Stability and Reliability of Detection Thresholds for Human A-Beta and A-Delta Sensory Afferents Determined by Cutaneous Electrical Stimulation

64 Journal of Pain and Symptom Management Vol. 25 No. 1 January 2003 Original Article Stability and Reliability of Detection Thresholds for Human A-Beta and A-Delta Sensory Afferents Determined by Cutaneous Electrical Stimulation Christine N. Sang, MD, MPH, Mitchell B. Max, MD, and Richard H. Gracely, PhD Department of Anesthesia (C.N.S.), Massachusetts General Hospital, Harvard Medical School,Boston, Massachusetts, and Pain and Neurosensory Mechanisms Branch (M.B.M., R.H.G.), National Institute of Dental Research, National Institutes of Health, US Department of Health and Human Services, Bethesda, Maryland, USA Abstract 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 A 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 A axons. This study assessed the stability and reliability of a method of electrical stimulation (ES) useful for clinical evaluation. Monopolar constantcurrent 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 repositioning 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 Address reprint requests to: Christine N. Sang, MD, MPH, Department of Anesthesia, Clinics 3, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA. Accepted for publication: February 25, 2002. 2003 U.S. Cancer Pain Relief Committee 0885-3924/03/$ see front matter Published by Elsevier. All rights reserved. PII S0885-3924(02)00541-9

Vol. 25 No. 1 January 2003 Electrical Stimulation in the Evaluation of Pain 65 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 A input may contribute to allodynia. J Pain Symptom Manage 2003;25:64 73. 2003 U.S. Cancer Pain Relief Committee. Published by Elsevier. All rights reserved. Key Words Electric stimulation, pain threshold, pain measurement, sensory thresholds, primary afferent, hyperesthesia Introduction Quantitative sensory testing (QST) is proving to be an indispensable tool to advance the classification of specific disorders 1 3 and illuminating underlying mechanisms of chronic pain. QST uses controlled sensory stimulation to extend components of the neurological examination to evaluate the function of primary afferents that mediate innocuous and painful sensation, and central processes that further modify the character and sensitivity of these primary afferents. A striking feature in many neuropathic and inflammatory pain conditions is mechanical allodynia, during which lightly touching the skin evokes an intense pain sensation. Calibrated nylon monofilaments are commonly used to quantify sensations evoked by controlled punctate pressure, although the adequate stimulus in many conditions may involve movement, or temporal summation such as provided by a vibratory stimulus. Unfortunately, mechanical stimuli identify the presence of mechanical allodynia, but do not identify the neural substrate mediating the allodynia. Pain triggered by light touch may be due to either altered central processing of A low-threshold mechanoreceptor (A LTM) input or activation of sensitized peripheral nociceptors. A LTM involvement in mechanical allodynia has been inferred from several lines of evidence with methods using electrical stimuli, such as intraneural stimulation, 4 cutaneous stimulation over a nerve supplying an area of spontaneous pain, 5,6 and cutaneous stimulation distant from any major nerves. 7 Inferences about mechanisms of dynamic mechanical allodynia from data in humans have been based on results of studies using transcutaneous electrical stimulation to differentiate between A and A stimulation. 8 13 These methods are based on the assumption that, since the largest fibers (A LTMs) are the most sensitive to electrical stimulation, they are the only fibers activated at detection-threshold levels of stimulation. 14 As stimulus intensity is increased further, A fibers are ultimately recruited and C-fibers are activated at even higher intensities. A -mediated allodynia is identified when electrical current evokes pain at or close to detection threshold, a current strength which activates only A fibers and causes an innocuous tactile sensation under normal conditions. In addition, the activation circumvents receptor transduction and thus is independent of receptor processes such as sensitization or suppression. 