Sensory perception testing by monofilaments in the digits of controls and workers with havs
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| Table 1
95th percentiles of sensory perception thresholds for the digits of men and women office workers a Bootstrapped 95% percentile confidence interval b One male was ambidextrous so their results (10 digits) were excluded when investigating hand dominance Men Women
Number of sub-
jects Number of digits 95th percentile 95% confidence interval a Number
of sub- jects
Number of digits 95th percentile 95% confidence interval a All 155 1549
0.16 (0.16–0.16) 144 1440 0.16
(0.16–0.16) Age
< 30 years 18 180 0.16 (0.16–0.16) 37 370 0.07
(0.07–0.07) 30–39 years 32 320
0.16 (0.16–0.16) 32 320 0.16
(0.12–0.16) 40–49 years 54 540
0.16 (0.16–0.16) 37 370 0.16
(0.16–0.16) 50 + years 51 509
0.16 (0.16–0.40) 38 380 0.16
(0.16–0.16) Digit
Little 155
309 0.16
(0.16–0.16) 144 288
0.07 (0.07–0.16) Ring 155
310 0.16
(0.16–0.16) 144 288
0.07 (0.07–0.16) Middle 155
310 0.16
(0.16–0.16) 144 288
0.16 (0.07–0.16) Index 155
310 0.16
(0.16–0.16) 144 288
0.16 (0.16–0.16) Thumb 155
310 0.16
(0.16–0.40) 144 288
0.16 (0.16–0.16) Hand dominance b Dominant 154 769
0.16 (0.16–0.16) 144 720 0.16
(0.16–0.16) Non-dominant 154 770
0.16 (0.16–0.16) 144 720 0.16
(0.12–0.16) Hand dominance and digit b Dominant Little 153 153
0.16 (0.16–0.16) 144 144 0.16
(0.07–0.16) Dominant Ring 154 154
0.16 (0.16–0.16) 144 144 0.07
(0.07–0.16) Dominant Middle 154 154
0.16 (0.16–0.16) 144 144 0.16
(0.07–0.16) Dominant Index 154 154
0.16 (0.16–0.16) 144 144 0.16
(0.16–0.16) Dominant Thumb 154 154
0.40 (0.16–0.40) 144 144 0.16
(0.16–0.16) Non-dominant Little 154
154 0.16
(0.16–0.16) 144 144
0.07 (0.07–0.16) Non-dominant Ring 154 154
0.16 (0.16–0.16) 144 144 0.07
(0.07–0.16) Non-dominant Middle 154
154 0.16
(0.16–0.16) 144 144
0.07 (0.07–0.16) Non-dominant Index
154 154
0.16 (0.16–0.16) 144 144 0.16
(0.07–0.16) Non-dominant Thumb 154
154 0.16
(0.16–0.40) 144 144
0.16 (0.16–0.16) International Archives of Occupational and Environmental Health 1 3
The relationship between the SWM threshold and the total number of thermal and vibration threshold abnormali- ties in 120 hands of workers with HAVS is shown in Fig. 2 . There was an increase in the number of abnormal thresh- olds as the SWM bend force increased, reaching a plateau at about 2 g-f. A fitted linear spline with a knot at 2 g-f indicated that, when the SWM bend force was < 2 g-f, the number of abnormal QST thresholds approximately doubled with each 1 g increase in force (95% CI 1.56–2.58). Similar curves were obtained when the SWM thresholds were com- pared separately with the number of abnormal thermal or vibration results. The AUC for abnormalities of thermal and vibration per- ception in the index and little fingers in the same hand of workers with HAVS was 0.78 (95% CI 0.67–0.87) and for abnormalities of thermal or vibration perception was 0.84 (95% CI 0.74–0.90). For the former, sensitivity was 100% when the SWM bend force was 0.4 g-f, decreasing as the bend force increased. Specificity increased as the SWM bend force increased reaching 100% at ≥ 10 g-f. A SWM cut-off of ≥ 1.0 g-f gave a sensitivity of 79% and a specificity of 64% Table 2 Frequency distribution of sensory perception thresholds for the digits of office workers and heavy manual workers a One person missing result for one finger b Two people missing three results in total Threshold (g–f) Office workers ( n = 300)
Heavy manual workers ( n = 115)
Frequency Percent
Cumulative percent
Frequency Percent
Cumu- lative
percent 0.04
1144 38.1
38.1 90 7.8 7.8 0.07
1359 45.3
83.5 239
20.8 28.7
0.16 462
15.4 98.9
467 40.7
69.4 0.40
27 0.9
99.8 184
16.0 85.4
0.60 7 0.2 100.0 102
8.9 94.3
1.0 0 0.0 100.0 44 3.8 98.2 1.4
0 0.0
100.0 14 1.2 99.4 2.0
0 0.0
100.0 7 0.6 100.0 Total
2999 a 100.0 1147 b 100.0 Table 3 Agreement between the first and second tests for identical digits of office workers and heavy manual workers Values in cells show the number of digits with the specified results Perfect agreement Differ by one filament Result at first test (g-f) Result at second test (g-f) Total digits
0.04 0.07
0.16 0.40
0.60 0.04
102 19 3 0 0 124 0.07 29 38 19 0 0 86 0.16
4 15 54 5 0 78 0.40 0 0 4 4 0 8 0.60
0 0 3 1 0 4 Total 135
72 83 10 0 300
Table 4 Frequency distribution of sensory perception thresholds for the digits of workers with HAVS a Threshold (g–f) Frequency Percent Cumu-
lative percent
0.04 4 0.6 0.6 0.07
41 6.6
