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Pathophysiological mechanisms of neuropathic pain: comparison of sensory phenotypes in patients and human surrogate pain models

Vollert, Jana,b,*; Magerl, Walterb; Baron, Ralfc; Binder, Andreasc; Enax-Krumova, Elena, K.a,d; Geisslinger, Gerde,f; Gierthmühlen, Jannec; Henrich, Florianb; Hüllemann, Philippc; Klein, Thomasb; Lötsch, Jörne; Maier, Christopha; Oertel, Brunof; Schuh-Hofer, Sigridb; Tölle, Thomas, R.g; Treede, Rolf-Detlefb

doi: 10.1097/j.pain.0000000000001190
Research Paper
Global Year 2018

As an indirect approach to relate previously identified sensory phenotypes of patients suffering from peripheral neuropathic pain to underlying mechanisms, we used a published sorting algorithm to estimate the prevalence of denervation, peripheral and central sensitization in 657 healthy subjects undergoing experimental models of nerve block (NB) (compression block and topical lidocaine), primary hyperalgesia (PH) (sunburn and topical capsaicin), or secondary hyperalgesia (intradermal capsaicin and electrical high-frequency stimulation), and in 902 patients suffering from neuropathic pain. Some of the data have been previously published. Randomized split-half analysis verified a good concordance with a priori mechanistic sensory profile assignment in the training (79%, Cohen κ = 0.54, n = 265) and the test set (81%, Cohen κ = 0.56, n = 279). Nerve blocks were characterized by pronounced thermal and mechanical sensory loss, but also mild pinprick hyperalgesia and paradoxical heat sensations. Primary hyperalgesia was characterized by pronounced gain for heat, pressure and pinprick pain, and mild thermal sensory loss. Secondary hyperalgesia was characterized by pronounced pinprick hyperalgesia and mild thermal sensory loss. Topical lidocaine plus topical capsaicin induced a combined phenotype of NB plus PH. Topical menthol was the only model with significant cold hyperalgesia. Sorting of the 902 patients into these mechanistic phenotypes led to a similar distribution as the original heuristic clustering (65% identity, Cohen κ = 0.44), but the denervation phenotype was more frequent than in heuristic clustering. These data suggest that sorting according to human surrogate models may be useful for mechanism-based stratification of neuropathic pain patients for future clinical trials, as encouraged by the European Medicines Agency.

Sensory profiles of human surrogate models of neuropathic pain and neuropathy underline the mechanistic relevance of heuristically found sensory phenotypes in patients suffering from neuropathic pain.

aDepartment of Pain Medicine, BG University Hospital Bergmannsheil GmbH, Ruhr-University Bochum, Bochum, Germany

bCenter of Biomedicine and Medical Technology Mannheim (CBTM), Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany

cDivision of Neurological Pain Research and Therapy, Department of Neurology, Universitätsklinikum Schleswig-Holstein, Campus Kiel, Kiel, Germany

dDepartment of Neurology, BG University Hospital Bergmannsheil GmbH, Ruhr-University Bochum, Bochum, Germany

eInstitute of Clinical Pharmacology, Pharmazentrum Frankfurt/ZAFES, University Hospital of Goethe-University, Frankfurt am Main, Germany

fFraunhofer Institute for Molecular Biology and Applied Ecology (IME), Project Group Translational Medicine and Pharmacology (TMP), Frankfurt am Main, Germany

gDepartment of Neurology, Klinikum Rechts der Isar, Technische Universität München, Munich, Germany

Corresponding author. Address: Neurophysiologie, Zentrum für Biomedizin und Medizintechnik Mannheim, Medizinische Fakultät Mannheim, der Universität Heidelberg, Ludolf-Krehl-Str.13-17, 68167 Mannheim, Germany. Tel.: +49 621/383-9926; fax: +49 621/383-9921. E-mail address: Jan.Vollert@rub.de (J. Vollert).

Sponsorships or competing interests that may be relevant to content are disclosed at the end of this article.

