– Written by Marcelo Bordalo, Mohamed Abdelatif Djadoun, Maryam Al-Naimi, Eduardo Yamashiro, Paulo Victor Partezani Helito, Qatar

INTRODUCTION

The development of ultrasonography (US) and magnetic resonance imaging (MRI), which provide high-resolution images of soft tissues, has made it possible to apply diagnostic imaging to the study of peripheral nerves. Thus, the imaging diagnosis of compressive neuropathies has developed significantly in the last 15 years, allowing the evaluation of millimeter-sized nerves. We will review the role of imaging methods in compressive neuropathies, demonstrating their main applications.

IMAGING TECHNIQUES

MRI

MR neurography enables visualization of the peripheral nerve anatomy, contours, fascicular arrangement, signal, and caliber, as well as alterations of these features¹. In neuropathies, the nerve remains isointense on T1-weighted images but is hyperintense relative to muscle on fluid-sensitive sequences due to increased endoneurial free water. The nerve may appear fusiform or present focal thickening that is usually proximal or at the level of compression, although nerve compression may not necessarily demonstrate morphologic or signal changes, despite clinical evidence of neuropathy. The injured nerve may also present distortion of the normal fascicular architecture (Figure 1) ².

Signal abnormalities in denervated muscles may be demonstrated. In acute denervation, prolongation of T2 relaxation times can occur as early as 4 days after nerve injury and present as high-signal intensity on fluid-sensitive sequences (Figure 2). Chronic denervation may lead to atrophy and fat replacement of the muscles, optimally seen on T1-weighted images (Figure 3) ³.

Ultrasound

On US, it is possible to see hypoechoic fascicles surrounded by hyperechoic joint tissue and collagen (epineurium). The set of fascicles has another thin layer that surrounds them, also hyperechoic, which consists of the perineurium. On the longitudinal plane, a wavy appearance is observed, with multiple parallel hypoechoic bands (fascicles/bundles) separated by hyperechoic lines (perineurium) (Figure 4).

US can be used to detect and demonstrate the site and cause of compression. The main US findings are thickening of the nerves in the fibrous tunnels and an abrupt change in caliber, in addition to hypoechogenicity and loss of the fascicular pattern (Figure 5). In a dynamic examination, there is a reduction in sliding movement in some cases, and eventually, comparison with the corresponding contralateral nerve can help in detecting more subtle signs⁴.

However, it must be remembered that a normal nerve appearance does not exclude the diagnosis of neuropathy.

COMPRESSIVE NEUROPATHIES

There are anatomical regions in which segments of peripheral nerves are vulnerable or predisposed to becoming trapped or suffering chronic compression. Neural compression occurs especially in osseofibrous tunnels, but it can occur at points where the peripheral nerve passes between muscles or through a band of fibrous tissue.

In changes caused by chronic compression, damage to the nerve occurs progressively or intermittently, sometimes with periods of remission and exacerbation. As a result of repeated trauma or chronic ischemia, fibrosis can occur within and around neural fascicles. In more serious cases, the nerve fibers may be destroyed, and the neural trunk may be replaced by fibrosis if surgical decompression is not performed. The diagnosis of compression neuropathies is traditionally made based on clinical history findings, physical examination, and, at times, electrodiagnostic studies, including electromyography, nerve conduction studies, and somatosensory evoked potentials. US and MRI have frequently been used to complement the investigation of these neural compressions, although they are not always able to directly demonstrate the cause of compression. When using these two imaging methods, knowledge of the basic anatomy of peripheral nerves and innervation territories is essential. The relative advantage of US is that it allows examination of the entire limb or a large part of the limb relatively quickly, as long as a radiologist is trained to evaluate peripheral nerves. More recently, the possibility of imaging tests, particularly via MRI, to identify signs of muscle denervation in the specific territory of a given nerve or branch has been emphasized. MRI has greater sensitivity in identifying muscle changes and has the advantage of being noninvasive compared to electromyography.

SPECIFIC COMPRESSIVE NEUROPATHIES

Suprascapular Nerve

The most common cause of suprascapular nerve injury is nerve entrapment in the suprascapular and/or spinoglenoid notch⁵.

The suprascapular nerve runs across the superior border of the scapula into the suprascapular notch, under the scapular superior transverse ligament (or suprascapular ligament). The nerve then runs posteriorly, supplies motor branches to the supraspinatus muscle, and enters the spinoglenoid notch under the scapular inferior transverse ligament (or spinoglenoid ligament), supplying motor branches to the infraspinatus muscle (Figure 6).

MRI is useful in evaluating the course of the nerve and patterns of muscle denervation. Entrapment of the nerve in the suprascapular notch, beneath the supraspinatus muscle, may lead to edema and/or atrophy of both the supraspinatus and infraspinatus muscles. More distal entrapment in the spinoglenoid notch will result in selective denervation of the infraspinatus (Figure 7).⁶ Visualization of the superior transverse ligament and its relationship to the nerve is also possible.

Axillary Nerve

Quadrilateral space syndrome (QSS) is a rare condition caused by compression of the axillary nerve and the posterior circumflex artery in the quadrilateral space. The clinical manifestations of this condition may be confusing, and MR is useful in its diagnosis.

Quadrilateral space compression may be static or dynamic. MR findings of QSS include increased teres minor and deltoid muscle signal intensity (on T2-weighted images) and/or fatty atrophy (on T1-weighted images) (Figure 8). Isolated teres minor atrophy can be seen in 3% of all routine MR studies and may be associated with other conditions⁷. MR angiography can also be useful in diagnosing QSS and demonstrating occlusion of the posterior circumflex artery during abduction and external rotation of the arm⁸. Direct visualization of the axillary nerve is possible with new MR neurography techniques (Figure 9).

