Normal and sonographic anatomy of selected peripheral nerves. Part II: Peripheral nerves of the upper limb

Abstract

The ultrasonographic examination is frequently used for imaging peripheral nerves. It serves to supplement the physical examination, electromyography, and magnetic resonance imaging. As in the case of other USG imaging studies, the examination of peripheral nerves is non-invasive, well-tolerated by patients, and relatively inexpensive. Part I of this article series described in detail the characteristic USG picture of peripheral nerves and the proper examination technique, following the example of the median nerve. This nerve is among the most often examined peripheral nerves of the upper limb. This part presents describes the normal anatomy and ultrasound picture of the remaining large nerve branches in the upper extremity and neck – the spinal accessory nerve, the brachial plexus, the suprascapular, axillary, musculocutaneous, radial and ulnar nerves. Their normal anatomy and ultrasonographic appearance have been described, including the division into individual branches. For each of them, specific reference points have been presented, to facilitate the location of the set trunk and its further monitoring. Sites for the application of the ultrasonographic probe at each reference point have been indicated. In the case of the ulnar nerve, the dynamic component of the examination was emphasized. The text is illustrated with images of probe positioning, diagrams of the normal course of the nerves as well as a series of ultrasonographic pictures of normal nerves of the upper limb. This article aims to serve as a guide in the ultrasound examination of the peripheral nerves of the upper extremity. It should be remembered that a thorough knowledge of the area's topographic anatomy is required for this type of examination.

Introduction

Ultrasonography is a commonly used method for imaging peripheral nerves of the upper limb. The typical image of a peripheral nerve in USG is distinctive and has been described in detail in part I of this article. The current part presents the normal anatomy and ultrasonographic picture of the brachial plexus, and the spinal accessory, suprascapular, axillary, musculocutaneous, radial and ulnar nerves, plus includes relevant ultrasonographic images.

Anatomy and ultrasonographic imaging of selected peripheral nerves of the upper limb

Accessory nerve (nervus accessorius)

The spinal accessory nerve, or the cranial nerve XI, leaves the cranium through the lateral part of the jugular foramen and divides into two terminal branches: internal and external. Only the external branch is subject for the USG study. It has a diameter of approximately 1 mm, runs posterior to the interior jugular vein (in 2/3 cases) or anterior to it (in 1/3 cases). Next it descends obliquely, behind the styloid process of the temporal bone and the posterior belly of the digastric muscle. Further in its course it reaches the SCM muscle – at the inferior border of its upper third segment – corresponding to the angle of the mandible and the C3 vertebra^(1–4). The nerve, or rather the external branch of CN XI, should be sought at this point with USG. The probe should be applied transversely (fig. 1 A) and moved cranially or caudally. The external branch of CN XI pierces the SCM muscle or runs diagonally below it, inferiorly and laterally (fig. 1 B). Its further segment runs superficially, under the fascia, through the posterior triangle of the neck, bordered anteriorly by the SCM muscle, posteriorly by the trapezius muscle, and inferiorly by the clavicle. Then it reaches the lateral border of the trapezius muscle approximately 2 cm above the clavicle and courses below the muscle. The spinal accessory nerve is not easily identified due to its small diameter and very large number of anatomical variants. The most common indication for its examination in clinical practice is suspected injury secondary to cervical lymph node biopsy, which would lead to atrophy of the trapezius and SCM muscles.

Fig. 1.

A. The place for perpendicular positioning of the probe at the level of the cricoid cartilage of the larynx, on the belly of the SCM muscle. B. The accessory nerve (arrow) and above it the wide flat belly of the SCM muscle, deeper to the nerve is the middle scalene muscle is visible

Brachial plexus (plexus brachialis)

The brachial plexus consists of 5 roots (in proper anatomic nomenclature referred to as the anterior rami of spinal nerves C5–T1), which in their course merge into 3 trunks (superior, middle and inferior), thereafter divide into 3 anterior divisions and 3 posterior divisions, which join to form 3 cords (posterior, lateral and medial) (fig. 2 A)^(2–6).

