The median nerve passes in the distal upper arm between the brachialis and the medial intermuscular septum, with the brachial artery sitting lateral to it.

At the elbow, the median nerve sits underneath the lacertus fibrosus and then typically passes between the superficial (humeral) head and the deep (ulnar) head of the pronator teres.

Motor branches to the palmaris longus, flexor carpi radialis, flexor digitorum superficialis, and flexor digitorum profundus typically branch from the median nerve in an ulnar direction proximal to the pronator teres.

Under the pronator teres, the AIN branches in a radial direction from the median nerve, and both pass underneath the fibrous arcade of the flexor digitorum superficialis.

The surgeon should be cognizant of the cutaneous nerves passing over the antecubital and proximal forearm region. Damage to these nerves can result in numbness and paresthesia as well as symptomatic neuromas in the forearm.

Anomalous muscles and nerve branches may be present, the most common of which is the so-called Martin-Gruber anastomosis.

Compression of the median nerve in the proximal forearm is rare compared with carpal tunnel syndrome.

Median nerve compression in the proximal forearm has been labeled as either pronator syndrome or anterior interosseous syndrome.

The true incidence of median nerve compression in the proximal forearm is difficult to ascertain, as is the relative contribution of the various potential impinging structures.

Numerous studies have shown that the most common causes of median nerve compression in the region of the elbow and proximal forearm seem to be fascial bands and muscular anomalies of the pronator teres and the fibrous arcade of the flexor digitorum superficialis.³,⁶

Anterior interosseous syndrome caused by nerve compression must be differentiated from Parsonage-Turner syndrome or mononeuritis.

Compression of the median nerve in the forearm often is transient due to excessive physical activity or swelling from injury.

Recovery from Parsonage-Turner syndrome can be prolonged, but the prognosis usually is good without surgical decompression.

The natural history and prognosis of pronator syndrome is not well understood.

Classically, pronator syndrome presents as paresthesia in the median nerve distribution with minimal or no weakness. The patient also may complain of pain localized to the proximal forearm that is increased with activities. There may be a focal area of increased pain localizing to the specific area of compression.

AIN syndrome presents as diminished motor function of the index (and long) flexor digitorum profundus, flexor pollicis longus, and pronator quadratus without injury or specific known cause.

Physical examinations to perform include the following:

Electrodiagnostic studies are often not helpful in pronator syndrome. Numerous studies have shown that symptoms and outcome of surgery do not correlate well with electrodiagnostic studies.

In anterior interosseous syndrome, electrodiagnostic studies will confirm denervation of the anterior interosseous muscles.

Electrodiagnostic studies are most valuable in the diagnosis of proximal median nerve compression for ruling out carpal tunnel syndrome.

Ultrasonography and magnetic resonance imaging (MRI) are valuable tests for identifying space-occupying lesions such as lipomas or ganglions.

Plain radiographs of the proximal forearm and elbow may reveal a supracondylar process or anatomic variation.

In the acute phase, rest, immobilization, and avoidance of aggravating activities, such as repetitive pronation and heavy gripping, should be recommended.

Forearm stretching exercises can be tried in chronic cases.

Modalities such as ultrasound and electrostimulation have been advocated, although there is limited validation of their usefulness.

Nerve gliding and nerve mobilization remain controversial.

Spontaneous recovery does appear to occur in the majority of patients with anterior interosseous syndrome, although recovery can take up to 12 months.⁷,¹⁰