Role of high-resolution ultrasound and magnetic resonance neurography in the evaluation of peripheral nerves in the upper extremity

Upper extremity peripheral nerves are best assessed with high-frequency (>15 MHz) linear probes⁽⁵⁾. Nerves are composed of alternating hypoechoic nerve fascicles, which are usually of similar size and echogenicity, and echogenic connective tissue, corresponding to the endoneurium, perineurium and interfascicular epineural fat. This arrangement has a “honeycomb” pattern when scanning a normal nerve in the transverse/short axis (Fig. 2). Long-axis imaging of the nerve shows a longitudinal fascicular pattern. As opposed to nerves, tendons usually present with numerous, thinner, and more densely packed echogenic lines alternating with hypoechoic lines, resulting in a fibrillar pattern that helps differentiate them from nerves⁽⁶⁾.

Fig. 2.

Sonographic findings of abnormal nerves may depend on whether there is noncompressive or compressive neuropathy. In noncompressive neuropathies, there is usually involvement of a longer nerve segment compared to compressive neuropathies, where the nerve appears larger proximal to the site of compression, often with fascicular thickening, and relatively abrupt flattening at the site of compression. Abnormal nerves may demonstrate loss of the normal fascicular pattern with decreased nerve echogenicity secondary to edematous intraneural changes, and may be associated with increased vascularity on Doppler ultrasound, irrespective of the cause⁶. Newer microvascular Doppler techniques detect slower blood flow, which may better identify abnormal intraneural and perineural hyperemia.

Neuropathies of motor nerves often result in muscle denervation changes that are specific to the involved nerve, and the pattern of muscle abnormalities is a secondary finding that can suggest abnormality of a particular peripheral nerve. Acute muscle changes may be difficult to detect on US. However, US is excellent at detecting atrophy and fatty replacement of muscles in the innervated territory. Both US and MRN are complementary examinations for evaluation of peripheral neuropathy. US has some advantages over MRN. These include examining the nerve in real time, the relative shorter scan time, the ability to adapt the field of view of the exam depending on the clinical indication, and the capability to provide dynamic imaging of the nerve with movement and assessing the nerve mobility in cases of entrapment⁽⁷⁾. Furthermore, US provides a quick comparison with the contralateral side, which can serve as an internal control in cases of unilateral symptoms, to better detect subtle nerve and muscle abnormalities. Finally, US can be used to guide perineural anesthetic injection, a well-established technique amongst anesthesiologists to achieve regional anesthesia for upper extremity surgery⁽⁸⁾. Radiologists also often perform upper extremity peripheral nerve perineural injections for diagnostic purposes utilizing local anesthetic or for therapeutic purposes utilizing steroids⁽⁹⁾.

Shear-wave elastography (SWE) measures the propagation speed of shear waves generated within tissues to provide quantitative information about tissue stiffness, and can be applied in peripheral nerve imaging⁽¹⁰⁾. SWE can be utilized to assess the mechanical properties of peripheral nerves and detect areas of nerve compression. Kantarci et al. demonstrated that patients with carpal tunnel syndrome have significantly higher median nerve stiffness at the carpal tunnel inlet, and SWE seems to be a reliable diagnostic method with good reproducibility for this condition⁽¹¹⁾.

Prior to scanning particular peripheral nerves, radiologists and sonographers must know the anatomy in detail, including nerve branching patterns, and be familiar with common anatomic variants. It is advised to start the examination from known anatomic landmarks near the nerve, with the evaluation initially performed in the short axis. Once the nerve is identified, it can be tracked proximally and distally using the so-called “elevator technique” (Video 1). If pathology is encountered (abnormal echogenicity, size or fascicular pattern), then the examination can be focused on that particular segment. Subsequently, the nerve and any associated pathology can be evaluated in the long axis. Ultrasound gel should be used liberally, especially at skin folds. The image quality should be optimized by changing the focal zone and the depth of insonation. Occasionally, particularly for smaller nerves that may be difficult to see reliably in the short axis, rotating the US probe into the long-axis plane after a small segment of the nerve is identified in the short axis may be helpful to characterize the nerve. Finally, caution should be used when applying transducer pressure over the nerve of interest, since this may prevent detecting abnormal nerve movement during stress maneuvers, or reduce blood flow to abnormal nerve segments on Doppler imaging.

