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Practice Parameter: Carpal Tunnel SyndromeTable 14. Median SNAPWrist to Palm Amplitude Ratio in CTS.AuthorDi Guglielmo andcolleagues 59Year 1997Number of Normal Hands(subjects)88 (69)Normal Subject’s Age: Mean(range)40 (20 to 86)Number of CTS Hands(subjects)294 (198)CTS Subject’s Age: Mean46 (13 to 84)(range)Technique: ConductionDistanceWrist Stimulation SitePalm Stimulation SiteRecording SiteMinimum Temperature 32°CAnatomicallandmarks1 cm to 2 cmproximal to wristcrease3 cm distal to wristcreaseD2Amplitude (wrist) ± SD 42 ± 19 µVAmplitude (palm) ± SD 45 ± 21 µVWrist to Palm Amplitude Ratio± SDCriteria for Abnormal Value0.8 ± 0.2Abnormal Value 0.4 ms / >0.5 ms81% / 97% (actual)Sensitivity of Abnormal Value 81% / 54%The median sensory conduction study was performed by placingthe recording ring electrodes on the middle finger and stimulatingthe median nerve at 9 points separated by 1-cm intervals beginning2 cm proximal to the wrist crease and ending 6 cm distal to thewrist crease. The time (ms) from the stimulus artifact to the peak ofthe SNAP was measured for each stimulation site and thedifference between the peak latency for successive SNAPscalculated. Two years later, Nathan and colleagues, 182 in a studythat also met 6/6 AAEM CTS LIC, localized the slowing ofconduction most commonly to the an area 3 to 4 cm distal to thewrist crease.*Written communication.Specificity equals the percentage of reference subjects’ hands withnormal results and was either “actual” based on analysis of the testdata from the reference population or an “estimate” based on thestatistical distribution of test data from the reference population.Sensitivity equals the percentage of CTS patients’ hands withabnormal results calculated from the test data on the CTSpopulation.CTS = Carpal Tunnel Syndrome SD = Standard DeviationSNAP = Sensory Nerve Action Potentialby Nathan and colleagues, 181 a short-segment incrementalstimulation technique (antidromic inching test or AIT)initially described by Kimura. 130 Though time consuming,the landmark studies by Kimura 128,130 localized theS944 CTS Literature Review© 2002 American Association of Electrodiagnostic Medicine
Practice Parameter: Carpal Tunnel Syndromeabnormality of median sensory conduction in most CTSpatients to the distal edge of the carpal ligament, and thisfinding has been confirmed by Nathan and colleagues, 182White and colleagues, 264 Imaoka and colleagues, 106 andSeror. 226 Because a frequency distribution of thesegmental latency differences in the normal subjectsshowed a skewed distribution of data, Nathan 181 used acontingency table to evaluate the sensitivity andspecificity of 2 different criteria of abnormality (0.4 msand 0.5 ms). Although Nathan 181 recommends use of the0.4 ms criterion of abnormality, only the 0.5 ms criterionprovides specificity (97%) comparable to the other testspresented in this review.Imaoka and colleagues 106 in a study that met 5 of the 6AAEM CTS LIC, used a special linear grid of 9 surfaceelectrodes at 15 mm intervals (a total of 8 neighboringpairs of electrodes) to record the median SNAPsimultaneously across the wrist with stimulation of themedian nerve at the elbow. With a criterion of deviationby 0.6 ms or more from a predicted peak latency valuebased on measurements of peak latencies recordedproximal to the wrist crease, Imaoka 106 and colleagues(1992) reported a specificity of 99% compared to normalsubjects (mean + 3 SD) and a high test sensitivity (87% inmild CTS).Seror, 226 in a study that met 4 of the 6 AAEM CTS LIC,used an orthodromic inching test (OIT) with stimulationof the third digit and measuring the peak latency of theSNAP recorded with a bipolar fixed distance (22 mm)surface electrode moved centimeter by centimeter from apoint 4 cm proximal to the distal wrist crease to a point 6cm distal to the distal