2 Thus, because activation of A LTMs occurs at currents well below those needed to activate A and C axons, Price et al. 6 and Gracely et al. 7 have proposed the use of electrical stimulation of the skin to identify A -mediated mechanical allodynia. ES has been useful to detect mechanisms of mechanical allodynia since it is noninvasive method to activate afferent fibers by direct depolarization of axons. Several laboratories have taken advantage of this property and used ES to distinguish whether mechanical allodynia is due to sensitization of central neurons or sensitization of peripheral receptors; 3 5,7,15 for example, if the faintest detectable current strength, which only activates A LTM fibers, causes pain, central nervous system mechanisms are implicated. Cutaneous electrical stimulation has demonstrated additional potential diagnostic utility in several studies. Pain threshold and pain tolerance measures have been selectively reduced in syndromes such as temporomandibular dysfunction, cervico-brachial syndrome, and fibro-

66 Sang et al. Vol. 25 No. 1 January 2003 myalgia. 11,16,17 Recent human and animal models of neural inflammation have shown increased sensitivity to A -strength electrical stimulation that coincides with the time course of the inflammation. 18,19 This accumulating evidence for clinical utility is matched by methodological advantages. Notermans 20 proposed the use of ES to determine pain threshold because 1) it is easily expressed in physical terms (milliamperes, ma); 2) it has little chance of damaging tissues; and 3) it produces an easily recognizable and easily definable pricking pain sensation. Although ES is a simple non-invasive psychophysical method, a number of factors have hindered its acceptance as a tool in pain research. The practical application of electrical stimulation as a diagnostic method in individual patients depends critically on reliability; variation in skin resistance and local physiological changes can potentially alter the results of electrical testing. 21,22 Like other sensory methods involving threshold measurements, this method is potentially limited by interaction with time or threshold intensity. Thus the basic stability and repeatability of electrical skin stimulation must be assessed if the method is to gain wide clinical use as a diagnostic sensory test. This study examined the influence of pulse duration, and both temporal and spatial variability at cutaneous sites distant from major nerves or at sites directly over major nerves. The methods simulated a clinical assessment: the skin was not pretreated, constant current stimuli were delivered through commercially available saline sponge electrodes using an ascending Method of Limits, and, in Experiment III, electrodes were removed and reapplied for each trial. left forearm along a transverse line 5 cm distal to the antecubital crease (Figure 1): 1) 1.2 to 3.5 cm (depending on forearm diameter) medial to the ulnar nerve; 2) over the ulnar nerve; 3) midway between the ulnar and median nerve; 4) over the median nerve; and 5) 1.2 to 3.5 cm lateral to the median nerve. Sites 2 and 4 were identified by moderate intensity electrical stimulation evoked sensations projected to the hypothenar eminence and midpalm, respectively. Monopolar constant-current rectangular pulses were delivered at 0.5 Hz through bipolar 6 mm diameter commercially available (Dantek, Medtronic) saline-sponge electrodes spaced 23 mm apart (center-to-center), applied along the axis of the limb with the cathode directed proximally. Subjects were seated with their arms resting outwards and supinated, and electrodes were applied at the level of the heart. 23 To avoid stimulation of nociceptors at suprathreshold intensities, an ascending series of stimulus intensities was delivered in 0.01 ma steps to each site, starting at zero and gradually increasing until a sensation was evoked (DT) and further increased in increments of 0.01 0.05 ma until the sensation changed to definite pain (PT). This sequence constituted a DT-PT trial. Subjects were also asked to describe the sensations evoked by the electrical stimuli at detection and pain thresholds. Experiment I. Temporal Stability and Effect of Pulse Duration Coupling of an electrical conductor to the peripheral nervous system involves a number Methods Ten healthy normal volunteers (age 23 35) were studied in each of three experiments, evaluated under uniform conditions including ambient noise, moisture, seating, and time of day. The electrically evoked pain threshold is not influenced by skin temperature, especially when the ambient temperature is maintained between 20 and 25 C. 20 The ambient temperature in the current study was maintained within 23 1 C for all experiments. Five equi-spaced sites were identified on a selected area of skin on the volar aspect of the Fig. 1. Schematic of left arm marked by five equispaced sites. Sites 2 and 4 lie over the a) ulnar and b) median nerves, respectively.

