Constant Current Stimulation Research Articles

Correlation Between Low Triggered Electromyographic Thresholds and Lumbar Pedicle Screw Malposition

A retrospective analysis of 1078 spinal surgical procedures with lumbar pedicle screw placement at a single institution. Based on previously established normative values, triggered electromyographic stimulation (TrgEMG) was re-examined to evaluate its efficacy in determining screw malposition. Threshold values for confirmation of intraosseous placement of pedicle screws with EMG stimulation is controversial. TrgEMG threshold values for 4857 pedicle screws placed from L2 to S1 from 1996 to 2005 were analyzed. An ascending method of constant current stimulation was applied to each pedicle screw to obtain a compound muscle action potential (CMAP) from lower extremity myotomes. Previously determined threshold value normative data from a published clinical series of 233 screws were as follows: 0 to 4 mA, high likelihood of pedicle wall breach; 4 to 8 mA, possible pedicle wall breach; >8 mA, no pedicle wall defect. A total of 7.74% (376 of 4857) of all screws tested had threshold values <8.0 mA. A total of 19.1% (72 of 376) of these were <4.0 mA: 54% (39 of 72) were repositioned (26) or removed (13) while the remaining 33 screws were left in place following repalpation. A total of 80.9% (304 of 376) had thresholds between 4 and 8 mA: 17.4% (53) were repositioned (38) or removed (15). Nine screws had thresholds of <or=2.8 mA and were either repositioned or removed following confirmation of a medial wall breach. A total of 74.5% (280 of 376) of all screws with thresholds <8.0 mA were verified as correctly placed by repalpation/radiography and therefore left in place. The probability of a medial breach pedicle screw detected by triggered EMG stimulation increases with decreasing triggered EMG thresholds: 0.31% for >8.0 mA, 17.4% for 4.0 to 8.0 mA, 54.2% for <4.0 mA, and 100% for <2.8 mA. At 2.8 mA, triggered EMG has a specificity of 100%, with sensitivity of 8.4%; at 4.0 mA, specificity of 99% and sensitivity of 36%; and at 8.0 mA, 94% specificity and 86% sensitivity. TrgEMG is an adjunct technique and should always be used in conjunction with palpation and radiography to optimize safe pedicle screw placement.

Urethral and Bladder Current Perception Thresholds: Normative Data in Women

Given increased evidence of sensory dysfunction in lower urinary tract pathology, we determined normative current perception threshold values in the lower urinary tract of asymptomatic women. After receiving institutional review board approval women without lower urinary tract symptoms underwent current perception threshold testing of the urethra and bladder using a Neurometer constant current stimulator. Current perception threshold values were determined at 3 frequencies, including 2,000 Hz (corresponding to A-beta fibers), 250 Hz (corresponding to A-delta fibers) and 5 Hz (corresponding to C fibers). A total of 48 women with a mean age of 38 years (range 23 to 67) underwent current perception threshold testing. Normative values were established for the urethra and bladder at 2,000, 250 and 5 Hz. Median urethral current perception thresholds at 2,000, 250 and 5 Hz were 1.2 (IQR 0.76-1.5), 0.45 (IQR 0.33-0.56) and 0.11 mA (IQR 0.07-0.24), respectively. Median bladder current perception thresholds at 2,000, 250 and 5 Hz were 4.1 (IQR 2.0-6.3), 2.3 (IQR 0.87-5.5) and 1.4 mA (IQR 0.22-2.9), respectively. Urethral and bladder current perception thresholds increased significantly with subject age at all 3 frequencies (p<0.0005). Prior pelvic surgery was associated with an increased bladder current perception threshold at all 3 frequencies (p<0.005) but not with the urethral current perception threshold. We report urethral and bladder current perception thresholds for a large sample of asymptomatic women. These reference values may help elucidate changes in afferent nerve function in women with lower urinary tract dysfunction.

