high-resolution sonography of compressive neuropathies of the wrist

Pictorial Essay

High-Resolution Sonography of CompressiveNeuropathies of the Wrist

Stefano Bianchi, MD,1* Xavier Montet, MD,1 Carlo Martinoli, MD,2 Florent Bonvin, MD,1

Jean Fasel, MD3

1Division of Radiodiagnostics, Hopital Cantonal Universitaire, Rue Micheli-du-Crest 24, CH-1211,Geneva 14, Switzerland2Department of Radiology R, University of Genoa, Italy3Departement de Morphologie, Centre Medical Universitaire de Geneve, Geneva, Switzerland

Received 15 April 2003; accepted 29 July 2004

Compressive neuropathies (CN) occur when aperipheral nerve (PN) is subject to excessive

and longstanding pressure.1–3 The increasinginterest in CN of the wrist can be explained bytheir growing frequency, financial consequencesdue to work absence, and new diagnostic possibil-ities. Although classically based on electrodiag-nostic studies, evaluation of CN is now possiblewith noninvasive imaging modalities, such as MRIand sonography. Recent advances in sonographictechnology allow accurate evaluation of the handand wrist, of small PNs, and of possible causes ofCN.4–9 This article reviews the normal anatomy ofPNs and the basic pathophysiology of CN; thenormal anatomy of the carpal and Guyon’s tun-nels, together with a description of their sono-graphic examination and sonoanatomy; and thesonographic appearances of the main conditionscausing wrist CN.

NORMAL ANATOMY AND SONOANATOMYOF PNs

The normal nerve is composed of nerve fibersinvested by the endoneurium, a thin layer of con-nective tissue, which packed together compose thenerve fascicles. The fascicles are surrounded bythe perineurium. The size and number of fascicles

that form a nerve depend on the individual nerve,distance from the nerve origin, and amount ofpressure to which the nerve is subjected. Thenerve trunk envelope is called the superficial epi-neurium. The interfascicular epineurium is foundamong fascicles. The endo- and perineurium arethin membranous structures, whereas the epineu-rium is a thick sheath containing loose connectivetissue with elastic fibers and vessels.

High-frequency sonography can accurately visual-ize the echotexture of normal PNs, both in vitro andin vivo.10–12 On longitudinal sonograms, they appearas multiple hypoechoic parallel but discontinuouslines separated by hyperechoic bands. On transversescans, the hypoechoic areas correspond to oval orrounded images, encircled by a hyperechoic back-ground (Figure 1). Histologically, the hypoechoicbands correspond to the fascicles that run longi-tudinally within the body of the nerve, while thehyperechoic background reflects the interfascicularepineurium. The number of fascicles detectable onsonograms is smaller than their actual number, andthe number of fascicles visualized decreases with thefrequency of the transducer utilized.

PATHOPHYSIOLOGY OF CN

The PNs are elastic structures that can adapt anddeform secondary to external forces. They run inthe extremities surrounded by fat and connectivetissue, which reduce friction between the nervesand adjacent rigid anatomic structures. Whencrossing joints, PNs are usually contained in osteo-fibrous tunnels that prevent their dislocation dur-ing articulation. Because of their relatively fixed

Correspondence to: S. Bianchi

*Current address: Fondation et Clinique de Grangettes, Chene-

Bougeries, Switzerland

J Clin Ultrasound 32:451–461, 2004; Published online in Wiley

InterScience (www.interscience.wiley.com).DOI: 10.1002/jcu.20065

� 2004 Wiley Periodicals, Inc.

VOL. 32, NO. 9, NOVEMBER/DECEMBER 2004 451

situation within these tunnels, however, the nervesare subject to compression by external forces.

