Measurement of small fibre pain threshold values for the early detection of diabetic polyneuropathy

Tryptophan metabolism and small fibre neuropathy: a correlation study

Small nerve fibres located in the epidermis sense pain. Dysfunction of these fibres decreases the pain threshold known as small fibre neuropathy. Diabetes mellitus is accompanied by metabolic changes other than glucose, synergistically eliciting small fibre neuropathy. These findings suggest that various metabolic changes may be involved in small fibre neuropathy. Herein, we explored the correlation between pain sensation and changes in plasma metabolites in healthy Japanese subjects. The pain threshold evaluated from the intraepidermal electrical stimulation was used to quantify pain sensation in a total of 1021 individuals in the 2017 Iwaki Health Promotion Project. Participants with a pain threshold evaluated from the intraepidermal electrical stimulation index <0.20 mA were categorized into the pain threshold evaluated from the intraepidermal electrical stimulation index-low group (n = 751); otherwise, they were categorized into the pain threshold evaluated from the intraepidermal electrical stimulation index-high group (n = 270). Metabolome analysis of plasma was conducted using capillary electrophoresis time-of-flight mass spectrometry. The metabolite set enrichment analysis revealed that the metabolism of tryptophan was significantly correlated with the pain threshold evaluated from the intraepidermal electrical stimulation index in all participants (P < 0.05). The normalized level of tryptophan was significantly decreased in participants with a high pain threshold evaluated from the intraepidermal electrical stimulation index. In addition to univariate linear regression analyses, the correlation between tryptophan concentration and the pain threshold evaluated from the intraepidermal electrical stimulation index remained significant after adjustment for multiple factors (β = -0.07615, P < 0.05). These findings indicate that specific metabolic changes are involved in the deterioration of pain thresholds. Here, we show that abnormal tryptophan metabolism is significantly correlated with an elevated pain threshold evaluated from the intraepidermal electrical stimulation index in the Japanese population. This correlation provides insight into the pathology and clinical application of small fibre neuropathy.

Electrical promontory stimulation test using a portable peripheral nerve stimulator with an ear canal electrode

OBJECTIVE

An intact cochlear nerve is necessary for successful cochlear implantation (CI). Although the promontory stimulation test (PST) using a promontory stimulator (PS) and a transtympanic needle electrode is invasive, it is still commonly used to verify cochlear nerve function. PSs are currently unavailable because they are no longer manufactured; however, considering that PST continues to be beneficial in certain situations, alternative equipment is needed. The PNS-7000® (PNS) was developed as a neurologic instrument for stimulating the peripheral nerves. This study investigated the usefulness of the ear canal stimulation test (ECST) using PNS with a silver ball ear canal electrode, which is a new noninvasive alternative technique to the PST.

METHODS

ECST was performed from November 2013 to December 2018 using PS and PNS for patients with severe to profound sensorineural hearing loss. The electrical threshold, most comfortable loudness level, uncomfortable loudness level, dynamic range, and gap detection were measured in the ECST. The results of these measured PNS items were compared with PS.

RESULTS

ECST was performed in 61 ears of 35 patients (age, 59.9 ± 20.1 years) using PS and PNS. The sound sensation was elicited in 51 (83.6%) and 52 (85.2%) ears with PS and PNS, respectively. All items excluding GAP were measured in 46 (75%) and 43 (70%) ears at 50 and 100 Hz, respectively. GAP was measured in 33 ears by the ascending and descending methods using PS and PNS. Spearman's rank-order correlation coefficient revealed a significant positive linear correlation between the PS and PNS results in all measurements. No significant difference was found between the PS and PNS thresholds in all measured items.

CONCLUSIONS

PNS is a useful instrument for performing ECST as a new alternative to PS. ECST using a silver ball electrode is a less invasive and easier test than PST.

Effects of anesthetics on nociceptive sensory evoked potentials by intraepidermal noxious electrical stimulation of A-δ fibers

PURPOSE

Generation of nociceptive sensory evoked potentials (NEPs) by selective stimulation of nociceptive intraepidermal nerve fibers is a simple technique which could be used as intraoperative nociception monitor. We evaluated the effects of remifentanil, propofol and sevoflurane on NEPs by this technique.

