Electrodiagnostic Localization of Severe Carpal Tunnel Syndrome
Abstract & Commentary
Source: Boonyapisit K, et al. Lumbrical and interossei recording in severe carpal tunnel syndrome. Muscle Nerve. 2002;25:102-105.
Carpal tunnel syndrome is the most frequent motive for patient referrals to EMG laboratories in the United States. In 2-10% of cases, using standard techniques, a measurable response is unobtainable following both median motor and sensory nerve stimulation. These record from the thenar eminence (abductor pollicis brevis) and fingers 1 to 4 digital nerves, respectively. How might one localize the lesion to the wrist in this situation?
Twenty-eight hands were tested in 23 patients with absent, routine, median motor, and sensory nerve responses. Median distal motor latency to the second lumbrical was directly compared to the ulnar distal motor latency to the first palmar/second dorsal interosseous muscle over the same 8-10 cm distance. Active (recording) electrode placement was just lateral to the midpoint of the third metacarpal. Additionally, 92.8% (26 of 28 hands) demonstrated significant prolongation (normal < 0.4 ms) of the lumbrical-interosseous latency difference (range, 3.9-16.7 ms). The 2 remaining hands demonstrated no response. When routine recordings reveal no median response, this measurement can be valuable in localizing median neuropathy to the carpal tunnel.
Commentary
Obesity doubles the risk of developing carpal tunnel syndrome (Am J Epidemiol. 1990;132:1102-1110) but, based on body mass indices (weight in kg/height2 in meters), it does not correlate with severity. Conversely, wrist index (wrist depth/wrist width in mm) and older age do have a correlation with severity (Muscle Nerve. 2002;25:3-7). Interestingly, and for unclear reasons, patients with carpal tunnel syndrome are more likely to have shorter but wider palms, and shorter third digits than controls (Muscle Nerve. 2001;24:1607-1611). Presumably, this adversely alters the relationship of the median nerve to the carpal ligament and structures in the tunnel and may lead to compression injury.
Tinel’s sign, Phalen’s test, the reverse Phalen’s test, and the carpal compression test are the techniques most familiar to neurologists searching for carpal tunnel syndrome. However, hand elevation appears to be a simpler and yet more provocative and sensitive test (Ann Plast Surg. 2001;46:120-124). One hundred eighteen patients possessing 200 hands with carpal tunnel syndrome had hand elevation tests which were more sensitive and specific (75.5% and 98.5%, respectively) than Phalen’s test or Tinel’s sign. Results for Phalen’s test were 67.5% and 91.0%, respectively, and for Tinel’s sign 67.5% and 90.0%, respectively. All those in favor of the hand elevation test, raise your hand! —Michael Rubin.
Rubin, MD, Associate Professor of Clinical Neurology, New York Presbyterian Hospital-Cornell Campus, is Assistant Editor of Neurology Alert.
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