Sensory perception testing by monofilaments in the digits of controls and workers with havs

Conclusions

  SWM are a useful screening tool for detecting sensory loss in the digits of workers exposed to HTV.

Keywords

  Hand-arm vibration syndrome · HAVS · Monofilaments · Sensory perception · Neuropathy

Introduction

The diagnosis of sensory neuropathy in the digits of workers 

exposed to hand-transmitted vibration (HTV) is challeng-

ing. The use of Semmes–Weinstein monofilaments (SWM) 

has been recommended by the Health and Safety Execu-

tive (HSE) in Guidance L140 on hand-arm vibration (HSE 

2005

) and in the latest classification of hand-arm vibration 

syndrome (HAVS) (Poole et al. 

2019

). SWM may also be 

used by diabetologists and hand surgeons. Their use has 

been endorsed by the American Peripheral Neuropathy 

Association because of their utility in clinical settings (PNA 

1993

), but their validity in determining sensory neuropathy 

in workers with HAVS is uncertain. Quantitative sensory 

perception tests (QST) of thermal and vibration perception 

are also used to do this, but they are time consuming and 

only available in a few specialised centres.

The SWM method relies on the principle that a nylon 

filament will buckle when compressed according to its 

length, diameter, and the type of material used to make it. 

The force of application is then limited by the buckling load. 

Once buckled, the force imparted by the filament should be 

constant.

Monofilaments work by stimulating light-touch and mecha-

noreceptors in the epidermis and dermis of the skin. Stimula-

tion causes ion exchange in the receptor, which then sends an 

action potential along myelinated (A fibre) and non-myelinated 

(C fibre) afferent nerves to the dorsal ganglia of the spinal cord 

and then up to the somatosensory cortex of the brain (Guy-

ton and Hall 

2016

). Some monofilaments are ‘soft tipped’ to 

avoid stimulating nociceptors in the skin and force overshoot. 

 

*

  C. J. M. Poole 

 [e-mail]

1

  Centre for Workplace Health, Health and Safety Executive, 

Harpur Hill, Buxton SK17 9JN, UK

2

  Analysis and Data Group, Health and Safety Executive, 

Harpur Hill, Buxton SK17 9JN, UK

---

 

International Archives of Occupational and Environmental Health

1 3

A full set of SWMs has 20 monofilaments with bend forces 

that range from 0.008 to 300 gram-force (g-f), but hand and 

foot sets can be purchased with five filaments in a set.

Unfortunately, their method of use has not been standard-

ised and some methods are better suited to a laboratory than a 

medical clinic. Methods that have been used include forced-

choice staircase algorithms with filaments applied in ascend-

ing, descending or random order, sometimes with additional 

auditory cues and ‘catch’ trials. The threshold can be taken 

as the lowest force felt, or the mean between the lowest felt 

and the next lowest monofilament, or the mean of a series 

of applications (PNA 

1993

; Berquin et al. 

2010

; Tracey et al. 

2012

). Testing relies on the co-operation of the subject and 

may therefore be described as a psychophysical test.

Monofilaments have been shown to vary in performance 

by make (Booth and Young 

2000

; Lavery et al. 

2012

) and 

to decrease in bend force with an increase in ambient tem-

perature or humidity (Werner et al. 

2011

; Haloua et al. 

2011

) 

as well as with repeated loading (Booth and Young 

2000

; 

Lavery et al. 

2012

). In one study, tolerance (± 10%) of a 

calibrated 10 g-f monofilament was found to vary with the 

number of times the filament was buckled, with only 80% 

of monofilaments remaining in the tolerance range after 100 

compressions (Booth and Young 

2000

).

The sensory perception threshold of the digits in nor-

mal healthy subjects has been shown to increase with age 

(Thornbury and Mistretta 

1981

; Schulz et al. 

1998

), but the 

effect of sex or handedness is less clear cut (Thornbury and 

Mistretta 

1981

; Schulz et al. 

1998

; Collins et al. 

2010

). In 

one study, mean sensory perception by digit in men over age 

55 ranged between 0.27 (little finger) to 0.40 (thumb) g-f 

(Schulz et al. 

1998

). The epidermis of the skin is known to 

thicken and harden with heavy manual work, so it would be 

reasonable to expect the sensory perception threshold to rise 

in such workers, particularly if gloves are not worn. There 

is one small study which showed heavy and moderate work 

to be associated with a significantly higher SWM threshold 

than light work (Birke et al. 

2000

).

Our study was undertaken to ascertain normal sensory 

perception by sex, age, hand and digit using SWM in office 

workers and the effect of heavy manual work on sensory per-

ception. The results were compared with a group of work-

ers diagnosed with neurological HAVS. The relationship 

between SWM and QST in workers with HAVS was also 

investigated.