Ultrasound assessment on selected peripheral nerve pathologies. Part I: Entrapment neuropathies of the upper limb - excluding carpal tunnel syndrome.

Ultrasound (US) is one of the methods for imaging entrapment neuropathies, post-traumatic changes to nerves, nerve tumors and postoperative complications to nerves. This type of examination is becoming more and more popular, not only for economic reasons, but also due to its value in making accurate diagnosis. It provides a very precise assessment of peripheral nerve trunk pathology - both in terms of morphology and localization. During examination there are several options available to the specialist: the making of a dynamic assessment, observation of pain radiation through the application of precise palpation and the comparison of resultant images with the contra lateral limb. Entrapment neuropathies of the upper limb are discussed in this study, with the omission of median nerve neuropathy at the level of the carpal canal, as extensive literature on this subject exists. The following pathologies are presented: pronator teres muscle syndrome, anterior interosseus nerve neuropathy, ulnar nerve groove syndrome and cubital tunnel syndrome, Guyon's canal syndrome, radial nerve neuropathy, posterior interosseous nerve neuropathy, Wartenberg's disease, suprascapular nerve neuropathy and thoracic outlet syndrome. Peripheral nerve examination technique has been presented in previous articles presenting information about peripheral nerve anatomy [Journal of Ultrasonography 2012; 12 (49): 120-163 - Normal and sonographic anatomy of selected peripheral nerves. Part I: Sonohistology and general principles of examination, following the example of the median nerve; Part II: Peripheral nerves of the upper limb; Part III: Peripheral nerves of the lower limb]. In this article potential compression sites of particular nerves are discussed, taking into account pathomechanisms of damage, including predisposing anatomical variants (accessory muscles). The parameters of ultrasound assessment have been established - echogenicity and echostructure, thickness (edema and related increase in the cross sectional area of the nerve trunk), vascularization and the reciprocal relationship with adjacent tissue.

Introduction

US is just one of the methods for imaging entrapment neuropathies, post-traumatic changes to nerves, nerve tumors and postoperative complications to nerves. In the case of all the above-mentioned, the nerve outline in cross section and longitudinal section, its inner echostructure and vascularization are assessed.

In chronic entrapment nerve pathology, edema and hyperemia are visible proximally to the compression site (fig. 1 A). On the longitudinal section, hourglass narrowing is visible and in cross section, using the so-called lift technique, it is possible to notice a change in shape from segment to segment – from an oval towards a more rounded shape, or even to a complete circle. Corresponding to this site there occurs a decrease in echogenicity, which is a sign of the presence of edema.

Fig. 1

A. Chronic neuropathy of the median nerve at the level of the carpal canal: compression site (arrow), cephalad nerve hyperemia symptoms. B. Effusive inflammation of the tendinous sheath of extensors (asterisk – effusion, arrow – median nerve). C. Intraoperative image of the synovial membrane hypertrophy. D. Swollen branch of thenar of the thumb (arrows), shaped by tumor (in histopathological examination – lipoma; asterisks)

The direct neighborhood of the nerve should always be assessed in detail with regard to any particular pathology which may have acted on the nerve through “mass effect”, such as intra-articular effusion (with a bulging of the joint recesses) (figs. 1 B, C), benign hyperplastic lesions (fig. 1 D) or osteophytes (osteophytosis). A comparative assessment with the opposite side can be very helpful, particuloklarly in doubtful cases. Indirect information about nerve condition can be achieved based on an image taken of the muscles innervated by it (if the nerve is not purely sensory of course). Muscles lose mass as a result of chronic neuropathy (fig. 1 E), undergoing degeneration, fatty atrophy, which manifests in US by an increased echogenicity and a loss of fiber structure(. The provocation of symptoms (radiating pain or parestesias) is important and can be achieved by the precise palpation in the area of the visualized pathology. US, as stated, enables dynamic assessment, this can be particularly important in postoperative conditions, where there might arise complications when pinching at the nerve is hampered by surrounding scar tissue. In terms of diagnosis, these two latter features distinguish US among other methods of diagnostics, e.g. magnetic resonance imaging (MRI).

Medial nerve

Pronator teres muscle syndrome

In the case of medial nerve pathology at the level of entry between the humeral and ulnar heads of the pronator teres muscle, edema and nerve hypervascularization is searched for proximally to the compression site. When there are no symptoms in the contralateral limb, one should compare the morphology of both nerves (taking into account their cross sectional areas before entry under the muscle and at the site of the suspected compression) and the echostructure of the pronator teres muscles, regarding the presence of hematomas, post-traumatic fibrosis or hypertrophy. Benign tumors such as lipomas and hemangiomas or productive bony lesions (posttraumatic, degenerative) may be responsible for the “mass effect”. Attention should be drawn to a very rare anatomical variant related to the presence of a supracondylar process within the anterior-medial area of the distal epiphysis of the humorous. It constitutes the area Struthers’ ligament attachment, in the direction towards the medial condyle of the humerus and the medial intermuscular septum of the arm(. This ligament, intersecting the medial nerve can be the reason of its compression. The process and ligament are easily identified in US, however the examination always demands X-ray verification.