High field strength systems (3 tesla (T) or higher) with strong magnetic gradients and coils dedicated to the anatomy of interest help to optimize imaging⁽¹²⁾. MRN consists of a combination of “anatomic”, high-resolution, nonfat-suppressed (NFS) anatomical sequences, such as T1-weighted (T1W) or proton density-weighted (PDW) sequences, and fat-suppressed (FS), high resolution fluid-sensitive sequences, such as T2-weighted (T2W) or short tau inversion recovery (STIR) sequences in several planes. Most commonly, these are performed with two-dimensional (2D) fast spin echo techniques. An anatomic sequence is commonly performed at least in the short-axis plane and is useful to contrast the difference between the nerve and surrounding fat. This helps to see the morphology and caliber of the nerve and its nerve fascicles, as well as its relationships to surrounding structures. Fat-suppressed, fluid-sensitive sequences accentuate the contrast differences between nerves and background soft tissues to detect signal abnormalities that can indicate nerve pathology, and many centers use heavily T2-weighted, high echo time sequences to increase the contrast between the nerves and background tissues. These sequences are usually performed in multiple planes to better characterize nerve findings in areas of complex anatomy. Since nerves and blood vessels often travel in neurovascular bundles and both structures can be hyperintense to background tissues on fluidsensitive sequences, it can be difficult to discriminate between them, especially more distally as the nerves become very thin. The NFS anatomic sequence can serve as a roadmap to determine the location of target nerves on more T2 contrasted, FS pulse sequences.

Three-dimensional (3D) spin echo sequences may also be helpful to depict nerves with optimal planes of imaging and can be performed with or without fat-suppression and with various weightings. Some clinically available 3D techniques, including three-dimensional inversion recovery turbo spin echo (3DIRTSE), nerve-SHeath signal increased with INKed rest-tissue RARE Imaging (SHINKEI) and post contrast 3D STIR Sampling Perfection with Application optimized Contrast (SPACE) sequences, have vascular suppression to help better visualize the nerves⁽¹³⁾. The MRN protocol used at our institution is summarized in Tab. 1.

As in US, a normal nerve has a fascicular pattern on MRN, with signal intensities similar to skeletal muscle on T1W images and similar to or slightly brighter than muscle on T2W images. In both compressive and noncompressive neuropathies, there is loss of the normal fascicular pattern of the nerve with increased signal on fluid-sensitive sequences. In cases of entrapment neuropathies, there is enlargement of the compressed nerve proximally with focal flattening at the site of compression. MRN is better than US at showing subacute muscle denervation such as diffuse muscular edema, which is reversible, and detecting irreversible atrophy and fatty infiltration when there is chronic muscle denervation.

Functional MRN with the use of diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) can be complementary to morphologic MRN and provide pathophysiologic information about the functional integrity of peripheral nerves or water molecule flow within nerve axons, and to assess surgical procedures⁽¹⁴⁾. Specific technical adjustments can be performed to obtain high-quality images of peripheral nerves, which, when added to the routine morphologic MRN sequences improve the detection and characterization of peripheral neuropathies. Quantitative parameters, derived especially from DTI neurography, including fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity, allow detection of signal intensity changes in injured or repaired peripheral nerves, and provide pathophysiologic information about the extracellular space, fiber organization, axonal flow, or myelin integrity. Functional MRN has the potential to improve treatment selection, monitor therapeutic response, and predict the healing potential of injured peripheral nerves.

The axillary nerve arises from the posterior cord of the brachial plexus. At the level of the shoulder, it splits into anterior and posterior divisions within the quadrilateral space after descending inferolaterally. The posterior division is usually located more superficially and is firmly related to the inferior glenoid rim⁽¹⁵⁾. The quadrilateral space is bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, medially by the long head of the triceps muscle, and laterally by the medial aspect of the humerus.

Axillary nerve compression most commonly occurs as it passes through the quadrilateral space, which is known as quadrilateral space syndrome (QSS), and the most frequent cause of QSS is a fibrous band between the long head of the triceps muscle and the teres major muscle or the humerus. Other causes include space-occupying lesions within the quadrilateral space, such as paralabral cysts arising from the inferior glenoid labrum, varicose veins, lymphadenopathy, ganglion cysts, benign tumors, such as lipomas, axillary schwannomas, and humeral osteochondromas, and muscle hypertrophy, especially of the triceps muscle in competitive overhead athletes⁽¹⁵⁾. In addition to QSS, the axillary nerve can sustain traumatic injuries resulting from anterior glenohumeral dislocation, proximal humeral fracture, direct anterolateral blow to the deltoid muscle or compression by maintaining an extended abducted and externally rotated position of the arm during surgery or by the prolonged uses of the crutches⁽¹⁶⁾. Usually, patients experience paresthesias and pain in the posterolateral area of the shoulder, exacerbated by abduction and external rotation, with loss of motor function and muscle atrophy in more advanced cases⁽¹⁷⁾.

