Effects of Age ... Nerve Conduction

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Associations among measures of median, ulnar, and sural nerve conduc-tion and age, skin temperature, sex, and anthropometric factors were eval-uated in a population of 105 healthy, asymptomatic adults without occupa-tional exposure to highly repetitive or forceful hand exertions. Height wasnegatively associated with sensory amplitude in all nerves tested (P ,19326*38 kin t e m p e r a t ~ r e , ' ~ ' ~ . ~ ~height,7,2 1 32935 and finger circumference.3 Asquare-shaped wrist has been negatively associatedwith distal median nerve conduction. Factors forwhich associations with specific nerve conductionmeasures were found to be statistically significant,biologically plausible, and clinically importantwere used to develop formulas which predict nor-mal values.

MATERIALS AND METHODS

Population. Participants were salaried employeesin a large corporation. Prior to age- and sex-strat-

Nerve Conduction Measures MUSCLE &NERVE October 1992 1095


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ified ra ndom selection, employees on jobs thoughtto have repetitive or forceful ha nd exertions wereexcluded (e.g., secretaries, word processors). Of27 1 randomly selected e mp l~ ye es ,~ 49 wereavailable for participation (Table 1). Of these, 75refused to participate and 69 were excluded, leav-ing 105 study participants. Exclusion criteria wereneurological symptoms, a self-reported history ofa systemic illness or disorder affecting the centralor peripheral nervous systems, or occupational ex-posure to forceful or repetitive hand exertions.Participants returned a self-administered ques-tionnaire which included information on age,height, medical history, occupational history, andsymptoms in the handlwrist during the precedingyear.

Physical Evaluation. All subjects had a screeningneurologic examination performed by a neurolo-gist (J. A.) and an additional examination of thehand and wrist performed by a nurse (B. S.). In-dex finger circumference was measured at themiddle of the proximal phalanx with a narrowcloth measuring tape. The wrist anterior- poste-rior and medial- lateral diameters were measuredwith engineering calipers at the distal wristcrease. 8 Wrist ratio was calculated as the ante-rior- posterior diameter divided by the medial-lateral diameter.

Table 1. Subject selection criteria.

Consent form and questionnaire

Less:

n = 271ent to randomly selected employees

Unavailable for participation ( n = 22)Refused to participate (n = 75)

Medical exclusion* n = 28)>21 oz. ethanol/week orself-described problem n = 2)Numbness or paresthesia in handoccurring > 3 imes or lasting >1 week oroccurring within prior week n = 18)

n = 249n = 174

Excluded n = 69):

Pain in volar hand/wrist n = 2)

Current job has 2 4 h/d withrepetitive/forceful hand exertions n = 14)Prior job, within previous 2.5 years, withrepetitive/forceful hand exertions n = 5) v

Study population n = 105~ ~

*Med ical reasons for exclusion were diabetes mellitus, rheumatoidarthritis, abnormal thyroid function within previous 10 years, carpaltunnel syndrome, cervical disk d isease (or symptoms of paresthesias inhands radiating from the neck) pregn ancy, renal disease, hemophilia,prior radial mononeuropathy, prior cerebral infarction, chem othera py, oruse of an ticonvulsant medication

Nerve Conduction Studies. Nerve conductionstudies were performed on the dominant hand ofall subjects by the same certified electromyogra-phy technician. Using a TECA T D - 5 electromyo-graphy machine and standard TECA surface elec-trodes, standard techniques of supramaximalpercutaneous nerve stimulation and surface re-cording were employed. Anatomic landmarksand standardized stimulation to recording elec-trode distances are shown in Table 2. Skin tem-peratures were monitored at the proximal creaseof digits I 1 and V and at the midcalf. Tempera-ture was maintained above 32C in the hand, andabove 30C in the calf, with an electric heatingpad.

Sensory nerve action potential and compoundmuscle action potential amplitudes were measuredfrom the baseline to the negative peak. Sensory la-tencies were measured to the onset of the negativedeflection and the negative peak. Motor latencies

were measured to the onset of the initial negativedeflection. Median motor conduction velocity forthe forearm was calculated using proximal and di-stal onset latencies. Terminal sensory conductionvelocities were calculated by dividing the distal dis-tance by the onset latency. F-wave latencies weremeasured as the minimal latency in a series of fiveresponses.

Statistics. Students t test was used to comparethe difference between means for quantitativevariables. Least squares regression was used formodel building and hypothesis testing. The Pear-son product moment was calculated for correla-tions of continuous variables. A P-value of c0.05was used to define statistical significance. Formodel building, criteria for including an addi-tional independent variable were that its coeffi-cient have a P-value 0.05. Data formany of the nerve conduction measures did notfollow a normal distribution. Prior to hypothesistesting, several transformations (natural log,square root, and inverse) were evaluated. Trans-formations resulting in a more normal distribu-

tion (Kolmogorov D-statistic an d visual inspectionof histogramsbox plots) were used in the regres-sion models. A model building approach was usedfor hypothesis testing. The independent variableswere added in an order based on the strength ofpreviously reported evidence for biologically plau-sible an d significant associations: age, skin temper -ature, height, dominant index finger circumfer-ence, dominant wrist ratio, and sex.