Median Nerve

Lacertus syndrome is caused by compression of the median nerve at the level of the elbow, usually underneath the lacertus fibrosus, between the superficial (humeral) and deep (ulnar) heads of the pronator teres muscle or at the origin of the flexor digitorum superficialis muscle. The MR findings of lacertus syndrome are related to space-occupying lesions along the course of the nerve and, especially, signal abnormalities and/or atrophy in the flexor pronator muscle group. Recently, intrinsic MR alterations within the median nerve in entrapment syndromes have been described with the use of high-resolution sequences⁹.

Carpal tunnel syndrome (CTS) is the most common compressive neuropathy of the upper extremity, with a prevalence of 3% in the general population¹⁰. MRI is accurate in 90% of CTS cases and predicts surgical benefits for these patients¹¹. The most specific MR signs of CTS are proximal median nerve enlargement, flattening of the median nerve in the carpal tunnel, volar bowing of the flexor retinaculum, and T2 hypersignal in distal median nerve branches (Figure 10). Isolated T2 hypersignal of the median nerve within the carpal tunnel has been reported to have a lower specificity¹². Recently, diffusion tensor imaging has been reported in various studies as a potential tool in the diagnosis and evaluation of CTS treated conservatively¹³. Quantitative analysis is possible in CTS, and fractional anisotropy values of the nerve below 0.5 are considered abnormal¹⁴. MR neurography also plays an important role in the postsurgical evaluation of CTS by detecting incomplete flexor retinaculum release and signal alterations within the nerve (Figure 11).

Several quantitative criteria can be measured in imaging exams and that have been described in the literature to aid in the diagnosis of CTS, but the most commonly used criterion is the cross-sectional area of ​​the median nerve, particularly in the US literature (Figure 12).

Most authors suggest that the upper limit of normality of the median nerve is between 9 and 12 mm². The value of 15 mm² has been suggested by some authors as the upper limit of normality for the cross-sectional area of ​​the median nerve, serving to differentiate mild from severe cases of the disease¹⁵.

Ulnar Nerve

Ulnar nerve compression is the most common neuropathy of the elbow and is secondary to dynamic or anatomic factors. In the elbow, the most common compression site is the cubital tunnel, where compression occurs as a result of decreased volume during elbow flexion. The MR findings include increased signal intensity on T2-weighted images, thickening of the nerve (Figure 13), and obliteration of fat tissue surrounding the nerve on T1-weighted images.

Ulnar nerve subluxation is a common feature and is present in up to 16% of asymptomatic individuals. In a minority of cases, it may cause compressive neuropathy. Subluxation can be seen on routine MRI; however, elbow flexion allows better visualization of nerve dislocation (Figure 14)¹⁶. It is easier to identify underlying instability of the nerve dynamically, and US is a suitable method for studying this condition and possible snapping syndromes (Figure 15)¹⁷. It is advisable to use dynamic flexion and extension maneuvers and maneuvers to evaluate flexion against resistance, which increases the chances of making the diagnosis.

Ulnar nerve compression can occur at the wrist level. Within Guyon’s canal, the ulnar nerve divides into superficial sensory and deep motor branches. On MR, the normal anatomy is well depicted on T1-weighted images, and bifurcation of the superficial and deep nerve branches is adequately seen. MRI is useful in detecting mass lesions along the course of the ulnar nerve branches (Figure 16). Ulnar nerve fascicular pattern and caliber alterations are better visualized on US.

Radial Nerve

Radial nerve entrapment is a lesser common type of compression and frequently associated with trauma. In the arm, the radial nerve can be compressed at the humeral spiral groove as it crosses the lateral intermuscular septum, where it enters the anterior compartment of the arm. With 3D volumetric sequences, MR neurography can depict radial nerve thickening in the lateral intermuscular septum, as well as caliber and signal abnormalities (Figure 17)^18,19. At the level of the elbow joint, the radial nerve bifurcates into the sensory branch of the radial nerve (SBRN), and the posterior interosseous nerve (PIN), a pure motor branch. The PIN passes under the arcade of Frohse, a fibrous arch present in 35% of individuals and formed by the proximal thickened edge of the superficial head of the supinator muscle, which is the most common site of compression of the PIN²⁰. Compression of the PIN may lead to a denervation pattern in the muscles innervated by the PIN, namely, the supinator, extensor digitorum communis, extensor digitorum minimi, and extensor carpi ulnaris (Figure 18).

Common Peroneal Nerve

The peroneal and tibial divisions of the sciatic nerve usually split at the level of the popliteal fossa to form the common peroneal and tibial nerves. The common peroneal nerve is more commonly compressed as it courses by the fibular neck due to its more superficial location. Intraneural ganglia can develop from the proximal tibiofibular joint into the articular branch of the common peroneal nerve. MR neurography and US enable visualization of nerve abnormalities such as thickening and T2 hypersignal/hypoechogenicity^21,22 (Figure 19).

SUMMARY

MR neurography and high-resolution US are excellent techniques for the evaluation of peripheral nerves. Knowledge of currently available techniques is essential to perform state-of-the-art MR and US examinations. To provide an accurate diagnosis, physicians must also be familiar with the basic clinical aspects of nerve entrapment and, especially, with anatomic and pathologic aspects related to the nerves.

Marcelo Bordalo M.D., Ph.D.

Chief of Radiology

Mohamed Abdelatif Djadoun M.D.

 Radiology Fellow

Maryam Al-Naimi M.D.

Radiology Consultant

Eduardo Yamashiro M.D.

Radiology Consultant

Paulo Victor Partezani Helito M.D.

Radiology Consultant

Aspetar Orthopedic and Sports Medicine Hospital

Doha,Qatar

Contact: marcelo.bordalo@aspetar.com

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Header image by Singapore 2010 Youth Olympic Games (Cropped)