Received October 09, 2017

Received in revised form January 26, 2018

Accepted February 14, 2018

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1. Introduction

Quantitative sensory testing (QST) in accordance with the DFNS (German Research Network on Neuropathic Pain) protocol assesses the sensory function of afferent myelinated A-beta- and unmyelinated and thinly myelinated C- and A-delta-fibers3 for both loss (ie, hypoesthesia) and gain of function (ie, hyperalgesia and allodynia).45 Quantitative sensory testing plays an important role for diagnosing patients with neuropathic pain19,56 and can offer indirect insights into underlying mechanisms of pathophysiology.7,15,18,27

Peripheral neuropathic pain is induced by partial nerve damage, leaving 2 classes of nociceptors as candidates to cause the pain: damaged nociceptors and surviving undamaged nociceptors.11 Damaged nociceptors are responsible for sensory loss due to denervation, but possibly also for ongoing pain due to ectopic activity either arising in the periphery or in denervated second-order neurons. Surviving nociceptors can be peripherally sensitized by inflammatory processes related to denervation and reinnervation which may cause hyperalgesia. Input from both damaged and undamaged nociceptors can induce central sensitization which may also cause hyperalgesia. We thus have 4 candidate mechanisms to model in human surrogate models of peripheral neuropathic pain: denervation, ectopic activity, peripheral sensitization, and central sensitization. Models of ectopic activity relate to ongoing pain, whereas the other 3 mechanisms relate to altered perception of evoked pain.

There are no established human surrogate models of ectopic activity and ongoing pain,33 but human surrogate models for denervation, peripheral sensitization, and central sensitization have been used for pharmacological studies.8,33,42 The aim of this study was to estimate the prevalence of 3 distinct types of predefined mechanisms (denervation, peripheral sensitization, and central sensitization) in a large number of healthy subjects submitted to various surrogate models of experimental pain (both previously published8,16,20,25,34,37,40,41,53,62 and unpublished data) using a sorting algorithm developed in patients with peripheral neuropathic pain. Each of the predefined mechanisms was represented by a pair of established surrogate models. This served the purpose of emphasizing those QST characteristic prototypical for the underlying mechanism and minimizing idiosyncrasies of individual surrogate models:

  • (1) Transient functional denervation by nerve compression or topical lidocaine, targeting either myelinated A-fibers or unmyelinated C-fibers, or both.
  • (2) Peripheral sensitization by inducing primary hyperalgesia (PH) by topical application of capsaicin or by ultraviolet B (UVB) radiation; these models commonly induce heat and pressure hyperalgesia.
  • (3) Central sensitization by inducing secondary hyperalgesia (SH) by cutaneous electrical high-frequency stimulation (HFS) or intradermal injection of capsaicin; they induce mostly mechanical hyperalgesia to pinprick.

We then used an algorithm that had been developed for heuristic sorting of QST profiles of neuropathic pain patients to sort sensory profiles of human surrogate models into mechanistically defined groups,60 including 5 additional human surrogate models with less clearly defined, controversial, or mixed underlying mechanisms. The final aim was to assign neuropathic pain patients to presumed underlying mechanisms according to their similarity to the mechanism-related profiles derived from the human surrogate models.

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2. Methods

2.1. German Research Network on Neuropathic Pain

The German Research Network on Neuropathic Pain (DFNS, http://www.neuropathischer-schmerz.de) was established in 2002 to investigate mechanisms and treatments of neuropathic pain. Quantitative sensory testing of human surrogate models of neuropathic pain using the complete DFNS QST protocol has since been conducted at the following sites: Ruhr-University Bochum; Goethe-University Frankfurt; Medical Faculty Mannheim, Heidelberg University; University of Schleswig-Holstein Campus Kiel; and Technical University of Munich. Appropriate ethics committee approval has been obtained at all sites. Subjects were included according to our developed guidelines,24 and quality of the included QST data was assured by standardized training courses for each investigator of each center and verified by regular audits.61 Each center and investigator used identical equipment and standardized instructions. A recent analysis of heterogeneity between centers has shown that data of various centers can be analyzed as a homogenous data set, if these quality criteria are fulfilled.59

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2.2. Surrogate models

The following surrogate models were included in the analysis (Table 1):

  • (1) A-fiber-conduction blockade by sustained pressure on the nerve trunk (unpublished data, methods as in Ref. 65)
  • (2) Topical lidocaine 5% patch34
  • (3) Topical capsaicin application; using cream, watery solution, or patch16,40,41
  • (4) Ultraviolet B light irradiation at 3 times minimal erythema dose (published and unpublished data, methods as in Refs. 25 and 43)
  • (5) Intraepidermal capsaicin injection (unpublished data, methods as in Refs. 36 and 43)
  • (6) Cutaneous electrical high-frequency stimulation28,32,37
  • (7) Muscle electrical high-frequency stimulation53
  • (8) Topical application of capsaicin solution and lidocaine16
  • (9) Topical 40% menthol application9,62
Table 1

Table 1

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2.3. Quantitative sensory testing