In entrapment neuropathies, nerve changes andclinical symptoms are dependent on both the degreeof compression and its duration. Low pressureapplied for a short time is relatively well tolerated,and complete recovery can be expected after with-drawal of the external force. Conversely, high pres-sure for long periods of time can severely alter themorphology and function of nerves and eventuallycan result in irreversible damage. External compres-sion on a PN causes internal structural changes.First, impaired venous flow leads to increased intra-neural interstitial pressure that results in reversibleintraneural edema, mainly found at the level of theconnective tissue. In more prolonged compressions,ischemia due to damage of the vasa nervorum, thesmall vessels that are responsible for vascularizationof the nerve trunk, leads to irreversible internalfibrosis. This culminates in degeneration of themye-lin sheath and axons. Such morphologic changesalter the function of nerve conduction, with subse-quent sensory or motor impairment depending onthe type of nerve affected.

Wrist CNs aremainly a consequence of nerve com-pression inside inextensible fibro-osseous tunnels.The most frequent is the carpal tunnel syndrome(CTS), which follows entrapment of the mediannerve (MN) inside the carpal tunnel (CT). Entrap-ment of the ulnar nerve (UN) at the Guyon’s tunnel(GT) is less frequent.

NORMAL ANATOMY

The main nerves of the wrist are the MN and UN.The MN enters the hand via the CT, a fibro-osseous

canal located at the volar aspect of the wristand delimited dorsally by the carpal bones and theinextensible volar transverse carpal ligament (TCL).The TCL is a thick fibrous structure that continuesthe superficial fascia of the forearm andmergeswiththe palmar aponeurosis of the hand. Anatomically,2 layers compose the TCL. The superficial layer isformedby thepalmaris longus tendon that continuesinto the palmar aponeurosis. The deep layer is madeby transverse fibers forming a rectangular ligament,3–4 cm wide, that inserts into the tuberosity of thescaphoidand thepisiformand into the tubercle of thetrapezium and the hook of the hamate. A verticalseptum originates from the inferior radial aspect oftheTCLand formsa small lateral fibrous tunnel thathouses the flexor carpi radialis tendon. The CT con-tains the 8 tendons of the flexor digitorum super-ficialis and profundus and the tendon of the flexorpollicis longus. Synovial sheaths surround all thesetendons and facilitate their gliding during flexionand extension of the fingers. Although the arrange-ment of the synovial tendon sheaths varies, there areusually 2. The lateral sheath surrounds the flexorpollicis longus, while the larger medial one envelopsthe remaining tendons.A variety of internalmusclesmay be found inside the CT, resulting from a morecranial insertion of the lumbricales or distal inser-tion of flexor digitorum anomalous bellies. Atapproximately 5 cm from the proximal wrist crease,the MN leaves the palmar cutaneous branch thatsupplies the proximal portion of the palm of thehand.13 The branch initially remains bound to themain trunk; after it becomes detached, it runsbetween the palmaris and flexor carpi radialis ten-don and then reaches the hand, running superficialto the TCL. Knowledge of the anatomy of the super-

FIGURE 1. In vivo normal sonographic appearance of peripheral nerves. Transverse (A) and longitudinal (B) sonograms obtained at the level of the

carpal tunnel show the median nerve (arrow) as a hypoechoic structure composed of multiple anechoic nerve fascicles (arrowheads), surrounded

by the hyperechoic perineurium. In part B note the fibrillar pattern of the flexor tendons (FT), which differs from that of the MN.

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452 JOURNAL OF CLINICAL ULTRASOUND

ficial branch is important because this small struc-ture can be inadvertently injured during CT release.The MN enters the tunnel along with the flexortendons. Inside the CT, it lies just under the retina-culum, superficial and parallel to the flexor tendonsof the second and third fingers and medial to theflexor pollicis longus tendon. Distal to that, thenerve gives off sensory branches to the first, second,and third fingers and the radial half of the fourthfinger anda thenarmotorbranch,which supplies thethenarmuscles. Depending on the origin and course,3 types of thenar motor branches can be described:extraligamentous, subligamentous, and transliga-mentous. The different types must be recognized toavoid inadvertent surgical sectioning. Some ana-tomic variations (bifid MN and persistence of themedian artery) can cause CTS and can affect thesurgical approach. These variations can be easilydetected on sonograms.