Transcranial electric stimulation for intraoperative motor evoked potential monitoring: Stimulation parameters and electrode montages

Objective To evaluate the efficacy of constant current transcranial electric stimulation (TES) parameters for eliciting muscle motor evoked potentials (MEPs) in the abductor pollicis brevis muscles (APB) and the tibialis anterior muscles (TA). The following parameters were tested intraoperatively: interstimulus interval (ISI), individual stimulation pulse duration within a train of five stimuli. Different montages of stimulating electrodes were assessed for effectiveness and focality. Further, reference values for APB and TA motor thresholds in neurosurgical patients with normal motor status under total intravenous anesthesia were determined. Methods Motor thresholds of contralateral muscle MEPs were determined at 0.1, 0.2, 0.4, and 0.5 ms pulse duration and ISIs of 2, 3, 4, and 5 ms using a train of five monophasic constant current pulses with C3/C4 (27 patients). The stimulating electrodes were positioned at C1, C2, C3, C4, Cz, and Cz+6 cm. Different montages were used to determine the most effective and the most focal stimulation montages for the APB and TA muscles (30 patients). Eighty-six patients with clinically normal motor function were studied for motor threshold reference values. Results The prolongation of the pulse duration has the strongest effect to decrease the motor threshold, which proportionally increases the delivered charge. The lowest stimulation threshold to elicit muscle MEPs in the APB and TA muscles is achieved with a train of stimuli consisting of an individual stimulus pulse duration of 0.5 ms. An ISI of 4 ms gave the lowest motor thresholds, but did not reach statistical significance compared to 3 ms. The stimulating electrode montage C3/C4 (C4/C3) allows for the lowest stimulation thresholds, but the vigorous muscle contractions it has is a disadvantage. The most focal stimulating electrode montages for the contralateral APB muscles are C3/Cz and C4/Cz, respectively, and for the TA muscles Cz/Cz+6 cm. Conclusions In adult neurosurgical patients with a normal motor status under total intravenous anesthesia, an individual pulse duration of 0.5 ms and an ISI of 4 ms provide the lowest motor thresholds. Pragmatically, C1/C2, resp., C2/C1 montage provides monitorable responses in both APB and TA muscles at reasonable stimulation thresholds without inducing movements disturbing surgery and especially microdissection. If the most focal hemispheric stimulation for the distal upper extremity muscles is required, the use of C3 or C4 referenced to Cz is recommended. Significance The stimulation parameters within a train of five pulses with an individual pulse duration of 0.5 ms and an ISI of 4 ms provide the lowest motor threshold. These data confirm not only studies for D wave recovery but also provide optimal stimulation parameters for intraoperative near threshold stimulation.

Mechanisms of Electrode Induced Injury. Part 1: Theory

ABSTRACT.Electrodes are the essential elements of clinical neurophysiology both in recording of neural activity and in functional electrical Stimulation of the nervous system. Therefore it is important to understand the potential complications of using electrodes. In this paper, the factors that influence the chance of electrode related injury are discussed from a theoretical standpoint. The mechanical factors, especially pressure related injury, are discussed first, followed by a discussion of injury that is of chemical origin such as contact dermatitis. Next, the ways in which electrical currents flowing from electrodes can cause injury including: Joule heating, electroporation, electroconformational denaturation, and excitatory neurotoxicity are discussed. The differential effects of constant current and constant voltage Stimulation on tissue heating are examined, as are the effects of the conductivity and geometric structure of the stimulated tissue. Finally, the effects of electrochemical reactions are discussed both in the context of surface and implanted electrodes.

Predicted current densities in the brain during transcranial electrical stimulation

Objective We sought an electrical modeling approach to evaluate the potential application of finite element method (FEM) modeling to predict current pathways and intensities in the brain after transcranial electrical stimulation. Methods A single coronal MRI section through the head, including motor cortex, was modeled using FEM. White matter compartments with both anatomically realistic anisotropies in resistivity and with a homogeneous resistivity were modeled. Current densities in the brain were predicted for electrode sites on the scalp and after theoretical application of a conductive head restraint device. Results Localized current densities were predicted for the model with white matter anisotropies. Differences in predicted peak current densities were related to location of stimulation sites relative to deep sulci in the brain and scalp shunting that was predicted to increase with inter-electrode proximity. A conductive head restraint device was predicted to shunt current away from the brain when a constant current source was used. Conclusions The complex geometry of different tissue compartments in the head and their contrasting resistivities may jointly determine the strength and location of current densities in the brain after transcranial stimulation. This might be predictable with FEM incorporating white matter anisotropies. Conductive head restraint devices during surgery may be contraindicated with constant current stimulation. Significance Individually optimized tcMEP monitoring and localized transcranial activation in the brain might be possible through FEM modeling.

Discrimination is not impaired by excision of the anal transition zone after restorative proctocolectomy.