METHODS

Patients undergoing general anesthesia were assigned to groups in two studies. A-δ fiber selective NEPs were recorded. Study 1: NEPs were recorded at control, under anesthetics administration: remifentanil at an effect-site concentration (Ce) of 1.0 ng/mL (n = 10), propofol at Ce of 0.5 µg/mL (n = 10), or sevoflurane at 0.2 minimum alveolar concentration (MAC) (n = 10), and recovery from the anesthetics. Study 2: NEPs were recorded at control and under administration of higher dose anesthetics: propofol at Ce of 0.5 and 1.0 µg/mL (n = 10) or sevoflurane at 0.2 and 0.5 MAC (n = 10). A P-value < 0.016 was considered statistically significant in multiple analyses.

RESULTS

Study 1: Remifentanil at Ce of 1.0 ng/mL significantly suppressed the amplitude of NEPs (mean amplitude (standard deviation) of control vs. remifentanil administration: 16.8 µV (3.8) vs. 10.1 µV (2.5), P < 0.001). Propofol and sevoflurane did not suppress the amplitude significantly. Study 2: Propofol at Ce of 0.5 and 1.0 µg/mL and sevoflurane at 0.2 and 0.5 MAC did not suppress the amplitude significantly.

CONCLUSION

The amplitude of A-δ fiber selective NEPs was suppressed by remifentanil but not propofol or sevoflurane. NEPs with intraepidermal electrical stimulation can assess the analgesic effect of anesthetics.

CLINICAL TRIAL NUMBER

UMIN000038214 REGISTRY URL: https://center6.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000043328.

Corneal characteristics of Mongolian population with type 2 diabetic peripheral neuropathy in inner Mongolia, China: an assessment using corneal confocal microscopy

OBJECTIVE

To quantify corneal nerve fiber parameters in a Mongolian population with diabetic peripheral neuropathy (DPN) by corneal confocal microscopy.

METHODS

This study conducted a comprehensive evaluation of 114 participants from Hulunbuir between January 2020 and December 2021. The participants included healthy controls, Mongolian and Han patients with type 2 diabetes mellitus. Demographic, medical, and laboratory data were collected, and neuropathy was evaluated by confocal corneal microscopy. And compare various parameters between Han and Mongolian were performed using SPSS software.

RESULTS

The average waist circumference of Mongolian diabetic patients was larger than that of Han diabetic patients (P < 0.05). The mean HbA1c of Mongolian was 9.30 (8.15, 10.30) %, and that of Han was 8.30 (7.20, 9.40) % (P = 0.023). The average values of Corneal Nerve Fiber Density (CNFD), Corneal Nerve Fiber Length (CNFL) and corneal nerve branch density (CNBD) in Mongolian diabetic patients were significantly lower than those in Han diabetic patients (P < 0.05). The correlation coefficient between CNFL and age was - 0.368. ROC results show that CNBD has a certain diagnostic value for DPN in Mongolian patients with type 2 diabetes and the optimal cut-off point value is 24.99(no./mm2), the sensitivity is 80.0%, and the specificity is 77.8%.

CONCLUSION

The corneal confocal microscopy could possibly represent a promising adjuvant technique for the early diagnosis and assessment of PDN in Mongolian T2DM patients.

Selective stimulation of nociceptive small fibers during intraepidermal electrical stimulation: Experiment and computational analysis

Electrical stimulation of skin nociceptors is gaining attention in pain research and peripheral neuropathy diagnosis. However, the optimal parameters for selective stimulation are still difficult to determine because they require simultaneous characterization of the electrical response of small fibers (Aδ- and C-fibers). In this study, we measured the in vivo electrical threshold responses of small fibers to train-pulse stimulation in humans for the first time. We also examined selective stimulation via a computational model, which combines electrical analysis, and terminal fiber and synaptic models, including the first cutaneous pain C-fiber model. Selective stimulation of small fibers is performed by injecting train-pulse stimulation via coaxial electrodes with an intraepidermal needle tip at varying pulse counts and frequencies. The activation Aδ- or C-fibers was discriminated from the differences in reaction time. Aδ-fiber elicited a pinpricking sensation with a mean reaction time of 0.522 s, and C-fiber elicited a tingling sensation or slight burning itch with a mean reaction time of 1.243 s. The implemented multiscale electrical model investigates synaptic effects while considering stimulation waveform characteristics. Experimental results showed that perception thresholds decreased with the number of consecutive pulses and frequency up to convergence (five pulses or 70 Hz) during the selective stimulation of Aδ- and C-fibers. Considering the synaptic properties, the optimal stimulus conditions for selective stimulation of Aδ- vs. C-fibers were train of at least four pulses and a frequency of 40-70 Hz at a pulse width of 1 ms. The experimental results were modeled with high fidelity by incorporating temporal synaptic effects into the computational model. Numerical analysis revealed terminal axon thickness to be the most important biophysical factor affecting threshold variability. The computational model can be used to estimate perception thresholds while understanding the mechanisms underlying the selective stimulation of small fibers. The parameters derived here are important in exploring selective stimulation between Aδ- and C-fibers for diagnosing neuropathies.

