Reproducibility of current perception threshold with the Neurometer®vs the Stimpod NMS450 peripheral nerve stimulator in healthy volunteers: an observational study
Current methods of assessing nerve blocks, such as loss of perception to cold sensation, are subjective at best. Transcutaneous nerve stimulation is an alternative method that has previously been used to measure the current perception threshold (CPT) in individuals with neuropathic conditions, and various devices to measure CPT are commercially available. Nevertheless, the device must provide reproducible results to be used as an objective tool for assessing nerve blocks.
We recruited ten healthy volunteers to examine CPT reproducibility using the Neurometer® and the Stimpod NMS450 peripheral nerve stimulator. Each subject’s CPT was determined for the median (second digit) and ulnar (fifth digit) nerve sensory distributions on both hands – with the Neurometer at 5 Hz, 250 Hz, and 2000 Hz and with the Stimpod at pulse widths of 0.1 msec, 0.3 msec, 0.5 msec, and 1.0 msec, both at 5 Hz and 2 Hz. Intraclass correlation coefficients (ICC) were also calculated to assess reproducibility; acceptable ICCs were defined as ≥ 0.4.
The ICC values for the Stimpod ranged from 0.425-0.79, depending on pulse width, digit, and stimulation; ICCs for the Neurometer were 0.615 and 0.735 at 250 and 2,000 Hz, respectively. These values were considered acceptable; however, the Neurometer performed less efficiently at 5 Hz (ICCs for the second and fifth digits were 0.292 and 0.318, respectively).
Overall, the Stimpod device displayed good to excellent reproducibility in measuring CPT in healthy volunteers. The Neurometer displayed poor reproducibility at low frequency (5 Hz). These results suggest that peripheral nerve stimulators may be potential devices for measuring CPT to assess nerve blocks.
Reproductibilité du seuil de perception du courant avec les stimulateurs des nerfs périphériques Neurometer®vs. Stimpod NMS450 chez des volontaires sains: une étude observationnelle
Dans le meilleur des cas, les méthodes actuelles pour évaluer les blocs nerveux, telles que la perte de perception de la sensation de froid, sont subjectives. La stimulation nerveuse transcutanée est une autre méthode, qui a été utilisée par le passé pour mesurer le seuil de perception du courant (CPT en anglais) chez les personnes atteintes de neuropathies. Il existe plusieurs dispositifs pour mesurer le CPT. Toutefois, le dispositif doit être en mesure de fournir des résultats reproductibles si l’on veut pouvoir l’utiliser comme outil objectif d’évaluation de la curarisation.
Nous avons recruté dix volontaires sains pour étudier la reproductibilité du CPT lors de l’utilisation des stimulateurs des nerfs périphériques Neurometer® et Stimpod NMS450. Le CPT de chaque participant a été déterminé dans les territoires sensitifs des nerfs médian (deuxième doigt) et ulnaire (cinquième doigt) de chaque main, en réglant le Neurometer à 5 Hz, 250 Hz et 2000 Hz et le Stimpod à des durées d’impulsion de 0,1 msec, 0,3 msec, 0,5 msec et 1,0 msec, à 5 Hz et 2 Hz. Les coefficients de corrélation intraclasse (CCI) ont également été calculés afin d’évaluer la reproductibilité; les CCI ont été définis comme étant acceptables à ≥ 0,4.
Les valeurs CCI pour le Stimpod se situaient entre 0,425 et 0,79, selon la durée d’impulsion, le doigt et la stimulation; les CCI pour le Neurometer étaient de 0,615 et 0,735 à 250 et 2000 Hz, respectivement. Ces valeurs ont été considérées comme acceptables; toutefois, le Neurometer était moins efficace à 5 Hz (les valeurs de CCI pour le deuxième et le cinquième doigt étaient de 0,292 et 0,318, respectivement).
Globalement, le Stimpod a affiché une reproductibilité bonne à excellente pour mesurer le CPT chez des volontaires sains. Le Neurometer a montré une reproductibilité médiocre à basse fréquence (5 Hz). Ces résultats suggèrent que les stimulateurs de nerfs périphériques pourraient constituer des dispositifs potentiels de mesure du CPT pour évaluer les blocs nerveux.
External electrical currents can be applied by various commercially available devices for electrodiagnostic sensory nerve testing, allowing determination of the sensory nerve Current Perception Threshold (CPT), defined as the lowest current that can be detected. The Neurometer® (Neurotron, Inc., Baltimore, MD, USA) is an example of such a device; it has been used clinically to detect peripheral neuropathy in diabetic individuals.1-7 By measuring CPT and CPT trends, transcutaneous nerve stimulation has been suggested as an effective tool in diagnosing and monitoring conditions associated with worsening peripheral neuropathy (i.e., increasing CPT values), such as diabetes mellitus or vibration-induced neuropathy.2
Current standard techniques for determining nerve block effectiveness are limited by the inherent subjectivity of patient self-reporting (e.g., cold test, pinprick tests). Commonly used assessment methods, including loss of cold sensation and, to a lesser extent, motor function impairment, are useful for assessing neurologic function following a block. Nevertheless, these methods essentially provide an “all or none” response and are difficult to quantify when predicting the block trend. Nerve stimulation therefore presents a practical method for evaluating block effectiveness or trend of onset after performing the regional block.
