Effect of amplifier gain setting on distal motor latency in normal subjects and CTS patients.
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.


CONCLUSIONS
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.
Goldfarb
Adina R
AR
Department of Neurology and Neuroscience, Peripheral Neuropathy Center, Weill Medical College of Cornell University, 635 Madison Avenue, Suite 400, New York, NY 10022, USA.
Saadeh
Peter B
PB
Sander
Howard W
HW
D000328:Adult; D000368:Aged; D000669:Amplifiers, Electronic / Q000592:standards; D002349:Carpal Tunnel Syndrome / Q000175:diagnosis* / Q000503:physiopathology; D004568:Electrodiagnosis / Q000295:instrumentation* / Q000379:methods*; D004576:Electromyography / Q000295:instrumentation / Q000379:methods; D006225:Hand / Q000294:innervation / Q000503:physiopathology; D006801:Humans; D008475:Median Nerve / Q000503:physiopathology*; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000503:physiopathology; D009431:Neural Conduction / Q000502:physiology*; D011826:Radial Nerve / Q000502:physiology; D011930:Reaction Time / Q000502:physiology*; D012016:Reference Values; D014459:Ulnar Nerve / Q000502:physiology