Due to the nerve’s small size and deeper location, US usually has a limited role in diagnosing axillary neuropathy. The axillary nerve supplies the deltoid, teres minor and triceps lateral head muscles, and fatty atrophy of these muscles should suggest neuropathy of this nerve. Although it can be difficult to see on US, this nerve courses adjacent to the posterior circumflex humeral artery, which can be used as a sonographic landmark and can be identified by color Doppler imaging⁽¹⁸⁾. Dynamic US can be helpful to detect cases of compression that occur exclusively during abduction and external rotation, and loss of flow in the axillary artery on Doppler imaging can also serve as an additional, indirect sign of axillary nerve compression. Comparison with the contralateral side may help confirm the diagnosis in less obvious cases⁽¹⁹⁾. MRN or computed tomography angiography (CTA) can show similar vascular findings and requires image acquisition in the neutral position and during arm movement. Additionally, US-guided axillary nerve block is frequently used as a pain-relieving technique.

MRN can help to detect the direct signs of axillary neuropathy, including nerve edema or changes in caliber, as well as any space-occupying lesions. While space-occupying lesions in the quadrilateral space are readily visible on MRN, fibrous bands can be difficult to see directly and may be inferred if there is an abrupt caliber change in the nerve. MRN is superior to US at detecting the pattern of deltoid and teres minor muscle denervation edema reflecting axillary neuropathy, though isolated posterior branch neuropathy may only result in teres minor involvement (Fig. 3). Sofka et al. showed isolated teres minor denervation is not uncommon in asymptomatic patients undergoing routine shoulder MR imaging, which may be due to history of surgical intervention, rotator cuff trauma or instability that has likely caused stretching of the posterior branch of the axillary nerve.

Fig. 3.

The musculocutaneous nerve (MCN) is a mixed nerve that arises from the lateral cord of the brachial plexus; it receives fibers from the C5–C7 nerve roots and provides motor innervation to the coracobrachialis, biceps brachii, and brachialis muscles, as well as sensory innervation to the lateral aspect of the forearm. At the axilla, the nerve is lateral to the axillary artery and the median nerve. The nerve then travels anteriorly to pierce the coracobrachialis muscle, after which it descends between the biceps brachii and brachialis muscles to terminate as the lateral cutaneous nerve of the forearm⁽²⁰⁾. Similar to other nerves, the MCN can be damaged by trauma, such as fractures or lacerations, or tumors. The MCN can be injured anywhere along its course, but it is most commonly injured as it traverses the coracobrachialis muscle, which can occur due to hypertrophy of the muscle resulting from repetitive strain^(21,22). Additionally, cases of MCN neuropathy have been reported following anterior humeral dislocations⁽²³⁾. US can identify the MCN along its course from the axilla to the elbow, in addition to the lateral antebrachial cutaneous nerve. Furthermore, US has proven valuable in detecting a diverse range of abnormalities, such as neuromas and traumatic injuries to the nerve⁽²⁴⁾. US of the MCN can be performed with the extremity in mild abduction and the hand in supination or the arm in abduction with the hand in supination positioned behind the head (Fig. 4). Similar to other neuropathies, MRN can help detect signal and size changes of the nerve along with target muscle denervation.

Fig. 4.

In the cubital fossa, the MCN courses lateral to the biceps brachii tendon, giving rise to the LACN, a pure sensory nerve innervating the lateral aspect of the forearm. The LACN emerges at the lateral border of the biceps brachii muscle and courses along the lateral border of its tendon adjacent to the cephalic vein. It pierces the antebrachial fascia at a variable location and then courses subcutaneously distally. Its superficial location makes the nerve prone to compressive injury, traction and iatrogenic injuries. Frequent causes of neuropathy of the LACN include repetitive and forceful pronation in athletes involved in throwing sports, compression due to tourniquet use, phlebotomy in the antecubital fossa, incorrectly placed blood pressure cuffs, and positioning during general anesthesia⁽²⁵⁾. The nerve can be also injured secondary to rupture of the distal biceps brachii tendon. Additionally, because of its superficial location, the LACN can be injured by penetrating trauma (Fig. 5). Clinically, it presents with pain and dysesthesia of the lateral aspect of the forearm, which can extend anywhere from the elbow to the wrist.

