MSK-05: Elbow Nerve Entrapments and Adult Fractures — Part 1 (Part 1 of 2)

Section 1: Pronator Syndrome and the Four Median Compression Sites

~4:52 – Pronator Syndrome and the Four Median Compression…

Bottom line: the median nerve at the elbow and proximal forearm passes through four sequential potential compression sites in proximal-to-distal order, namely the ligament of Struthers (an anatomical variant attaching to a supracondylar spur), the lacertus fibrosus (bicipital aponeurosis), the two heads of the pronator teres, and the fibrous arch of the flexor digitorum superficialis. Pronator syndrome shares the median sensory distribution of carpal tunnel syndrome but is distinguished by the absence of nocturnal symptoms and by sensory loss over the thenar eminence, because compression sits proximal to the palmar cutaneous branch. The anterior interosseous nerve (AIN) is a pure motor branch of the median nerve, and an isolated AIN palsy produces inability to make the OK sign because the flexor pollicis longus and the lateral half of the flexor digitorum profundus are paralyzed.

The median nerve navigates four potential compression sites as it travels from the distal arm into the proximal forearm. Memorizing the sites in proximal-to-distal order anchors the entire workup: the differential diagnosis, the imaging strategy, and the surgical target if release becomes necessary.

The first site is the ligament of Struthers, an anatomical variant present in a minority of patients. When present, the ligament connects a supracondylar spur (a small bony hook on the anteromedial distal humerus) to the medial epicondyle, with the median nerve passing underneath. Plain radiograph of the distal humerus reveals the spur. If a board stem mentions a supracondylar spur on radiograph together with median nerve symptoms, the answer is the ligament of Struthers. The second site is the lacertus fibrosus (bicipital aponeurosis), a thick fibrous sheet running off the biceps tendon beneath which the median nerve dives toward the antecubital fossa. The third site is the pronator teres muscle itself: the median nerve passes between the humeral and ulnar heads, and hypertrophy from repetitive pronation activities (screwdrivers, wringing towels, repeated forearm pronation at a manual workstation) compresses the nerve. This is the most common site of compression in pronator syndrome and the one that gives the syndrome its name. The fourth site is the arch of the flexor digitorum superficialis, where the median nerve passes under the fibrous proximal origin of that muscle.

The clinical presentation of pronator syndrome is a deep, aching pain in the proximal volar forearm with numbness in the median distribution: thumb, index finger, middle finger, and the radial half of the ring finger. The fingertip sensory map is identical to carpal tunnel syndrome, which is precisely why two specific discriminators carry the entire diagnostic logic. The first discriminator is the night symptoms pattern. Carpal tunnel syndrome classically wakes patients at 2 AM shaking their hand to chase away tingling that has built up while sleeping with wrists flexed; pronator syndrome does not produce nocturnal symptoms. The second discriminator is the palmar cutaneous branch of the median nerve, which exits the trunk proximal to the carpal tunnel and provides sensation to the thenar eminence and central palm. In carpal tunnel syndrome this branch is spared because it passes above the transverse carpal ligament rather than through the tunnel, so thenar palm sensation is preserved. In pronator syndrome the compression sits proximal to the branch point and the palmar cutaneous branch is involved, producing numbness over the thenar eminence. A stem describing numbness on the palm itself, not just the fingers, localizes the lesion proximal to the wrist and points to pronator syndrome.

The anterior interosseous nerve (AIN) is a pure motor branch of the median nerve that arises 5 to 8 cm distal to the medial epicondyle as the median nerve runs through the cubital fossa. It supplies three muscles: the flexor pollicis longus, the lateral half of the flexor digitorum profundus (to the index and middle fingers), and the pronator quadratus. An isolated AIN palsy produces a specific bedside finding. The patient cannot make the OK sign by opposing the thumb pulp to the index finger pulp because the flexor pollicis longus cannot flex the thumb interphalangeal joint and the flexor digitorum profundus to the index finger cannot flex the distal interphalangeal joint. The thumb and index finger meet pulp-to-pulp in a pinch rather than tip-to-tip in a circle. There is no sensory loss because the AIN is purely motor. This is the pinch sign or Kiloh-Nevin sign in older textbooks. Isolated AIN palsy can be idiopathic, often a Parsonage-Turner-like neuralgic amyotrophy that resolves over many months, or due to direct compression in the proximal forearm.

Workup for proximal median nerve compression includes a plain radiograph of the distal humerus to evaluate for a supracondylar spur, and electrodiagnostic studies (electromyography and nerve conduction studies) to localize the site of compression and exclude carpal tunnel syndrome as the actual driver. Treatment is primarily conservative with activity modification, particularly elimination of the repetitive pronation that provoked symptoms. Stretching of the forearm flexors and pronator group is prescribed. If conservative management fails after a reasonable trial of at least three months, surgical release of the offending compression site is indicated.