Indian J Physiol Pharmacol 1996; 40(4): 372-376

MEDIAN NERVE SOMATOSENSORY EVOKED POTENTIALSIN RUBBER FACTORY WORKERS

VIJAYKUMARAND O. P. TANDON*

Department ofPhysiology,

University College ofMedical Sciences and GTE Hospital,

Delhi - 110095

(Received on May 23,1996)

Abstract: Somatosensory evoked potentials (SEPs) on median nerve stimulation werestudiell in 38 mbber factory workers to detect possible subclinical impairments in thesomato~"nsory pathway as a rc~ult of exposure to mbber factory chemicals. Resultsshowed trend towards the prolongation of latencif!~ of all the major peaks of SEP suchas Pi, N1, P2, N2, P3, N3 in exposed workers besirle~ changes in peak to peak aropliturles(higher Pi - N1 and lower N2 - P3). These changes may be attributed to synergisticadion of solvents anrl other mbber factory chemicals on the generators of SEPs locatedhigher U[J :'It the thalamo-cortical level.

Key words: somatosensory evoked potentials

rubber factory environment

median nerve

factory workers

INTRODUCTION

The manufacture of rubber products suchas tyres and tubes requires use of wide varietyof chemicals (approx, 500) grouped into variouscategories according to the purpose for utilisation(1). Many of these chemicals such as solventsare known neurotoxic agents as they readilycross the blood-brain barrier because of beingnonpolar and having affinity for lipid rich tissueof brain. Long term exposure to organic solventsmay cause impairment to both central andperipher' I nervous system (2). Chronic and lessreadily noticed adverse effects caused by longterm exposure to chemicals have become ofprimary interest in occupational medicineespecially where improvements of workingconditions have led to decreased prevalence ofacute and severe intoxicants (3). This hasnecessitated the need for development of newmethods which enable the early detection ofneurotoxic effects of exposure to variouschemicals. Neuropsycho-Iogical tests have notproven to be of definite value in differentiatingneurotoxic disAase from naturally occurring

*Corrcspondinl! !\ulh,'t

neurologic diorder as many factors can interferewith neuropsychologic evaluation of patientssuch as personality, cultural background, priordisease states, drug and alcohol abuse, primorbidIQ (4).

This problem is circumvented by using non­invasive electrophysiological methods such asevoked potentials. These potentials can measureeffects of toxins on the central nervous system(CNS). These can provide reliable informationon the integrity of the sensory pathwaysfrom the periphery to the cortex (5). Since theworkers in the rubber factories are exposed todifferent forms of chemicals which are eitherused as ingredients or by products produced byvarious processes and reactions, a study wasconducted to detect possible subclinicalimpairments in the somatosensory pathwayusing middle latency somatosensory evokedpotentials This can help in localising site ofimpairment in CNS.

METHODS

The study was carried out on 38 male rubber

Indian J Physiol Pharmacol 1996; 40(4) SEPs in Rubber Factory Working 37::l

factory workers and 11 control subjects (notexposed to rubber factory environment) in themiddle age group ranging from 18-55 years. Themean duration of occupational exposure torubber factory environment was 47.76 months(range 1 to 240 months). The subjects werebrought to the neurophysiology laboratory andgiven adequate rest. They were explained aboutthe purpose of the study and the technique.The median nerve somatosensory evokedpotentials (SEPs) were recorded usingNeuropack Machine (Nihon Kohden, Japan).Detailed methodology for recording SEP havealso been published by us elsewhere (6, 7). Theelectric stimulation was given to right mediannerve at wrist with the help of an electronicstimulator delivering a constant square wavecurrent of 0.1 msec duration at a frequency of 2Hz. The intensity of stimulation produced adetectable twitch of the right thumb. The evokedresponses were recorded by using silver, silverchloride disc electrodes on the contr lateralparietal scalp 2 cm posterior to the C3 positionas per international 10-20 placement system (8).The ear electrode Al was taken as the referenceas the ear lobes are relatively free from corticalelectrical activities. A filter setting of2 Hz -1 KHzand an electrode impedance of <5000 Ohm wasused. 256 evoked responses were averaged by theinbuilt computer in the Evoked Potential RecorderCMEB 5200 series, Nihon Kohden, Jrpan).

The absolute peak latencies and amplitudesof positive potentials (P) and negative potentials(N) from 15 to 60 msecs of the application ofstimuli were analysed. These values in theexposed group were compared with those ofcontrol group.

RESULTS

The physical characteristics of rubberfactory workers and their comparison withcontrol group are given in Table 1. Thedifferences between exposed and control groupwere found to be statistically nonsignificant.Table II and III respectively show measuredvalues of absolute peak latencies (msec) andamplitudes (pv) of major peaks PI, Nl. P2, N2,P3 and N3 in rubber factory workers andcontrol group. There is a trend towards theprolongation of all the peak latencies in rubberfactory workers. But it did not reach statisticalsignificance. Peak to peak amplitude P1-N1 washigher in exposed group (P<O.OOlJ. But N2-P3showed reduced amplitude in exposed group(P<0.05). Correlation coefficients of absolutepeak latencies with duration of occupation,height, weight and surface area in exposedgroup are shown in Table IV. The latency ofpeak N1 (N20) indicated statistically significant(P<O.Ol) positive correlation wi.th duration ofoccupation.

TABLE I: Physical chanlcteristics oJf nlbber factory workers (Mean ± SD) and control group.

