Normal and sonographic anatomy of selected peripheral nerves. Part I: Sonohistology and general principles of examination, following the example of the median nerve

Abstract

Ultrasonography is an established method for imaging peripheral nerves. It serves to supplement the physical examination, electromyography, and magnetic resonance imaging. It enables the identification of post-traumatic changes of nerves, neuropathies secondary to compression syndromes, inflammatory or neoplastic nerve lesions as well as the evaluation of postoperative complications. In certain situations, this technique is the imaging method of choice. It is increasingly used in anesthesiology for regional anesthesia. As in the case of other ultrasound imaging studies, the examination of peripheral nerves is non-invasive, well-tolerated by patients, and relatively inexpensive. This article presents the histological structure of peripheral nerves and their appearance in ultrasonography. It also presents the examination technique, following the example of the median nerve, and includes a series of diagrams and ultrasound images. The interpretation of the shape, echogenicity, thickness and vascularity of nerves is described, as well as their relation to the surrounding tissues. The “elevator technique”, which consists of locating a set nerve at a characteristic anatomic point, and following it proximally or distally, has been explained. The undisputed benefits of the ultrasound examination have been presented, including its advantages over other diagnostic methods. These advantages include the dynamic component of the ultrasound examination and the possibility of correlating the patient's symptoms with the ultrasound images. As an example, the proper anatomy and the ultrasonographic appearance of the median nerve were described. This nerve's course is presented, its divisions, and characteristic reference points, so as to facilitate its location and identification, and enable subsequent use of the aforementioned “elevator technique”. This article opens a series of publications concerning anatomy, technique of examination and pathologies of peripheral nerves.

Introduction

Ultrasonography (USG) is a valuable method for imaging peripheral nerves, complementing routinelyperformed diagnostic studies, including the physical examination, electromyography and magnetic resonance imaging. In certain situations, USG becomes the imaging method of choice; these include evaluating nerves of small diameter (less than 1 mm) such as cutaneous nerves, or in cases of diffuse neuropathies, when unlike MRI, ultrasonography allows for the assessment of even very long nerve trunks and their branches. As in other types of USG studies, the USG diagnostics of peripheral nerves is noninvasive, well-tolerated by patients, and is relatively inexpensive. However, this diagnostic technique requires substantial experience and a thorough knowledge of the nerves’ topographic anatomy⁽¹⁾. It consists of both the static and dynamic evaluation of nerves, the latter during passive or active movements of the extremities; both components are important in the context of diagnosing musculoskeletal disease with USG.

The first reports of the use of USG in the evaluation of peripheral nerves appear in 1992 describing a case of carpal tunnel syndrome^{(2, 3)}. A breakthrough in ultrasound diagnostics of peripheral nerves occurred during the last several years, with the introduction of modern transducers, whose high frequencies allowed for the imaging of fine nerves and their branches^{(1, 4–6)}.

Technique for ultrasonographic imaging of peripheral nerves

For the imaging of peripheral nerves, a linear probe with a frequency greater than 12-14 MHz and a resolution less than 0.3 mm is used (fig. 1). In the case of obese patients or the evaluation of deeply located nerves, a convex probe may be used, thus ensuring a deeper penetration of the ultrasound waves^{(6, 7)}. Although the improved range of the imaging signal corresponds to a poorer image resolution, the image quality is still sufficient for the precise monitoring of nerve injection in regional anesthesia.

Fig. 1.

Linear broad-spectrum probes with frequencies of 4–13 MHz and 6–18 MHz as well as a convex broad-spectrum probe 1–8 MHz, all used for the ultrasonographic assessment of peripheral nerves

When studying very superficial nerves, distancing add-ons, made of gelous agar, are helpful (fig. 2). Such equipment improves the imaging of set nerves both by improving or eliminating the poor contact between the probe and uneven bony surfaces, as well as by imaging the nerve at the level of the ultrasound wave focus. In particular, such adjuncts are useful in the evaluation of fine nerves of the wrist.

Fig. 2.

Agar-based distancing adaptor

Ultrasonographic picture of the peripheral nerves (sonohistography)

The basic unit of the peripheral nerve – the neuron – composed of a nerve fiber surrounded by the endoneurium, is too thin to reflect an ultrasound beam, and thus is not visible in USG imaging. Only groups of nerve fibers which form nerve bundles surrounded by the perineurium may be pictured with this technique. The perineum contains collagen fibers, fibroblasts, blood and lymphatic vessels, and thus forms a layer sufficiently thick to reflect ultrasound waves. This structure possesses mechanical properties which protect the nerve from stretching and tearing^{(4, 6–8)}. Nerve bundles combine to form the trunk of a peripheral nerve, which is surrounded by the epineurium, seen clearly in USG as a hyperechogenic layer^{(4, 6, 7)}.

