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Effect of Hand Size on the Stimulation Intensities Required for Median and Ulnar Sensory Nerve Conduction Studies

Published:November 30, 2012DOI:https://doi.org/10.1016/j.apmr.2012.11.029

      Abstract

      Objectives

      To examine the effect of hand size on median and ulnar sensory nerve conduction study (NCS) stimulation intensities and pain scores.

      Design

      Prospective, single group design to compare main outcomes by using a standard distance of 14cm versus the proximal wrist crease in 3 different hand sizes.

      Setting

      Electrodiagnostic laboratory in a department of rehabilitation medicine.

      Participants

      Healthy volunteers (N=25) aged 20 to 30 years.

      Interventions

      Hand size was determined, based on the distance between the proximal wrist crease and the base of the long finger, resulting in 3 groups (≤11cm, >11–12cm, >12cm) with 12 hands per group. Antidromic median and ulnar sensory NCSs were performed. The nerves were randomly stimulated at the proximal wrist crease and 14cm from the recording electrode.

      Main Outcome Measures

      Supramaximal stimulation intensities and 10-cm visual analog scale (VAS) pain scores at each stimulating site were recorded and compared.

      Results

      Thirty-six hands from 25 young healthy volunteers were studied. There was no correlation between the body mass index (BMI) and stimulation intensity, and BMI and VAS (r<0.3) in both median and ulnar nerves. Overall analysis showed that the stimulation intensity and VAS at 14cm were significantly greater than at the proximal wrist crease. Subgroup analysis showed the same result in all groups for the median sensory NCS, but in only the small hand group for the ulnar sensory NCS.

      Conclusions

      When the same distance is used for NCSs regardless of patient size, smaller individuals required greater stimulation and reported greater discomfort. This may reflect greater nerve depth and suggests that one size fits all may not be the best approach with NCSs.

      Keywords

      List of abbreviations:

      BMI (body mass index), NCS (nerve conduction study), VAS (visual analog scale)
      Although electrodiagnosis has been found useful in diagnosing a number of neuromuscular disorders, a recognized drawback is the pain incurred during testing. Much has been written about pain during needle examination
      • Richardson J.K.
      • Evans J.E.
      • Warner J.H.
      Information effect on the perception of pain during electromyography.
      • Jan M.M.
      • Schwartz M.
      • Benstead T.J.
      EMG related anxiety and pain: a prospective study.
      • Buckelew S.P.
      • Conway R.C.
      • Shutty M.S.
      • et al.
      Spontaneous coping strategies to manage acute pain and anxiety during electrodiagnostic studies.
      • Strommen J.A.
      • Daube J.R.
      Determinants of pain in needle electromyography.
      • Khoshbin S.
      • Hallet M.
      • Lanbeck R.
      Predictors of patients' experience of pain in EMG.
      • Walker W.C.
      • Keyser-Marcus L.A.
      • Johns J.S.
      • Seel R.T.
      Relation of electromyography-induced pain to type of recording electrodes.
      ; however, only a few articles have addressed pain during nerve conduction studies (NCSs).
      • Gans B.M.
      • Kraft G.H.
      Pain perception in clinical electromyography.
      • Wee A.S.
      • Boyne R.L.
      • Abernathy S.D.
      • Nick T.G.
      Pain perception to nerve conduction and needle electromyographic procedures.
      Sensory NCS techniques often specify the same distance irrespective of patient size.
      • Jablecki C.K.
      • Andary M.T.
      • So Y.T.
      • Wilkins D.E.
      • Williams F.H.
      Literature review of the usefulness of nerve conduction studies and electromyography for the evaluation of patients with carpal tunnel syndrome.
      • Stevens J.C.
      AAEM minimonograph #26: the electrodiagnosis of carpal tunnel syndrome.
      • Johnson E.W.
      • Melvin J.L.
      Sensory conduction studies of median and ulnar nerves.
      • Dumitru D.
      • Amato A.
      • Zwarts M.
      Nerve conduction studies.
      • Weber R.
      Motor & sensory conduction and entrapment syndrome.
      • Delisa J.A.
      • Lee H.J.
      • Lai K.S.
      • Spielholz N.
      Manual of nerve conduction velocity and clinical neurophysiology.
      • Buschbacher R.M.
      Manual of nerve conduction studies.
      This seems logical to allow for a comparison of values, but it may not be anatomically logical. The median and ulnar nerves are closest to the surface at the wrist and become deeper as they run more proximally (fig 1).
      • Sinnatamby C.S.
      Last's anatomy regional and applied.
      • Johnson D.
      • Ellis H.
      Pectoral girdle and upper limb.
      • Dean D.
      • Herbener T.E.
      Cross-sectional human anatomy.
      The same distance in a smaller extremity may give a stimulation site with greater nerve depth and hence may require a larger stimulation intensity to achieve a supramaximal response. The purpose of this study was to examine the effect of hand size on median and ulnar sensory NCS stimulation intensities and pain scores.
      Figure thumbnail gr1
      Fig 1Cross-sectional anatomy showing position of median and ulnar nerves at wrist and distal forearm level. Abbreviations: A, anterior; L, lateral; M, medial; n., nerve; P, posterior.