Anal sensation has been assessed using a constant current stimulator in 21 patients after restorative proctocolectomy (15 J pouch, 6 W pouch). The anal transition zone (ATZ) has been excised in 15 patients but preserved in 6. Results were compared with 14 age- and sex-matched patients with ulcerative colitis (UC) and 14 controls. Median threshold values in the lower, mid and upper zones of the anal canal in pouch patients were: 6.8 mA, greater than 15.0 mA and greater than 15.0 mA respectively compared with 3.9 mA (P less than 0.01), 5.7 mA (P less than 0.01) and 11.4 mA (P less than 0.01) in UC and 3.5 mA (P less than 0.01), 4.2 mA (P less than 0.01) and 11.4 mA (P less than 0.01) in controls. The mid and upper zone threshold anal sensation was significantly lower when the ATZ had not been removed and except in the lower zone did not differ from UC or controls (median threshold sensations when ATZ was preserved were: lower zone, 5.8 mA; mid zone, 5.5 mA; upper zone, 7.5 mA). Paired studies in eight patients showed that excision of the ATZ was associated with a significant impairment in anal sensation (mid zone: 5.7 versus greater than 15.0 mA, P less than 0.05; upper zone: 7.5 versus greater than 15.0 mA, P less than 0.05) whereas no change in anal sensation was recorded when the ATZ was preserved. Despite these physiological changes, no clinical benefit could be identified in patients in whom the ATZ was preserved. Discrimination was normal in all except one patient. The incidence of soiling, nocturnal incontinence and failure to defer defaecation for more than 1 h was recorded in 3/15 patients (20 per cent) whose ATZ had been removed, compared with 1/6 patients (17 per cent) in whom the ATZ had been preserved. We conclude that excision of the ATZ does not eliminate the ability to discriminate and does not increase the risk of impaired continence after ileo-pouch anal anastomosis.

Anal sensation after restorative proctocolectomy for ulcerative colitis.

The hypothesis that anal sensation might be better after restorative proctocolectomy with end-to-end ileoanal anastomosis than after mucosal proctectomy with endo-anal anastomosis was tested in this study. Anal sensation was measured in 14 patients before operation, 16 patients after restorative proctocolectomy with end-to-end anastomosis (RP + EEA) and 13 patients after mucosal proctectomy with endo-anal anastomosis 1 cm above the dentate line (MP + EAA). Threshold electrosensitivity was measured in the upper, mid and lower anal canal by means of a bipolar constant current stimulator probe. The 'recto'-anal inhibitory reflex was tested, and the patients' ability to discriminate between flatus and faeces and to release flatus 'safely' was assessed by interview. In the upper anal canal, threshold sensitivity was significantly greater in patients who had undergone MP + EAA than in patients who had undergone RP + EEA (P less than 0.05). In the mid and lower anal canal, electrosensitivity in the three groups of patients did not differ significantly. Twelve patients (75 per cent) regained the 'recto'-anal reflex after RP + EEA, but after MP + EAA only three patients (23 per cent) did so (P less than 0.02). Thirteen patients after RP + EEA could release flatus safely without fear of faecal leakage, compared with only four after MP + EAA (P less than 0.02). The proportions of patients in these two groups who said they were able to discriminate flatus from faeces did not differ significantly. Anal sensation and discriminatory function are significantly better after end-to-end ileoanal anastomosis than after mucosal proctectomy with endo-anal anastomosis.

Motor Evoked Potential Monitoring during Cerebral Aneurysm Surgery: Technical Aspects and Comparison of Transcranial and Direct Cortical Stimulation

This study evaluates technical aspects, handling, and safety of intraoperatively applied transcranial electrical stimulation (TES) and direct cortical stimulation (DCS) for eliciting muscle motor evoked potentials (mMEPs) during cerebral aneurysm surgery. In 119 patients undergoing cerebral aneurysm surgery, mMEPs were evoked by a train of five stimuli with individual pulse duration of 0.5 milliseconds, a repetition rate of 2 Hz, and constant current anodal stimulation. The maximal stimulation intensity was 240 mA for transcranial and 33 mA for direct stimulation. mMEPs were recorded continuously from the abductor pollicis brevis, from tibial anterior muscles bilaterally, and from the biceps brachii and extensor digitorum communis muscles contralateral to the side operated on. In 118 (99%) of 119 patients, transcranially evoked mMEPs were monitorable for the vascular territory of interest. DCS was performed successfully in 95 (95%) of 100 patients. In 86 (99%) of 87 patients with internal carotid artery, middle cerebral artery, or posterior circulation aneurysms, mMEPs from upper-extremity muscles were obtained with DCS. In 11 (55%) of 20 patients with anterior communicating artery, anterior cerebral artery, or pericallosal aneurysms, mMEPs from the lower-extremity muscles could be recorded. The incidence of seizures was 0.84% for TES and 1% for DCS. Minor and inconsequential subdural bleeding after positioning of the strip electrode occurred in 2%. The cogent comprehensive combination of transcranial and direct cortical electrical stimulation allows for the continuous mMEP monitoring of the cerebral vascular territory of interest in 99% of the patients with cerebral aneurysms. Unwarranted effects of electrode placement and stimulation are rare and without clinical consequences.