Nerves may be assessed in the transverse or longitudinal sections. In the longitudinal view, the peripheral nerve is seen as several parallel hyperechogenic lines – representing the perineurium – between two more prominent and also hyperechogenic layers of the epineurium. This image resembles that of an electric cable (fig. 3 A). Whereas in the transverse section, the nerve resembles a honeycomb, within which are visible tiny round and hypoechogenic areas representing the nerve bundles with hyperechogenic rims of the epineurium (fig. 3 B)^{(1, 4, 5, 7–9)}.

Fig. 3.

A. Longitudinal view of the median n erve midway in the forearm (nerve indicated by arrows). B. Transverse section of the median nerve at the same level, known as the “honeycomb” view

The transverse image is much more frequently used in clinical practice, as it allows for the nerve to be examined by the so-called “elevator technique” (explained later) along a very long segment. As an example, nerve bundles in the upper limb may be visualized from the level of the brachial plexus root to that of the proper palmar digital nerves. The only short fragment inaccessible to the USG study is that passing below the clavicle, as this bone obscures the image of the underlying soft tissues (fig. 4).

Fig. 4.

Acoustic shadow of the clavicle with a neurovascular bundle laying behind (arrow)

The aforementioned “elevator technique” consists of finding the set nerve at a characteristic anatomic point and “tracking it” either proximally or distally (figs. 5 A–C)⁽¹⁰⁾. In this way it is possible to assess the nerve's shape, echogenicity, thickness, its relation to the surrounding tissues, the surface area of the nerve and its vasculature^{(9, 11)}. If an abnormality is seen in the transverse view, the nerve should be examined in the longitudinal view, although it may be difficult to obtain a good image, particularly of nerves with a nonlinear course, and thus this view may be limited to short segments. Hence peripheral nerves are always evaluated in the transverse view while the longitudinal view is only used in certain fragments.

Fig. 5.

A–C. Successive images of moving the probe (in the axis of the limb) using the “elevator technique” along the course of the median nerve

The ultrasonographic picture of nerves changes from hypo- to hyperechogenic as they are followed more peripherally; this fact is due to an increasing amount of connective tissue between the nerve bundles. It has been assumed that nerves are not anisotropic. However, the property of anisotropy is seen in cases of nerves with large cross-sections^{(4, 6, 7, 9)}. The shape of a nerve may also be different and vary between individuals: round, oval, triangular, or irregularly shaped, which may change further under compression by the probe or with the movement of a neighboring muscle. Moreover, a nerve may change its shape along its course, for example from a triangular to a round cross-section^{(4, 6, 7)}. Anatomic variants of nerves should also be remembered, including bifid or even trifid variants of the median nerve (fig. 6)⁽⁴⁾.

Fig. 6.

A bifid median nerve (arrow) in the carpal tunnel

For localizing fine and deeply-seated nerves, characteristic anatomic reference points are used. These are often large vessels accompanying the nerves, which may be seen via Doppler imaging^(5–7).

Motor and motor-sensory nerves may be evaluated indirectly by analyzing the skeletal muscles which they innervate. In case of chronic denervation, by comparing the image to the contralateral side, muscular atrophy may be evident as a decrease of the muscle's volume and fatty infiltration, which increases its echogenicity⁽⁴⁾. Examples include injury of the suprascapular nerve which is manifested by degenerative changes of the subscapularis muscle (fig. 7), or trauma to the long thoracic nerve (which is rarely visualized through USG in healthy persons), which may be seen in the anterior dentate muscle – by assessing the state of dents of the anterior dentate muscle it is possible to determine the level of the injury to the nerve⁽⁴⁾. Unfortunately, an indirect method of diagnosing neuropathies is unreliable in the elderly population, in whom there is a progressive generalized atrophy of muscles, impeding the localization or the reliable assessment of the peripheral nerves.

Fig. 7.