Vol. 25 No. 1 January 2003 Electrical Stimulation in the Evaluation of Pain 67 of interfaces and resistive-capacitive elements. As a result, the effects of narrow pulses are variably diminished, while the cumulative effects of monopolar stimulation, which is accentuated by long pulse durations, results in polarization that also diminishes the effects of stimulation. Although the use of constant current stimulation controls for varying resistances, constant current methods do not control for the adverse effects of capacitive elements. Experiment I assessed the influence of these capacitive elements by determining the effect of pulse duration on variability of detection threshold and pain threshold. Stimuli with pulse durations of 1, 2, and 5 msec were delivered at 0.5 Hz to the site midway between the ulnar and median nerves (Site 3). Ten DT-PT trials were determined at each pulse duration. The shortest pulse duration that demonstrated maximal sensitivity with minimal variance would likely offer the best compromise between the adverse effects of short and long pulse durations. Experiment II. Temporal Stability and Spatial Variability Without Effect of Repositioning, Effect of Electrode Site In previous studies, we were concerned that, due to spatial variability in sensitivity, attempts to reposition the electrode at the same location would introduce unacceptable variability. To test the best case variance, and to assess whether phenomena such as habituation and sensitization affect measurement of detection and pain thresholds, DT and PT were determined by delivering 2 msec stimuli to each of the 5 sites at 0.5 Hz. This experiment also addressed the difference in sensitivity at defined spatial locations: over, adjacent, and between major cutaneous nerves. Each DT-PT trial was administered at each site 6 times in succession without moving the electrode. Experiment III. Effect of Repositioning the Electrode and Electrode Site This experiment replicated the clinical situation in which sensitivity is evaluated before and after a pain control intervention, or evaluated in longitudinal measurements of disease progression. To determine whether different sites have different detection and pain thresholds, and whether repositioning the electrode caused more variability than leaving it in the same place (as in Experiment III), each DT-PT trial was administered by delivering 2 msec stimuli to each of the 5 sites. Each subject was randomized to a different sequence of the 5 sites (based on a 5 5 Latin square design), with the same sequence of sites repeated 6 times. Statistical Analysis Data were processed using Stata 4.0 (College Station, TX). Thresholds were computed from the mean of 10 (Experiment I) or 6 (Experiments II and III) complete trials. The PT to DT ratios (PT:DT) were determined for each set of sensory thresholds. Data were analyzed using one-way ANOVA to evaluate the effect of pulse duration, site, and repositioning, and repeated measures ANOVA to evaluate temporal variability. Analysis by Shapiro-Wilk test 24 indicated that departures from normality were insignificant and would meet the assumptions for an ANOVA. In addition, Bartlett s Test 25 demonstrated homogeneity of variances for each set of variables analyzed by ANOVA. When normality was an issue, parametric data analysis was supplemented by nonparametric analysis. Matched-Sample Sign Test was used to determine equality of medians for those subjects who returned for both Experiments I and II. A probability value of less than 0.05 was considered to be significant. Coefficients of determination (R 2 ) for the regression models were adjusted for unnecessary variables in the model. Results Each of the subjects described the sensation at detection threshold as a tingle, tap, or flutter, and the sensation at pain threshold as a pinprick, sting, or electric shock that ceased immediately at termination of the stimulus. Using intraoperative nerve trunk stimulation and recording, Collins 14 observed that A activation produced pricking sensations lasting for the duration of the stimulus, while C-fiber activation produced prolonged burning pain. Collins also observed that the current required to activate A fibers was considerably less than that needed to activate C-fibers. Thus, we inferred that our subjects reports of pain sensation evoked by the least stimulus current represented activation of A fibers without C-fiber involvement. A total of 20 subjects participated in three experiments, each involving 10 subjects. Two subjects participated in all three experiments,