The standardized protocol of the DFNS was used for QST, which assesses the function of small and large afferent fibers. The protocol has been described in detail previously;52 in brief, the following parameters were tested: thermal detection thresholds for the perception of cold (cold detection threshold [CDT]) and warmth (warm detection threshold), paradoxical heat sensations (PHSs) during assessment of thermal sensory limen of alternating warm and cold stimuli, thermal pain thresholds for cold and hot stimuli, tactile mechanical detection threshold (MDT) and vibration detection threshold, pain summation to repetitive pinprick stimuli (wind-up ratio), mechanical pain sensitivity (MPS) including thresholds for pinprick (mechanical pain threshold) and blunt pressure (pressure pain threshold), and a stimulus–response–function for pinprick sensitivity (MPS) during which dynamic mechanical allodynia (DMA) was assessed by stroking light touch stimuli. For all parameters, negative (loss of function) as well as positive (gain of function) phenomena were assessed.

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2.4. z-Transformation

An initial analysis of 180 healthy subjects revealed that all QST parameters are either normally or log-normally distributed (with the exception of DMA and PHS).51 To compare individual QST data between subjects regardless of sex-, age-, or body-site differences, the DFNS proposed presenting QST values as z-values.44,48,51,52 These values are normalized to the mean and SD of reference data obtained in healthy subjects so that a z-value of zero represents the mean value of a healthy control cohort matched in age and sex, and the respective body region.44,48,51,52 Z-scores above zero indicate gain of function when the subject was more sensitive to the test stimuli compared with the population mean (hyperesthesia or hyperalgesia), whereas z-scores below zero indicate loss of function referring to a lower sensitivity of the subject (hypoesthesia or hypoalgesia). Although individual z-scores are considered as abnormal if beyond ±1.96,51 for groups of patients, z-scores of ±1.0 have been shown to be of clinical significance, as they include a relevant number of patients with abnormal values.45

Dynamic mechanical allodynia and PHS, which do not normally occur in healthy subjects and do not fit any known distribution, can thus not be z-transformed. They are usually presented as percentages of presence, but for use in statistical analysis, they can be transformed to pseudo-z-values: DMA can be coded into a 0/2/3-variable representing no DMA (coded as 0), DMA with average pain ratings below 1 on a 0 to 100 numerical rating scale (coded as +2), and DMA with average pain ratings between 1 and 100 (coded as +3). Paradoxical heat sensation can be transformed to a binary 0/2-variable showing absence (coded as 0) or presence (coded as +2) of pathological values.

As many surrogate models were applied at the volar lower arm or upper thigh, which are not standardized reference areas, we also performed intraindividual comparisons either relative to the same area before treatment, or, if these data were unavailable, to the contralateral untreated side using effect sizes (Cohen d 13). This measure normalizes changes in the mean value before and after treatment to SD. There are no general interpretations of “good” effect sizes, but it is often considered that effect sizes below 0.3 can be considered small, above 0.5 medium, and above 0.8 are commonly considered as large treatment effects.12

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2.5. Pattern-based sorting algorithm

As sorting method, we applied a method developed for assigning patients to sensory phenotypes developed in our previous work, where we also showed general validity of the method.5,60 The same method was applied here for 2 reasons: first, continuity to our work in patients suffering from neuropathic pain would allow for higher comparability between the hypothesis-driven approach in human surrogate models in the present article, and our earlier, hypothesis-free work in patients suffering from neuropathic pain. Second, and more importantly, the probabilistic version of our previously published sorting algorithm allows for any given patient to be assigned to more than one sensory phenotype (or to present with more than one underlying mechanism); our previous article established a robust cut-off criterion for this type of sorting.60 In this study, the probabilistic sorting was used to assign individual patients to appropriate mechanisms of pathophysiology.

Based on 6 surrogate models with a clearly described mechanism (A-fiber block by nerve compression or by topical lidocaine 5% for small fiber block, topical capsaicin or UVB radiation for PH, and intraepidermal capsaicin injection or cutaneous HFS for SH), we developed an algorithm determining whether an individual QST profile is most similar to denervation, PH, or SH. We used a method we recently established in patients suffering from neuropathic pain,5,60 applying a standard normal distribution, stripped from the normalization factor, so that the resulting value ranges on a scale from 0% to 100%.60

For each participant n, over the QST profile of the z-values of i = 13 parameters, a probability can be calculated, showing a percentage of similarity to the prototypic QST profile of fiber block, PH, or SH:

With i = one of 13 QST parameters, m = one of 3 prototypic sensory profiles and

being the SD of the ith QST parameter for the mth mechanism in the defining data set,

being the mean z-value of the same QST parameter and mechanism in the defining data set, n = the nth participant in a set of participants, and finally

being the z-value found in the nth participant for the ith QST parameter. By averaging the probability over the 13 QST parameters, we quantify the similarity of the individual subject's QST profile to the mean profile of each of the 3 mechanisms.60 The resulting probability value is ranging from 0% to 100% and can be calculated for m = 3 mechanisms.

