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Peripheral Nerve Imaging: The Carpal Tunnel
The most common peripheral nerve compression syndrome is compression neuropathy of the median nerve at the carpal tunnel. The median nerve innervates the wrist and sends cutaneous sensory branches to innervate the palm, the palmar aspect of the first three digits, and the radial half of the fourth digit. The median nerve also innervates the superficial flexors of the first three rays, as well as the pronator teres, the flexor pollicis longus and brevis, the abductor pollicis, the lateral aspect of the flexor digitorum profundus, the pronator quadratus, the first and second lumbricals, and the interossei of the hand. Compression of the median nerve within the carpal tunnel causes median nerve dysfunction, including numbness and paresthesias of the tips of the thumb, index, and middle finger. The condition is frequently bilateral. Women are affected more often than men.
The diagnosis of carpal tunnel syndrome is typically made using the history and physical examination along with electrophysiologic testing. Imaging techniques, specifically ultrasonography and MRI, are used in equivocal cases to provide anatomic information not achievable by electrophysiologic testing alone. Current ultrasound techniques using high-frequency linear-array transducers of 7 to 15 MHz provide exquisite display of the carpal tunnel, the contained tendons and vessels, and the superficially situated median nerve.
ANATOMY
The carpal tunnel is a fibro-osseous tunnel on the volar aspect of the wrist. The volar border of the carpal tunnel is formed by the flexor retinaculum. The flexor retinaculum extends from the scaphoid and the tubercle of the trapezium on the radial aspect of the wrist to the pisiform and the hook of the hamate on the ulnar aspect. Normally, the flexor retinaculum is nearly straight or slightly convex ventrally. The floor of the carpal tunnel is formed by the carpal bones, predominantly the capitate, trapezoid, and part of the hamate ( Fig. 28-1 ).
The 10 structures that traverse the carpal tunnel include the four tendons of the flexor digitorum superficialis, the four tendons of the flexor digitorum profundus, the tendon for the flexor pollicis longus, and the median nerve. The eight digital flexor tendons are enclosed within a common synovial sheath. The single tendon for the flexor pollicis longus is contained within its own synovial sheath along the radial aspect of the other flexor tendons within the carpal tunnel. The median nerve resides just deep to the flexor retinaculum and abuts upon its inner surface. It is usually located on the radial side of the flexor digitorum superficialis.
Guyon's canal is a separate fibro-osseous canal formed by the transverse and palmar carpal ligaments at the ulnar aspect of the wrist. It lies adjacent to, but separate from, the carpal tunnel and contains the ulnar artery, ulnar vein, and ulnar nerve.
IMAGING
Ultrasonography
Ultrasound exquisitely displays the median nerve at the wrist and distinguishes it from the adjacent tendons on the basis of four features: (1) the nerve's intrinsic echotexture, (2) persistence of that texture in all planes (isotropy), (3) superficial position of the median nerve within the carpal tunnel, and (4) ready differentiation between the moving tendons and the relatively nonmoving median nerve during finger flexion.
Intrinsic Echotexture
In-vitro studies at 15 MHz document that peripheral nerves display a distinct “speckled” fascicular pattern that derives from their internal structure. Each nerve fiber is invested by the endoneurium. These fibers group into fascicles that are invested by perineurium. The fascicles then group into nerve trunks that are invested by epineurium. The epineurium surrounds the entire nerve and extends inward, around the fascicles, as the interfascicular epineurium, binding the nerve together. The endoneurium and perineurium are too thin for ultrasound to resolve as separate structures. The epineurium, however, is a thick sheath containing loose connective tissue, elastic fibers, and vessels. Ultrasound resolves the epineurium well and thereby delineates the size, position, and configuration of the median nerve within the carpal tunnel.
In longitudinal sonographic sections the median nerve shows multiple hypoechoic, parallel-but-discontinuous lines separated by echogenic bands. In transverse sections, the nerve shows a round to elliptical contour and exhibits round hypoechoic spaces within an echogenic background. Correlation of ultrasound images with histologic sections demonstrates that the hypoechoic spaces correspond to the nerve fascicles whereas the echogenic background corresponds to the interfascicular epineurium. This ultrasound appearance is reproduced in clinical studies using high-frequency linear-array transducers of 10 to 15 MHz ( Fig. 28-2 ).
In ultrasound studies performed with lower-frequency transducers, the hypoechoic spaces within the nerves appear less defined and less numerous (i.e., blurred). This blurring may be due to poor lateral resolution with “coalescence” of adjacent structures of similar echogenicity, to reverberation artifact from hyperechoic stroma, and/or to inability to depict the nerve fascicles unless they are oriented perpendicular to the direction of the ultrasound beam.
Normal tendons display a fibrillar echotexture. When the tendon is insonated at right angles to its fibers, it shows strong, bright, highly ordered echoes reflecting from its fibers. In longitudinal section, these fibers form thick bands of tightly packed, parallel, linear echos. In transverse section, the fibers form round arrays of closely grouped, dot-like echoes (see Fig. 28-2 ). The differing echotextures of nerve and tendon enable the sonographer to distinguish the speckled fascicular median nerve from the hyperechoic fibrillar tendon in both transverse and longitudinal ultrasound sections.
Isotropy
In this context, isotropy signifies the unchanging appearance of a structure when the structure is examined from different directions. Anisotropy signifies that the appearance of the structure changes significantly with changing direction of examination. The median nerve is isotropic. Longitudinal and transverse sections through the median nerve display the same fascicular sonographic pattern. Naturally, that pattern appears “speckled” in transverse section and more linear in longitudinal section, but both planes of study show the fascicular pattern. Tendons are strongly anisotropic. Their echotexture changes markedly with changing sonographic sections. Because the tendon has a highly ordered structure with superimposed planes of collagen and septa, the characteristic fibrillar pattern is demonstrated only when the ultrasound beam is oriented perpendicular to the structural planes. Changing the angle of insonation by angling (“rocking”) the transducer to and fro causes great variability in the intensity and the sonographic pattern of the fibrillar echoes. As the angle of insonation deviates from perpendicular, the fibrillar structure becomes less intense and less distinct. At too great an angle of insonation, the sonostructure almost disappears, leaving “hollow cylinders” designated tendon ghosts ( Fig. 28-3 ). These differences in (an)isotropy enable the sonographer to distinguish the isotropic median nerve from the strongly anisotropic tendon.
Superficial Position of the Median Nerve
Within the carpal tunnel, the flexor tendons for the fingers and thumb are situated dorsally, close to the radius, ulna, and carpal bones. The median nerve is situated ventrally, just deep to the flexor retinaculum, toward the radial aspect of the tunnel. In the supinated, volar position used for ultrasonographic examination of the carpal tunnel, the median nerve characteristically lies “superficial” to the tendons, immediately deep to the flexor retinaculum ( Fig. 28-4 ).
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