Usefulness of Interdigital Nerve Conduction Studies of the Foot for the Detection of Early Polyneuropathy

ABSTRACT

BACKGROUND AND PURPOSE: Frequently the conventional nerve conduction studies (NCS) with testing usual peripheral nerves did not reveal any abnormalities in patients with early polyneuropathy. Recently Lee and Oh demonstrated that the interdigital sensory nerve conduction studies (NCS) of the foot were very sensitive in detecting early changes of neuropathy. The authors performed the study to see abnormal features of interdigital NCS in early phase of polyneuropathy and to understand the sensitivity of interdigital NCS in making diagnosis of polyneuropathy. METHODS AND MATERIALS The interdigital NCS of the foot were performed in 19 clinically suspected polyneuropathy with no abnormalities by the conventional NCS (group I), in 7 polyneuropathy with electrophysiological abnormalities by the conventional NCS (group II), and 10 normal controls without clinical or electrophysiological findings of polyneuropathy (group III). The interdigital NCS were done according to the original description of Oh et al ,using the near-nerve needle recording at the level of ankle with averaging technique. The maximal nerve conduction velocity (NCV), the largest negative NCV, the amplitude and the duration of compound nerve action potential (CNAP) were analysed in each recording of I,V digital and I-II, II-III, III-IV, IV-V interdigital nerves of the foot. RESULTS: The abnormal interdigital NCS findings were found in 9 out of 19 patients in group I (47.4% ) , 7 out of 7 patients in group II(100.0%) ,and none in group III.(0.0%). In group I, the most common abnormal findings were no recordable CNAP (77,7%) and the second were the decreased amplitude of CNAP (66.6%). The decreased NCV were found only in one case ( group I-19). The digital or interdigital nerves involvement were symmetrical and the number of involved branches ranged from one to twelve, being averaged to be 7. In group II, the most common abnormalities were no recordable or decreased amplitude of CNAP (85.7 %) and the decreased NCV were found in 1 out of 7. Three out of 7 showed abnormalities in all digital or interdigital nerves and the remaining four showed abnormal findings in at least 4 among 12 digital or interdigital nerves. CONCLUSION: The interdigital NCS with the near nerve needle and averaging technique increased the diagnostic sensitivity in those subjects with clinically suspected polyneuropathy (group I) and the most common abnormal features in the interdigital NCS were the amplitude changes of CNAP. Therefore the authors suggest that the interdigital nerves of the foot are involved earlier in the process of polyneuropathy and the interdigital NCS of the foot might provide enhanced sensitivity for detecting early polyneuropathy.