The smaller GT is located palmar and medial tothe CT.14 It is formed by the palmar carpal liga-ment, which forms the roof, and by the TCL, thepisiform, and the hook of the hamate, which consti-tute the walls. The tunnel houses the UN, the ulnarartery, and veins surrounded by loose connectivetissue and fat.

SONOGRAPHIC TECHNIQUE AND NORMALSONOANATOMY

At the level of the distal forearm, the MN and theUN can be detected by sonography.8,10,15 Knowledgeof the normal anatomy and of the nerves’ internalechotexture is necessary for their detection. Mostsonologists can detect both nerves at the wrist andthen by moving the transducer cranially can followthem in the forearm.Detection of the smallernerves,such as the superficial branch of the radial nerve, ismore difficult, requiring very high frequency trans-ducers and experience in PN evaluation.

Sonograms can accurately depict both tunnels andtheir contents.5,16 The CTmay be visualized on bothtransverse and longitudinal sonograms. If obtainedwith the appropriate transducers, transverse scansallow simultaneous evaluation of all intracanalstructures (Figure 2). Sonograms must be firstobtained proximal to the CT at the level of thepronator quadratus muscle to localize the differentanatomic structures before they enter the canal.Additionally, it is important to explore the cranialportion of theMNbecause in CTS the nerve is widerhere than it is inside the CT. The proximal portionof the CT is then examined. Identification of bone

FIGURE 2. In vivo normal sonographic appearance of the median nerve at the level of the wrist. schematic of the surface anatomy (A) and

transverse sonograms obtained from cranial (B), mid-point (C), and distal (D) locations are depicted. Cranially (B), the nerve (open arrow) lies

anterior to the pronator quadratus muscle (PQ). Note the ulnar nerve and ulnar artery (arrowheads). At the level of the proximal carpal tunnel (C),

the nerve lies inside the tunnel in a superficial location, between the transverse carpal ligament (arrows) and the flexor tendons (FT). Sc, scaphoid;

Pi, pisiform. At the distal end of the tunnel (D), the transverse carpal ligament appears rectilinear and the median nerve more flattened. TZ,

trapezium; H, hook of the hamate.

COMPRESSIVE NEUROPATHIES OF THE WRIST


landmarks is essential to the examination. Imagesobtained at the proximal side of the tunnel are ade-quate if they show the rounded appearance of boththe pisiform and distal scaphoid. Both bones arecovered by flexor tendons: the flexor carpi radialislies on the scaphoid, while the flexor carpi ulnarisinserts into the pisiform and, by its more superficialfibers, continues distally as the pisohamate liga-ment. The TCL appears at this level as a curvedhyperechoic band connecting the lower face of thepisiform to the scaphoid. Beneath the ligament arethe flexor tendons and the MN. Since the tendonsrunobliquely, the transducermustbe tilted craniallyto correctly depict them as hyperechoic structures.

Each tendon can be localized by its anatomicposition. The flexor pollicis longus tendons areeasily visualized as oval structures just deep andlateral to the flexor carpi radialis. Changes in be-havior during dynamic scanning may help in dif-ferentiating among the tendons. For example, theflexor digitorum profundus tendon of the third fin-ger can be discriminated from the others because ofits movements during passive flexion and extensionof the distal interphalangeal joint of the correspond-ing finger. The MN appears as an oval hyperechoicstructure containing multiple rounded formations.It lies very superficially, in a radial position, justunder the TCL. The internal echotexture of thenerve and its relatively fixed position during fingermotions are helpful in its detection. Bone land-

marks on the distal side of the tunnel are the flatvolar surface of the trapezium and the small palmarsurface of the hook of the hamate.

