Definition

There are three general areas in which the median nerve can become entrapped around the elbow and forearm. Because this chapter mainly deals with entrapment below the elbow and above the wrist, the most proximal and least frequent entrapment is not discussed but merely mentioned. Elbow median nerve entrapment is the compression of the nerve by a dense band of connective tissue called the ligament of Struthers, an aberrant ligament found immediately above the elbow. The topics discussed in this chapter are compression of the median nerve at or immediately below the elbow, where the pronator teres muscle compresses it, and compression distally of a branch of the median nerve—the anterior interosseous nerve.

Increased risk for pronator syndrome may be associated with individuals involved in repetitive elbow, wrist, and hand movements, such as chopping wood, playing racket sports, rowing, weightlifting, and throwing. However, pronator syndrome is four times more likely to affect women than men, suggesting that the dominant risk factor is an anatomic anomaly (structural variation) and not overuse. The dominant arm is most likely to be affected, particularly if the individual is heavily muscled (muscle hypertrophy). Pronator syndrome is most commonly diagnosed in individuals between the ages of 40 and 50 years [1]. Pronator syndrome is rare but is the second most common cause of medial nerve compression after carpal tunnel syndrome. Pronator syndrome is responsible for less than 1% of all median nerve entrapment disorders [2]. Anterior interosseous syndrome has a similar 1% occurrence rate in median nerve entrapment disorders [3].

Pronator Teres Syndrome

Pronator teres syndrome [4,5] is a symptom complex that is produced where the median nerve crosses the elbow and becomes entrapped as it passes first beneath the lacertus fibrosus—a thick fascial band extending from the biceps tendon to the forearm fascia—then between the two heads (superficial and deep) of the pronator teres muscle and under the edge of the flexor digitorum sublimis (Fig. 24.1). Compression may be related to a local process, such as pronator teres hypertrophy, tenosynovitis, muscle hemorrhage, fascial tear, postoperative scarring, anomalous median artery, or giant lipoma. The median nerve may also be injured by occupational strain, such as carrying a grocery bag or guitar playing, and by insertion of a catheter [4–13].

Anterior Interosseous Syndrome

The anterior interosseous nerve arises from the median nerve 5 to 8 cm distal to the lateral epicondyle [4,6,14]. Slightly distal to its course through the pronator teres muscle, the median nerve gives off the anterior interosseous nerve, a purely motor branch (Fig. 24.2). It contains no fibers of superficial sensation but does supply deep pain and proprioception to some deep tissues, including the wrist joint. This nerve may be damaged by direct trauma, forearm fractures, humeral fracture, injection into or blood drawing from the cubital vein, supracondylar fracture, and fibrous bands related to the flexor digitorum sublimis and flexor digitorum profundus muscles. In some patients, it is a component of brachial amyotrophy of the shoulder girdle (proximal fascicular lesion) or related to cytomegalovirus infection or a bronchogenic carcinoma metastasis. The nerve may be partially involved, but in a fully established syndrome, three muscles are weak: flexor pollicis longus, flexor digitorum profundus to the second and sometimes the third digit, and pronator quadratus [4,6,12–18].

In an acute compression, with unmistakable symptoms, the diagnosis is relatively simple to establish [5,14]. In many cases of intermittent, mild, or partial compression, the signs and symptoms are vague and nondescript. The most common symptom is mild to moderate aching pain in the proximal forearm, sometimes described as tiredness and heaviness. Use of the arm may cause a mild or dull aching pain to become deep or sharp. Repetitive elbow motions are likely to provoke symptoms. As the pain intensifies, it may radiate proximally to the elbow or even to the shoulder. Paresthesias in the distribution of the median nerve may be reported, but they are generally not as severe or well localized as the complaints in carpal tunnel syndrome. When numbness is a prominent symptom, the complaints may mimic carpal tunnel syndrome. However, unlike carpal tunnel syndrome, pronator teres syndrome rarely has nocturnal exacerbation, and the symptoms are not affected by a change of wrist position.

Anterior Interosseous Syndrome

The onset of anterior interosseous syndrome can be related to exertion, or it may be spontaneous. In classic cases of spontaneous anterior interosseous nerve paralysis, there is acute pain in the proximal forearm or arm lasting for hours or days. There may be a history of local trauma or heavy muscle exertion at the onset of pain. As mentioned, the patient may complain of weakness of the forearm muscles innervated by the anterior interosseous nerve. Theoretically, there should be no sensory complaints [6].

Findings may be ill-defined and difficult to substantiate in pronator teres syndrome [5,14]. The most important physical finding is tenderness over the proximal forearm. Pressure over the pronator teres muscle produces discomfort and may produce a radiating pain and digital numbness. The symptomatic pronator teres muscle may be firm to palpation compared with the other side. The contour of the forearm may be depressed, caused by the thickening of the lacertus fibrosus. Distinctive findings are weakness of both the intrinsic muscles of the hand supplied by the median nerve and the muscles proximal to the wrist and in the forearm with tenderness, Tinel sign over the point of entrapment, and absence of Phalen sign. Pain may be elicited by pronation of the forearm, elbow flexion, or even contraction of the superficial flexor of the second digit. Sensory examination findings are usually poorly defined but may involve not only the median nerve distribution of the digits but also the thenar region of the palm because of involvement of the palmar cutaneous branch of the median nerve. Deep tendon reflexes and cervical examination findings should be normal [7,12,13,15–19].

Anterior Interosseous Syndrome

To test the muscles that the anterior interosseous nerve innervates [5,14], the clinician braces the metacarpophalangeal joint of the index finger and the patient is asked to flex only the distal phalanx. This isolates the action of the flexor digitorum profundus on the terminal phalanx and eliminates the action of the flexor digitorum superficialis. There is no terminal phalanx flexion if the anterior interosseous nerve is injured.

Another useful test is to ask the patient to make the “OK” sign [20]. In anterior interosseous syndrome, the distal interphalangeal joint cannot be flexed, and this results in the index finger’s remaining relatively straight during this test (Fig. 24.3). The patient is asked to forcefully approximate the finger pulps of the first and second digits. The patient with weakness of the flexor pollicis longus and digitorum profundus muscles cannot touch with the pulp of the fingers, but rather the entire volar surfaces of the digits are in contact. This is due to the paralysis of the flexor pollicis longus and flexor digitorum profundus of the second digit. The pronator quadratus is difficult to isolate clinically, but an attempt can be made by flexing the forearm and asking the patient to resist supination. Sensation and deep tendon reflexes should be normal [7,12,13,15