As a simple way of categorizing patients into mechanisms, we suggest sorting each patient to the phenotype with the highest probability value:

  • (1) Calculate p i according to F(1) for each of the 13 QST parameters. Use μ and σ from Table 2 for nerve block (NB).
  • (2) Average the 13 probabilities. The resulting value is the probability for this patient to show the NB phenotype.
  • (3) Repeat steps 1 and 2, using μ and σ from Table 2 for PH and SH.
  • (4) Allocate the patient to the phenotype with the highest probability value.
Table 2

Table 2

To show reproducibility of the QST profile of the subgroups found, a random number generator was used to split the data set of the 6 defining surrogate models in half, using about 50% of the data to define the prototypic sensory profiles of each of the 3 mechanisms and the other half to verify correct allocation.

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2.6. Healthy sensory profile and deterministic and probabilistic algorithm

A fourth prototypical sensory profile is derived from the QST reference data sets in healthy subjects without any experimental models. According to the definition of the z-transformation, this profile will have a mean z-value of zero and an SD of one for each parameter. In our previous analysis,60 we showed that the algorithm can separate patients suffering from neuropathic pain from healthy subjects if a probability cut-off of 64% is applied. To this point, we use a deterministic approach, that is, each participant is sorted to exactly one mechanism. It is, however, our belief that different mechanisms can coexist; thus, with the cut-off determined for healthy subjects in our previous work transferred onto the surrogate models, we suggest 2 alternative versions, namely a deterministic or a probabilistic algorithm. The deterministic algorithm follows the steps below:

Calculate F(1) for each of the 13 QST parameters. Use μ and σ from Table 2 for healthy.

  • (1) Average the 13 probabilities. The resulting value is the probability for this patient to show a healthy profile.
  • (2) Repeat steps 1 and 2, using μ and σ from Table 2 for NB, PH, and SH.
  • (3) Allocate the participant to the phenotype with the highest probability value.

By contrast, in the probabilistic algorithm, steps 1 to 3 remain identical and step 4 is exchanged as follows:

  • (4) Sort the participant to all phenotypes with a probability above 64%. If the only probability above this cut-off is for being healthy or no phenotype reaches a probability above this cut-off, the sensory profile of the subjects cannot be distinguished from normal.

These 2 versions were used for all analyses below and are presented alongside, all QST profiles of all surrogate models underwent both versions of the algorithm.

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2.7. Comparison with sensory phenotypes in patients

In our recent work,5,60 we have described 3 distinct sensory phenotypes that appear in patients suffering from peripheral neuropathic pain of various etiology or clinical origin. To analyze how this previous hypothesis-free pattern searching method in patients suffering from neuropathic pain may relate to our new mechanistic sensory subgroup classification described above, the n = 902 patients from the initial cluster analysis study5 underwent the deterministic version of the new sorting algorithm and the result was compared with each patient's sensory phenotype in the deterministic version of the previous sorting algorithm developed for patients.60 Agreement between algorithms was assessed using Cohen kappa.21 Cohen kappa is a value that measures similarity between groups or concordance of ratings on a scale ranging from 0 (no similarity) to 1 (perfect identity). Although no universal guideline for interpreting Cohen kappa exists, as a rule of thumb, it was suggested that values below 0.4 would indicate poor similarity, between 0.4 and 0.75 good similarity, and above 0.75 excellent.21 We then applied the new probabilistic sorting algorithm to estimate the prevalence of the 3 predefined mechanisms in this cohort of patients; this allows each patient to be assigned to more than one mechanism.

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3. Results

The analysis comprised a total of n = 657 healthy subjects who participated in published and unpublished studies on human surrogate models: 9 distinct models (2 of them including areas of both PH and SH) at 5 centers (Table 1). Approximately 44% of subjects were female (291/657); sex distribution was not homogenous across models, as for the UVB, cutaneous HFS, and menthol models, predominantly men were recruited. Age ranges were mostly lower than in neuropathic pain populations. These factors were accounted for by normalizing all QST data to sex-specific and age-specific reference data.