Compared with the proximal side of the tunnel,the distal side is small due mainly to a decrease inthe laterolateral diameter. The distal TCL appearsthicker and more rectilinear than the proximal por-tion. At this level the tendons can hardly be differ-entiated because they are crowded inside thesmaller distal portion of the canal. TheMN appearsflatter than on the proximal side. Longitudinalsonograms show the flexor tendons as linear hyper-echoic structures with the typical internal fibrillarechotexture. The MN presents the characteristicfascicular internal appearance.11 Slight palmarflexion of the wrist will move the MN parallel tothe transducer and allow an accurate assessment ofits internal echopattern (Figure 3). Dynamic exam-ination during finger motion reveals only small dis-placement of the nerve compared with the widetendon movements. The occurrence of a bifid MN,found in as many as 2.8% of normal subjects,appears on sonograms as a splitting of the nerve atthe proximal level (Figure 4).17–19 In the distal por-tion of the canal, the 2 nerves are closer to 1 anotherand are difficult to distinguish. The occurrence ofan accessory artery (median artery) located close tothe MN inside the CT is a well-known anatomicvariant (Figure 5). The artery usually originates inthe forearm from the ulnar artery and then joins

FIGURE 3. In vivo normal sonographic appearance of the median nerve examined at the level of the wrist. Schematic of the surface anatomy (A)

and longitudinal sonograms obtained with the wrist in a neutral position (B) and slightly flexed (C). Note the superficial position of the median

nerve (arrows) inside the carpal tunnel. The flexor tendons (FT) lie deeper. In part B, the distal nerve is parallel to the probe and its internal

structure can be more accurately assessed. Cap, capitatum; L, lunate.

BIANCHI ET AL


the MN to enter the CT. Thrombosis of the medianartery has been associated with CTS in some cases.

Transverse sonograms of the GT are alsoobtained at both the proximal and distal sides(Figure 6). Proximal sonograms are acquired atthe level of the pisiform and show the thin palmarcarpal ligament overlying the pulsatile ulnar artery.With accurate inclination of the transducer, theUN is clearly visualized lying between the arteryand themedial pisiform. Distal sonograms show thesuperficial ulnar artery and branch of the UN over-lying the tip of the hook of the hamate. In a moremedial and posterior position, the deep ulnar arteryand the deep (motor) branch of the UN are sur-rounded by the hypothenar muscles. Longitudinalsonograms are of limited help in assessment of theGT. Accessory muscles located inside the GT canreduce the size of the canal and facilitate CNs; theabductor digiti minimi is the muscle most fre-

quently involved. It usually lies superficially withinthe tunnel and appears as a hypoechoic structure.The transverse diameters of the muscles in theaxial plane seem to correlate with the degree ofcompression of the UN.20

CTS

Sonographic changes in CTS have been describedextensively.21–24 They can be divided into 3 groups:(1) changes in the appearance of the MN, which arenonspecific and not correlated with the cause ofCTS; (2) changes in the appearance of the CT; and(3) changes related to the causes of compression.

Changes in Appearance of the MN

Regardless of the cause of compression, the MNpresents a swollen appearance at the proximal sideof the tunnel and flattens at the distal side (Figures 7

FIGURE 4. Anatomic variant of the median nerve: bifid nerve. Schematic of the surface anatomy (A) and transverse sonogram (B) obtained at the

level of the distal forearm show 2 nerves (arrows) that lie superficial to the flexor tendons (FT).

FIGURE 5. Anatomic variant of the median nerve: median artery. Schematic of the surface anatomy (A) and transverse power Doppler sonogram

(B) obtained at the level of the proximal carpal tunnel show the median nerve (open arrow) and the adjacent median artery (white arrow) with the

tunnel. The small ulnar artery (white arrowhead) and the ulnar nerve (open arrowhead) lie medial to the median nerve. Pis, pisiform.