68 Sang et al. Vol. 25 No. 1 January 2003 2 subjects participated in Experiments I and II, and 4 subjects participated in Experiments II and III. Experiment I. Effect of Pulse Duration on Detection Threshold and Pain Threshold Figure 2A shows the detection and pain thresholds at three different pulse durations. Each data point represents the mean threshold of the 10 trials for each individual subject at that pulse duration. There was a statistically significant inverse relationship between pulse duration and threshold for both detection (P 0.01) and pain (P 0.01). This figure indicates that the 2-msec duration offered the best compromise between sensitivity, shortest pulse duration, and minimal variability. Figure 2B presents the ratios of pain to detection thresholds at the three different pulse durations. For these 10 subjects, for all pulse durations, mean PT:DT ratio was never less than 6.4. Fig. 3. Experiment II. Time course of detection and pain thresholds for each site averaged over the 10 subjects. Each series of connected data points represents 6 successive determinations approximately 5 minutes apart of DT and PT, without moving the electrode. Z1 through Z5 correspond to sites 1 5 in Figure 1. Experiment II. Time Course of Detection and Pain Thresholds and Effect of Location The 2-msec pulse duration was chosen for the subsequent experiments. Figure 3 shows the time course of the detection and pain thresholds. Each series of connected data points represented 6 successive determinations of DT and PT for each specific site averaged over the ten subjects. There was no significant change over time for either DT or PT at any of the 5 sites. Figure 4 shows the ratio of the pain threshold to detection threshold plotted against detection threshold for the 300 pair of responses (5 sites 6 repetitions 10 subjects). Subjects are identified by unique plot symbols. These data show that a majority of the detection Fig. 2. Experiment I. Effect of pulse duration on detection threshold and pain threshold. A: Sensory thresholds at three different pulse durations. The solid points represent the mean PT s; the hollow points represent the mean DT s. Detection threshold error bars are too small to be visible. B: Ratios of pain to detection thresholds at three different pulse durations. Each data point represents the mean ( standard error) PT:DT ratio for the 10 subjects. Fig. 4. Pain/Detection (PT:DT) ratios for all trials in Experiment II plotted against detection threshold. Data from each subject are shown by a unique symbol. Each subject received 6 DT and PT trials at each of 5 locations for a total of 30 trials for each of the 10 subjects. The line indicates the maximum possible ratio with a stimulation limit of 10 ma. Data from 3 subjects with both detection thresholds greater than 8 and ratios near the limit are classified as outliers.

Vol. 25 No. 1 January 2003 Electrical Stimulation in the Evaluation of Pain 69 thresholds were less than 0.8 ma and the ratios were always greater than 2.0. Inspection of the figure indicates outlier data with detection thresholds greater than 0.8 ma and high ratios resulting from pain thresholds near the maximum stimulus intensity of 10 ma. These data represented the results from a single site in two subjects, and from all sites in a third subject. Mean detection threshold, including these outlier data, was 0.48 ma with a standard error of 0.023 for all 300 data points. Eliminating the outlier subjects reduces the mean and standard error to 0.34 0.010. In the absence of significant differences, the influence of time or location can be evaluated by averaging these points over either subjects or the independent variables and computing standard errors that permit comparison of these sources of variability. These standard errors were 0.021 over the 10 subjects, 0.002 over the 6 trials, and 0.057 over the 5 locations (0.036 subjects, 0.0026 trials, 0.024 location without outliers). Similarly, mean pain threshold of all of the data was 4.70 ma with an overall standard error of 0.14 and standard errors of 0.12 over the 10 subjects, 0.019 over the 6 trials, and 0.32 over the 5 locations. Without outliers the mean and standard error were 4.0 0.11, and the standard errors by source were 0.082 for subjects, 0.019 for trials, and 0.22 for location. Experiment III. Effect of Site and Repositioning on Detection Threshold and Pain Threshold Figure 5 shows the sensory thresholds at the five different sites. Each data point represents the average of 60 observations (6 DT-PT determinations from 10 subjects) at that site. There was no significant effect of site on either detection threshold or pain threshold. Moreover, there was no significant difference in variability between sites when placed over the nerves and elsewhere. However, inspection of the figures shows a trend for increased sensitivity over the nerve sites. This difference approached significance for both the detection (P 0.059) and pain (P 0.069) thresholds when the data were collapsed to nerve and non-nerve sites. Figure 6 shows the ratio of the pain threshold to detection threshold plotted against detection threshold for the 300 pair of responses collected in Experiment III. All detection thresholds were 0.7 ma or less and the mean pain/ detection threshold ratio was 10.9 0.25 with Fig. 5. Experiment III. Effect of site on detection threshold and pain threshold. A: Sensory thresholds at the 5 sites (depicted in Figure 1). The hollow points represent mean DT s; the solid points represent mean PT s. B: Ratios of pain to detection thresholds at the 5 sites. Each data point represents the mean