Comparison of the Response to Pulpal Sensibility Tests in Well-Controlled and Uncontrolled Type II Diabetes Mellitus Patients: A Cross-Sectional Study

Introduction

Diabetes mellitus is a metabolic disorder in which impairment of sensory fibers would be anticipated. The present study would assess the dental pulp response to cold and EPT sensibility tests in patients with type 2 diabetes, both well-controlled and uncontrolled.

Materials and Methods

One hundred two maxillary central incisors, belonging to participants aged 35–67 years, were included in this survey. At last, 51 diabetic patients were allocated to each group of well-controlled (HbA1C < 7) and uncontrolled (HbA1C ≥ 7). Electric and cold pulpal vitality tests were conducted for all teeth. Statistical analysis was performed with Student's t-test, the chi-square test, and the multiple linear regression model. A P value less than 0.05 was considered significant.

Results

Based on the results of this study, the mean value of response to EPT was 4.51 ± 2.06 and 4.41 ± 1.85 in well-controlled and uncontrolled diabetic patients, respectively. Also, the pulpal responses to cold and EPT tests had no significant differences between the two groups (P > 0.05).

Conclusion

Tooth responses to the cold and EPT sensibility tests were not different in well-controlled and uncontrolled diabetic patients. Despite no statistically significant correlation, male diabetic patients in the uncontrolled group showed a lower sensory response threshold to EPT compared to a well-controlled group.

The diversity and abundance of gut microbiota are associated with the pain sensation threshold in the Japanese population

Small fibre neuropathy (SFN) is an initial pathology of diabetic polyneuropathy (DPN). Serum lipopolysaccharide binding protein levels are positively correlated with the pain threshold in the foot, suggesting that the abundance of gut Gram-negative bacilli, which are a source of lipopolysaccharides, may be involved in the development of DPN. Furthermore, the abundance of the gut and oral microbiota is assumed to be involved in the pathogenesis of diabetes. Nevertheless, the association between SFN and the microbiota has not been clarified. A total of 1056 individuals were recruited in the 2018 Iwaki Health Promotion Project. Pain sensation was evaluated based on the pain threshold from intraepidermal electrical stimulation (PINT). Patients with PINT scores <0.15 mA were categorized into the low-PINT group (n = 718); otherwise, they were categorized into the high-PINT group (n = 283). Furthermore, each group was divided into the subjects with or without glucose tolerance based on HbA1c levels, fasting blood glucose levels and diabetic history. Principal coordinate analysis and α- and β-diversity of the microbiota were evaluated. The correlation between clinical and microbiota data was examined. Oral microbiota diversity showed no structural differences according to PINT scores, whereas principal coordinate analysis and α- and β-diversity revealed significant structural differences in gut microbiota (p < 0.01, p < 0.05 and p < 0.05, respectively), even after the participants with glucose intolerance were excluded (p < 0.01, p < 0.05 and p < 0.05, respectively). The relative abundance of the genus Bacteroides was significantly lower in high-PINT participants compared with low-PINT participants (10 ± 6.7% vs. 11.3 ± 7.0%, p < 0.01), even after the exclusion of subjects with diabetes and impaired fasting glucose (10.0 ± 6.5% vs. 11.2 ± 6.9%, p < 0.05). In univariate linear regression analyses, PINT was significantly correlated with metabolic syndrome parameters, eGFR, uric acid level and the abundance of Bacteroides. The correlation between Bacteroides and PINT scores remained significant after adjustment for multiple factors (β = -0.07181, p < 0.05). Changes of bacterial diversity and a low abundance of gut Bacteroides were correlated with elevated PINT scores in the Japanese population. This correlation may represent a new therapeutic option for SFN.

