Perceptual threshold (PT) to electrical stimulation has been shown to be a reliable method of testing somatosensory function in limb dermatomes (Davey et al. 2001). In this study we have measured and mapped PT to electrical stimulation of the skin over the trunk and compared it with two-point discriminative ability (TPDA) determined using callipers.

With local ethical approval and informed consent, 19 healthy volunteers (12 males; aged 21-24 years) were recruited. Five sites were tested on both the front and back of the thorax on both sides of the body. Tests were made 10 cm lateral to the midline at the following levels: upper thorax, middle of thorax above nipple, middle of thorax below nipple, lower thorax at the level of the umbilicus and at the level of the iliac spines. Electrical stimulation (0.5 ms pulse width; 3 Hz) was applied via a self-adhesive cathode at the stimulation site and an anode strapped to the right wrist. The current was turned up and down with the subject continuously reporting ‘yes’ they could feel the stimulus or ‘no’ they could not. PT was recorded as the lowest current at which the subject reported sensation. TPDA was measured at the same sites by applying two points of a pair of callipers to the skin and asking the subject to report whether they sensed one or two points in order to ascertain the smallest distance at which the subject could detect two points. Sites were tested in random order.

There was good correlation (linear regression; P < 0.05) between results from equivalent test sites on the left and right sides for both PT and TPDA. Average (± S.E.M.) values for PT were higher (ANOVA; P < 0.05) in men than women (right anterior: women 1.29 ± 0.08 mA; men 1.59 ± 0.08 mA; left anterior: women 1.26 ± 0.08 mA; men 1.70 ± 0.09 mA; left posterior: women 1.50 ± 0.07 mA; men 2.26 ± 0.11 mA; right posterior: women 1.55 ± 0.09 mA; men 2.28 ± 0.09 mA). TPDA was also, on average, higher (Student’s unpaired t test, P < 0.05) in men (40.6 ± 1.4 mm) than in women (36.4 ± 1.4 mm). Lower PT and TPDA (ANOVA, P < 0.05) were recorded on the anterior trunk than on the posterior trunk (men: PT front 1.64 ± 0.06 mA; PT back 2.27 ± 0.07 mA; TPDA front 41 ± 1.5 mm; TPDA back 47 ± 1.7 mm; women: PT front 1.27 ± 0.06 mA; PT back 1.50 ± 0.05 mA; TPDA front 32.9 ± 2.2 mm; TPDA back 41.9 ± 2.7 mm). Twelve subjects were re-tested within 3-18 days, six by a different observer and the remainder by the original observer. PT correlated well (linear regression; P < 0.05) between the two tests with the same observer testing (r ² = 0.543) and with a different observer (r ² = 0.427). TPDA did not correlate for the same observer (P > 0.05; r ² = < 0.001) and correlated poorly for different observers (P < 0.05; r ² = 0.136).

We conclude that PT is lower in women than in men and lower on the front of the trunk than on the back. PT has better repeatability, is quicker to perform than TPDA and may prove useful in the clinical assessment of patients following trauma.