PT:DT ratio for each individual subject. a range from 2.0 28.3. Mean detection threshold was 0.30 ma with an overall standard error of 0.007 and 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 of all of the data was 3.24 ma with an overall standard error of 0.093 and standard errors of 0.12 over the 10 subjects, 0.023 over the 6 trials, and 0.13 over the 5 locations. Comparison of the standard errors indicates that the thresholds were more stable over time than over location. Correlations supported this difference. The correlations among all possible pairs of the 6 trials averaged over the five locations varied from 0.89 0.98 (mean 0.943) for detection thresholds and 0.91 0.99 (mean 0.97) for pain thresholds. In contrast, correlations among all possible pairs of the five locations, averaged over the six trials, ranged from 0.17 0.97 (mean 0.56) for detection thresholds and 0.61 0.96 (mean 0.88) for pain thresholds. Thus thresholds were highly repeatable after repositioning the electrode but varied among location possibly due in part to the influence of underlying nerves. Detection and pain thresholds in Experiment III were compared using both differences and ratios. The mean differences between DT and PT at the 5 sites were 4.20, 3.42, 3.75, 3.22, and 3.30 ma. The mean PT:DT ratios at the 5 sites were 12.7, 12.3, 11.9, 11.8, and 9.7 (medial to lateral). In all three experiments, the mean ( SEM) PT:DT ratio in 900 trials at 5 sites in 20 subjects and 30 visits was 11.7 ( 0.6) (range 2.0 32.9). In all subjects, detection threshold current intensity at sites 2 and 4 (directly over ulnar and

70 Sang et al. Vol. 25 No. 1 January 2003 Fig. 6. Pain/Detection (PT:DT) ratios for all trials in Experiment III plotted against detection threshold. Data from each subject are shown by a unique symbol. Each subject received 6 DT and PT trials at each of 5 locations for a total of 30 trials for each of the 10 subjects. The line indicates the maximum possible ratio with a stimulation limit of 10 ma. Data from 3 subjects with both detection thresholds greater than 0.8 ma and ratios near the limit are classified as outliers. median nerves) evoked identical concurrent sensations referred to the hypothenar eminence and midpalm, respectively. In 210/240 trials (17/20 subjects) involving stimulation at these sites, the development of pain at the referred sites preceded the development of pain at the sites of contact with the electrodes, confirming that axons in passage are activated at lower current strengths than are local nociceptors. Discussion These results support the reliability of electrical stimulation at cutaneous sites both over and distant from peripheral nerves. There was no significant difference in detection thresholds at nerve and non-nerve sites. However, a trend of increased sensitivity over nerves suggests a subtle difference that may be demonstrated with a greater number of subjects. In addition to possible increased sensitivity, the nerve sites demonstrated a qualitatively different response. Stimulation at nerve sites evoked non-painful sensations at both the proximal electrode site and distally at the receptive field of the stimulated fibers. The threshold for detection at these two locations were similar. This equivalence suggests that the distribution of the A axons mediating the sensations is such that their sensitivity to electrical stimulation of the skin surface is similar at both distal end near the receptor and more proximally in the nerve trunk. However, if stimulating currents are increased to levels that evoke pain, this correspondence disappears. Pain is detected first in the distal referred region, and at higher levels, at the location of the electrode. We are not certain of the reason for this pattern of pain radiation. A possible explanation might be that when the electrode is placed over a nerve, the nociceptor axons within the nerve that innervate a distant site may be larger in diameter or more densely packed in the region activated by the electric current than the nociceptive axons that innervate the skin and soft tissue directly under the electrode. Larger fibers are more sensitive to electrical stimulation, while a dense concentration of fibers allows stimulation of a sufficient population required for perception of pain. These results support the strategy of testing the thresholds and evoked qualities of A stimulation by stimulating the proximal nerve innervating the test region. 5,6 This method avoids contact with the skin in the test regions and thus eliminates the confounds associated with this contact, such as touch-evoked pain observed in cases mechanical-allodynia. Inferences drawn from electrical stimulation of human skin depend on sufficient reliability, which until now has not been evaluated systematically. Temporal variability may be introduced by a number of mechanisms. Although low-intensity electrical stimulation activates afferent axons directly, bypassing receptor mechanisms, 2 habituation and sensitization may still influence the results of ES. 26,27 Electrical stimulation may introduce local physiological changes which may modify the ionic microenvironment and result in changes in skin resistance or impedance. 28,29 Similar to previous experiments, this study used constant current stimuli; hence, the intensity of the stimulating current was independent of skin resistance but not of impedance. 20 Tursky and Greenblatt 30 describe the electrode-skin circuit as two high impedance electrode-skin junctions connected by a subsurface low impedance network, whose configuration changes to a low impedance cir-