Medial nerve compression at the level of the elbow and proximal segment of the forearm can be caused by hypertrophic or fibrous aponeurosis of the biceps brachii muscle or fibrous proximal edge (tendinousarch) of the flexor digitorum superficialis muscle (FDS)(. Inflammation of the bicipitoradial bursa as well as inflammatory pathologies of the distal tendon of the biceps brachii muscle itself can also evoke a compression effect.

Kiloh-Nevin syndrome

Pathologies of the anterior interosseus nerve (AIN) are related to the presence of pathological tissue masses localized at a distance of about 6–8 cm from the medial epicondyle, on the anterior surface of the forearm(. It can be a post-traumatic hematoma, iatrogenic, benign tumors (lipomas, hemangiomas) or anatomical variant – fibrous attachment of the pronator teres muscle or flexor digitorum superficialis muscle (FDS III), accessory ligaments from FDS to the flexor pollicis longus muscle (FPL) or its accessory head. Diagnosis of this nerve can be problematic due to the difficulties related with visualization. Hence US is not performed in order to directly asses the nerve, but to search for, in the case of clinical suspicion of AIN neuropathy, the abovementioned pathological masses and degenerative atrophic lesions in the innervated muscles (FPL, part of flexor digitorum profundus – FDP, pronator quadratus muscle – PQ)( (fig. 2).

Fig. 2

Pronator quadratus muscle (P.Q.) atrophy on the right side, normal image in the contralateral limb on the left side; radial bone (R), ulna (U)

Ulnar nerve

There are two potential areas prone to compression on the ulnar nerve trunk, at the elbow and at the wrist (Guyon's canal).

Ulnar nerve compression at the elbow

Chronic compression on the ulnar nerve at the elbow is the second most common ailment, after entrapment neuropathy of the carpal tunnel syndrome(. The ulnar nerve can undergo compression in the area of the medial intermuscular septum perforation, at the level of the groove for the ulnar nerve or at the level of the entry to the proper ulnar nerve canal (that is between the heads of the flexor carpi ulnaris muscle)(.

In US the immediate neighborhood of the nerve which may be causing compression should be excluded. This might include – inflammation in the elbow and surrounding bursas, benign hyperplastic lesions, posttraumatic ossification and degenerative osteophytosis(. The cause of this neuropathy can also be the presence of an accessory muscle, anconeus epitrochlearis, thought by some surgeons and anatomists to be a muscular execution of Osborne's ligament( (fig. 3 A). It occurs in over 30% of population and most often is asymptomatic. Additional factors predisposing to the occurrence of the neuropathy within this section of the ulnar nerve are, a relatively less fatty tissue thickness, greater size of coronoid process (more often in men) and joint valgosity (more often in women)(.

Fig. 3

A. Anconeus epitrochlearis muscle (AE), compressing ulnar nerve (arrows). B. Ulnar nerve cross section at the level of the corpus of humerus bone (cross sectional area 5 mm2) and at the level of the groove for ulnar nerve (cross sectional area 17 mm2). C. Intraoperative image of chronic pressure neuropathy of ulnar nerve – the nerve with a clear narrowing site separated by the surgical tool. D. Longitudinal section of ulnar nerve (between the cross-hair pointers) as an example of a primary neuropathy at the level of the entry between the FCU heads (FCU c.h. – ulnar head, FCU c.u. – humeral head, Hum. – humerus bone). E. Ultrasound image of the atrophic lesions in hand muscles due to chronic ulnar nerve neuropathy (L – lumbricales muscles, IO – interossei)

For many years the upper limit of what was to be considered normal, relating to the area of ulnar nerve at the level of humeral epicondyle, was thought to be 7.5 mm2(. Currently the value of 8.3 mm2 is accepted or the ratio 1.5:1 of the area at the level of the groove to the arm level(. A great help in doubtful cases is comparison with the contralateral side (fig. 3 B). Obviously, as with each chronic neuropathy, other features of nerve image are analyzed, such as the loss of bundle structure and its hyperemia. The cross sectional area of the nerve is diagnosed above all else, as its assessment using the longitudinal application is usually less useful at this level, due to its non-linear course (figs. 3 C, D).