Parameter

Height (em)

Weight (kg)

Surface area (m2)

Duration of occupation(months)

Rubber factorywo,./'-ers

162.37 ± 4.92

53.33 ± r. 71

1.56 ± O.US

47.76 ± ~<9.7S

Controlgroup

164.09 ± 7.33

62.36 ± 13.60

1.67 ± 0.20

P value(group test)

0502

0.06

374 Kumar and Tandon Indian J Physiol Pharmacol 1996; 40(4)

DISCUSSION

This study was attempted primarily to detectsubclinical involvement of somatosensorypathway. The neural generators of various peaksof SEP have been explained elsewhere (7).

Different components of the SEPpredominantly reflect sequential activation of

neural generators excited by ascending volley(9) when the median nerve proprioceptive fibresIa and II cutaneous afferents are stimulated.The impulses from these afferent fibres ascendthrough the dorsal columns-leminiscus­thalamocortical system. Our results indicatetrend towards prolonged latencies of all thepeaks of SEP in rubber factory workers in

TABLE II: Mean and standard deviations of absolute peak latencies of 8EPs inrubber factory workers and their comparison with control group.

Variable Rubber factoryworkers

Controlgroup

P value(group t test)

Peak latencies(msec)

PINlP2N2P3N3

Peak to peakamplitude (f1v)

PI - Nl

Nl - P2

P2 - N2

N2 - PJ

P3 - N3

16.78 ± 2.23 15.82 ± 1.77 0.19321.01 ± 2.80 19.88 ± 1.81 0.21529.17 ± 4.90 27.47 ± 4.12 0.30234.30 ± 6.58 32.31 ± 6.15 0.37543.70 ± 7.81 42.22 ± 8.86 0.59351.25 ± 8.94 48.73 ± 11.43 0.444

TABLE ill The values (Mean ± 8D) of peak to peak amplitudes of 8EPs inrubber factory workers and their comparison with control group.

Rubber factory Control t valueworkers group (student 't' test)

0.887 ± 0.52 0.397 ± 0.255 3.55*

1.861 ± 0.993 1.632 ± 0.747 0.85

0.784 ± 0.597 0.553 ± 0.589 1.0976

1.228 ± 0.625 1.789 ± 0.854 2.0259**

0.694 ± 0.518 0.482 ± 0.121 1.350

*P<O.OOl; **p < 0.05

TABLE IV Correlation coeficient of absolute peak latencies of 8EPs with duration ofoccupation and other physical characteristics in rubber factory workers.

Peak latencies Duration of occupation Height Weight Surface area

PI 0.4505 0.2314 -0.1645 -0.0582N1 0.4635* 0.1723 -0.1130 -0.0382P2 0.4597 - 0.1628 -0.0901 -0.1717N2 0.3:328 -0.0565 - 0.1483 -0.1752P3 0.3248 -0.0347 0.0920 0.1163N3 0.3206 0.0620 0.1094 0.094

*p < 0.01

Indian J Physiol Pharmacol 1996; 40(4) SEPs in Rubber Factory Working 375

comparison with the control group (Table II).The latency of peak N 1 - the first corticalresponse to external stimulus (9, 10, 11) showedpositive correlation with the duration of exposure(Table IV). The prolongation oflatencies may beattributed to the synergistic neurotoxic actionof solvents and other rubber factory chemicalson the SEP pathway. Main constituents oforganic solvents used in making of tyres andtubes include naphtha, pentane, hexane,heptane, octane, benzene, toluene, xylene,isopropanol etc. These have been detected inambient environment of selected work areas (12).Solvent induced changes in SEP componentshave been reported by several workers (3, 13­17). The studies conducted by Deschmp et al(13) on subjects exposed to organic solventsindicated longer P22 and N35 latencies andamplitude N20-P22 than control groups. There.was increasing trend for N20 latency. The meanduration of exposure in that study was muchhigher (13.9 years) than our study (about 4years) Table I. This perhaps be the explanationfor the increasing latencies in the rubber factoryenvironment exposed workers not reachingstatistically significant level. This is furthersubstantiated by observation of significantcorrelation of latency of N1 with duration ofoccupation (Table IV).

Similarly other studies showed prolongationof latencies of median SEP in workers exposedto n-hexane but no exposure duration indicated(14) and mixed solvents with mean duration of

exposure 12 years (15). We have also foundstatistically significant increase in amplitude ofP1N1 (Table III) besides longer latency of N20in rubber factory workers. This increase inamplitude may be attributed to excitation dueto local irritation of generators as a result ofaccumulation of chemicals. Mutti et al observedsignificant prolonged latencies of P15, N20 andN26 and higher N20 - P22 amplitude in 15women exposed to n-hexane but the latephase (N41-N75) of cortical responses afterstimulation of median nerve led to decreasedamplitude (16). Our observations are inagreement with this.

The present study indicates that these SEPswhose generators are located higher up at thethalamocortical level are affected in the rubberfactory workers. Hence, these observations suggestthat exposure to rubber factory environment doesaffect the integrity of those generators and as aresult show changes in latency and amplitude ofvarious components of SEPs. But it is difficult topin point the culprit chemical (s) responsible forthese effects. Future studies involving quantitativeand qualitative analysis of ambient workplaceenvironment and their level in body fluids maythrow light on possible role of these neurotoxicantsin these cases.

ACKNO~DGEMENTS

We are thankful to Shri B. Manjhi forlaboratory help. Statistical assistance bycomputer centre ofUCMS, Delhi is acknowledgedwith thanks.

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