Patterns of nerve conduction abnormalities in severe carpal tunnel syndrome.

Nerve conduction study findings in severe carpal tunnel syndrome have seldom been systematically investigated. The authors determined the persistence of each nerve conduction study parameter in severe carpal tunnel syndrome. Subjects were prospectively-collected 30 hands from 23 carpal tunnel syndrome patients, with an orthodromic sensory nerve action potential (SNAP) amplitude of the median nerve of 1 muV or less, or with an compound muscle action potentials (CMAPs) of the abductor pollicis brevis (APB) of 2 mV or less. Compound muscle action potentials of APB and second lumbricalis (2L) and orthodromic and antidromic SNAPs at the index finger were measured. As results, 2L-CMAP was obtainable for all but one hand, APB-CMAP for 20, orthodromic SNAP for 18, and antidromic SNAP for 13 hands. Four hands with preserved APB-CMAPs lacked SNAPs, whereas three hands with preserved orthodromic or antidromic SNAPs lacked APB-CMAPs. The mixed-nerve action potential at the wrist following orthodromic palm stimulation was identified in only 3 of 18 hands investigated. Orthodromic SNAP has an advantage that contamination by CMAPs is entirely lacking, and the authors documented its much higher persistence than previous studies because of their more detailed methods. Such different criteria of no response of SNAP between researchers should be taken notice in the grading system of carpal tunnel syndrome.