Fig. 5.

In order to image the LACN, the US transducer should be positioned on the crease of the elbow; in this position, the short axis of the LACN can be visualized on the lateral side of the biceps brachii tendon. The cephalic vein is adjacent to the LACN and can be easily identified⁽²⁶⁾. Similar to other nerves, enlargement and increased echogenicity on US and increased T2 signal and enlargement of the nerve are seen in case of neuropathy.

The ulnar nerve originates from the medial cord of the brachial plexus from the C8 and T1 nerve roots. In the arm, it traverses the medial intermuscular septum (arcade of Struthers) and courses posteriorly along the medial aspect of the triceps muscle until it passes distally through the cubital tunnel at the elbow. The floor of the cubital tunnel is formed by the joint capsule and the posterior band of the ulnar collateral ligament; the roof is formed by the retinaculum between the olecranon and the medial epicondyle (Osborne ligament), and distally, the aponeurosis joins both heads of the flexor carpi ulnaris muscle⁽²⁷⁾. During elbow flexion, there is decreased cubital tunnel volume and ulnar nerve cross-sectional area. In the forearm, the ulnar nerve pierces the flexor carpi ulnaris fascia and descends beside the ulna toward the wrist, where it enters the hand via the ulnar canal (also known as Guyon’s canal). As a result, the most common sites of entrapment are at the cubital tunnel and at the ulnar canal (Fig. 6)⁽²⁸⁾. The ulnar nerve innervates the flexor carpi ulnaris muscle and the ulnar side of the flexor digitorum profundus muscle, the hypothenar muscles, the dorsal and palmar interosseous muscles of the hand, the adductor pollicis muscle, and the flexor digitorum profundus muscle. It provides sensation to the small finger, the radial half of the ring finger, and the ulnar side of the hand and wrist with known anatomic variations.

Fig. 6.

The radial nerve is one of the two terminal branches of the posterior cord of the brachial plexus, arising from the C5-T1 nerve roots, and it descends within the upper arm along the posterior part of the humerus through a depression called the spiral groove⁽³⁶⁾. It wraps around the distal humerus and courses along the lateral aspect of the humeral diaphysis until it reaches the radial tunnel. The radial tunnel is about 5 cm in length and located between the radiocapitellar joint proximally and the proximal edge of the supinator muscle distally. The tunnel is bounded laterally by the brachioradialis, and extensor carpi radialis longus and brevis muscles, and medially by the brachialis muscle. Approximately 3 cm distal to the lateral humeral epicondyle, the radial nerve divides into two branches, the superficial sensory radial nerve and deep motor branch, also called the posterior interosseous nerve (PIN), which passes between the two heads of the supinator muscle. The level of division of the radial nerve however can be variable. The cranial border of the superficial head of the supinator muscle is composed of a fibrous arch called the arcade of Fröhse. Distally, the PIN splits into superficial and deep branches and innervates the dorsal compartment of the forearm⁽³⁷⁾. The superficial branch of the radial nerve courses under the brachioradialis muscle between the heads of the extensor carpi radialis longus and the brachioradialis muscles to innervate the dorsoradial hand (thumb and index finger)⁽³⁸⁾. The radial nerve can be entrapped anywhere along its course, with possible involvement above or at the elbow (Fig. 7).

Fig. 7.

Injuries of the radial nerve at the elbow: radial tunnel syndrome (RTS) and posterior interosseous nerve syndrome (PINS)

Both RTS and PINS refer to the compression of the deep motor branch of the PIN at the level of the elbow within the radial tunnel. The main difference is the severity of compression and the clinical presentation, as RTS can result in sensory symptoms such as pain in the lateral elbow and forearm without motor deficit, which may be confused clinically with lateral epicondylitis. On the other hand, PINS presents with pain and motor weakness involving finger extension with preserved wrist extension⁽⁴²⁾. In this condition, the wrist extensors are not affected since the brachioradialis and the extensor carpi radialis longus muscles responsible for wrist extension are both supplied by the radial nerve above the elbow⁽⁴³⁾. Electromyography (EMG) evaluation is usually normal in the case of radial tunnel syndrome, but positive in the case of PINS. As with other cases of neuropathy, causes include extrinsic compression or nerve injury from repetitive microtrauma. The most common site of PIN injury is compression at the arcade of Fröhse. Other causes include nerve compression in the lateral elbow at the level of the radial head secondary to inflammatory arthropathy or fibrous adhesion anterior to the radiocapitellar joint, compression by an arcade of branches of the recurrent radial artery at the radial neck and a fibrous edge of the medial proximal extensor carpi radialis brevis (leash of Henry)⁽⁴⁴⁾.