Vol. 25 No. 1 January 2003 Electrical Stimulation in the Evaluation of Pain 71 cuit with the administration of an electrical stimulus. This is at least in part due to dilatation of local blood vessels resulting from the axon reflex 30,31 or the presence of perspiration which may provide a low resistance path through the skin, or act as a lateral current path to shunt current along the surface of the skin. 30 Results of this study show that estimation of DT and PT by electrical skin stimulation is not significantly affected by potential sensitization, habituation, galvanic skin response, or alterations in the electrical microenvironment. Spatial variability could be introduced by electrode repositioning or use of an electrode site close to the location of a nerve. This may be due to either histological variation of vessels and other factors contributing to differential skin resistance, or to neurohistological differences between sites. 22 Results from this study indicate that the reliability of electrical skin stimulation is independent of site. Excessive variability was observed in Experiment II in 3 subjects, 2 of whom participated in Experiment III and showed normal responses. This variability was due to insensitive trials that were confined to a single location in 2 subjects and found at all locations in 1 subject. This insensitivity could represent a region of hypesthesia in the 2 subjects and generalized hyperesthesia or a response bias in the third subject. A common and likely explanation is that the insensitivity was due to the preparation, such as shunting of the current across the surface of the skin because of perspiration, drying of the saline electrodes, or excessive skin oil (and high impedance) at particular sites. We monitored skin impedance in Experiment I but discontinued monitoring in Experiments II and III to more closely match a clinical evaluation. Impedance monitoring in future studies would distinguish between such effects, although the necessity of such monitoring would depend on the desired comparisons. The present data indicate that the PT:DT ratios are still greater than 2.0 in these outlier trials and pain evoked at a ratio less than this ratio would still indicate A involvement. Our data indicate that investigation of trials with detection thresholds greater than 0.8 ma would likely provide sufficient quality assurance in a clinical application. The finding that pain is never evoked by a current less than twice that required to detect sensation validates the claim in previous reports 5,6,15 that if electrical stimuli at detection threshold produces pain, A low threshold mechanoreceptors (A LTMs) would be implicated in the mechanism of these painful sensations. The finding that the ratios of pain threshold to detection threshold exceed 2.0 in each trial of every subject is consistent with findings by Collins et al. 14 that considerably higher current is required to activate A and C nociceptors. These results support the inference that, in patients with pain evoked by detection threshold intensities of cutaneous electrical stimulation, altered central nervous system processing is responsible for the misinterpretation of innocuous stimuli as pain. 5,7 It is possible that this rule may not apply to patients with nerve injury, as this study and other supportive data were derived from patients without nerve injury in the area of stimulation. Additional studies in a variety of clinical pain syndromes associated with allodynia are needed. The clinical utility of A assessment is not limited to cases of mechanical allodynia. Increasing evidence supports A LTM involvement in inflammatory pain. Application of inflammatory agents such as carrageenan and Freund s complete adjuvant to paws or nerve trunks of rats results in decreased withdrawal latencies to A LTM activation produced by mechanical or electrical stimulation (Eliav, personal communication). In studies of paw inflammation, both innocuous touch stimuli and noxious pinch stimulation produced greater neural and motor reflex responses. 32 In addition to increased A sensitivity, these studies also indicate that A fibers may contribute to the maintenance of the sensitized state. Gentle mechanical stroking or A -strength electrical stimulation increases A sensitivity further, and effect described as progressive tactile hypersensitivity. 19 Under these conditions of inflammation A fibers appear to switch phenotype, releasing substance P at their terminals and behaving like nociceptors in maintaining central sensitivity. 33 In these studies, electrical stimuli are used to both evaluate A function and as an adequate A stimulus in evoking progressive tactile hypersensitivity. Electrical stimuli are ideally suited for this latter role since stimulation is easily applied and quantified in comparison to the mechanical method of gentle stroking. Exclusive stimulation of A afferents may also reveal effects not found