FIGURE 6. In vivo normal sonographic appearance of the ulnar nerve examined at the level of the wrist. Schematic of the surface anatomy (A) and

transverse sonograms obtained cranial (B) and distal (C) to the Guyon’s tunnel. At a cranial level (B), the ulnar nerve (open arrowhead) lies next to

the ulnar artery (white arrowhead) inside the Guyon tunnel. Note that both structures are lateral to the pisiform (Pis). More distally (C), the

superficial sensitive branch (open arrowhead) and the corresponding artery (white arrowhead) are volar to the hook of the hamate (H), while the

deep branch (open arrow) lies medial to it, embedded within the hypothenar muscles (asterisks).

FIGURE 7. Carpal tunnel syndrome: median nerve shape changes. Longitudinal (A) and proximal (B) and distal (C) transverse sonograms obtained

at the level of the carpal tunnel in a patient suffering from carpal tunnel syndrome. Images show the enlarged hypoechoic proximal nerve (open

arrows), the flattened nerve inside the tunnel (white arrowheads), and the swelling of its distal portion (white arrows). In part C, note volar bulging

of the transverse carpal ligament. FT, flexor tendons.

and 8). A nerve cross-sectional area greater than0.09 cm2 at the proximal side of the tunnel hasbeen reported to be a valuable criterion for the diag-nosis of CTS. The flattening ratio is obtained on thedistal side. It is calculated by dividing the laterolat-eral diameter of the MN by its anteroposterior dia-meter. A value greater than 4 is associatedwithCTS.High-frequency transducers can also show intra-neural changes (ie, reduced echogenicity, a disorga-nized internal fascicular pattern) probably due toedema and fibrosis. Color Doppler imaging canshow increased signals in the perineural plexus andintraneurally due to acute neuritis.

Changes in Appearance of the CT

In the majority of patients, because of the increasedintracanal pressure in CTS, the TCL has a pro-

nounced convex appearance. This sign, the ‘‘bul-ging’’ of the TCL, may be found at either theproximal or distal side of the tunnel. Objectiveassessment of bulging can be obtained by measur-ing the distance from the most superficial point ofthe TCL to a line connecting the tips of the hook ofthe hamate and the tubercle of the trapezium. Adistance of 4 mm or more is considered abnormaland is associated with CTS. Sometimes increasedthickness of the TCL is noted together with focalcalcifications.

Demonstration of Causes of Compression

ThevastmajorityofCTSisduetononspecifictenosyn-ovitis of the flexor tendons. Sonograms demonstratetenosynovitis as a hypoechoic halo surrounding theflexor tendons due to fluid effusion and a thickened

FIGURE 8. Carpal tunnel syndrome: median nerve shape changes. Longitudinal (A) and transverse (B, C) sonograms obtained at a more distal level

than in Figure 7 show more clearly the swelling of the distal median nerve (white arrows). The longitudinal images show the ‘‘cobra head sign.’’

Cap, capitate; FT, flexor tendons; L, lunate.



synovium (Figure 9). Typically, tenosynovitis facili-tates visualization and differentiation of the tendonsrunning within the tunnel. Color Doppler imagingcan detect hyperemic changes within the inflamedsynovium.

Focal masses inside the tunnel are easilydetected on sonography. Ganglia are cysticlesions with a fibrous wall, filled by thick muci-nous fluid (Figure 10). They are depicted onsonograms as hypo- or anechoic expansiblelesions with multilobulated and sharply definedborders. Internal septa can be seen, leading to amultiloculated appearance. In older ganglia, theperipheral wall and internal septa can thicken toalmost completely fill the cavity. No internal flowsignals are depicted by color Doppler imaging.Amyloid deposits can be seen in longstandingrenal failure. The substance accumulates in apara-articular site and can cause CTS. On sono-grams, amyloid deposits appear as hypoechoic,ill-defined lesions. Clinical correlation is a pre-requisite for diagnosis. Sonography can also detect