Fig. 2.

The structure of the brachial plexus: A. model (from Ossan World of Anatomical Models); B. transverse processes of the C5 vertebra; C. C6; D. C7; their tuburcles (asterisk), the roots of the brachial plexus (arrows); SCM – sternocleidomastoid muscle

The clavicle divides the brachial plexus into supra- and infraclavicular parts.

Supraclavicular segment

The roots of the plexus are visible at the level of the intervertebral foraminae, between the anterior and posterior tubercles of the vertebral transverse processes (aside from the C7 vertebra which does not have an anterior tubercle and T1 which lacks both tubercles), in the vicinity of the vertebral artery and vein. In the USG examination the brachial plexus roots have a low and homogenous echogenicity, with no visible fascicles. Their ultrasonographic appearance is thus radically different from the image of more peripheral segments.

Each of the nerve roots runs laterally and inferiorly, in the direction of the posterior scalene fissure between the anterior and middle scalene muscles.

At this level the roots join to form 3 nerve trunks: the superior (formed from the merging of C5 and C6 roots), middle (being a continuation of the C7 root), and the inferior (from the merging of C8 and T1). The trunks can be identified using the “elevator technique”, moving the probe cranially towards the intervertebral foramen of the appropriate vertebra (point of nerve root exit). The identification of roots should occur in relation to the respective vertebrae, starting from the characteristic picture of C7, and remembering the gradual increase in distance between tubercles of the transverse processes as one moves to more distal vertebrae⁽⁵⁾ (fig. 2 B)^(1–3).

There are nerves coming off the anterior rami of the spinal nerves as well as the trunks of the plexus: the dorsal scapular, the long thoracic, the suprascapular and subclavian nerves.

The examination of the supraclavicular segment of the brachial plexus is best started with a transverse positioning of the probe at the level of the larynx, and more precisely at the most prominent cricoid cartilage. The probe should be moved laterally and posteriorly, until the lateral border of the SCM muscle and the anterior plus middle scalene muscles are visualized. Thereafter the angle of the probe should be changed from the transverse to the oblique, with the wave signal being directed distally (figs. 3 A, B).

Fig. 3.

The brachial plexus at the level of the scalene fissure: A. location for the transverse application of the probe; B. roots – arrows; SA – anterior scalene muscle; SM – middle scalene muscle

Infraclavicular segment

Further in their course, the trunks divide into 3 anterior and 3 posterior divisions, which then join together to form the lateral, medial and posterior cords, lying respectively lying lateral, medial and posterior to the axillary artery (figs 4 A, B). The exact points of the division and subsequent merging cannot be clearly identified in an USG study due to the large number of interconnections and partial coverage by the clavicle's acoustic shadow.

Fig. 4.

A. The location for placing the probe, above and parallel to the clavicle. B. Cords of the brachial plexus (arrow) adhering to the subclavian artery (TP), in the background the surface of the 1^st rib is visible as well as the dome of the pleura (star)

The probe should be placed transversely in the axillary fossa and moved along the axillary artery. The cords are visible in the axillary fossa under the pectoralis minor and major muscles. Posteriorly, they adhere to the subscapular muscle, teres major muscle, the tendon of the latissimus dorsi muscle as well as to the long head of the biceps brachii muscle. This part of the plexus has short and long branches^(1–10). Only the long branches, meaning the musculocutaneous, axillary, median, ulnar and radial nerves, may be subject to USG examination.

Suprascapular nerve (nervus suprascapularis)

The suprascapular nerve branches off the brachial plexus at the level of the scalene muscles and courses under the trapezius muscle towards the supraspinatus fossa, where it goes through the shallow suprascapular notch, and then through the spinoglenoid notch (fig. 5 A). These notches are respectively covered by the superior and inferior transverse scapular ligaments, although the latter is not always present.