      Methods

       Participants

      Volunteers were recruited by posting information throughout our hospital and medical school. All hands were divided into 3 groups, based on the distance between the proximal wrist crease and the base of the long finger: small, ≤11cm; medium, >11 to 12cm; and large, >12cm. There were 12 hands in each group. Subjects did not have deformities, a history of injury to the upper extremities, or an underlying disease that might affect the peripheral nerves. The hospital institutional review board approved the study, and informed consent was obtained.

       Interventions

      Hand size was determined by measuring the distance between the proximal wrist creases and the base of the long finger, with a flexible measuring tape. Sex, height, weight, and calculated body mass index (BMI) were recorded.
      With the use of a technique described by Buschbacher,
      • Buschbacher R.M.
      Manual of nerve conduction studies.
      antidromic median and ulnar sensory NCSs were performed with ring electrodes on the long finger and little finger, respectively, and the reference electrode was placed 4cm distal to the active electrode. Hand temperature, as measured on the dorsum of the hand, was deemed acceptable at >32°C. A Medelec Synergy electromyography machinea was used. Frequency bandpass was set at 20 to 20,000Hz. Stimulations were performed with a constant current stimulator set initially at .05 milliseconds. Supramaximal stimulation intensity was determined by seeking the smallest stimulus intensity to obtain a maximal amplitude response, then increasing by 20%. If the supramaximal intensity was not reached at a .05-millisecond duration, the pulse duration was increased to 0.1 milliseconds and the process was continued. Supramaximal responses were sought for the median and ulnar nerves at the proximal wrist crease and at 14cm from the proximal ring electrode, as measured by a flexible measuring tape. The choice of first and second stimulation site (proximal wrist crease vs 14cm) was made by alternating the order from subject to subject (eg, subjects 1, 3, and 5 would be started at 14cm, while subjects 2, 4, and 6 would be started at the proximal wrist crease).
      The visual analog scale (VAS) was recorded using 100-mm, nonhatched VAS pain score paper, with the anchors of 0 indicating no pain and 100mm indicating the worst pain imaginable.
      To avoid observer bias, all studies were performed by only 1 independent electromyographer throughout the study. This observer did not know the objective of the study and was informed to do the sensory NCS as our protocol mentioned above.

       Analysis procedures

      Supramaximal stimulation intensity, pulse duration, and VAS pain score were recorded. Stimulus pulse duration was not included in the analysis of the stimulation intensity levels. Data were entered into an Excelb spreadsheet and checked for errors. SPSS version 11.5c was used. The Pearson correlation was used to determine the correlation between BMI, stimulation intensity, and VAS. For the normal distribution data, the paired t test was used to compare the stimulation intensity and the VAS pain score between each stimulating site. The Wilcoxon signed-rank test was used to compare the nonnormal distribution data. Statistical significance was at P<.05.