On US, signs of neuropathy, including hypoechoic swelling of the PIN, are observed proximal to the site of compression (Fig. 8). Associated findings, like hyperechoic thickening of the arcade of Fröhse, radiocapitellar ganglion cysts, soft tissue masses and bicipitoradial bursitis, may suggest the cause of compressive neuropathy⁽⁴³⁾. Supinator muscle fatty atrophy is a common finding of PINS on both US and MRN, although MRN may help detect earlier stages of PINS if there is reversible muscle edema without muscle atrophy.

Fig. 8.

The median nerve is a mixed sensory and motor nerve, formed by the confluence of the medial and lateral cords of the brachial plexus with contributions from the C5-T1 nerve roots. It descends in the upper arm along the medial side of the brachial artery, and then crosses the artery to become lateral to the artery more distally. At the elbow, it courses under the bicipital aponeurosis, medial to the biceps brachii tendon and the brachial artery, and enters the forearm between the two heads of the pronator teres muscle, where it gives rise to the anterior interosseous nerve (AIN) (Fig. 9)⁽⁴⁹⁾. It passes deep to the sublime bridge, a fibrous sheath formed by the two heads of the flexor digitorum superficialis, and passes distally between the flexor digitorum superficialis and flexor digitorum profundus muscles, giving rise to the palmar cutaneous branch before it enters the carpal tunnel⁽⁵⁰⁾.

Fig. 9.

Anterior interosseous nerve syndrome (AINS)

AINS usually results from the compression of the AIN, and presents with acute pain in the proximal forearm followed by weakness on flexion of the thumb at its interphalangeal joint and the index finger at its distal interphalangeal joint. On physical examination, patients are unable to bring the tips of the index finger and thumb together in the shape of an “O”, which is called a positive Kiloh-Nevin sign⁽⁵⁵⁾. AINS could result from nerve compression due to repetitive elbow flexion and pronation, compression from a fibrous band of the superficial head of the pronator teres, an aberrant course of the radial artery, or thrombosed anterior interosseous vessels. Noncompressive causes, such as interstitial neuritis, can also be considered⁽⁵⁶⁾. Usually, EMG studies are normal in AINS, and direct signs of neuropathy are inconclusive on imaging due to the small size of the nerve⁽⁵⁷⁾. On MRN, fat-suppressed, fluid-sensitive images show diffuse edema of denervated muscles, especially the pronator quadratus muscle, while chronic denervation may result in muscle atrophy or fatty infiltration on T1W imaging (Fig. 10)⁽⁵⁸⁾. Finally, physicians should be aware of important upper limb anatomical nerve variations, like the Martin-Gruber anastomosis, which in cases of AINS can cause denervation edema in muscles not classically innervated by the AIN due to its anastomosis with the ulnar nerve⁽⁵⁹⁾.

Fig. 10.

Carpal tunnel syndrome (CTS)

CTS is the most common upper extremity entrapment neuropathy. The carpal tunnel is an osteofibrous canal in the volar wrist, bounded by the carpal bones forming the floor; the transverse carpal ligament, also called flexor retinaculum, forming the roof; the pisiform ulnarly; and the scaphoid tubercle radially. It contains the flexor tendons and the median nerve, which is usually the most superficial structure. Anything that decreases the space inside the carpal tunnel can cause median nerve compression. This includes compressive neuropathy, which includes multiple causes, like bone fractures, hematoma, soft tissue and nerve tumors, flexor bursitis, volar wrist synovitis, joint effusion and some anatomical variants that may cause compression of the median nerve^(60,61). There are multiple systemic conditions associated with CTS, including pregnancy, kidney failure, diabetes, hypothyroidism, and the use of oral contraceptives⁽⁶²⁾ although the cause can remain indeterminate. Usually, patients initially present with sensory symptoms, while motor symptoms are delayed. They commonly experience pain and paresthesias in the palmar aspect of the thumb, index and long fingers and radial half of the ring finger, and in more advanced cases, weakness of thumb abduction and opposition, reduced finger grip and thenar muscle atrophy⁽⁶³⁾.