Fig. 5.

A. Diagram of the course of the suprascapular nerve. B. Application of the probe in the lateral part of the supraspinatus fossa. C. Suprascapular nerve (arrow) besides the suprascapularis artery (T); scapular spine (triangle), glenoid fossa (asterisk)

The probe should be applied transversely to the posterior surface of the scapula so as to show on its lateral side the space between the glenohumeral joint along with the glenoid labrum (fig. 5 B). Moving the probe distally or proximally, one searches for the characteristic depression on the posterior contour of the scapula's surface. It is bordered medially by the spine of the scapula, laterally by the edge of the glenoid fossa, at the base pulses the suprascapular artery, at whose medial side passes the suprascapular nerve (fig. 5 C)^{(1, 9, 10)}. This vessel courses above the superior transverse scapular ligament and beneath the inferior one, while the nerve runs below both ligaments, adhering to the bony surface of the supraspinatus and then the infraspinatus fossa^{(2–7, 11)}.

Due to its small diameter, the suprascapular nerve is not always clearly visualized through USG. It is then helpful, using Doppler USG, to identify the suprascapular artery, to which the nerve adheres.

Axillary nerve (nervus axillaris)

The axillary nerve branches off the posterior cord at the level of the axillary artery, initially laying on the anterior surface of the subscapularis muscle, and then on the joint capsule of the glenohumeral joint. It runs distally towards the back along with the posterior humeral circumflex artery, winding around the humerus at the level of its surgical neck (fig. 6 A). It then crosses through the quadrangular space, which is bordered laterally by the bony surface of the surgical neck, inferiorly by the teres major muscle, superiorly by the teres minor muscle, and medially by the long head of the triceps muscle. Further along its course, the nerve divides into end branches^(2–5).

Fig. 6.

A. Diagram of the course of the axillary nerve; B. application of the probe parallel to the long axis of the arm at its posterolateral aspect; C. axillary nerve (arrow) running along the bony surface of the neck of humerus, below the inferior border of the teres minor muscle (TM); H – the humerus

Helpful in the ultrasonographic localization of this small, deeply-running nerve is the accompanying posterior circumflex artery, seen via Doppler. The probe may be applied at the axillary fossa (parallel to the posterior axillary line) or at the posterior surface of the scapula at the level of the surgical neck of the humerus, parallel to the long axis of the arm, where the lower edge of the teres minor muscle will be seen and pulsing below it the posterior circumflex artery (fig. 6 B). The nerve runs perpendicular to the long axis of the humerus, parallel to the artery and above it (fig. 6 C)^{(1, 9, 10)}. The second approach (the posterior surface of the scapula) is more comfortable for the patient, as there is no need for abduction of the extremity, which could often be limited by muscle contraction or pain.

Musculocutaneous nerve (nervus musculocutaneus)

The musculocutaneous nerve branches off the lateral cord of the brachial plexus at the border of the lateral head of the pectoralis minor muscle. Running laterally and inferiorly, it pierces the coracobrachialis muscle and upon reaching its lateral border, the musculocutaneous nerve descends between the biceps brachii and brachialis muscles, and into the lateral head of the biceps. At the level of the elbow joint, right above the cubital fossa, the nerve pierces the fascia and gives off a terminal sensory branch, innervating the skin on the radial part of the forearm as the lateral cutaneous nerve of the forearm^{(1–6, 9–11)}.

This nerve is not accompanied by any large arterial vessel, thus it is easiest to find it at the level of the lateral edge of the pectoralis minor muscle, where the nerve branches off the lateral cord. The probe should be applied transversely at the level of the axillary fossa, so as to find the axillary artery (fig. 7) and track the nerve with a triangular cross-section coursing lateral to this vessel to pierce the coracobrachialis muscle. After identifying the musculocutaneous nerve, further evaluation is through the “elevator technique”.

