Semmes–Weinstein monofilaments are a semi-quantitative test of sensory loss. The idea, developed by Josephine Semmes and Sidney Weinstein, was to measure touch-pressure in a standardized way by controlling the force of an applied stimulus to the skin [1]. For this they replaced the ‘hairs’ previously used by Von Frey with more durable plastic filaments of variable diameters. Applied to the skin until they bend the monofilaments deliver the same force irrespective of any difference in observer or their degree of unsteadiness. They have been mostly used by orthopaedic surgeons as a tool for sensibility testing in hand surgery rehabilitation. They most likely test the skin receptor known as the Merkel disc which responds to constant touch-pressure. The original ‘long’ test kit of 20 monofilaments was grouped by five descriptive terms which were delineated by comparing with other tests of sensibility in 200 patients with peripheral nerve problems [2]:

• Normal light touch.

• Diminished light touch (with retention of two point discrimination).

• Diminished protective sensation (difficulty with manual dexterity and abnormal two point discrimination).

• Loss of protective sensation (markedly decreased stereognosis and manual dexterity).

• Loss of deep protective sensation (as above plus a risk of injury).

Each monofilament is designated by a number which represents a given force and then grouped into colour-coded ranges. Weinstein developed an enhanced version with a textured rounded tip to create a more consistent area of skin surface contact with less likelihood of slippage (Weinstein Enhanced Sensory Test [WEST]) [1]. This is available as a set of five colour-coded monofilaments, attached to a handle, representing the five levels of sensory thresholds described above. This version is the most appropriate for an occupational health practice (Table 1). There is published age- and sex-matched normative data for WEST monofilaments (men and women age <55 years, 0.035 g-f; age >55 years, women 0.15 g-f, men 0.385 g-f) [3].

What should you use it for?

It is a requirement of the UK Control of Vibration at Work Regulations 2005 (CVAWR) to provide health surveillance for workers regularly exposed at or above an exposure action value of 2.5 m/s². This is usually undertaken by a tiered approach by occupational health practitioners to detect early cases of hand–arm vibration syndrome (HAVS). WEST monofilaments are regularly used to assess sensory perception in tier 3 and tier 4 health surveillance for large cohorts of workers. The normal practice in HAVS health surveillance is to use the 0.2 g-f monofilament as a cut-off to determine when sensory perception is reduced.

Relevance to occupational health practitioners

The guidance to the CVAWR advises on the management of cases of HAVS based on the severity of the condition and its functional effects. The severity of HAVS, as currently appears in the guidance to the CVAWR, is graded by the use of a modified Stockholm Workshop Scale (SWS). A key determinant of progression within the SWS is the development of stage 2 sensorineural (2SN). WEST monofilaments are a useful indicator of the ‘loss of sensory perception’ occurring in stage 2SN. More specialized tests at tier 5 are advocated by some, particularly when determining the suitability of ongoing vibration exposure, but these are only available in a few specialist centres and WEST monofilaments are, therefore, a key component in routine large volume HAVS surveillance.

Ease of use

Peripheral neuropathy in HAVS affects the digits so WEST monofilaments are applied to the glabrous skin of the volar surface of the fingers between the tip and the whorl. Ideally all fingertips should be tested, but at least the index and little fingers, to cover median and ulnar nerve distribution. This should be done in a quiet room with the hand supported on a flat surface or towel. The filament under test is rotated on the handle clear of the others. While the subject averts their eyes, keeping the monofilament to the perpendicular, it is slowly applied to the skin until it bends for one second at least three times until a response (two out of three positive responses indicates a normal result). It is best to start by familiarizing the test subject with the procedure and then begin at a suprathreshold level, i.e. a 0.2g-f and work towards lighter or heavier filaments. Once felt there is no need to move to heavier monofilaments. Heavier monofilaments only require one application. To ensure consistency a short click from an interval marker is used to alert the subject of the test start and finish during which they are asked to respond with a ‘yes’ when the filament is felt.

Time to master

There are a number of test protocols which have been used but a review by the Health and Safety Laboratory of the Health and Safety Executive suggested that an inability to detect a 0.2 g monofilament was suggestive of significant vibration-induced neurosensory deficit when compared to other quantitative sensory testing (sensitivity 74 and 73%, respectively) [4]. It is important not to bend the shaft too little or too excessively (a ‘C’ shape curve is too much) or ‘bounce’ the filament as it will affect the calibration. It is best to try and develop a smooth rhythm when performing the test. If the filament is removed too quickly or jerkily, it may lead to an erroneous response. The shafts should not be touched directly or left near a heat source and should be cleaned between subjects. Some also add a few sham or dummy tests if subject inconsistency is suspected.