Topical Capsaicin Response as a Phenotypic Measure in Patients with Pain

Patient phenotyping in clinical trials of chronic pain treatments: IMMPACT recommendations

There is tremendous interpatient variability in the response to analgesic therapy (even for efficacious treatments), which can be the source of great frustration in clinical practice. This has led to calls for “precision medicine” or personalized pain therapeutics (ie, empirically based algorithms that determine the optimal treatments, or treatment combinations, for individual patients) that would presumably improve both the clinical care of patients with pain and the success rates for putative analgesic drugs in phase 2 and 3 clinical trials. However, before implementing this approach, the characteristics of individual patients or subgroups of patients that increase or decrease the response to a specific treatment need to be identified. The challenge is to identify the measurable phenotypic characteristics of patients that are most predictive of individual variation in analgesic treatment outcomes, and the measurement tools that are best suited to evaluate these characteristics. In this article, we present evidence on the most promising of these phenotypic characteristics for use in future research, including psychosocial factors, symptom characteristics, sleep patterns, responses to noxious stimulation, endogenous pain-modulatory processes, and response to pharmacologic challenge. We provide evidence-based recommendations for core phenotyping domains and recommend measures of each domain.

Experimenter- and Infrared Thermography-Derived Measures of Capsaicin-Induced Neurogenic Flare Among Non-Hispanic White and Black Adults

OBJECTIVE

Capsaicin is a widely utilized experimental pain stimulus; however, few studies have reported on ethnic differences in pain responses to capsaicin. The present study used infrared thermography to 1) measure differences in capsaicin-induced neurogenic flare between non-Hispanic black (NHB) and non-Hispanic white (NHW) adults and 2) determine the association between neurogenic flare and secondary hyperalgesia.

METHODS

Fifty-four participants (NHB N = 28) underwent heat/capsaicin sensitization model procedures. Neurogenic flare was examined using experimenter (i.e., subjective) and thermography (i.e., objective) measurements. A typically nonpainful mechanical punctate probe was used to measure secondary hyperalgesia.

RESULTS

Ethnic groups did not significantly differ in age, sex, marital status, or personal income. Although experimenters rated a significantly wider area of capsaicin-related neurogenic flare among NHW compared with NHB participants (F1, 52 = 8.33, P = 0.006), thermography results showed no differences between groups in neurogenic flares (F1, 52 = 0.01, P = 0.93). Further, although NHB individuals reported greater average pain during the capsaicin procedures compared with NHW individuals (NHB = 58.57 [3.67], NHW = 46.46 [3.81]; F2, 51 = 5.19, P = 0.03), the groups did not differ in secondary hyperalgesia (F2, 51 = 0.03, P = 0.86), and ethnicity did not moderate the association between neurogenic flare and secondary hyperalgesia (F3, 50 = 0.24, P = 0.87).

CONCLUSIONS

Findings cautiously support the use of infrared thermography over subjective experimenter report when measuring neurogenic inflammation in diverse samples. However, infrared thermography should not be used as a diagnostic tool for pain, given the lack of association between these factors. Future research is warranted to replicate these findings in a larger and more diverse sample to determine accurate neurogenic inflammation measures across other ethnic minority populations.

Patient phenotyping in clinical trials of chronic pain treatments: IMMPACT recommendations

There is tremendous interpatient variability in the response to analgesic therapy (even for efficacious treatments), which can be the source of great frustration in clinical practice. This has led to calls for "precision medicine" or personalized pain therapeutics (ie, empirically based algorithms that determine the optimal treatments, or treatment combinations, for individual patients) that would presumably improve both the clinical care of patients with pain and the success rates for putative analgesic drugs in phase 2 and 3 clinical trials. However, before implementing this approach, the characteristics of individual patients or subgroups of patients that increase or decrease the response to a specific treatment need to be identified. The challenge is to identify the measurable phenotypic characteristics of patients that are most predictive of individual variation in analgesic treatment outcomes, and the measurement tools that are best suited to evaluate these characteristics. In this article, we present evidence on the most promising of these phenotypic characteristics for use in future research, including psychosocial factors, symptom characteristics, sleep patterns, responses to noxious stimulation, endogenous pain-modulatory processes, and response to pharmacologic challenge. We provide evidence-based recommendations for core phenotyping domains and recommend measures of each domain.

Structural and functional characterization of nerve fibres in polyneuropathy and healthy subjects

OBJECTIVES

Quantification of intraepidermal nerve fibre density (IENFD) is an important small fibre measure in distal symmetric polyneuropathies (DSP), but quantitative evaluation of additional structural and functional factors may help in elucidating the underlying mechanisms, and in improving the diagnostic accuracy in DSP. The literature reports a weak or moderate relationship between IENFD and spontaneous and evoked pain in neuropathies, but the relationship between functional and structural small fibre parameters in patients with DSP is unclear. The objectives of the current study, therefore, were to determine morphological and functional parameters related to small nerve fibres in subjects with distal symmetric polyneuropathy (DSP) and healthy controls, and to characterize the interplay among these parameters in these two groups.

MATERIALS AND METHODS

17 patients with painful DSP (≥4 on 0-10 numerical rating scale) and with symptoms and signs of small fibre abnormality (with or without large fibre involvement) and 19 healthy control subjects underwent comprehensive functional and structural small fibre assessments that included quantitative sensory testing, response to 30min topical application of 10% capsaicin and analysis of skin biopsy samples taken from the distal leg (IENFD, epidermal and dermal nerve fibre length densities (eNFLD, dNFLD) using global spatial sampling and axonal swelling ratios (swellings/IENFD and swellings/NFLD)).

RESULTS

DSP patients had reduced sensitivity to cold (median -11.07°C vs. -2.60, P≤0.001) and heat (median 46.7 vs. 37.70, P≤0.001), diminished neurovascular (median 184 vs. 278 mean flux on laser Doppler, P=0.0003) and pain response to topical capsaicin (median 10 vs. 35 on 0-100 VAS, P=0.0002), and lower IENFD, eNFLD and dNFLD values combined with increased swelling ratios (all P<0.001) compared to healthy controls. The correlation between structural and functional parameters was poor in DSP patients, compared with healthy controls. In healthy controls eNFLD and dNFLD, IENFD and eNFLD, IENFD and dNFLD all correlated well with each other (r=0.81; P≤0.001, r=0.58; P=0.009, r=0.60; P=0.007, respectively). In DSP, on the other hand, only eNFLD and dNFLD showed significant correlation (r=0.53, P=0.03). A diagnostic approach of combined IENFD and eNFLD utilization increased DSP diagnostic sensitivity from 82.0% to 100% and specificity from 84.0% to 89.5%.

CONCLUSIONS

This study presents a rigorous comparison between functional and morphological parameters, including parameters such as eNFDL and dNFLD that have not been previously evaluated in this context. The correlation pattern between functional and structural small fibre parameters is different in patients with DSP when compared to healthy controls. The findings suggest a more direct relationship between structure and function of nerve fibres in healthy controls compared to DSP. Furthermore, the findings suggest that combining IENFD with measurement of NFLD improves the diagnostic sensitivity and specificity of DSP.

IMPLICATIONS

Combining small fibre parameters may improve the diagnostic accuracy of DSP.