Fig. 7.

Application of the probe at the lower border of the anterior axillary line

Radial nerve (nervus radialis)

The radial nerve is the terminal branch of the brachial plexus’ posterior cord (fig. 8 A). It runs posterior to the axillary artery, then wraps like a spiral around the humerus. Initially it lies in the radial groove between the points of origin of the lateral and medial heads of the triceps muscle, covered from the back by the long head of this muscle. In the terminal part of the groove, the radial nerve pierces the intermuscular septum and courses on the anterior surface of the humerus towards the cubital fossa. There it runs between the brachialis and brachioradialis (BR) muscles, and divides into a superficial and deep branch.

Fig. 8.

A. Diagram of the course of the radial nerve. B. Application of the probe perpendicularly to the long axis of the arm, below the lower border of the deltoid muscle. C. The radial nerve (arrow) in the groove of the humerus between the lateral (CLT) and medial heads (CMT) of the triceps muscle; H – the humerus

Through USG it is possible to follow the above-described course of the radial nerve and its branches in its entirety. It is best to use the “elevator technique”, starting by imaging the nerve trunk of the radial nerve at the level of the radial groove in the humerus, where it can be easily identified. The probe is applied perpendicularly to the long axis of the humerus, midway along its shaft, on the posterolateral surface (fig. 8 B). Initially the nerve is seen in the groove, directly upon the bone surface (fig. 8 C), while more distally it courses away from the bone towards the intermuscular septae. The bifurcation of the radial nerve is clearly visible. The deep branch passes between the muscle fibers of the supinator muscle, separating them into two adjoined semilunar parts, having the characteristic shape of two touching halfmoon. As the nerve courses between these parts, it moves from being anterior and superior to posterior and distal. All the while, the deep part of the radial nerve is observed with the probe perpendicular to the long axis of the arm and then forearms (fig. 9 A, B). Below the lower border of the supinator muscle, the nerve branch changes its name to the posterior interosseous nerve. At this point, the nerve consists of individual bundles, and in normal conditions is rarely seen in an USG study⁽⁹⁾.

Fig. 9.

A. Positioning of the probe perpendicular to the long axis of the forearm at the level of the BR muscle. B. The deep branch of the radial nerve (arrow) between the superficial and deep parts of the supinator muscle (S); BR – brachioradialis muscle, H – the humerus

The superficial branch, just as thin as the deep one, is also accessible along its entire length to USG imaging. Initially it runs along the BR muscle, lateral to the radial artery (figs. 10 A, B). Then it courses anteriorly, crossing the supinator, pronator teres, FDS, and FPL muscles. Approximately 5 cm above the radial styloid process, the superficial branch of the radial nerve moves away from the radial artery, and pierces the fascia to course more superficially, between the tendons of the ECRL and the BR. Further along it crosses the first extensor compartment and the anatomical snuff box^{(1–6, 10, 11)}.

Fig. 10.

A. Application of the probe perpendicular to the long axis of the forearm, on the anterior surface of the BR muscle. B. The superficial branch of the radial nerve (arrow) beneath the anterior part of the BR belly, lateral to the radial artery (T). In the background the supinator muscle (S) is visible, and anterior to it a part of the pronator teres muscle (PT)

Locating the nerve at this level may be problematic. It is suggested to find it using the “elevator technique” from the trunk of the radial nerve or to take advantage of the radial artery, accompanying the nerve along a large portion of its course⁽⁹⁾.

