Proximal Median Neuropathy

Detailed Anatomy at the Antecubital Fossa

As the median nerve descends in the upper arm, it runs medial to the humerus and anterior to the medial epicondyle. In a very small number of individuals, a bony spur originates from the shaft of the medial humerus approximately 5–7 cm proximal to the medial epicondyle. In some of those individuals, a tendinous band known as the ligament of Struthers stretches between the spur and the medial humeral epicondyle. In the antecubital fossa, the median nerve travels just medial to the brachial artery ( Fig. 21.1 ). As it enters the forearm, it runs first beneath the lacertus fibrosus , a thick fibrous band that runs from the medial aspect of the biceps tendon and then over the proximal forearm flexor musculature. In most individuals, the median nerve then runs between the two heads of the pronator teres (PT) muscle to provide innervation to that muscle (one head originating from the distal humerus, the other from the ulna). In many individuals, there are fibrous bands within the two heads of the PT muscle. The anterior interosseous nerve is then given off posteriorly, approximately 5–8 cm distal to the medial epicondyle, after the median nerve passes between the two heads of the PT. As the median nerve runs distally, it passes deep to the flexor digitorum sublimis (FDS) muscle and its proximal aponeurotic tendinous edge, known as the sublimis bridge . Just distal to the medial epicondyle, the brachial artery bifurcates into the radial and ulnar arteries. As the median nerve runs between the two heads of the PT, the ulnar artery accompanies it but is usually deep to the ulnar head of the PT. After the PT, the ulnar artery and median nerve run adjacent in the proximal forearm. Just distal, the ulnar artery separates from the median nerve to run more medially where it joins the ulnar nerve in the mid-forearm.

Etiology

Median neuropathy in the region of the antecubital fossa has been described as a consequence of external compression from casting, trauma, venipuncture, and compressive mass lesions, including tumor or hematoma. Rare cases of brachial artery puncture and subsequent hematoma formation have led to compartment syndromes and subsequent injury of the proximal median nerve. Injury to the median nerve in the antecubital fossa from a needle puncture, although rare, is the second most common nerve reported injured from phlebotomy.

In addition, several other sites of proximal median entrapment have been reported ( Fig. 21.1 ). All are uncommon, and some remain controversial. The four major potential sites of entrapment are as follows:

• Median nerve entrapment may occur at the ligament of Struthers in the distal upper arm, where both the median nerve and brachial artery pass between this ligament and the humerus.

• More distally in the region of the antecubital fossa, the median nerve may become entrapped beneath a hypertrophied lacertus fibrosus.

• Further distally, the median nerve may become entrapped in the substance of the PT muscle, especially in individuals who have additional fibrous bands running through that muscle.

• More distally, the median nerve may become entrapped beneath the sublimis bridge of the FDS muscle.

Clinical

The clinical syndromes of proximal median neuropathy depend on the underlying etiology and lesion site.

Traumatic Lesions

In patients with traumatic lesions, there usually is an obvious, acute disturbance of median motor and sensory function. Significantly, sensory disturbance in proximal median neuropathy is noted in the entire median territory, including the thenar eminence, as well as the thumb, index, middle, and lateral ring fingers. This feature clearly distinguishes proximal median neuropathy from carpal tunnel syndrome (CTS), in which sensation over the thenar eminence is spared. Sensory loss over the thenar eminence occurs as the palmar cutaneous branch, which innervates the thenar eminence, leaves the median nerve proximal to the carpal tunnel. Depending on the site of the lesion, weakness may affect some or all of the proximal median-innervated forearm muscles, including the PT, FDS, flexor digitorum profundus (FDP) to digits 2 and 3, flexor carpi radialis (FCR), flexor pollicis longus (FPL), and pronator quadratus (PQ), as well as the distal median-innervated muscles, including the abductor pollicis brevis (APB), opponens pollicis (OP), and first and second lumbricals. Weakness of the FDP to digits 2 and 3, FDS and FPL often leads to a characteristic high median neuropathy posture, whereby the individual is unable to flex the thumb, index, and middle fingers ( Fig. 21.2 ).

Entrapment Syndromes

The symptoms and signs in the proximal median nerve entrapment syndromes are fairly nonspecific. Typically, there is pain or discomfort in the region of the entrapment. Unlike CTS, the symptoms are not exacerbated at night. The two major syndromes include (1) proximal entrapment of the median nerve at the ligament of Struthers and (2) median nerve entrapment more distally, either beneath the lacertus fibrosus, in the substance of the PT, or beneath the sublimis bridge ( Fig. 21.1 ). The latter three entrapment sites usually are referred to collectively as the pronator syndrome . Strictly speaking, the term may be reserved for nerve entrapment within the substance of the PT muscle proper. However, entrapment at any of these last three locations usually produces a similar clinical syndrome.

Ligament of Struthers Entrapment

Entrapment at the ligament of Struthers is a very rare syndrome whereby the median nerve is entrapped by a tendinous band running from the medial epicondyle to a bony spur on the distal medial humerus ( Fig. 21.3 ). The prevalence of such a supracondylar bony spur is approximated at 1%–2% of the population. The syndrome is characterized by pain in the volar forearm and paresthesias in the median-innervated digits, which are exacerbated by supination of the forearm and extension of the elbow. The radial pulse also may be attenuated with these maneuvers, as the brachial artery also runs with the median nerve under the ligament of Struthers. A bony spur may be palpable at the distal humerus. Weakness of the PT and other median-innervated muscles may occur, and subtle sensory loss may be noted in the median distribution, including the thenar eminence.

Pronator Syndrome

Although rare, the pronator syndrome occurs more often than entrapment at the ligament of Struthers. The PT muscle may be enlarged or firm, with a Tinel’s sign over the site of entrapment. Pain may radiate proximally and often is aggravated by using the arm, especially with repeated pronation/supination movements. Specific maneuvers that may produce symptoms of pain in the forearm and paresthesias in the median-innervated digits depend on the site of entrapment ( Fig. 21.4 ): resisted pronation with the elbow in extension (for the PT), resisted flexion of the proximal interphalangeal joint of the middle finger (for the sublimis bridge), and resisted flexion of the elbow with the forearm in supination (for the lacertus fibrosus). The sole finding of increased pain with these maneuvers is an unreliable sign, unless it is accompanied by median nerve territory paresthesias. Significant weakness or wasting of median-innervated muscles is rare, but mild weakness of the FPL and APB is not uncommon, with occasional involvement of the FDP to digits 2 and 3 and the OP. The PT muscle is usually spared. There may be occasional paresthesias radiating into the median-innervated digits, with subtle impairment of sensation in the median nerve distribution, including the thenar eminence.