anomalous muscles located inside the CT; theyappear as hypoechoic masses that exhibit thesame internal echotexture as normal peripheralmuscles. Dynamic examination is particularlyuseful in their assessment by showing the muscleentering and leaving the CT during flexion andextension of the fingers. The evaluation of ahypertrophied callus due to fracture of the distalradius is best evaluated by standard radiographyor computed tomography. Hypertrophied callusappears on sonograms as a cortical line displa-cing the adjacent soft tissues. A volar subluxed ordislocated semilunar is readily detected by sonog-raphy inside the CT by its characteristic outline.Neurogenic tumors located inside the CT are arare cause of CTS. They appear as hypoechoicmasses with well-defined borders located closeto the MN. Definite differential diagnosisbetween schwannomas and neurofibromas canbe difficult, if not impossible, on sonography.Schwannomas are usually eccentric to the nerve(Figure 11) and can show a high level of internal

FIGURE 9. Carpal tunnel syndrome: flexor tendon tenosynovitis. Longitudinal (A) and transverse (C and D) sonograms show an anechoic fluid

effusion (asterisks) located inside the flexor tendon sheath (FT). The median nerve (arrows) appears swollen proximally and thinned inside the

carpal tunnel. In part D, obtained at the proximal tunnel, note volar bulging of the transverse carpal ligament (open arrowheads; also evident in

part B and compression of the median nerve by tenosynovitis.

BIANCHI ET AL


color Doppler signals, while in neurofibromasnerve fascicles are shown to enter relatively avas-cular tumors.

Sonography also has a role in the assessment ofrecurrent symptoms after CTS release, whichmainly occur because of incomplete surgical sec-tioning of the TCL or postsurgical perineuralfibrosis. Incomplete sectioning of the TCL isusually limited to the distal portion. In thesecases, sonography usually can demonstrate thatthe proximal portion of the TCL is swollen andinterrupted, while the distal portion of the liga-ment appears continuous (Figure 12). Fibrosispresents as an irregular, ill-defined hypoechoicarea surrounding the MN. Dynamic examinationmay show adhesions between the MN and theadjacent structures.

GUYON TUNNEL SYNDROME

Compared with CTS, ulnar neuropathy at theGuyon’s canal is rare. Depending on the site ofcompression (cranial or distal to nerve bifurca-tion), various syndromes due to the involvementof either the main nerve trunk or its motor orsensitive branches have been described.

Chronic external pressure caused by repetitiveuse of tools during manual work or sport activities(such as biking), in which chronic stress is appliedon the ulnar aspect of the volar wrist, can causenerve entrapment (Figure 13). Focal masses arereadily detected on transverse sonograms. Suchmasses include ganglion cysts affecting the piso-triquetrum joint space, accessory muscles, andpseudoaneurysms of the ulnar artery.

FIGURE 11. Schwannoma of the median nerve. Longitudinal (A) and transverse (B) sonograms show an anechoic mass (arrows) adherent to the

median nerve (arrowheads) and displacing the flexor tendons (FT). The sonographic findings are typical for a nerve tumor. The eccentric,

peripheral position of the tumor strongly suggests a schwannoma (confirmed at surgery). L, lunate; R, radius.

FIGURE 10. Carpal tunnel syndrome: ganglion cyst. (A) Photograph of the affected site. (B) Longitudinal sonogram shows a well-defined,

multiloculated anechoic mass (asterisks) that displaces the median nerve (arrows) as well as the flexor tendons (FT). The sonographic appearance

is highly suggestive of a ganglion cyst (confirmed at surgery).



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FIGURE 13. Microtraumatic partial tear of the ulnar nerve. Transverse sonogram (A) obtained at the level of the pisiform (Pis) shows a focal

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FIGURE 12. Carpal tunnel syndrome: recurrence after incomplete sectioning of the transverse carpal ligament. Transverse sonograms obtained at

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BIANCHI ET AL


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high-resolution sonography of compressive neuropathies of the wrist

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