72 Sang et al. Vol. 25 No. 1 January 2003 in situations in which punctate stimuli are effective. In the rat chronic constriction model, monofilament stimulation reveals a consistent hypersensitivity, while electrical stimuli reveal a complex response of both hyper- and hyposensitivity (Eliav, personal communication). Our laboratory has recently found clinical evidence consistent with A involvement in inflammatory pain. 18 In an extensive sensory assessment of oral surgical patients at the time (48 hrs) of maximum postoperative inflammation, we observed increased sensitivity to electrical (and mechanical) stimuli applied to both the nerve territory of a single extracted lower third molar tooth and to territory of an adjacent nerve whose trunk was exposed to post extraction inflammation. Mechanical and electrical detection thresholds were normal in other divisions of the trigeminal nerve. Thermal testing of both detection and pain thresholds to cold and heat showed no effects at any location. These results suggest that the sole sensory sign of inflammatory pain may be an increased sensitivity that is restricted to A LTM afferents. Two other groups of investigators have reported a narrower spread than our mean ratio of 10:1 between group means of electrical pain and detection thresholds. Working with normals and patients with fibromyalgia and chronic shoulder and neck pain, Arroyo and Cohen, 16,17 Sheather-Reid and Cohen, 9 and Lautenbacher et al. 11 described ratios of between 1.5:1 and 3:1 at different skin sites. None of these articles reported pain/detection threshold ratios for individual patients. At least part of this difference may be due to differences in stimulation frequency. All of these other studies used 100 Hz trains of electrical pulses, while we gave single pulses spaced at two-second intervals. Our choice was based on our previous work with electrical stimulation of dental pulp. 34 Detection threshold in that study was constant across a broad range of frequencies, but pain threshold showed a U-shaped plot for current versus frequency, in which the lowest pain threshold and the narrowest spread between pain and detection threshold occurred at 100 Hz. The greatest spread occurred at 5 Hz, the lowest frequency tested. Single pulse constant current electrical skin stimulation is a robust technique for investigating the mechanisms of pain symptoms. Further studies are needed to confirm whether single pulses give a greater separation between pain and detection threshold than rapid trains of pulses. The limited temporal and spatial variability in young healthy subjects support its use as a quantitative sensory assessment tool. It can bypass receptors, stimulating A LTMs (or all A afferents) independently of peripheral processing such as receptor sensitization. It is quantifiable and easily applied with safe, battery-operated stimulators. Other sources of variability of electrical pain or detection thresholds that we did not explore (for example, age, sex, body size, or prolonged time intervals between testing sessions) may be considered in future studies. This mildly painful, atraumatic, noninvasive method may provide a useful adjunct to conventional QST assessment of painful conditions. Acknowledgments The authors thank Drs. Gary Bennett and Ian Gilron for reviewing the manuscript, and Fred Brown for his technical assistance. References 1. Bennett GJ. Neuropathic pain. In: Wall PD, Melzack R, eds. Textbook of Pain. Edinburgh: Churchill Livingstone, 1994:201 224. 2. Gracely RH. Studies of pain in normal man. In: Wall, PD, Melzack R, eds. Textbook of pain. Edinburgh: Churchill Livingstone, 1994:315 351. 3. Gracely RH, Price DD, Roberts WJ, Bennett GJ. Quantitative sensory testing in patients with CRPS-I & II. In: Janig W, Stanton-Hicks M, eds. Reflex sympathetic dystrophy a reappraisal, Seattle: IASP Press, 1996:151 172. 4. Torebjork HE, Lundberg LER, LaMotte RH. Central changes in processing of mechanoreceptive input in capsaicin-induced secondary hyperalgesia in humans. J Physiol 1992;448:765 780. 5. Price DD, Bennett GJ, Rafii A. Psychophysical observations on patients with neuropathic pain relieved by a sympathetic block. Pain 1989;36:273 288. 6. Price DD, Long S, Huit C. Sensory testing of pathophysiological mechanisms of pain in patients with reflex sympathetic dystrophy. Pain 1992;49: 163 173. 7. Gracely RH, Lynch SA, Bennett GJ. Painful neuropathy: altered central processing maintained dynamically by peripheral input. Pain 1992;51:175 194. 8. Wasner G, Baron R, Janig W. Dynamic mechanical allodynia in humans is not mediated by a central presynaptic interaction of A beta-mechanoreceptive and nociceptive C-afferents. Pain 1999;79:113 119.

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