= entrapment syndrome caused by chronic pressure on the median nerve within the carpal tunnel

Etiology:

1. intrinsic: flexor tendon tendinitis or tenosynovitis, infiltrative disorder, mass, cyst, muscle hypertrophy (from repetitive wrist / finger flexion), anomalous origin of lumbrical muscles
2. extrinsic: mass, carpal instability, Kienböck disease

Pathogenesis: probably ischemia with venous congestion (stage 1), nerve edema from anoxic damage to capillary endothelium (stage 2), impairment of venous and arterial blood supply (stage 3)

• nocturnal hand discomfort
• weakness, clumsiness, finger paresthesias

MR:

• “pseudoneuroma” of median nerve = swelling of median nerve proximal to carpal tunnel
• swelling of nerve within carpal tunnel
• increased signal intensity of nerve on T2WI
• volar bowing of flexor retinaculum
• swelling of tendon sheath ← tenosynovitis
• mass(es) within carpal tunnel
• marked enhancement ← nerve edema ← breakdown of blood-nerve barrier
• no enhancement (= ischemia) provoked by wrist held in an extended / flexed position