Figure 1 shows z-profiles of the 6 models that were chosen for defining phenotype means. As human surrogate models of NBs, both nerve compression and topical lidocaine led to substantial loss in thermal detection threshold and MDT (Fig. 1A). For CDT and MDT in the A-fiber compression block, this loss was almost complete, reaching a mean z-value beyond −5, that is, beyond 5 SDs of normal detection, whereas for topical lidocaine, z-values were around −2 indicating highly significant but partial sensory loss. Unlike topical lidocaine, selective A-fiber block was also associated with signs of sensory gain, namely the frequent occurrence of pinprick hyperalgesia and PHSs.

Figure 1

Figure 1

As human surrogate models of PH, topical capsaicin and UVB sunburn both induced substantial heat and mechanical hyperalgesia (Fig. 1B). Heat hyperalgesia was more pronounced for capsaicin, whereas mechanical hyperalgesia was more pronounced for UVB. Capsaicin also induced loss of cold detection and cold pain.

As human surrogate models of SH, cutaneous HFS and intradermal capsaicin injection led to mechanical hyperalgesia and thermal sensory deficits (Fig. 1C). Mechanical hyperalgesia and DMA were more pronounced in the capsaicin injection model.

Table 3 compares mean z-scores normalized to published reference data of healthy subjects with effect sizes (Cohen d) in intraindividual comparison with untreated control areas. Intraindividual comparisons mostly confirmed the patterns of negative or positive sensory signs of z-values, but the loss of thermal detection for intradermal capsaicin and cutaneous HFS (Fig. 1C) seems to be overestimated in z-values due to the nonstandard test area.

Table 3

Table 3

The QST profiles of the remaining models are presented in Figure 2. Figure 2A shows the QST profiles of surrounding skin from 2 models where SH is either controversial (UVB) or known to be mild (topical capsaicin). Although the area of SH of topical capsaicin displays sensory loss in the z-profile, these effects are much smaller in the intraindividual comparisons with untreated skin, again suggesting an overestimation in z-values due to the nonstandard test area (Table 3). Topical menthol has been introduced to induce cold hyperalgesia, and muscle HFS to induce hyperalgesia of deep tissue (Fig. 2B); sensory changes in both models were mild. The combined application of topical capsaicin and lidocaine (Fig. 2C) shows the combined effects of lidocaine and capsaicin-induced PH, that is, the capsaicin-induced mechanical hyperalgesia is abolished, whereas the thermal and tactile loss is exaggerated.

Figure 2

Figure 2

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3.1. Sorting algorithm

By random number assignment, 49% of subjects from the 6 surrogate models with clearly defined mechanism (topical lidocaine 5%, A-fiber block, topical capsaicin, UVB radiation, capsaicin injection, and cutaneous HFS) were assigned to the training data set (n = 265), which defined mean values and SDs for the sorting algorithm (Table 2). Individual allocation by the deterministic sorting (DET) algorithm replicated the a priori assignment of surrogate models in 79% of the cases for training set and in 81% of the cases for the test set (remaining 279 subjects from the same surrogate models). Cohen kappa coefficient of agreement (scale: 0 = random classification, 1 = perfect agreement between methods) was 0.54 (95% confidence interval [CI]: 0.43-0.65) for the training set and 0.56 (95% CI: 0.46-0.67) for the test set; both values may be categorized as “good,” although no universal guideline for interpreting Cohen kappa exists.21 Most common shifts were PH or SH to NB (18% and 27%, respectively), and least common shifts were NB to SH and secondary to PH (both <1%). Shifts between original and algorithmic assignment are shown in Table 4.

Table 4

Table 4

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3.2. Deterministic and probabilistic sorting

The z-profiles of each sensory subgroup are shown in Figure 3. Forty-one subjects showed a sensory profile that was most similar to untreated healthy skin, although part of a surrogate model (Fig. 3A). The subjects sorted to the NB profile were mostly characterized by loss of cold and mechanical detection (CDT and MDT, representing A-delta and A-beta fiber function, respectively, Fig. 3B). Loss of warm detection was less pronounced compared with cold detection because the topical lidocaine block was less effective in comparison with the A-fiber block. Loss of vibration detection was not detectable, which is due to a limitation of the A-fiber block, which only affects a small area, whereas vibration is poorly localized and can be sensed by rapidly adapting mechanoreceptors (Pacinian corpuscles) situated beyond this area. Primary hyperalgesia (Fig. 3B) was characterized by pronounced heat hyperalgesia and moderate mechanical hyperalgesia, whereas SH presented most prominently by mechanical hyperalgesia and some loss of thermal detection.