Low cost general-purpose microcontroller based multi-channel neuromuscular stimulator

A constant current stimulator for high-impedance loads using a microcontroller is presented. The designed muscle stimulator has the advantage that all the parameters such as amplitude, duration and time period can be adjusted. It can give up to eight independent channels of output at a time with adjustable parameters. It has got the advantage of having a user-friendly interface. The facility for storing the stimulation parameters and making it available for future reference has made. A prototype produces stimulus of amplitude and pulsewidth within 0 ≤ Iskin ≥ 20 mA and 1 ms ≤ Tpulse ≥ 250 ms, respectively. Pulse-repetition spans from 0.1–1000 Hz. Worst case ripple is less than 3% at Iskin = 1 mA. Maximum power consumption is 6.5 W per channel at Iskin = 20 mA.

Urethral Sensitivity in Incontinent Women

Objectives: The aim of this study was to ascertain whether frequent voiding and urge incontinence are associated with supersensitivity to electrical stimulation in the posterior urethra. Methods: Current perception thresholds (CPT) were tested at four stimulus frequencies (1, 3, 20, and 100 Hz; duration 0.5 ms) using a square-wave constant current electrical stimulator connected to ring electrodes on a urethral catheter. The strength of the current at the first tingling sensation was regarded as the CPT. CPT analysis and cystometry were performed on 61 women (ages 28–89 years). Results: CPTs were significantly higher at lower than at elevated stimulus frequencies, and they were also generally higher in old than in younger patients. Seven women repeated the CPT test after two months, and the thresholds were unchanged. There were no significant differences in sensitivity between patient groups with stress incontinence, urge, or mixed symptoms. Moreover, CPT was not significantly related to bladder volume at first sensation of filling. Conclusion: Measuring CPT is an easy and reproducible method of testing urethral sensibility, but our results do not support the suspicion that urethral hypersensitivity is involved in increased voiding frequency and urge incontinence.

Excitability of chronic hemiparetic muscles: determination of chronaxie values and strength-duration curves and its implication in functional electrical stimulation

Central nervous system disorders affect the anatomy and physiology of the lower motoneuron. This fact has an impact on the stimulation parameters, especially on the duration of the stimulating impulses, for functional electrical stimulation in chronic hemiparetic patients. The aim of this study was thus to test the excitability and to determine chronaxie values and strength-duration curves of weak wrist and finger extensor muscles and spastic finger and wrist flexor muscles in the hemiparetic arm. Twelve patients with chronic hemiplegia (>6 months after the onset of the cerebral lesion) participated in the study. A constant current stimulator was used. As to chronaxie values no significant differences were found between the extensor muscles (mean+/-SD: 0.44+/-0.16 ms) and flexor muscles (mean+/-SD: 0.36+/-0.22 ms). A moderate variability was seen for both extensor muscles (0.2-0.8 ms) and flexor muscles (0.1-0.9 ms). These values are well within the normal range determined for innervated muscles. All strength-duration curves were completely normal for each muscle. We conclude that in chronic hemiparetic muscles, impulses of the same duration can be used as in muscles of healthy subjects.

