Where can median nerve entrapment occur?
CTS is easily the most common entrapment neuropathy, with a prevalence of 0.2% to 1%. Nine flexor tendons and the median nerve pass through the carpal tunnel, which is narrowest at its mid-portion. CTS occurs when the median nerve is compressed by the flexor retinaculum/transverse carpal ligament at the wrist, producing characteristic nocturnal dysesthesias (70%), but occasionally progressing to sensory loss and weakness of thumb abduction. Pain can radiate into the proximal arm (40%). This condition is bilateral in half of patients and occurs with increased frequency in occupations associated with high levels of repetition and force (meatpackers, shellfish packing, and musicians). Additionally, patients with more metabolically compromised nerves (diabetics, alcoholics) or those experiencing more swelling (pregnancy) or synovitis (RA) are at increased risk.
The anterior interosseous nerve syndrome occurs when this nerve, a purely motor branch of the median nerve, is compressed 6 cm distal to the lateral epicondyle. The resulting loss of distal thumb and index finger flexion produces a characteristic flattened pinch sign (inability to form an “O”). Sensation is normal. The pronator teres syndrome occurs when the median nerve is compressed by the pronator teres muscle at the forearm, resulting in proximal volar forearm pain that is worsened by grasping and resistive pronation of the forearm. Patients may have numbness of the thumb and the index finger, thumb weakness, and writer’s cramp.