Figure 3

Figure 3

Figure 4 (6 defining models) and Figure 5 (5 additional models) show results of deterministic and probabilistic sorting into the 3 prototypic mechanistic profiles. In the probabilistic sorting algorithm, the percentage of sensory profiles compatible with being from normal skin increased to 187 (28%) vs 41 (6%) in the deterministic version of the algorithm. The highest frequency of healthy profiles was found in muscular HFS and topical menthol, which also had the mildest sensory changes in their averaged QST profiles (Fig. 2B).

Figure 4

Figure 4

Figure 5

Figure 5

Quantitative sensory testing profiles compatible with the NB profile increased to 352 cases (54%) in the probabilistic algorithm vs 178 (27%) in the deterministic version. Apart from the defining models of A-fiber block and topical lidocaine, the highest frequency of the NB profile was found in cutaneous HFS and in the menthol model.

Quantitative sensory testing profiles compatible with PH increased from 380 (58%) in the deterministic to 470 (72%) in the probabilistic algorithm. Beyond the defining models, this profile was frequent in the skin area surrounding the UVB radiation model (where profiles compatible with SH were rare), topical menthol, and muscular HFS.

Quantitative sensory testing profiles consistent with SH were more than twice as frequent in the probabilistic sorting (n = 134; 20%) than in the DET (n = 58; 9%). Beyond the defining capsaicin injection and cutaneous HFS, this profile was only found in the skin area surrounding topical capsaicin in a relevant frequency (deterministic: 27% and probabilistic: 65%) confirming that SH occurs also in this model.

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3.3. Comparison with sensory phenotypes in patients

Figure 5F shows DET and probabilistic sorting results for 902 neuropathic pain patients that were the basis of the original cluster analysis (Baron et al. 2017), and Table 5 compares mechanistic sorting results with original heuristic cluster allocation. In 65% of the cases, patients from the sensory loss phenotype were sorted to NB, patients from the thermal hyperalgesia phenotype to the PH pattern, and patients from the mechanical hyperalgesia phenotype to the SH QST pattern (Table 5). This corresponded to a Cohen kappa coefficient of agreement (scale: 0 = random classification, 1 = perfect agreement between methods) of 0.44 (95% CI: 0.28-0.60), showing a substantial degree of agreement.21 The most prominent shifts were from the 2 hyperalgesia phenotypes in the heuristic patient clustering into the mechanistic phenotype consistent with NB.

Table 5

Table 5

Interestingly, in the probabilistic sorting (Table 5), fewer patients were sorted to the NB profile (512 vs 600 in DET), whereas relatively more patients were sorted to PH (271 vs 186 in DET) or SH (198 vs 116 in DET). But, the most prominent shift was still towards the profiles consistent with NBs. Still, only one-third of the patients could be uniquely assigned to 1 of the 3 mechanisms. Another third of the patient QST profiles was consistent with multiple mechanisms (n = 282), most frequently a combination including the NB phenotype. The last third was not sufficiently distinct from a normal skin QST phenotype to be assigned to any mechanism (n = 279).

Figure 6 illustrates the similarity in QST profiles grouped either according to the original cluster analysis (Fig. 6A) or the mechanistic sorting (Fig. 6B). Although the profiles of patients sorted according to both algorithms are highly similar, the profiles of patients and surrogate models in the mechanistic sorting display some interesting differences (compare Fig. 3 with Fig. 6B): patients sorted to the NB exhibit hypoalgesia, whereas in surrogate models, pain thresholds are normal or show mild gain. Patients sorted to NB also have loss of vibration detection threshold, which could not be modeled in the A-fiber block. For PH, the models display almost no cold, but an isolated heat hyperalgesia, whereas patients show both cold and heat hyperalgesia. For SH, participants under surrogate models have more gain in mechanical pain threshold than in MPS, which is reversed in patients.

Figure 6

Figure 6

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4. Discussion

These multicenter data confirmed principal properties of NB (sensory loss), PH (heat hyperalgesia), and SH (pinprick hyperalgesia) that have been already reported in human and animal experiments.8,35,42,47,55 They also revealed additional sensory alterations in these human surrogate models that had not been previously described, that is, sensory gain in NBs and sensory loss in hyperalgesia models. This probably reflects a selection or reporting bias in previous studies that had not assessed these sensory functions.