Ulnar nerve (nervus ulnaris)

This nerve arises from the medial cord of the brachial plexus (fig. 11 A). Initially it runs in the medial bicipital groove of the biceps brachii muscle, posterior and medial to the brachial artery, gradually moving away from it (fig. 11 B, C). Midway down the humerus, it lies superficially, pierces the medial intermuscular septum and moves posteriorly towards the extensor compartment. In the groove for the ulnar nerve on the posterior aspect of the humerus, it is only covered by a thickened fascia and a thin layer of skin and subcutaneous tissue (figs. 12 A, B). After leaving this groove, the ulnar nerve again moves towards the flexor compartment, this time of the forearm, crossing between the humeral and ulnar heads of the FCU, then runs medially to the ulnar artery in the layer between the FCU and FDP muscles (figs. 13 A, B). Reaching the wrist, it passes through Guyon's canal. This triangular canal is bordered anteriorly by superficial fibers of the transverse ligament and more distally the palmaris brevis muscle as well. Its posterior border is the deep layer of transverse ligament fibers and the pisohamate ligament, while the pisiform bone forms its lateral border (figs. 14 A, B). Within Guyon's canal, the ulnar nerve divides into superficial and deep branches. The superficial branch is well visualized via USG on the surface of the hook of the hamate. It runs superficially and divides into the proper palmar digital nerve (along the radial side of the 5^th digit) and the common palmar digital nerve (directed to the ulnar side of the 5^th digit and the radial side of the 4^th). The deep branch, aside from its initial segment, is not visible in USG.

Fig. 11.

A. Diagram of the course of the ulnar nerve. B. Application of the probe transversely at the inferior edge of the anterior axillary line. C. The neurovascular bundle in the proximal part of the arm, with nerves indicated by arrows: median – M, ulnar – U, radial – R, musculocutaneous – MC; the brachial artery (T), the biceps brachii muscle (B), the coracobrachialis muscle (CB)

Fig. 12.

A. Application of the probe transversely at the level of the groove for the ulnar nerve in the humerus. B. Ulnar nerve (arrow) within the groove for the ulnar nerve in the humerus, bordered by the medial epicondyle of the (CMH) and the ulna (U)

Fig. 13.

A. Transverse application of the probe in the proximal part of the forearm on its anteromedial aspect. B. The ulnar nerve (arrow) beside the ulnar artery (T), in the layer between the FCU and FDP muscles

Fig. 14.

A. Transverse application of the probe at the level of Guyon's canal. B. The ulnar nerve (arrow) as well as the ulnar artery (T); pisiform bone (P)

Further in the forearm, the ulnar nerve gives off the dorsal branch of the ulnar nerve, which passes to the dorsal side in the distal third of the forearm. It runs beneath the FCU tendon, wraps around the shaft of the ulna, and then at the head of the ulna pierces the fascia between the 4^th and 6^th extensor compartments. On the dorsum of the hand, it divides into 3 branches, running to the fingertips as the dorsal digital nerves^(2–7).

Due to its large diameter, superficial course and characteristic location, the ulnar nerve is easily visualized with USG^{(1, 9, 10)}. It is best to begin at the level of the groove for the ulnar nerve in the humerus or at the level of Guyon's canal, applying the probe transversely and moving it proximally or distally. Reference points, as the brachial and ulnar artery, may be useful in its identification.

A crucial element of the ulnar nerve's USG examination is a test of its stability in the groove for the ulnar nerve of the humerus (fig. 15). This dynamic USG examination technique consists of applying the probe across the groove when the elbow joint is in maximum flexion from initial full extension. The groove may be easily found through palpation, as the medial epicondyle of the humerus and the edge of the ulna are easily detectable.

Fig. 15.

A. USG picture of a slipped ulnar nerve (arrow): proper location (left) and displacement from the groove (right); humerus (star). B. Intraoperative picture of the slipped nerve. C. The ulnar nerve again within the groove for the ulnar nerve of the humerus

Abbreviations

BR

brachioradialis muscle

ECRB

extensor carpi radialis brevis

ECRL

extensor carpi radialis longus

FCR

flexor carpi radialis

FCU

flexor carpi ulnaris

FDP

flexor digitorum profundus

FDS

flexor digitorum superficialis

FPL

flexor policis longus

PL

palmaris longus

SCM

sternocleidomastoid muscle

USG

ultrasonography