Several groups have reported the application of transcutaneous nerve stimulation with the Neurometer to assess different local anesthetic regimens used in regional anesthesia by comparing CPT before and after application of the anesthetic.8-10 Also, stimulation of the sural nerve with an electric current has been used to assess epidural fentanyl injection.11 Peripheral nerve stimulators, which are commonly used for locating nerve structures, can also facilitate transcutaneous stimulation via adhesive gel pads or a percutaneous electrode guidance probe,12 but these devices have not been well studied for measuring CPT. Furthermore, to be clinically useful, the CPT results obtained by transcutaneous nerve stimulation devices must be reproducible. We hypothesized that the CPT values obtained with the peripheral nerve stimulator would show acceptable reproducibility. Here, we examined a common peripheral nerve stimulator, Stimpod NMS450, vs the Neurometer to determine whether transcutaneous nerve stimulation can provide reliable reproducible CPT measurements in our study population of healthy volunteers. The primary outcome of this study was reproducibility of CPT measurements using the peripheral nerve stimulator. Intraclass correlation coefficients were calculated to examine reproducibility between two separate trials with each of the devices.
Following approval from our institutional Research Ethics Board on July 4, 2012 and written consent from participants, we rerecruited ten volunteers for our pilot study. Participants were healthy adults (> 18 yr of age) with no implanted electrical devices (e.g., pacemaker, spinal cord or peripheral nerve stimulator) or history of neuropathic lesions or polyneuropathic conditions. The study was performed in August 2012 at the University of Alberta Hospital, Edmonton, Alberta, Canada.
Once the electrodes were attached, an escalating current was delivered from the Neurometer (accuracy up to 0.001 mA) to the second digit of the first hand at frequencies of 2,000 Hz, 250 Hz, and 5 Hz, in that order. The member of the research team performing the experiment was responsible for manually controlling the current. The subjects were asked to report when they felt an “electrical” sensation in their finger. To correct for a possible delayed response in a subject’s perception of the stimulus and also to gain a more precise CPT reading, the current was reduced gradually until the subject could not detect it and then the current was increased gradually until the stimulus was perceptible. This last reading was defined as the CPT.
Next, a ground electrode (Red Dot, 3 M Health Care, St Paul, MN, USA) was placed on the subject and connected to the Stimpod, and the subject’s second digit was stimulated with current (accuracy ± 5%) at a frequency of 5 Hz and pulse widths of 0.1 msec, 0.3 msec, 0.5 msec, and 1.0 msec, in that order, followed by stimulation at 2 Hz and the same order of pulse duration. Again, a member of the research team increased the current manually until the subject could detect an “electrical” sensation, which was recorded as the CPT. This sequence was repeated for the fifth digit of the same hand.
For all volunteers, a duplicate set of data was collected for both hands using both devices in the same order and sequence as the first set of recordings. A new pair of electrodes was used for each digit. There was no rest period between measurements apart from changing the electrodes prior to obtaining the duplicate set of data. Data were entered into a Microsoft® Excel (Microsoft Corp., Redmond, WA, USA) spreadsheet by a member of the research team.
Mean and standard deviation values were calculated in Microsoft Excel. One-way random intraclass correlation coefficients (ICCs) were calculated to assess reproducibility between trials. Briefly, CPT values were determined for each device at each setting for each digit for each volunteer. The values at each site were then averaged among the ten volunteers. The values of the right and left digit were then averaged to obtain the mean ICC for that digit. Upper and lower confidence interval (CI) values were determined in a similar fashion. For sample size calculation, the lowest ICCs we would accept were 0.4 (considered borderline “good” according to Fleiss),13 and the expected ICCs of the devices were assumed to be 0.8 (“excellent”). We assumed that the current threshold for both hands was similar in healthy volunteers. Based on a β value of 0.2 and α value of 0.05 for two measurements for each of the left and right hands for the second digit (total n = 4), the estimates of sample size (K) for the intraclass correlation would be 8.0 using the method described by Walter et al.14 The same parameters were used for measurements of the fifth digit (n = 4). Intraclass correlation coefficients and 95% CIs were calculated using SPSS® version 20 (IBM Corp., Armonk, NY, USA).
Current perception threshold values for each device and setting
Stimpod 5 Hz
Stimpod 2 Hz
Mean (SD) (mA)†
Mean (SD) (mA)†
The data generated by our study show that the CPT obtained using the Stimpod can be reliably reproduced. Importantly, this reproducibility is consistent for both the second and fifth digits of both hands and most frequency/pulse duration combinations.