In a randomized split-half analysis, a sorting algorithm previously validated for sensory profiles of neuropathic pain patients60 led to reproducible sorting of surrogate model sensory profiles into patterns defined a priori according to known mechanisms. Sorting of 902 neuropathic pain patients into these mechanistic phenotypes led to a similar distribution as the original heuristic clustering,5,60 laying the basis for a mechanism-based treatment approach.64

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4.1. Nerve blocks as human surrogate model of transient functional denervation

The sensory profile of human surrogate models for transient functional denervation (ie, A-fiber compression block and topical lidocaine) was characterized by pronounced loss in thermal and mechanical detection combined with PHS. These patterns have been reported in previous single-center studies.34,65 Consistent with its selective effect on myelinated nerve fibers, compression NB had larger effects on MDT and CDT than on warm detection threshold.28,65 Pinprick stimuli are often perceived as less painful in complete A-fiber block,28,65 whereas mild pinprick hyperalgesia has been associated with preserved A-delta fiber function2,29 Topical lidocaine had mild effects compared with complete conduction block by regional anesthesia.22,33 Interestingly, in contrast to the present data, intradermal lidocaine 0.01% and 0.1% was reported to induce also a heat hyperalgesia probably due to sensitization of TRPV1 and TRPA1 receptors.38,39,49,63

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4.2. Primary hyperalgesia as human surrogate model of peripheral sensitization

The profile of surrogate models of peripheral sensitization (topical capsaicin or UVB radiation) was characterized by pronounced hyperalgesia to heat, pressure, and pinprick pain. Primary nociceptive afferents are easily sensitized to heat stimuli, but much less so to von-Frey or pinprick stimuli.55 Peripheral sensitization to heat may be explained by phosphorylation of the heat-gated cation channel TRPV1,58 but has also been shown to be induced by TRPA1 agonist allyl-isothiocyanate.1 Although peripheral sensitization to blunt pressure has been reported before,31 pinprick hyperalgesia in these models may indicate additional central sensitization induced by enhanced peripheral nociceptive input to the spinal cord. Sensory loss occurred mostly in the topical capsaicin model and was restricted to thermal detection thresholds; this has been discussed previously10,16 and may reflect desensitization by the TRPV1 agonist capsaicin, which is the intended mode of action clinically.26

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4.3. Secondary hyperalgesia as human surrogate model of central sensitization

Human surrogate models of central sensitization (intradermal capsaicin and cutaneous electrical HFS) were characterized by pronounced pinprick hyperalgesia, but also pronounced thermal sensory loss. Combined studies in monkeys and humans using intradermal capsaicin had shown pronounced increases in both wide dynamic-range and high-threshold spinal neuron output despite unchanged A- and C-nociceptor input.6,54 Tactile sensory loss has been reported in human surrogate models of SH and also in patients with pinprick hyperalgesia.23 An inverse spinal gate with small fiber input inhibiting processing of large fiber input46,66 has been suggested as a mechanism. Thermal sensory loss is a new finding, suggesting that sensory loss in chronic pain patients does not necessarily have to be due to structural changes, but may also be a functional sign, and hence potentially sensitive to analgesic treatment regimes.

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4.4. Other human surrogate models

The presence of a zone of SH surrounding topical capsaicin and UVB is controversial in the published literature.8,33,40,41 Our data confirmed sensory profiles compatible with SH surrounding topical capsaicin but not UVB, which mostly had characteristics of PH or NB instead. These findings suggest that topical capsaicin is useful to study both PH and SH, whereas the UVB model seems to be limited to PH.

Muscle HFS was introduced as a model for plasticity of deep pain, and the overlying skin did not exhibit profiles compatible with SH. Our data were from the lower back; more pronounced effects might occur when other muscles, for example, in the lower limb or face are stimulated.

Topical menthol had only mild effects with some characteristics of NBs and PH. Its main value is the pronounced cold hyperalgesia, but the current data do not make a strong suggestion as to whether this is due to peripheral or central sensitization or both.

The sensory profile of topical lidocaine plus topical capsaicin displayed an almost perfect combination profile of lidocaine block and capsaicin-induced PH, but it lacks the distinct mechanical hyperalgesia characterizing SH. This finding suggests that sensory profiling may be able to identify combined mechanisms and to distinguish between contributions of peripheral vs central sensitization also in patients with neuropathic pain.

