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Effect of amplifier gain setting on distal motor latency in normal subjects and CTS patients
Adina R Goldfarb1, Peter B Saadeh, Howard W Sander
1Department of Neurology and Neuroscience, Peripheral Neuropathy Center, Weill Medical College of Cornell University, 635 Madison Avenue, Suite 400, New York, NY 10022, USA.
Objective:
To determine normative cutoffs and sensitivities for median distal latency (MDL), median-thenar to ulnar-thenar latency difference (TTLD), and median-thenar to ulnar-hypothenar latency difference (THLD) at various amplifier gains for carpal tunnel syndrome (CTS) electrodiagnosis. A prior study utilized only an amplifier gain of 0.2 mV/division.
Methods:
Abnormal cutoffs for MDL, TTLD and THLD were determined based on 34 control hands at gains of 0.2, 0.5, 1, 2, and 5 mV. Diagnostic sensitivities were determined for 50 patients (80 hands) with clinically and electrodiagnostically defined CTS.
Results:
At a gain of 0.2 and 0.5 mV/division, abnormal cutoffs for MDL, THLD, and TTLD were: 3.7, 1.2, and 0.8 ms. At gains of 1, 2, and 5 mV the abnormal cutoffs were 4, 1.2, and 1 ms. The sensitivities at gains of 0.2, 0.5, 1, 2, and 5 mV for MDL, THLD, and TTLD were: 65, 66, 53, 57, 61/86, 83, 88, 86, 86/91, 91, 76, 73, 59.
Conclusions:
MDL and THLD sensitivities are gain-independent. THLD is substantially more sensitive than MDL at all gains. TTLD sensitivity is maximized with 0.2 and 0.5 mV gains.
Significance:
TTLD and THLD increase diagnostic sensitivity with minimal additional effort. TTLD sensitivity is maximized with 0.2 or 0.5 mV gains. The electromyographer's preferred gain may be used.
