F163. Role of ultrasonography in severe distal median nerve neuropathy

Abstract

Patients with severe carpal tunnel syndrome may present with atrophy of thenar muscles and loss of sensations in the median nerve distribution. Nerve conduction studies do not provide precise localization especially when compound muscle action potential (CMAP) is absent over the abductor pollicis brevis (APB) and the 2nd lumbrical and no senssory action potential is recordable. In such patients, presumptive diagnosis of entrapment at the carpal tunnel is made based upon topography of denervation observed on needle EMG; however, denervation limited to the APB points to median nerve involvement distal to the branching of anterior interoseous nerve and not necessarily to the carpal tunnel. This study was aimed at determining how often a localization other than carpal tunnel results when ultrasound imaging is used as a complementary test. Patients with a clinical diagnosis of severe carpal tunnel syndrome seen in 2 years with the following criteria were included in the study: 1. Absent CMAP over APB and 2nd lumbrical on stimulation of the median nerve at the wrist and the elbow; 2. Absent sensory potentials over digits on median nerve stimulation at the wrist; 3. Needle EMG showing denervation of APB with normal findings in flexor pollicis longus, pronator teres, first dorsal interosseous and extensor indicis. Patients with diabetes mellitus and other causes of neuropathy or those with prior history of carpal tunnel release were excluded. Ultrasonic studies were done with 8–18 MgHz probe. CSA of median nerve was measured at the level of distal wrist crease and mid forearm (short axis view). The nerve was also studied in the long axis view across the carpal tunnel measuring the diameter at the carpal tunnel inlet and at the narrowest part of the nerve within the carpal tunnel. Measurements included cross sectional area (CSA) at wrist and forearm as well as diameter at the carpal tunnel inlet and within carpal tunnel. Wrist/forearm CSA ratio was calculated. Ratio between diameter at the carpal tunnel inlet and the smallest diameter within the tunnel was also calculated. A total of 18 patients were included consisting of 10 males and 8 females; age range was 30–91 with a mean of 70. 12 involved the right and 6 the left hand. CSA at wrist was 22.5 ± 7.4 mm2. Wrist forearm ratio was 2.8 ± 0.9. Ratio between diameter at inlet and within carpal tunnel was 0.49 ± 0.1. In 16 patients ultrasonic imaging confirmed entrapment at carpal tunnel. In 2 patients abnormality was found proximal to the carpal tunnel, one instance of neuroma and one of dislocated lunate. In 16 out of 18 patients presenting with no CMAP over APB and 2nd lumbrical and with absent SNAPs, presumptive diagnosis of CTS was further supported by ultrasonic imaging. In 2 patients additional diagnostic features were detected by ultrasonic imaging.