It should be noted that dynamic assessment regarding the luxation of the nerve above the groove (asymptomatic in about 20% of population) might indicate a predisposition towards slow nerve damage. This phenomenon occurs in cases of congenital lack of Osborne's band or luxation of the medial head of the triceps muscle(.

In the assessment of chronic ulnar nerve neuropathy one should pay attention to the presence of degenerative changes in the area of the forearm muscles, innervated by the nerve (flexor carpi ulnaris – FCU, FDP III, IV) and hand (thenar muscles and III, IV, lumbrical muscle, interosseous muscle I, abductor pollicis brevis muscle – AbdPB, flexor pollicis brevis muscle – FPB)(.

Guyon's canal syndrome

As distinct from the level of the elbow, in Guyon's canal syndrome, nerve compression rarely occurs and idiopathic neuropathies at this level are within the area of casuistics(. In the majority of cases Guyon's canal syndrome is connected to the presence of abnormal tissue masses (lipomas, ganglions, ulnar artery aneurysma). The reason may also be accessory muscles, e.g. accessory digiti minimi abductor (fig. 4). It is present in 25% of population and most often is asymptomatic, only predisposing towards neuropathic development(.

Fig. 4

Ulnar nerve at the level of Guyon's canal (arrow), accessory abductor digiti minimi muscle (asterisks), OP – pisiform bone

Careful attention should be paid to the hook of the hamate bone, particularly within the context of trauma medical interview. Its fracture is easily identified using US (in contrast to X-ray or even MRI). The hook constitutes a solid, non-elastic basis on which the superficial branch of the nerve may undergo compression and repeated microdamage.

This situation occurs for example in cyclists and in people working with hammers(.

US assessment is based first of all on the search for the pathologies mentioned above. The principles of US assessment of the nerve trunk are the same as in the other pressure neuropathies.

Radial nerve

The radial nerve trunk can become compressed at two levels: at the axilla (e.g. in patients using crutches that fit to the armpit and used for a long time and in workers operating a metal press) and the spiral groove of the humerus (at the level of 1/3 of the shaft of the bone)(.

Radial nerve neuropathies at the level of the humerus bone shaft

The most frequent pathology of the radial nerve at this level is fracturing to the humerus and the consequences of treatment (discussed in more detail in part III of the article).

A rare non-traumatic reason of neuropathy at the arm level is compression of the nerve caused by the lateral head of triceps brachii muscle (e.g. in post-injection fibrosis or in badly planned training at a gym).

In US one should search for lesions in the nerve trunk (edema expressed by the change in cross sectional area and hyperemia). Assessment using the US transducer, both transversely and longitudinally, may be useful.

Supinator muscle syndrome

The profound branch of the radial nerve (PIN) may undergo chronic compression at the level of the supinator muscle. The most frequent reason is the fibrous band (fascia thickening) referred to as the arcade of Frohse (fig. 5 A), fibrosis of the edge of the extensor carpi radialis brevis muscle (ECRB) or vascular malformation causing the narrowing of this nerve(. The arcade of Frohse occurs, according to some sources, in nearly half of the population, nevertheless it very rarely causes compression of the nerve. The vessels which may are the radial recurrent arteries (along the supinator muscle) or fine blood vessels (so-called Henry's loop)(. Other rarer reasons causing chronic compression to the profound branch of the radial nerve are: pathological tissue masses in the direct vicinity (lipomas, hemangiomas and ganglia), inflammatory processes of the joint and iatrogenic damage (trapping postoperative scars, metal implants).

Fig. 5

A. Intraoperative image of PIN, exit from supinator muscle (asterisk) with a visible branch to ECRB (arrow). B. Acute bending angle (large arrow) along PIN (small arrows), at the level of the entry between profound (Sup2) and superficial part (Sup1) of the supinator muscle. C. Smooth physiological nerve bending angle at the same level

In the case of chronic compression, the nerve is initially swollen proximally to the compression site, which in US examination is visible in cross section by the sudden change in shape of the nerve (from oval to circular) and an increase in its diameter. A longitudinal application shows not only the angular flexion of the nerve but also the widening of its outline proximally to the site. An essential element during assessment is performing a dynamic examination consisting of slow forearm supination during which the nerve flexion degree markedly increases (fig. 5 B). A mild angle of flexion is observed in normal cases, while in a pathological situation the angle of flexion is very sharp (fig. 5 C). The necessity of comparing all sites, which seem incorrect, should be remembered. As in each chronic neuropathy of a motor nerve (and PIN is a purely motor nerve) one can additionally use the assessment of the innervated muscles (supinator muscle, ECRB, extensor carpi ulnaris muscle – ECU, extensor digitorum muscle – ED, extensor digiti minimi muscle – EDM, abductor pollicis longus muscle – AbdPL, extensor pollicis longus muscle – EPL, extensor pollicis brevis muscle – EPB, extensor indicis muscle – EI)(.

