Median Nerve

Normal Anatomy

The median nerve supplies the radial side of the flexor portion of the forearm and hand, and is formed by the blending of the lateral and medial cords of the brachial plexus. It contains both motor and sensory fibers from the C5, C6, C7, C8 and T1 nerve roots. The median nerve descends in the arm in close relationship to the brachial artery. It has no branches at the level of the arm. At

Ulnar Nerve Transposition
Fig. 12. Subcutaneous anterior transposition of the ulnar nerve. Axial T1-weighted image shows a mildly swollen ulnar nerve (arrow) surrounded by a thickened fascial sling and scar tissue (arrowheads).

the elbow, the median nerve is found medial and parallel to the brachial artery as it travels in between the bicipital aponeurosis (lacertus fibrosus) and parochialism muscle. The median nerve then courses into the forearm deep to the pronator teres muscle [35]. The median nerve supplies all superficial ventral muscles of the forearm, except for the flexor carpi ulnaris muscle, including the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis.

Approximately 2 to 5 cm below the level of the medial epicondyle, in close proximity to the bifurcation of the brachial artery into the radial and ulnar arteries, the median nerve gives off the anterior interosseous nerve. This courses over the flexor digitorum profundus and along the interosseous membrane toward the wrist joint. All the ventral deep muscles of the forearm, including the radial half of the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus muscles, are innervated by the anterior interosseous nerve, except for the ulnar half of the flexor digitorum profundus, which is supplied by the ulnar nerve.

MRI Anatomy

Prominence of the pronator teres and parochialism muscles often obliterates the perifascial fat planes, making it difficult to visualize the median nerve on MR images at the level of the elbow (see Fig. 8). In the proximal forearm, easier identification can be achieved as the nerve travels in the fat plane between the superficial humeral and deep ulnar heads of the pronator teres muscle and underneath the lacertus fibrosus.

Pathology: Pronator Syndrome

The pronator syndrome is the most common cause of median nerve entrapment at the elbow. Elbow fractures and dislocations [44], accessory muscles such as Gantzer's muscle (the accessory head of the flexor pollicis longus muscle), soft tissue masses, and dynamic forces at the elbow have also been implicated. Honeymooner's paralysis is related to median nerve compression secondary to prolonged pressure of a lover's head against a partner's forearm.

Clinical signs of median neuropathy include weak pronation of the forearm, weak flexion, and radial deviation of the wrist associated with thenar atrophy and inability to oppose or flex the thumb.

Numbness in the median nerve distribution with repetitive pronation/supination of the forearm, but not with flexion and extension of the elbow, is a frequent presenting symptom of pronator teres syndrome. On physical examination, forearm pain elicited by resistance to pronation or resistance to isolated flexion of the third and fourth proximal interphalangeal joints can be observed. Electromyographic studies may show only mildly reduced conduction velocities.

There are four potential compression sites within the spectrum of pronator teres syndrome: (1) the supracondylar process/ligament of Struthers, (2) the lac-ertus fibrosus, (3) the pronator teres muscle, and (4) the proximal arch of the flexor digitorum superficialis muscle [45].

The supracondylar process syndrome is the least common compression neuropathy of the median nerve. The supracondylar process or avian spur arises from the distal humerus about 5 to 7 cm above the elbow joint (Fig. 13) [46]. The lack of radiographic identification of the supracondylar process does not exclude the presence of compressive median neuropathy caused by the ligament of Struthers. This structure, which spans from the supracondylar process to the medial epicondyle, may cause median nerve, and to a lesser extent ulnar nerve entrapment [46]. The compression is worsened with extension and supination.

The median nerve travels deep to the lacertus fibrosus at the level of the elbow. This latter structure emanates from the distal bicipital tendon coursing obliquely over the pronator-flexor group of muscles to insert on the antebra-chial fascia. Compression of the pronator muscle and median nerve may be secondary to a thickened lacertus fibrosus.

The most frequent etiology for pronator syndrome is dynamic compression of the median nerve between the superficial humeral and deep ulnar heads of the pronator teres muscle [47]. In up to 50% of anatomic specimens, fibrous bands can be found dorsal to the humeral head or to the nerve itself. Median nerve compression by these bands is more pronounced in pronation and elbow extension, when the distance between the two heads of the pronator teres muscle is narrowed. The fibrous arch of origin of the flexor digitorum superficialis muscle is the most distal as well as the second-most common site of median nerve compression at the elbow.

When there is clinical suspicion of pronator syndrome, MRI examination of the upper extremity should cover all potential compression sites from the distal one third of the arm to the proximal two thirds of the forearm [48]. Denerva-tion edema or atrophy in the flexor-pronator group of muscles, including the

Rotator Cuff Mri Image
Fig. 13. Supracondylar process. Lateral radiograph of the elbow depicts an osseous spur emanating from the distal humerus approximately 5-7 cm above the elbow joint.

pronator teres, the flexor carpi radialis, the palmaris longus, and the flexor dig-itorum superficialis muscles, can be seen in patients who have pronator syndrome (Fig. 14). Space-occupying lesions such as nerve sheath tumors (Fig. 15), ganglions, and bursae can also be depicted.

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