On US, which is the gold standard imaging test for CTS, the median nerve is visualized as a hypoechoic, fasciculated, oval structure with similarly sized hypoechoic fascicles when seen in the short axis, and US has similar sensitivities and specificities as EMG studies in diagnosing CTS but can better characterize the nerve and surrounding anatomical structures⁽⁶⁴⁾. The diagnosis of CTS on US is based on measuring the cross-sectional area of the median nerve by tracing the hypoechoic nerve, excluding the echogenic perineurium. Numerous studies have examined the cross-sectional area of the nerve, yielding variable findings. However, the generally accepted upper normal limit is 11–2 mm². Another method for assessing nerve size involves measuring the cross-sectional area at two different locations: the carpal tunnel, where the nerve is largest, and the proximal margin of the pronator quadratus muscle. A difference of more than 2 mm² between these two levels has high sensitivity and specificity for the diagnosis of CTS (Fig. 11). If the nerve is bifid, each part must be measured separately, and the sum of the measurements used. A pathologic condition is indicated if the cross-sectional area difference exceeds 4 mm², and the upper normal limit for cross-sectional area is also around 11–12 mm^2(65,66). Although, using the cross-sectional area difference is more reliable in diagnosing neuropathy compared to using the cross-sectional area of the nerve at a specific site with a cut off of 11–12 mm², as the nerve might be larger in asymptomatic individuals. A persistent median artery (PMA), which is an anatomic variant representing a remnant of the embryological median artery, occasionally persists into adulthood as a sizeable artery that can be seen accompanying the median nerve. It is often associated with bifid median nerves, and has been reported in up to half of the bifid median nerve cases⁽⁶⁷⁾ (Fig. 12). A PMA can contribute to CTS particularly when enlarged or thrombosed⁽⁶⁸⁾. In the scenario of surgical treatment for CTS, the PMA is usually carefully avoided during the surgery because of the risk of vascular compromise of the digits and the increased risk of bleeding when the artery is injured. The PMA should be reported whenever imaging of the carpal tunnel is performed, particularly in a preoperative setting.

Fig. 11.

Fig. 12.

The RBMN is a pure motor branch that originates from the median nerve and supplies motor innervation to the thenar muscles. Three variants of the course of this nerve to reach the thenar muscles have been described: the extraligamentous type with radial or ulnar branching when the RBMN arises close to the distal end of the transverse carpal ligament and follows a retrograde course to reach the thenar eminence; the subligamentous type, in which the RBMN arises within the carpal tunnel and maintains its course deep to the transverse carpal ligament to reach the thenar eminence; and the transligamentous type, when the nerve arises within the carpal tunnel and then pierces the transverse carpal ligament to reach the thenar eminence⁽⁶⁹⁾. Compression of the recurrent branch can lead to pain, numbness, and weakness in the thenar eminence. Etiologies of RBMN compression include repetitive manual labor, trauma, and space-occupying lesions (Fig. 13). Injuries to this nerve can lead to great dysfunction in the opposability of the thumb, and the nerve has been referred to as the “million-dollar nerve” since iatrogenic RBMN injuries can lead to large malpractice settlements.

Fig. 13.

The proper digital nerves provide innervation of the digits and are the terminal branches of the radial, median, and ulnar nerves beyond the carpal tunnel and the Guyon’s canal⁽⁷⁰⁾. Pathology of the digital nerves is termed digitalgia paresthetica. It is most commonly posttraumatic, given the superficial location of the nerves, and can be caused by penetrating and non-penetrating injuries. Other etiologies include inflammatory, infectious or tumoral causes. Digital amputation can lead to stump neuromas, which can be painful⁽⁷¹⁾. Perineural fibrosis of the ulnar proper digital nerve of the thumb has been reported in bowlers and has been named “bowler’s thumb”. This condition has also been reported in baseball players and is thought to be secondary to repetitive valgus microtrauma that results in nerve traction⁽⁷²⁾. The imaging appearance of digital neuropathy is similar to other nerves (Fig. 14). Imaging of digital nerves can be more challenging given their small sizes, and US is often the first-line modality given its higher spatial resolution. A hockey stick US transducer may be preferred to use whenever possible due to its small footprint and the small curved skin surfaces around the base of the thumb. MRN should be adapted to imaging digital nerve by using small coils, small field-of-view, and high-resolution sequences.

Fig. 14.