Quantitative sensory testing

QST consists of procedures requiring a power source, where the intensity and characteristics of the stimuli are well controlled and where the detection threshold is determined in parametric units that can be compared with established 'normal' values (Arezzo 2003). QST measures can be used to identify the sensory modalities affected and to estimate the magnitude of the deficit. QST measures of vibration, thermal and pain thresholds have proven valuable in identifying diabetic patients with subclinical neuropathy (Boulton et al. 2004), track progression (Bril 2003) and risk of foot ulceration (American Diabetes Association 1988). In addition, QST measures have played a key role as primary efficacy endpoints in a series of multicentre clinical trials evaluating the prevention or treatment of diabetic polyneuropathy (Okamoto et al. 2002; Ekberg et al. 2003).

The strengths of QST are well documented (Arezzo 2003) and include:

1. the accurate control of stimulus characteristics

2. the ability to assess multiple modalities

3. the use of well established psychophysical procedures to enhance sensitivity

4. the capacity to measure function over a wide dynamic range of intensities, thus supporting the evaluation of multiple degrees of neuropathy

5. the ability to measure sensation at multiple anatomical sites, enabling the exploration of a potential distal-to-proximal gradient of sensory loss

6. for most measures, the availability of data from large, age-matched, 'normal' comparison groups

The limitations of QST are also clear. No matter what the instrument or procedure used, QST is only a semiobjective measure, affected by the subject's attention, motivation and cooperation, as well as by anthropometric variables such as age, sex, body mass and history of smoking and alcohol consumption (Gerr & Letz 1994; Gelber et al. 1995). Expectancy and subject bias are additional factors that can exert a powerful influence on QST findings (Dyck et al. 1998). Further, QST is sensitive to changes in structure or function along the entire neuroaxis from nerve to cortex; it is not a specific measure of peripheral nerve function (Arezzo 2003).

QST testing for vibratory and cooling thresholds receives a class II rating as a diagnostic test. It is designated as safe, effective and established. Thus QST is accepted and commonly used in clinical trials of diabetic neuropathy.

The biothesiometer is a hand held electromagnetic vibrator with a stimulating probe (12 mm diameter) that vibrates at 100 Hz. The stimulating probe is placed on the site to be tested, usually the big toe or the finger, and rests on its own weight (300 g). In a four-year prospective study (Young et al. 1994), patients with baseline threshold elevated above a fixed value (i.e. 25 V with the biosthesiometer) were seven times more likely to develop foot ulcers. This observation is supported by a recent evaluation of 187 type 2 diabetic patients, which used multivariate logistic regression to document that an elevated VPT score was the strongest predictor of foot ulceration (i.e. relative risk of 25.40 (Kastenbauer et al. 2001)). The strength of the relationship between elevated VPT and foot ulceration is illustrated by the finding, in 1,035 type 1 and type 2 diabetic patients, that each 1 unit increase in vibration threshold (voltage scale) at baseline increased the hazard of foot ulceration by 5.6 % over a one year study period (Abbott et al. 1998).

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