Stability and reliability of detection thresholds for human A-Beta and A-delta sensory afferents determined by cutaneous electrical stimulation.

Activity in primary afferent fibers that usually mediate fine touch can evoke sensations of pain in conditions in which there is sensitization of central neurons. Input from these large diameter Abeta afferents may also sustain and exacerbate these central mechanisms. The role of these fibers in clinical pain syndromes can be evaluated by applications of electrical stimuli that preferentially activate Abeta axons. This study assessed the stability and reliability of a method of electrical stimulation (ES) useful for clinical evaluation. Monopolar constant-current rectangular pulses were delivered to 5 equi-spaced sites on the volar aspect of the left forearm along a transverse line 5 cm distal to the antecubital crease. Current intensity was gradually increased to determine detection threshold and pain detection threshold. This study determined: 1) Effect of pulse duration (1, 2, and 5 msec); 2) the variation of detection threshold and pain threshold over repeated stimulation; 3) the effect of electrode position with respect to distance from the trunk of underlying ulnar or median nerves; and 4) the effect of re-positioning the electrode on variability of detection threshold and pain threshold. There was no significant variability over time for either detection threshold (DT) or pain threshold (PT) at any of the 3 pulse durations tested. There was also no significant effect on variability of shifting the electrode between sites, nor was there a significant difference in variability between sites when placed either over or adjacent to peripheral nerves. Under simulated clinical conditions of electrode re-positioning, the mean detection threshold in 300 trials and ten subjects was 0.30 mA with an overall standard error of 0.007, standard errors of 0.014 over the 10 subjects, 0.003 over the 6 trials, and 0.012 over the 5 locations. Similarly, mean pain threshold in these 300 trials was 3.24 +/- 0.093, with standard errors of 0.12 over the 10 subjects, 0.023 over the 6 trials, and 0.13 over the 5 locations. Mean ratio of pain threshold divided by detection threshold ratio was 10.9 +/- 0.25 with a range of 2.0-28.3. Single pulse, constant current electrical stimulation of the skin at threshold levels is a quantifiable and reliable sensory method that is repeatable within and between testing sessions. Our results suggest that in skin unaffected by allodynia, a ratio of the two sensory thresholds (pain threshold and detection threshold) of less than 2.0 is uncommon. We propose that, in the presence of mechanical allodynia, a pain threshold/detection threshold of less than 2.0 suggests that altered central nervous system processing of Abeta input may contribute to allodynia.