Median nerve instability in the wrist: Ultrasound diagnosis.

Instability of the nerves, which does not follow traumatic injury, is relatively rare. The ulnare nerve is most typically involved. Median nerve instability is far less common. The identification of instability of the median nerve in the wrist is definitely an indication for ultrasound examination before carpal tunnel release.

The median nerve, in the carpal tunnel, runs between the tendons for the second and third fingers of the flexor digitorum superficialis and the tendon for the second finger of the flexor digitorum profundus, and it is surrounded by a synovial sheath or bursa on the ulnar side.

It runs immediately beneath the flexor retinaculum or transverse carpal ligament, and it is covered by a layer of celluloadipose tissue that adheres to the adjacent ulnar bursa.

The flexor retinaculum is located between the distal radius to the base of the third metacarpal; it is inserted medially on the pisiform and the hamate and laterally on the tuberosity of the scaphoid and on the os trapezium.

The flexor retinaculum has three sections: the proximal (thin and continuous with the deep investing forearm fascia), the midportion or transverse carpal ligament proper (thickest) and the distal (composed of an aponeurosis between the thenar and hypothenar muscles, which is thin again).

Despite its high intrinsic stiffness and strength, the flexor retinaculum is not a major factor in the maintenance of the stability of the carpal arch, which is contrary to the intercarpal ligaments that form the carpal tunnel floor and are essential for the stability of the carpal tunnel.

Instability of the nerves, which does not follow traumatic injury, is relatively rare and almost exclusively affects the ulnar nerve in the cubital tunnel; ulnar nerve instability is often asymptomatic.

In our case image, during wrist flexion, the median nerve (arrow) dislocates, radial to ulnar, with respect to the flexor digitorum superficialis muscle and tendons, particularly in respect to those for the second and third fingers (Figure 1), to return to its natural location with the subsequent extension (Figure 2) (Supporting Information Video S1). The probe was placed at the proximal carpal tunnel parallel to the flexor retinaculum. The wrist was examined following a clinical diagnosis of carpal tunnel syndrome (pain in the first three fingers and ulnar side of the fourth, exacerbated by movement); however, the median nerve did not appear enlarged, there were no macroscopic alterations, and the normal fasciculated aspect was maintained, relatively common ultrasound appearance in carpal tunnel syndrome. To our knowledge, the instability of the median nerve in the wrist has never been described previously.

FIGURE 1

Axial sonograms show, during wrist flexion, the median nerve (arrow) dislocates, radial to ulnar, with respect to the flexor digitorum superficialis muscle and tendons (FDS) (A–C). FCR, flexor carpi radialis; RA, radial artery

FIGURE 2

Axial sonograms show, during wrist extension, the median nerve (arrow) returns to its natural location (A–C). FCR, flexor carpi radialis; FDS, flexor digitorum superficialis; RA: radial artery

The etiology is unknown, but it is logical to think that it is due to an insufficiency of celluloadipose tissue that adheres the median nerve to the adjacent ulnar bursa, rather than an insufficiency of the transverse carpal ligament.

If the changes in median nerve mobility in carpal tunnel syndrome are well described in the medical literature, it is possible that median nerve hypermobility can cause carpal tunnel syndrome.

Therefore the instability of the median nerve in the wrist could be a cause of carpal tunnel syndrome, but surely the identification of instability of the median nerve in the wrist is definitely an indication for ultrasound examination before carpal tunnel release.

CONFLICT OF INTEREST

No conflict of interest to be disclosed.

Supplementary video S1 clip. During wrist flexion, the median nerve dislocates, radial to ulnar, with respect to the flexor digitorum superficialis muscle and tendons, particularly in respect to those for the second and third fingers (arrow), and returns to its natural location with the subsequent extension (arrow).

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