7.3 0.16
175 28.3
35.5 0.4
89 14.4
49.9 0.6
80 12.9
62.8 1.0
95 15.3
78.2 1.4
52 8.4
86.6 2.0
34 5.5
92.1 4.0
27 4.4
96.4 6.0
11 1.8
98.2 > 10
11 1.8
100.0 Total
619 a 100.0 International Archives of Occupational and Environmental Health 1 3 for abnormalities of thermal and vibration perception. The corresponding sensitivity and specificity for abnormalities of thermal or vibration perception was 68% and 89%, respec- tively. Table 5 shows sensitivities and specificities of SWM thresholds for detecting abnormalities of QST in workers with HAVS. Discussion The sensory perception thresholds, as measured by SWM, in the digits of heavy manual workers not exposed to HTV was found to be significantly higher than that of office work- ers. This is probably because of thickening or hardness of the skin, but sensory neuropathy from trauma to the hands cannot be excluded. To date clinicians have been advised to take 0.2 g-f as the cut-off from normal (Lawson 2018 ), but
by so doing they may be misdiagnosing thick or hard skin as stage 2 neurological HAVS. From these data the cut-off from normal, or 95th percentile for male heavy manual workers should be 1.0 g-f, and for men ≥ 50 years 1.4 g-f. The latter threshold is in keeping with the 95th percentile of 2 g-f for heavy manual workers not exposed to HTV in Italy (Poole et al. 2019
). Based on this, the regression line in Fig. 2 and the AUC analysis, the optimum time for practitioners to refer workers exposed to HTV to specialised centres for thermal and vibration sensory perception tests would appear to be when the mean SWM bend force in two digits of a hand, ideally not supplied by the same nerve, is ≥ 1.0 g-f. In this way, SWM can be used as a screening tool for the more sophisticated and expensive tests of thermal and vibra- tion perception. Lowering the SWM threshold for referral would increase the sensitivity for identifying abnormalities of thermal and vibration perception but reduce its specificity. Lowering the threshold by one filament to ≥ 0.6 g-f would take into consideration the reliability of the method in that 97% of intra-subject results were identical or differed by one filament. The overlap in sensory perception of some office work- ers and heavy manual workers may be because some of the heavy manual workers wore gloves, in which case their skin would be expected to be like that of an office worker. The absence of a significant difference in sensory perception by hand or digit indicates that these factors do not have to be taken into consideration when determining abnormality in workers with potential neuropathy. The increased sensitiv- ity of the digits of women office workers < 30 years may have occurred by chance, but as this finding is biologically plausible it is likely to be a true finding. The overlap in sensory perception thresholds of the heavy manual workers not exposed to HTV and workers with HAVS could be expected as some of the workers with HAVS had an early stage of HAVS with only neurological symp- toms and some of the heavy manual workers would have had thick or hard skin or sub-clinically damaged hands. By comparison, reduced sensory perception has been reported in the feet where the highest SWM threshold was found over HAVS
Heavy manual Office
0 .1 .2 .3 .4 .5 0 .1 .2 .3 .4 .5 0 .1 .2 .3 .4 .5 0.04 0.07 0.16 0.4
0.6 1 1.4 2 4 6 ≥10 Proportion of digits Sensory preception threshold (g-f)
Semmes–Weinstein monofilament sensory perception thresh- olds of the digits of office workers, heavy manual workers and work- ers with HAVS Fig. 2 Best fit Poisson regression line between Semmes Weinstein monofilament thresholds and the number of abnormal thermal and vibration perception thresholds in the hands of workers with HAVS International Archives of Occupational and Environmental Health 1 3
the heels and the lowest over the arches in keeping with the thickness and hardness of the skin (Strzalkowski et al. 2015 ). Recent work has shown quantitative sensory tests (QST) to be unaffected by the thickness of the skin in the digits (Lundstrom et al. 2018 ). Doctors have been recom- mended to use SWM (HSE 2005