To Reveal the Impact of a Constant Current Repetitive Electrical Epidural Stimulation on Regeneration of the Contused Rat Spinal Cord

To Reveal the Impact of a Constant Current Repetitive Electrical Epidural Stimulation on Regeneration of the Contused Rat Spinal Cord

Inhibitory effect of iodophenpropit, a selective histamine H 3 antagonist, on amygdaloid kindled seizures

The effect of histamine H 3 antagonist, iodophenpropit on amygdaloid kindled seizures in rats was studied in comparison with those of other H 3 antagonists. Under pentobarbital anesthesia, the rats were fixed to a stereotaxic apparatus and bipolar electrodes were implanted into the amygdala. Electrodes were connected to a miniature receptacle, which was embedded in the skull with dental cement. To cause kindled seizures, electrical stimulation was applied to the amygdala bipolarly every day by a constant current stimulator, and electroencephalogram and convulsive behavior were observed. Drug effects were estimated in rats showing generalized kindled seizures. Intraperitoneal injection of H 3 antagonists, iodophenpropit, thioperamide, AQ0145 and clobenpropit, resulted in a dose-related inhibition of amygdaloid kindled seizures. The effect of iodophenpropit on amygdaloid kindled seizures was more potent than those of thioperamide, AQ0145 and clobenpropit. In conclusion, iodophenpropit may be useful for the treatment of partial epilepsy and/or secondary generalized seizures in humans.

Intracerebroventricular administration of histamine H3 receptor antagonists decreases seizures in rat models of epilepsia

The effects of histamine H3 antagonists on amygdaloid kindled and maximal electroshock seizures in rats were studied to determine their potential as new antiepileptic drugs. Under pentobarbital anesthesia, rats were fixed to a stereotaxic apparatus and a stainless steel guide cannula for drug administration was implanted into the lateral ventricle. In amygdaloid kindled seizures, electrodes were implanted into the right amygdala and electroencephalogram was recorded bipolarly; stimulation was applied bipolarly every day by a constant current stimulator and continued until a generalized convulsion was obtained. In the maximal electroshock (MES) seizure test, electroconvulsion was induced by stimulating animals through ear-clip electrodes, and the durations of tonic and clonic seizures were measured. Thioperamide, clobenpropit, iodophenpropit, VUF5514, VUF5515 and VUF4929 caused a dose-dependent inhibition of both seizure stage and afterdischarge (AD) duration of amygdaloid kindled seizures. The duration of tonic seizure induced by MES was also inhibited by H3 antagonists, but the duration of clonic seizures were unchanged. Among the H3 antagonists tested, clobenpropit and iodophenpropit were somewhat more potent than the other drugs on amygdaloid kindled seizures and MES seizures, respectively. These results indicate that some H3 antagonists may be useful as antiepileptic drugs, especially for secondary generalized seizures and/or tonic-clonic seizures in humans.

A multi-channel telemetry system for brain microstimulation in freely roaming animals

A system is described that enables an experimenter to remotely deliver electrical pulse train stimuli to multiple different locations in the brains of freely moving rats. The system consists of two separate components: a transmitter base station that is controlled by a PC operator, and a receiver-microprocessor integrated pack worn on the back of the animals and which connects to suitably implanted brain locations. The backpack is small and light so that small animal subjects can easily carry it. Under remote command from the PC the backpack can be configured to provide biphasic pulse trains of arbitrarily specified parameters. A feature of the system is that it generates precise brain-stimulation behavioral effects using the direct constant-voltage TTL output of the backpack microprocessor. The system performs with high fidelity even in complex environments over a distance of about 300 m. Rat self-stimulation tests showed that this system produced the same behavioral responses as a conventional constant-current stimulator. This system enables a variety of multi-channel brain stimulation experiments in freely moving animals. We have employed it to develop a new animal behavior model (“virtual” conditioning) for the neurophysiological study of spatial learning, in which a rat can be accurately guided to navigate various terrains.

Pregnancy‐related Increases in Sensory Perception Thresholds Are Not Correlated with Serum Progesterone Levels

Why pregnant women require smaller doses of anesthetic agents still remains speculative. One hypothesis proposes that pregnancy raises sensory perception thresholds, perhaps through a progesterone-mediated effect. This study was undertaken in order to quantify any changes in sensory perception thresholds after parturition and to correlate these changes with the expected decrease in postpartum serum progesterone levels. Nineteen gravid women scheduled to undergo an elective Cesarean section consented to participate. Sensory current perception threshold (CPT) testing was performed before and 7 days after an elective Cesarean section. CPT was defined as the minimum amount of constant current stimulation that can be reproducibly detected at a particular frequency. CPT values were determined on the distal phalanx of the nondominant index finger at 2000 Hz, 250 Hz, and 5 Hz monofrequency stimulation. Seven women permitted serum progesterone level determinations at the prepartum and the postpartum CPT testing sessions. Parturition resulted in a statistically significant decrease in the sensory CPT at all 3 frequencies tested, (p < 0.05). However, there was no significant correlation between the postpartum reduction in serum progesterone levels and the observed postpartum decrease in CPT for any frequency, (r < 0.5). CPTs are significantly reduced after parturition. However, this reduction does not appear to be significantly correlated with the reduction in serum progesterone levels.