Wartenberg's disease

The superficial branch of the radial nerve (RSNR) may undergo chronic prickling at the point of intersection with the cephalic vein or at the point of intersection with the l extensor compartment at the level of the wrist. Nevertheless, the most frequent neuropathic reason of this branch are iatrogenic damage related to the vein cannulation, steroid injections to the I extensor compartment and to surgical procedures. Sometimes nerves can be damaged as a result of wearing a too tight watchstrap or as a result of the compression caused by handcuffs (“detainee paralysis”)(. RSNR is a purely sensory nerve thus the accessory assessment parameter can include a trial of pain radiation, carried out by compression within the visualized site. Because of its small size only the nerve diameter is analyzed in US examination, which is easier to measure than the area. It should be compared with the section above, the assessed area or with the opposite limb. Due to this reason hyperemia has little criteria of credibility.

Suprascapular nerve

The suprascapular nerve is rarely chronically damaged. However, when pathology does occur the most frequent reasons are lesions narrowing the size of the glenoid-spine notch: transverse ligament thickening, osteophyte formations in the scapular notch and benign tumors – all are perfectly visible in US(.

The best criterion of nerve assessment is measurement of diameter. The remaining elements (hyperemia, cross sectional area) are not used because of the small size and profound location of the nerve. For direct assessment one can use thickness measurement and supraspinatus and infraspinatus muscle echogenicity. One should always compare the morphology of the area with the contralateral side.

Thoracic outlet syndrome (TOS)

TOS is a syndrome of symptoms caused by chronic brachial plexus compression (mainly its inferior trunk) and infraclavicular vessels at the level of the neck and the space under the pectoralis minor muscle(.

The etiology of TOS are: incorrect anatomical relationships at the level of natural “isthmus”, through which passes the brachial plexus (the scalene muscles slit), space between I rib and the clavicle and the space under the pectoralis minor muscle(.

Scalene muscles slit

In US examination one should locate a cervical rib (fig. 6) – the bone extension of the transverse process of C7 vertebrae, most often on the left side, more rarely – bilaterally. The rib is visible as a contrasting, well-defined structure connected with the VII cervical vertebra. Only in 10% of patients does it compress the plexus, making the scalene muscle slit narrower. A fully developed cervical rib is the reason for most vascular complications, and its nucleus or fibrous band (going from the rib or transverse process) – raises neurological complications.

Fig. 6

Cervical rib with a clear acoustic shadow (asterisk), brachial plexus trunks (arrows)

Another subject for the assessment of this area are the scalene muscles(. Hypertrophy may occur due to sport or occupational overload and as a result of undergoing traumas – to their fibrosis. In the case of unilateral TOS syndrome, a comparative assessment with the opposite side will be helpful. For compression reasons one should also take into account anatomical variants of costal attachment of these muscles, including overlapping of the attachments of scalenus anterior and scalenus medius muscles, the presence of sickle-shaped attachment of scalenus anterior muscle and finally the presence of the accessory scalenus minimus muscle(.

Space between I rib and the clavicle

Incorrect relation between the clavicle and I rib can be the result of post-traumatic lesions, more rarely congenital malformation. US assessment of this area is very difficult because of lack of translucency of bony elements.

Space under pectoralis minor muscle

Wright's syndrome results from compression on the neurovascular bundle by the pectoralis minor muscle tendon or by coracoid process. In US examination the anatomy of the area should be compared with the opposite side in search for developmental anomalies, benign hyperplastic lesions, post-traumatic lesions or degenerative lesions and calcified lymph nodes. US assessment of the nerves themselves is particularly difficult and consists in visualizing the compression/ deformation area by longitudinal transducer application. The patient should remain seated during the examination because supine positions change mutual anatomic relations. It is recommended to perform the examination in different upper limb arrangements, particularly by abduction(.

Conclusion

The presented study indicates the usefulness of US in the diagnostics of entrapment neuropathy of the upper limb. Lesions most often occur at the level of the wrist (median nerve) and elbow (ulnar nerve). These nerves are perfectly visible using US examination and the method has credible assessment criteria correlating with neurophysiological studies. In the case of small or profoundly localized nerves, the auxiliary elements of US assessment are the observation of indirect symptoms – lypodystrophy lesions in the innervated muscles or pain or parestesias provoked by compression in the area of the pathology.

US examination visualizes the etiology of compression syndrome, facilitating the planning of surgical procedure.