wrist crease to provide 11measurements; an abnormality was defined as aconduction delay greater than 0.36 ms based on controlstudies with a range of 0.20 to 0.34 ms. Seror 226concluded that most CTS patients can be diagnosed withother methods and that the more time consuming“inching” technique is needed to confirm the diagnosis inonly about 5% of all CTS patients.Seror, 224 in a study that met 4 of the 6 AAEM CTS LIC,concluded that the OIT was superior to the AIT becausethe OIT was more sensitive than the AIT, the stimulationsite and intensity was unchanged during the study whichensures that the same nerve fibers are evaluated, and thestimulation intensity is less and better tolerated by thepatient compared to AIT.Comparison of Carpal Tunnel Segment Median MixedNerve Conduction to More Proximal (Forearm) MixedNerve Conduction or Distal (palm to digit) MedianSensory Nerve Conduction. The possible usefulness ofcomparing CVs of different segments of the same nerveto demonstrate focal conduction slowing to minimizeintersubject variability has been evaluated with mediansensory and mixed nerve conduction in CTS patients. Theresults of 2 studies that met all 6 AAEM CTS LIC arepresented in Table 16. The first by Scelsa 221 concludedthat comparison of the median palm to index fingersensory CV to the median carpal tunnel mixed nerve CVdemonstrated a sensitivity to detect CTS (87%) that wassignificantly greater than the sensitivity (61%) ofcomparison of the median forearm CV to the mediancarpal tunnel CV with similar specificities (98% and 96%,respectively). The second by Kuntzer 140 demonstrated anintermediate sensitivity (69%) with similar specificity(99%) by comparison of the peak latency of the mixednerve median palm to wrist segment to the peak latencyof the median sensory palm to D2 segment. There are also2 studies that meet all 6 AAEM CTS LIC by Padua andcolleagues 188,189 that described the usefulness of a ratio ofthe orthodromic sensory CV for 2 segments of the mediansensory nerve (D3 to palm/palm to wrist) to diagnoseCTS with high sensitivity: 1996, 98% and 1997, 97%.In 1985, Kimura and Ayyar 131 reported a 100% incidenceof abnormalities in CTS patients if the ratio of the medianantidromic sensory CV across the wrist to the mediansensory CV across the forearm was calculated. However,in a study that met 4 of the 6 AAEM CTS LIC, Rosen 214noted that the quotient of the median antidromic mixednerve CV in the carpal tunnel segment to the antidromicmedian sensory CV in the forearm segment (70%) wasless sensitive than median mixed nerve palm to wristconduction (100%).Buschbacher, 33 in a study of 258 normal subjects listed inTable 1, noted that 50% of the wrist to digit mediansensory peak latency is attributable to the wrist-palmsegment, a finding that agrees well with the ratio of thepeak latency for the 2 segments reported by Kuntzer: 1400.98 ± 0.17 in Table 16.Comparison of Median Sensory Nerve Conduction toUlnar or Radial Sensory Nerve Conduction in the SameLimb. In theory, the biologic variation in speed of nerveconduction from person to person due to age and geneticdifferences can be controlled by comparison of the speedof nerve conduction in 1 nerve to another nerve in thesame limb. 69,243 This comparison principle underlies thebasis for development of the sensory NCSs reported inTables 17, 18, 19, and 20.Comparison of Median and Ulnar Sensory NerveConduction Between Wrist and Digit. Table 17presents the results of a study that met all 6 AAEMCTS LIC by Kuntzer 140 who determined the differencebetween the median and ulnar nerve peak latencymeasurements with orthodromic stimulation (14-cmconduction distance) in CTS patients and normalcontrol subjects and found the percentage of CTSpatients with abnormal values was 61%. Stetson, 242 in aMuscle & Nerve Supplement X 2002 S945
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