      Results

      The demographic data of participants are shown in table 1. Thirty-six hands from 25 subjects, 10 men and 15 women, were included in the study. Subjects were allowed to choose whether 1 or 2 hands would be tested. Subjects were healthy volunteers aged 20 to 30 years with a BMI in the normative range. The World Health Organization defined normative BMI for Asian populations as 18.5 to 24.9kg/m2.
      • WHO expert consultation
      Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies.
      Table 1Demographic data of the participants
      Subject No.Age (y)SexHand SizeHeight (cm)Weight (kg)BMI (kg/m2)
      126FSmall1586024.0
      225FSmall1524419.0
      329FSmall1626223.6
      427FSmall1624918.7
      527FSmall1554719.6
      629FSmall1564719.3
      728FSmall1605019.5
      822MMedium1726020.3
      926FMedium1675319.0
      1027FMedium1614718.1
      1126FMedium1624617.5
      1228FMedium1555221.6
      1321FMedium1605119.9
      1428FMedium1585421.6
      1523FMedium1605722.3
      1625FMedium1706422.2
      1723MLarge1807623.5
      1823MLarge1705519.0
      1923MLarge1858224.0
      2029MLarge1736822.7
      2128MLarge1827823.6
      2225MLarge1757223.5
      2330MLarge1706522.5
      2424MLarge1787624.0
      2521MLarge1827723.3
      Abbreviations: F, female; M, male.
      The Pearson correlation analysis showed no correlation between the BMI and stimulation intensities (r<0.3), and BMI and VAS (r<0.3) in both nerves, so the regression analysis was not continued.
      Table 2 shows a comparison of the overall stimulation intensity and the overall VAS between the proximal wrist crease and 14cm. The stimulation intensity and VAS at 14cm were significantly greater than at the proximal wrist crease. Some hands (median nerve, 7 hands; ulnar nerve, 5 hands) required a longer pulse duration at 14cm, from .05 to 0.1 milliseconds.
      Table 2Comparison of overall stimulation intensity and overall VAS between proximal wrist crease and 14cm
      NerveProximal Wrist Crease14cmMean DifferenceT ScoreP
      Median (n=36)
       Stimulation intensity (mA)42.1±13.766.8±15.824.7±17.28.1<.001
       VAS (mm)40.2±20.352.8±22.412.6±17.74.3<.001
      Ulnar (n=36)
       Stimulation intensity (mA)49.5±14.459.7±16.310.1±17.43.5.001
       VAS (mm)44±22.252±22.18±12.73.8.001
      NOTE. Values are mean ± SD or as otherwise indicated.
      Table 3, Table 4 show a subgroup analysis of each hand size group. The stimulation intensity and VAS at 14cm were significantly greater than at the proximal wrist crease in all groups for the median sensory NCSs, but only in the small hand group for the ulnar sensory NCSs.
      Table 3Comparison of stimulation intensities between proximal wrist crease and 14cm in each hand size group
      Stimulation Intensity (mA)
      Proximal Wrist Crease14cm
      NerveHand SizeMedianRangeMedianRangeZP
      MedianSmall (n=12)38.824–59.170.548.4–97.8−3.1.002
      Medium (n=12)39.624.6–66.368.236–80.4−3.1.002
      Large (n=12)42.627–84.669.446.3–99−3.1.002
      UlnarSmall (n=12)41.125.5–61.262.740.5–77.1−3.1.002
      Medium (n=12)46.835.1–7954.636.3–83.2−1.6.09
      Large (n=12)60.129.7–7466.235.2–94.5−1.3.18
      Table 4Comparison of VAS pain scores between proximal wrist crease and 14cm in each hand size group
      VAS (mm)
      Proximal Wrist Crease14cm
      NerveHand SizeMedianRangeMedianRangeZP
      MedianSmall (n=12)4218–696333–100−2<.05
      Medium (n=12)41.56–585019–77−3.1.002
      Large (n=12)3810–775010–82−2.2.03
      UlnarSmall (n=12)51.515–8861.536–91−2.6.01
      Medium (n=12)43.58–794212–82−1.5.12
      Large (n=12)36.516–803920–80−1.5.12

