Pronator syndrome is an uncommon entrapment neuropathy of the median nerve just below the elbow that is caused by compression of the median nerve by a variety of anatomic abnormalities including (1) the ligament of Struthers from an anomalous supracondylar process to the medial epicondyle, which may compress the median nerve; (2) the pronator teres muscle; (3) a fibrous arch in the flexor digitorum superficialis (FDS) of the middle finger; (4) posttraumatic hematoma; (5) soft tissue masses, for example, lipomas; (6) prolonged external compression from crush injuries or tourniquet; (7) fractures of the elbow (e.g., Volkmann fracture); and rarely (8) the bicipital aponeurosis (the lacertus fibrosus) (Table 61.1; Fig. 61.1). These anatomic abnormalities may work alone or together to compromise the median nerve as it passes through the forearm toward its most common site of entrapment, the carpal tunnel.

Patients suffering from pronator syndrome will experience a dull, aching pain with movement-induced dysesthesias radiating from the site of compression both proximally to the elbow and distally to the anterior forearm. Patients suffering from pronator syndrome frequently complain of a heavy or tired sensation in the muscles of the forearm and clumsiness of the affected extremity. Unlike carpal tunnel syndrome, nighttime symptomatology is uncommon. The onset of pronator syndrome can be acute following twisting injuries to the elbow or as a result of direct trauma to the area overlying the median nerve. More commonly, the onset of pronator syndrome is insidious and is usually the result of misuse or overuse of the elbow joint and proximal forearm from repetitive activities like cleaning fish, sculling, or chopping wood. If this entrapment neuropathy is not treated, pain and functional disability may become more severe, and ultimately, permanent weakness of the finger flexors may occur.

Physical findings in patients suffering from pronator syndrome will exhibit weakness of the intrinsic muscles of the forearm and hand innervated by the median nerve. The median nerve will often be tender to palpation at the site of entrapment, and a Tinel sign may be present. Hypertrophy of the pronator teres muscle may be noted. Patients suffering from pronator syndrome frequently exhibit positive functional muscle testing that can help localize the site of median nerve entrapment (Fig. 61.2). If flexion of the elbow against resistance between 120 and 135 degrees of elbow flexion causes significant pain, the source of the nerve entrapment is the ligament of Struthers. If the pain is due to compression of the median nerve by the pronator teres muscle, the patient will experience pain on resisted pronation of the forearm with the wrist flexed to relax the flexor digitorum superficialis muscle. If the site compression of the median nerve is the proximal arch flexor digitorum superficialis muscle, resisted flexion of the FDS to the middle finger will exacerbate the pain. If the site of the nerve entrapment is the bicipital aponeurosis (lacertus fibrosus), the patient will experience pain on active flexion of the elbow against resistance with the arm in pronation. Electromyography and nerve conduction velocity testing will aid in determining the exact location of median nerve entrapment in patients who present clinically with signs and symptoms consistent with pronator syndrome. These electrodiagnostic tests will also help distinguish the various causes of pronator syndrome from isolated entrapment of the anterior interosseous nerve. Furthermore, it should be remembered that cervical radiculopathy and median nerve entrapment may coexist as the so-called “double crush” syndrome. The double crush syndrome is seen most commonly with median nerve entrapment at the wrist or with carpal tunnel syndrome but has been reported with the median nerve.

Plain radiographs, ultrasound imaging, and magnetic resonance imaging are indicated in all patients who present with pronator syndrome in order to confirm the clinical diagnosis of pronator syndrome as well as to rule out occult bony pathology and to identify occult fractures, masses, or tumors
that may be responsible for compromise of the median nerve (Fig. 61.3). Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood count, uric acid, sedimentation rate, and antinuclear antibody testing.