Comparative images of the normal infraspinatus muscle (left) and the same muscle with signs of neurogenic atrophy (right) in a patient with chronic compression of the suprascapular nerve (infraspinatus muscle – asterisk, the dorsal surface of the scapula – arrows)

An indisputable benefit of the USG examination is the possibility of confronting the USG image with the patients’ symptoms, by checking if the place of the visualized pathology is compatible to the location of pain, is it located at the point of entry or radiation (which occurs with neuromas)^{(4, 12)}. Another advantage of USG study over other imaging techniques is dynamic examination of peripheral nerves enabling diagnostics of a number of pathologies, what will be the subject of the following publications.

Proper anatomy and ultrasonography of peripheral nerves – example of the median nerve

The median nerve forms on the anterior surface of the axillary artery from branches of the lateral and medial cords of the brachial plexus, at the pectoralis minor's inferior border (fig. 8 A). In the arm, the nerve runs in the medial bicipital groove of the biceps brachii muscle, initially lateral to, then anterior and distally medial to the brachial artery (figs. 8 B, C). In the cubital fossa, along with the brachial artery, the median nerve crosses deep to the bicipital aponeurosis to enter the forearm between the humeral and ulnar heads of the pronator teres muscle (figs. 9 A, B). At this level, the nerve gives off the anterior interosseous nerve, and then descends in the fascial plane between the FDS and FDP (figs. 10 A–C).

Fig. 8.

A. Diagram of the course of the median nerve. B. Application of the probe perpendicular to the long axis of the forearm, in the median aspect of the cubital fossa. C. The median nerve (arrow) below the belly of the biceps femoris muscle (triangles), medial to the brachial artery (asterisk)

Fig. 9.

A. Application of the probe parallel to the long axis of the proximal forearm. B. The longitudinal cross-section of the median nerve (arrows) coursing posterior to the humeral head of the pronator teres muscle (asterisk)

Fig. 10.

A. Transverse application of the probe at the distal end of the forearm and longitudinal placement at the radial aspect of the forearm. B. Transverse cross-section of the median nerve (arrow), with the pronator quadratus muscle (triangle) and the radius (asterisk) seen in the background. C. Longitudinal section of the median nerve (arrow) between the FDS and FDP muscle bellies

It passes the wrist through the carpal tunnel, before dividing into terminal branches (figs. 11 A–C). These are the three common palmar digital branches (digits 1–3) running deep to the superficial palmar arterial arch along the flexor tendons. At the level of the metacarpophalangeal joint, these divide into seven proper palmar digital nerves, which run toward the fingertips along the radial and ulnar aspects of the proximal and middle phalanges, superficially to the proper palmar digital arteries.

Fig. 11.

A. Transverse application of the probe at the level of the carpal tunnel. B. Model showing the nerve coursing below the transverse ligament of the carpal tunnel (from Ossan World of Anatomical Models). C. The median nerve at the level of the carpal tunnel (arrow) between the scaphoid (asterisk) and pisiform (triangle) bones

It should be mentioned that branches to the thenar muscles run separately or with the common palmar digital nerves, and along the FPL tendon sheath^{(8, 10, 13–16)}.

The anterior interosseous nerve, directly after branching off the median nerve trunk, runs towards the interosseous membrane, lateral to the anterior interosseous artery. It is covered by the FPL muscle and the belly of the FDP muscle. In the distal part of the forearm, it is covered by the pronator quadratus muscle. To identify this fine nerve, we can use the Doppler option to image the artery which accompanies it.

The median nerve gives off one more important branch which may be visualized in the ultrasonographic study, this is the palmar branch of the median nerve. Its branching point is variable, but with the USG probe it may be sought in the distal third of the forearm. It passes between the FCR and PL tendons, then pierces the fascia usually slightly proximal to the flexor retinaculum. To locate this small branch, it is necessary to track the median nerve's course in transverse views and search for its branches.

The USG study of the median nerve is easy and allows for an assessment of its entire course. In the arm, the nerve courses along the brachial artery, easily identified with the Doppler option. In the forearm, it is very well seen between the flat bellies of the FDS and FDP muscles. It is best though to begin an examination of the median nerve at the carpal tunnel, where in the transverse view it appears as an oval structure adherent to the flexor retinaculum, and manifests minor anisotropy. The probe should be applied transversely and moved along the limb's axis, slightly medial to the midline and along the anterior surface of the forearm^{(4–7, 11–17)}.

Abbreviations

FDP

flexor digitorum profundus

FDS

flexor digitorum superficialis

FPL

flexor policis longus

MRI

magnetic resonance imaging

PL

palmaris longus

USG

ultrasonography