      Discussion

      The results of this study have demonstrated that the stimulation site can affect the patient's level of comfort during NCSs. Although this was a small study, a larger group of subjects is not likely useful, since this study showed statistically and clinically important differences between the populations.
      In this study, the stimulus pulse duration was not included in the analysis of the stimulation intensity levels. Seven hands in the median sensory NCS required a 0.1-millisecond pulse duration at the 14-cm site. All 7 hands also required greater stimulation intensities at 14cm than at the proximal wrist crease. Five hands in the ulnar sensory NCS required a 0.1-millisecond pulse duration at the 14-cm site. Four of them required lower stimulation intensities at 14cm than at the proximal wrist crease. Increasing pulse duration from .05 to 0.1 milliseconds may confuse analysis of the stimulation intensity levels. However, the pain scores were higher at 14cm than at the proximal wrist crease in all cases.
      Sensory NCS techniques usually specify the same distance irrespective of patient size.
      • Jablecki C.K.
      • Andary M.T.
      • So Y.T.
      • Wilkins D.E.
      • Williams F.H.
      Literature review of the usefulness of nerve conduction studies and electromyography for the evaluation of patients with carpal tunnel syndrome.
      • Stevens J.C.
      AAEM minimonograph #26: the electrodiagnosis of carpal tunnel syndrome.
      • Johnson E.W.
      • Melvin J.L.
      Sensory conduction studies of median and ulnar nerves.
      • Dumitru D.
      • Amato A.
      • Zwarts M.
      Nerve conduction studies.
      • Weber R.
      Motor & sensory conduction and entrapment syndrome.
      • Delisa J.A.
      • Lee H.J.
      • Lai K.S.
      • Spielholz N.
      Manual of nerve conduction velocity and clinical neurophysiology.
      • Buschbacher R.M.
      Manual of nerve conduction studies.
      Using the same distance for individuals of different sizes may not be the best approach. A standard 14-cm distal distance in a small individual may require stimulation at a site in the forearm where the nerve is deeper under flexor forearm muscles and there may be more subcutaneous tissue.
      The median and ulnar nerves come closer to the surface as they approach the wrist. At the proximal wrist crease, both nerves are situated at the most anterior surface. The median nerve lies laterally to the flexor pollicis longus tendon, while the ulnar nerve lies in front of the flexor retinaculum. At the distal forearm level, the median nerve lies behind and lateral to the flexor carpi radialis and flexor digitorum superficialis muscles, respectively (see fig 1).
      • Sinnatamby C.S.
      Last's anatomy regional and applied.
      • Johnson D.
      • Ellis H.
      Pectoral girdle and upper limb.
      • Dean D.
      • Herbener T.E.
      Cross-sectional human anatomy.
      This study demonstrated that a larger stimulation intensity was required at 14cm than at the proximal wrist crease, which is consistent with what would be expected anatomically. The VAS pain scores also indicated that subjects had greater discomfort when the 14-cm stimulus site was used. By using a shorter distance, especially in small hands, subjects had significantly less pain. Kelly
      • Kelly A.M.
      Does the clinically significant difference in visual analog scale pain scores vary with gender, age, or cause of pain.
      reported for a 10-cm VAS, a difference in pain scores of at least 0.9cm is clinically significant. In this study, the average differences of pain scores in the small hand group (for both median and ulnar sensory NCSs) and in the medium hand group (for ulnar sensory NCSs) were more than 0.9cm, which were clinically significant.
      This study found, as did the study by Wee et al,
      • Wee A.S.
      • Boyne R.L.
      • Abernathy S.D.
      • Nick T.G.
      Pain perception to nerve conduction and needle electromyographic procedures.
      that BMI has no significant relationship to pain from the NCS test. Therefore, the difference in pain scores in this study was due to the different sites of stimulation and the different stimulation intensities. The difference in pain scores was less in the ulnar sensory NCSs than in the median sensory NCSs because there was less of an increase in the stimulation intensity between the 2 sites.

       Study limitations

      This study used a small, controlled population of healthy, young, nonobese adults. Further studies are needed to examine what might be even greater effects in older and more obese populations.

      Conclusions

      This study found that stimulation at the wrist can result in less pain than stimulation at 14cm, especially for smaller people. Neither technique has been shown to be more sensitive or specific for critical diagnoses such as carpal tunnel syndrome. Therefore, there are few reasons why clinicians should subject patients to the increased pain of the 14-cm technique. By increasing comfort during NCSs, clinicians and scientists will improve the overall patient experience, perhaps resulting in more accurate and complete testing for the individual, and an improved perception of this useful examination by those outside the electrodiagnostic field.

      Suppliers

      • a.
        Medelec; Natus Medical Incorporated, 1501 Industrial Rd, San Carlos, CA 94070.
      • b.
        Microsoft Excel; Microsoft Corp, One Microsoft Way, Redmond, WA 98052.
      • c.
        SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.

      Acknowledgments

      We thank Jacqueline J. Wertsch, MD, and Andrew J. Haig, MD, for review and critique of the manuscript, and Priti Mahajan, MD, for technical help.

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