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Table 2. Description of nerve conduction measurements ( n = 105) mean * standard deviatlon (range)Recording electrodes

(interelectrode Conduction velocity(m/s)erve Stimulate distance) Amplitude (p V or mV) Latency (ms)

Median sensory Wrist

Wrist

Midpalm, midthenarcrease

Median motor Wrist

Antecubital fossa, overbrachial pulse

proximalMedian F response Wrist; cathode

Ulnar sensory Wrist

Wrist

Medial midpalm

Sural sensory Midcalf

Digit /I, proximal anddistal phalanxes;30-40 mm apart

(140 mm)Digit IV, proximal anddistal phalanxes;30-40 mm apart(140 mm)

Wrist, between PLand FCR tendons(80 mm)

Thenar muscle(70 mm)

Thenar muscle(measured)

Thenar muscle

Digit V, proximal and

distal phalanxes;30-40 mm apart(140 rnm)

Digit IV, proximal distalphalanxes; 30-40mm apart (140 mm)

Medial wrist (80 mm)

Lateral malleolus(140 rnm)

32.7 i 11.4(14-68)

17.7 * 8.6(5-50)

115 f 41.5(40- 225)

12.5 f 3.1(4-19)

28.9 * 11.6(9-60)

19.0 c_ 7.3(6-38)

20.8 8.4

17.5 t 7.7(8-52)

(6-48)

3.0 t 0.2(2.6- .8)

3 0 t 0.3(2.5-4.1)

1.8 *0.2(1.4-2.4)

3.2 i 0.4(2.4- .2)

27.1 *2 . 22.8 * 0.2

(21.6-32.6)

(2.5- .7)

2.9 0.2(2.4- 3.9)

1.7 0.1

3.4 0.3(1.5 .2)

(2.9- 4.9)

60.2 4.9(47-70)

56.7 2.9(50-64)

63.0 -+ 4.1

(48 4)

52.2 -t 5.3(36-64)

*PL = palmaris longus, FCR = flexor carpi radfalfs

RESULTS

Population Description. Th e mean +SD) age formen was 42.7 (& 12.6) years an d for women it was37.8 (k10.0) years ( P < 0.05); a e and sex distri-butions are repo rted elsewhere: Distributions ofheight, dominant hand index finger circumfer-ence, and dominant hand wrist ratio, stratified bysex, are summarized in Table 3. As a group,women were shorter and had a smaller finger cir-

3 .

Table 3. Population distribution of height, dominant hand index

finger circumference, and wrist ratio, by sex.Measure Female ( n = 43) Male (n = 62) P-value

Height (cm) 163.8 * 7.1 180.1 6.4 0.0001(150- 180) (168- 193)

Dominant index finger 6. 2 0.4 7.2 f 0.4 0.0001Circumference (cm) (5.2-7.0) (6.4-8.1)Dominant wrist ratio 0.68 & 0.03 0.68 0.03 NS

(0.62-0.7 6) (0.61 0.78)

Vaiues e xpressed in mean SO (range)

Nerve Conduction Measures

cumference than men, but there was considerableoverlap between the two sexes. Skin temperatureat the index finger had a range of 32.0 to 35.5 Cwith a mean of 34.0 -+ 0.82. For the fifth digit, therange was 32.5 to 36.0 C (34.1 * 0.76), and atmidcalf the range was 30.0 to 37.0 C (33.6 * 1.3).Nerve Conduction Measures. Nerve conductionmeasurement distributions are shown in Table 2with results for the median to ulnar comparisonmeasures in Table 4. A description of the modelbuilding process is repo rted elsewhere ; resultsare shown in Tables 5 and 6. Age was significantly

associated with all sensory amplitude measures ( P< 0.001) and all conduction velocity and latencymeasures P < 0.05) except sural conduction ve-locity. There was no consistent pattern of associa-tion between digit temperature and median or ul-nar nerve conduction, likely because of thenarrow range of temperatures maintained as partof the study protocol. There was a strong associa-tion between temperature and sural conduction

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Table 4. Descriptionof median to ulnar comparison measurements (n = 105).~~

Percentile

Measurement Mean t SD 95th 99th Range

Medran minus ulnar distal sensory latency (digit vsV) 0 1 t o 2 0 0 4 o a 0 3- 0 a

Median to ulnar amplitude ratio (digit llidigitV) 1 2 t 0 3 4 i a 2 0

Median minus ulnar distal sensory latency (digit IV) 0 1 ? 0 2 5 0 5 0 9 - 04 -1 2Median minus ulnar sensory latency midpalm (wrist) 0 1 k 0 1 7 0 4 0 6 -0 2 0 6

0 7-3 0

velocity ( P < 0.001), probably related to the widerrange of temperatures at the midcalf.

Height was negatively associated with sensoryamplitude in all nerves tested ( P < 0.05), and pos-itively associated with median and ulnar sensorydistal latencies ( P < 0.01) and sural latency ( P

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Effects of Age ... Nerve Conduction