Application of constant current, low voltage electrical stimulation systems to pig carcasses and its effects on pork quality

This study examined the effectiveness of a constant current, low voltage electrical stimulation system on improving pork quality when applied to pigs at 2 min post-exsanguination. A total of 48 female Duroc×Large White/Landrace pigs of 85–90 kg liveweight were randomly allocated immediately prior to slaughter to one of four constant current electrical stimulation treatments: control (no electrical stimulation), 50, 200 and 400 mA. Stimulation was applied to pig carcasses at 2 min post-exsanguination for 30 s. No differences ( P>0.05) in WB shear force values, muscle lightness or PSE incidence of pork M. longissimus lumborum (LL) was found due to electrical stimulation treatment. Muscle pH of the LL muscle was lower ( P<0.001) in carcasses in the 200 and 400 mA treatments compared to those from carcasses in both the 50 mA and control treatment groups, when measured at the various time points from 40 min to 8 h post-slaughter. Although carcasses stimulated with 200 and 400 mA had higher percentage drip loss ( P<0.05) and purge ( P<0.001), this was not found to impact WB shear force values, muscle lightness or PSE incidence.

Nerve stimulation with an electrode of finite size: differences between constant current and constant voltage stimulation

Differences between constant current and constant voltage nerve stimulation are controversial. To elucidate this controversy, exact solutions are found for the electrical potential and current of a conducting electrode of finite size placed near a boundary of altered conductivity. Substantial differences in the effects of a finite and a point stimulator are predicted. This was strongly dependent on the stimulator–boundary distance, the conductivity of the media, and the curvature of the boundary. The difference between constant voltage and constant current stimulation was smaller than the effects of changes in medium conductivity and electrode distance. A poorly conducting boundary layer surrounding the stimulator minimized these differences.

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 Aβ 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 Aβ 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 Aβ input may contribute to allodynia.

Are Ordinal Data Produced by Pain Threshold Assessments Reliable and Do they Demonstrate Responsiveness to Treatment?

Abstracts 431 I Lund, J Kowalski, T Lundeberg Karolinska Institutet, Stockholm, Sweden The use of visual analogue scale data as numerical has beencriticised, as the size of the distance between the differentsteps is unknown. Its reliability and use at a group level havealso been discussed (Svensson, 2000). An alternative pain assessment is pain thresholdmeasuring. Pain thresholds have been shown to begenerally lower within but also outside the primary painlocation in patients with osteo-arthritic knee pain. Themeasured thresholds were lower compared to healthycontrols and were shown to increase following successfultreatment (Wilder-Smith et al , 2001). This may indicate thatpain threshold changes could be a measure of painalleviation and be evaluated anywhere, eg in the hand. Thedata created from threshold measurements could beregarded as comprising an ordered structure but withoutmetric properties. In this study we wanted to examine whether the ordinaldata produced by such a technique are reliable and ifresponsiveness to treatment would be demonstrated.To measure pain thresholds a PainMatcher® (CefarMatcher AB, Sweden) constant low current (15 mA)stimulator delivered electrical stimulation to the skin of athumb and index finger at random velocity and with afrequency of 10 Hz and raised intensity by increased pulsewidth. Electrical thresholds for sensory detection and pain weremeasured in 93 healthy volunteers (64 women, 29 men; 19-42 years) and in nine women with hip osteo-arthritits(48-70 years). Test-retest reliability was assessed repeatedlyfor two days. Responsiveness was measured before and after treatment with transcutaneous electrical stimulation in the healthy volunteers and before and six months afterhip replacement surgery in the patients with osteo-arthritis.Statistical hypothesis testing was done by Friedmananalysis of variance and calculated by STATISTICA 5.5.Statistical analyses of the rank-invariant methods bySvensson were evaluated by SYSRAN 1.0 for Matlab 6.0. The rank-invariant analyses showed no statistical evidencefor systematic disagreement in the test of reliability.Responsiveness to treatment following intervention wasdetected as systematically increased thresholds both inhealthy volunteers and patients. The measuring techniquewith PainMatcher could be a valuable tool for clinical painevaluation.