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4.5. Mechanistic significance of heuristically found patient phenotypes

The sorting according to surrogate model profiles was applied to our previously published patient data, which leads to a similar distribution as the original heuristic clustering. This supports previous mechanistic interpretations of the clinically found phenotypes: the thermal hyperalgesia patient phenotype shows strong overlap with surrogate models of PH. This supports previous interpretations as irritable nociceptor18 and peripheral sensitization.57 Both evoked and ongoing pain is likely to be due to surviving nociceptors in these patients.

The mechanical hyperalgesia patient phenotype shows a strong overlap with surrogate models of SH, which supports an interpretation of this phenotype to be a phenotype of reorganization or central sensitization. Substantial thermal sensory loss suggests that also damaged nociceptors are involved, generating ongoing pain and inducing central sensitization.4

The sensory loss patient phenotype shows a strong overlap with experimental NBs. These blocks were frequently used as tools to identify normal sensory function of fiber classes (A vs C), but not yet widely recognized as mimicking aspects of neuropathic pain.7,33 Both the clinical phenotype and the surrogate models are dominated by loss of small and large fiber functions. This supports an interpretation as denervation or deafferentation, where central neurons may develop denervation super-sensitivity to other inputs.14

Roughly one-third of the patients were assigned a NB phenotype according to the surrogate models vs one of the hyperalgesia phenotypes in the heuristic patient cluster analysis. This systematic shift is consistent with the presence of partial nerve damage in most of the neuropathic pain conditions that may lead to coexistence of denervation and sensitization of the remaining pathways.11 These data cover, however, only peripheral neuropathic pain, and to this point, we cannot make any extrapolations onto, for example, central pain, nociceptive pain, or deep pains.

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4.6. Limitations and technical considerations

The validity of the prototypic mechanistic profiles is partly demonstrated by the accuracy of allocation of the training set data to the appropriate a priori profile of roughly 80%. This shows general robustness, but also that the deterministic algorithm is not perfect and should be used with some caution on individual basis. For a better separation of mechanism-specific sensory profiles from idiosyncrasies of individual experimental models, more human surrogate models of PH and SH (eg, burn injury and surgical incision) and of temporary functional denervation (eg, limb ischemia and capsaicin-induced defunctionalization) should be studied using the methods of this article.

The models in this analysis do not explicitly cover the endogenous pain-modulating systems30 nor ectopic activity. Descending modulation may contribute to the SH phenotype, but might also exhibit yet another sensory profile. Ectopic activity may have been present in any of our models, but we have neither positive nor negative evidence about it.

The percentage of sensory profiles compatible with normal variability was higher in the surrogate models than in the patients (29% vs 20%). We think that this is due to the fact that for ethical reasons, all manipulations in humans were relatively minor (as compared to patients and animal models50).

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4.7. Summary and conclusions

With these prototypical sensory profiles for 3 predefined mechanisms (denervation, peripheral, and central sensitization), we provide a well-studied mechanistic background for our previously described heuristic sensory phenotype clusters. Using the probabilistic sorting according to human surrogate model profiles, patients suffering from neuropathic pain can be tentatively stratified in future studies to presumed underlying mechanisms. The European Medicines Agency encourages such an approach in its new guideline17 as a step towards increasing response rates in clinical trials by a mechanism-based treatment approach towards neuropathic pain.64 It should be noted, however, that the 3 classes of human surrogate models studied here likely represent combined rather than single mechanisms. This, however, is likely true also for studies in awake behaving animals. Therefore, a reverse translation approach may be useful for developing novel analgesic medications: they should initially be tested in animal models of NB, PH, or SH. Medications effective on these phenotypes can easily be validated in human surrogate models and then transferred to subgroups of neuropathic pain patients.

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Conflict of interest statement

The authors have no financial or other relationships that might lead to a conflict of interest.

This study was supported by the German Research Network on Neuropathic Pain (DFNS).

The EUROPAIN project is a public-private partnership and has received support from the Innovative Medicines Initiative Joint Undertaking under grant agreement no 115007, resources for which are composed of financial contribution from the European Union's seventh framework programme (FP7/2007–2013) and European Federation of Pharmaceutical Industries and Associations (EFPIA) companies' in kind contribution. The NEUROPAIN project is an investigator-initiated European multicentre study with R. Baron as principal investigator and 10 coinvestigator sites, supported by an independent research grant from Pfizer Ltd.

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Acknowledgements

The authors thank all consortia for building up the basis of this study by patient and subject recruitment and assessment.

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Keywords:

Quantitative sensory testing; German Research Network on Neuropathic Pain; Menthol; A-fiber block; UVB; High-frequency electrical stimulation; Capsaicin; Lidocaine; Human surrogate pain models

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