), but their validity has been uncertain because of the absence of normative data and their unknown relationship with the results of QST. These ques- tions have now been answered. Furthermore, most clinicians in a community clinic setting will have access to SWM, but not to QST, so the optimum time to refer a worker for QST is important to know. A weighted kappa statistic of 0.63 for repeated intra-digit testing with SWM indicates moderate to substantial reli- ability (Landis and Koch 1977 ). This was achieved when the same trained tester was used for each population. This is reassuring, but it should be noted that the technique for the use of SWM needs to be taught and practised. Higher reliability has been reported for the plantar surface of the great toe with coefficients > 0.9 with more complex testing algorithms lasting 20 min (Tracey et al. 2012 ). The method of application should be standardised, and the bend forces of the filaments validated at regular intervals. Fatigue and deterioration in bend force of the filaments with repeated use is less relevant in HAVS practice as normally only a few workers will be tested in any one day. Care needs to be taken to avoid stimulating nociceptors and the duration of each filament’s contact with the skin of a digit needs to be long enough to stimulate light touch and mechanoreceptors. Unless their use is well taught and practised, then the reli- ability of results between practitioners is likely to be poor. More sophisticated methods of application can be used in a laboratory setting, such as the method of limits, or multiple applications of the same force, or mechanical methods of application, but such accuracy is probably unnecessary when SWM are being used to screen workers for more accurate testing.
The strength of this work is that relatively large popu- lations of workers have been studied in a standardised way with the same SWM by the same investigators. It is not known how a different technique with a more complex algorithm would affect results, but the method described is quick and easy to use in a busy clinic setting and suitable for screening. We used the mean of the two digits with the highest SWM thresholds and compared them with the QST results for the index and little fingers because this is how clinical testing is undertaken. We do not believe that com- paring SWM and QST results of only the index and little fingers would have made an appreciable difference to our results. As with all psychophysical methods, conscious bias cannot be eliminated, but these data should help with its identification. For the office workers and the workers with
Sensitivity and specificity of SWM thresholds for detecting abnormalities of thermal and/or vibration perception in the same hands of workers with HAVS a Mean threshold of the two digits with the highest thresholds SWM threshold (g–f)
a Abnormalities of thermal and vibration per- ception in the hand Abnormalities of thermal or vibration perception in the hand Sensitivity (%) Specificity (%) Sensitivity (%) Specificity (%) ≥ 0.15
100.00 2.11
100.00 4.26
≥ 0.2 100.00
4.21 100.00
8.51 ≥ 0.3
100.00 23.16
94.81 38.30
≥ 0.4 100.00
30.53 89.61
44.68 ≥ 0.5
93.10 41.05
81.82 57.45
≥ 0.6 89.66
53.68 74.03
72.34 ≥ 0.7
89.66 58.95
72.73 80.85
≥ 0.8 86.21
62.11 70.13
85.11 ≥ 1.0
79.31 64.21
67.53 89.36
≥ 1.2 68.97
71.58 55.84
91.49 ≥ 1.4
65.52 77.89
50.65 97.87
≥ 1.5 62.07
78.95 48.05
97.87 ≥ 1.7
62.07 80.00
46.75 97.87
≥ 2.0 51.72
84.21 37.66
97.87 ≥ 2.5
41.38 87.37
31.17 100.00
≥ 2.7 34.48
87.37 28.57
100.00 ≥ 3.0
31.03 88.42
25.97 100.00
≥ 3.5 13.79
91.58 15.58
100.00 ≥ 4.0
10.34 92.63
12.99 100.00
≥ 10 3.45
96.84 5.19
100.00 International Archives of Occupational and Environmental Health 1 3 HAVS, SWM testing took place in controlled ambient tem- peratures and humidity. Testing of the heavy manual workers took place at the workplace in portacabins where the ambi- ent conditions could not be controlled, but we do not believe this made a substantial difference to our results. Based on this research, clinicians should be careful to purchase hand- sets of SWM that include filaments with bend forces in the range 0.2–2.0 g-f and not a standard WEST handset in the range 0.07–200 g-f. Conclusion We have shown that heavy manual work increases the sen- sory perception thresholds of the digits in the hands as meas- ured by SWM. The intra-subject reliability of SWM is good when a simple forced-choice method is used by the same assessor. In workers with HAVS, abnormalities of thermal and vibration perception increase as the SWM threshold increases up to a bend force of 2 g-f. We recommend that workers exposed to HTV have their digits screened with SWM and are referred for QST when the mean SWM bend force in two digits is ≥ 0.6 g-f.
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