63 ± 1.12 and 2.60 ± 0.65, respectively, and was significant [Table 3].

Table 3

Anthropometric parameters and visible cords in corner-artery distance of <4 and ≥4 cm

DISCUSSION

The ICBPB was developed and verified primarily in anatomical dissections of cadavers and then applied to clinical settings. The anatomical study of the brachial plexus may be different in living body due to conservation, changes in volume of blood vessels close to nerves, and removal of fat.[789] Similarly computed tomography and magnetic resonance imaging findings may also differ from actual positions because of imaging with arms alongside the body as this is not the ideal position for ICBPB.

Localization of brachial plexus can be done by means of surface landmarks, elicitation of paraesthesia, neurostimulation, or ultrasound guidance. Ultrasound has revolutionized the method of giving blocks by altering it from a blind to visible technique. By direct visualization of the needle tip, target nerve, and the spread of local anesthetic as it is injected, ultrasound can increase the efficacy of the block.[10] Complications such as nerve injury and intravascular injection can still occur despite the use of ultrasound.[10] Moreover, sometimes, there is failure of blocks under ultrasound guidance also. This could be explained by limits of two-dimensional ultrasound imaging,[11] difficulties in visualization of needle tip,[12] or anatomical variability among individuals.[131415]

We observed the relationship between the positions of axillary artery, and the cords were similar to that found by Di Filippo et al.[16] They observed that the mean position of different cords related to the artery was 77° for medial, 316° for lateral, and 179° for posterior. Maximum positional variability was observed in medial cord which was similar to our findings. In their study, all the three distinguishable cords were seen only in 36% of the patients while three cords were visible in 64.5% patients in our study. This discrepancy may be due to the use of lower frequency (7.5 MHz) ultrasound probe used by them.

Posterior cord was visible only in 71% of the study population. This may be due to its position behind the vessels and greater distance from the skin as compared to other cords and more so in obese patients.

Abnormal vascular formations in the infraclavicular region are common. Two or more axillary veins were observed in 12.5% of the patients and two axillary arteries in 1.5%. These multiple vessels may interfere with the procedure and can cause accidental vascular injection.

Field visibility correlated with anthropometric characteristics and in patients with high BMI, and less number of cords was visible. Thus, it is suggested to use a mixed technique (ultrasound + nerve stimulator) in patients with higher BMI. We have also considered the distance between apical corner of the ultrasound image and axillary artery which corresponds to the minimal path for the needle for block. A distance of ≥4 cm will create technical difficulties with a 5-cm needle usually used for the procedure. This distance correlated with anthropometrical parameters and it is concluded that needle selection should be done taking due consideration of the anthropometrical parameters.

CONCLUSION

Our topographic study at lateral infraclavicular fossa has found that cords are clustered all around the axillary artery with variability in their positions along with abnormal vascular formations. This sonoanatomic knowledge can be helpful in improving safety and success of nerve blocks. Cord variations at this position can make the procedure technically difficult as multiple injections and more volume of drug will be needed for the block. Recently, there is growing interest in costoclavicular approach of infraclavicular block. At the costoclavicular space and in contrast to lateral infraclavicular fossa, the cords are relatively superficial, clustered together, exhibit a triangular arrangement, and share a consistent relationship with one another.[17] Therefore, we encourage a formal topographic study comparing the variations of cords at costoclavicular and lateral infraclavicular fossa in future.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.