First digit macrodactyly and carpal tunnel syndrome caused by giant median nerve with macrodystrophia lipomatosa

Abstract

A 63-year-old man reported numbness in his left first, second and third digits for 2 years. Initially, he found that 'shaking out' his left hand helped, but this benefit did not last. He underwent decompression of left-sided carpal tunnel syndrome 2 years earlier and his symptoms worsened after surgery, with more sensory loss and pain. He worked as a carpenter, and had injured his left thumb with nails on multiple occasions without feeling pain. On examination, he had an enlarged left thumb (figure 1), though he appeared unaware of this; his other digits were normal in size. The median nerve was palpable at the wrist, but Tinel's test was negative. There was a mild weakness of abductor pollicis brevis and opponens brevis, with reduced sensation over the fingertips of the lateral three digits and palm.Electrophysiological testing confirmed median nerve entrapment at the carpal tunnel, with absent sensory potentials at the third and fourth digits, with prolonged distal motor latency (5.6 ms), and decreased motor compound muscle action potential at the carpal tunnel (compound muscle action potential 2.7 mV). Proximal to the carpal tunnel inlet, the conduction velocities were normal (47.3 m/s at the forearm and 46.7 m/s at the upper arm). Electrodiagnostic testing of the ulnar nerve was normal. We did not perform needle electromyography. Ultrasound scan showed a hypertrophic enlarged median nerve along the entire tract in the left forearm (figure 2), but the main enlargement was at the wrist, where the cross-sectional area was 136 mm 2 (normal <12). There were no abnormalities in the ulnar and radial nerve, and plexus brachialis. A T1-weighted MR scan with gadolinium of the left arm showed an enlarged median nerve with lipomatous infiltration (figure 3), which is quite specific for macrodystrophia lipomatosa.The differential diagnosis of hypertrophied nerves has been discussed previously. 1 The pathophysiology that causes nerve hypertrophy can be very diverse, and includes entrapment neuropathies, leprosy, hereditary neuropathies and inflammatory neuropathies. Important sonographical features of nerve hypertrophy are diffuse or focal thickening and Figure 1 The patient's enlarged left thumb, with other digits of normal size.Figure 2 Ultrasound scan of the median nerve at the wrist, showing hypertrophic enlarged nerve with characteristic honeycomb structure.