Intraclass correlation (ICC) can be used to evaluate strength of association.15 Intraclass coefficient values range from one to zero, indicating 100% and no reproducibility, respectively. In this study, the ICC values of the Neurometer at 5 Hz fell outside the range of acceptable reproducibility for both the second and fifth digits, whereas all the other ICCs were in the “good” or “excellent” range. Even so, these conclusions are based on the results and analysis presented here, and they are applicable to the devices and settings tested in this study.
The Neurometer uses a transcutaneous electrical current for CPT measurement. The device has previously been tested successfully for CPT reproducibility in various study populations;3,16-18 therefore, we decided to utilize the Neurometer as a benchmark device to compare with a peripheral nerve stimulator. At low frequency (5 Hz), the Neurometer showed poor reproducibility according to ICC calculations. On the other hand, our data show that the Stimpod possesses an acceptable level of reproducibility at similar frequencies (5 Hz or less), suggesting that it could potentially be an alternative tool for assessing nerve blocks if there is a significant magnitude of change in CPT before and after a block.19,20 The fact that the Neurometer did not provide reproducible results at a low frequency in our study is not surprising because such poor reproducibility has been reported previously.21 In fact, most of the information supporting Neurometer reproducibility comes mainly from the manufacturer. In contrast, a published review of the literature regarding the Neurometer found “…little published information about reliability of test results between operators and the replicability of results between testing times”.22 The analyses presented here show that Neurometer CPT measurements at 5 Hz fell into the poor or unacceptable range of reproducibility for the purposes of this study when compared with the Stimpod at similar frequencies.
In terms of practicality, the Neurometer is expensive and cumbersome and does not lend itself to portability in its current form (Fig. 1). Moreover, it is not readily available in regional block areas of hospitals or operating theatres. In contrast, peripheral nerve stimulators are portable and relatively inexpensive, and they are routinely available in anesthetizing locations within hospitals. Since these devices are also used for nerve localization and testing,12,23 anesthesiologists are already familiar with their functions and use. Peripheral nerve stimulators deliver time-adjustable pulsed direct current and current-controlled and frequency-adjustable stimuli. It has been suggested that the pulse duration used in direct current peripheral nerve stimulators has some bearing on whether motor or sensory nerves are stimulated. In particular, a pulse duration of ≤ 0.1 msec is suggested to stimulate motor neurons more selectively without undue sensory discomfort,24 whereas a pulse duration of ≥ 0.3 msec is considered to be more selective for sensory nerves,12 although this distinction remains unclear in clinical practice.25 The ICCs of the Stimpod at a frequency of 2 Hz and a pulse width of 0.3 msec fell within the “good” and “excellent” ranges of reproducibility for the second and fifth digits, respectively, potentially making this a useful combination of settings for CPT testing.
Regarding limitations, since we performed our study by systematically testing the Neurometer prior to testing the Stimpod, it is possible that a non-random difference in mean scores across trials may have been introduced. This could introduce a learning effect that could be enhanced by lack of training or lack of a period of familiarization. In order to examine this possibility, we created bipartite graphs for CPT values for each device/setting/digit showing each volunteer’s CPT readings for the first and second tests (Fig. 2). In our view, based on these graphs, no obvious learning effect was introduced between the first and second tests in our study, as we cannot detect any trend to suggest that testing the Neurometer first had any positive effect on perception of the Stimpod stimulus.
This work is also limited because the optimal frequency and pulse duration for stimulating peripheral sensory nerves has yet to be established, although we consider our study to be a reasonable and valid starting point towards determining this value. Another factor that may influence our results is the positioning of our volunteers while CPT values were being recorded. A variance in CPT values has been shown between subjects in a horizontal position vs a tilt-up position,26 although the clinical significance of this finding remains unclear.27
Further studies are required to determine if CPT, as measured by a peripheral nerve stimulator, would be clinically reproducible and clinically useful to monitor the progression of sensory neural blockade after a peripheral nerve block. Measurement of CPT may be used in addition to, or in place of, applying ice to monitor for loss of cold sensation. This is especially relevant, as C fibres are presumed to respond optimally to both low frequency (5 Hz) electrical current and cold stimuli.9 Furthermore, it may be possible through further study to determine a percent increase in CPT that correlates with loss of cold sensation. The results presented here suggest that a peripheral nerve stimulator can be used at low frequency to obtain CPT with acceptable reproducibility; however, since most commercially available peripheral nerve stimulators can be set to a frequency of 1 Hz or 2 Hz (but not 5 Hz), it is likely to be more clinically relevant to consider using these frequencies when planning future studies.
The authors sincerely thank Zakiya Dhanani, Jordan Leung, Mark Rockley, and Jenkin Tsui for their contributions to the study design and data collection. The corresponding author (B.T.) is supported in part by a Clinical Scholar Award from the Alberta Heritage Foundation for Medical Research (AHFMR), a Smiths Medical Canada Ltd. Canadian Research Award in Pain Research and Regional Anesthesia from the Canadian Anesthesiologists’ Society (CAS), and a CAS/Abbott Laboratories Career Scientist Award. This study was supported in part by educational funding from the Department of Anesthesiology and Pain Medicine, University of Alberta.
Conflicts of interest
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