Evaluation of the Response to Pulpal Sensibility Tests (Cold, EPT) in Anemic and Healthy Women

Introduction

The prevalence of anemia has been reported high in the female population in Iran. Anemia can be asymptomatic or can present in a variety of symptoms, especially when serum Hb values decrease. The present study would assess dental pulp response to cold and EPT sensibility tests in anemic and healthy women.

Materials and Methods

One hundred twenty maxillary central incisors belonging to participants aged 18–58 years were included in this survey. 60 patients had anemia (Hb ≤ 12.5) with/without medication (30 in each group) and 60 women as controls had no anemia (Hb > 12.5) with/without medication (30 in each group). Electric and cold pulpal sensibility tests were performed for all teeth. Statistical analysis was performed with t-student, Chi-square test, and two-way ANOVA. The significance level was set as p < 0.05.

Results

According to the results of this study, the mean value of hemoglobin in anemic and healthy women was 11.5 and 14.08 g/dl, respectively. The mean value of response to EPT in anemic women with and without medication was 3.21 and 3.14, respectively. The mean value of response to EPT in healthy women with and without medication was 3.81 and 3.58 g/dl, respectively. The mean value of time delay response to cold test was 3.03 and 2.82 s in anemic patients with/without medication. Also, the mean value of time delay response to cold test was 2.80 and 2.93 s in healthy women with/without medication. The pulpal responses to EPT tests had significant differences between anemic and healthy women (p=0.043). There were no significant differences between anemic and healthy women considering time delay response to the cold test (p=0.077).

Conclusion

Based on the results of the present study, tooth response to the EPT  sensibility test may alter in anemic patients despite medication. The evidence from this preliminary study suggests that peripheral neuropathy in anemic patients could affect pulpal sensibility tests.

Novel surface electrode design for preferential activation of cutaneous nociceptors

Objective Small area electrodes enable preferential activation of nociceptive fibers. It is debated, however, whether co-activation of large fibers still occurs for the existing electrode designs. Moreover, existing electrodes are limited to low stimulation intensities, for which behavioral and physiological responses may be considered less reliable. A recent optimization study showed that there is a potential for improving electrode performance and increase the range of possible stimulation intensities. Based on those results, the present study introduces and tests a novel planar concentric array electrode design for small fiber activation in healthy volunteers. Approach Volunteers received electrical stimulation with the planar concentric array electrode and a regular patch electrode. Perception thresholds were estimated at the beginning and the end of the experiment. Evoked cortical potentials were recorded in blocks of 30 stimuli. For the patch, stimulation intensity was set to two times perception threshold (PT), while three intensities, 2, 5, and 10 times PT, were applied with the planar concentric array electrode. Sensation quality, numerical-rating scores, and reaction times were obtained for each PT estimation and during each block of evoked potential recordings. Main results Stimulation with the patch electrode was characterized as dull, while stimulation with the planar concentric array electrode was characterized as sharp, with increased sharpness for increasing stimulus intensity. Likewise, NRS scores were higher for the planar concentric array electrode compared to the patch and increased with increasing stimulation intensity. Reaction times and ERP latencies were longer for the planar concentric array electrode compared to the patch. Significance The presented novel planar concentric array electrode is a small, non-invasive, and single-use electrode that has the potential to investigate small fiber neuropathy and pain mechanisms, as it is small fiber preferential for a wide range of stimulation intensities.

The insulin-degrading enzyme as a link between insulin and neuropeptides metabolism

We have applied a recently developed HPLC-MS enzymatic assay to investigate the cryptic peptides generated by the action of the insulin-degrading enzyme (IDE) on some neuropeptides (NPs) involved in the development of tolerance and dependence to opioids. Particularly, the tested NPs are generated from the NPFF precursor (pro-NPFF (A)): NPFF (FLFQPQRF) and NPAF (AGEGLSSPFWSLAAPQRF). The results show that IDE is able to cleave NPFF and NPAF, generating specific cryptic peptides. As IDE is also responsible for the processing of many other peptides in the brain (amyloid beta protein among the others), we have also performed competitive degradation assays using mixtures of insulin and the above mentioned NPs. Data show that insulin is able to slow down the degradation of both NPs tested, whereas, surprisingly, NPAF is able to accelerate insulin degradation, hinting IDE as the possible link responsible of the mutual influence between insulin and NPs metabolism.

Exploring Nociceptive Detection Thresholds Combined with Evoked Potentials in Patients with Diabetes Mellitus

There is a lack of diagnostic tools that can objectively measure small fiber neuropathy (SFN) in patients with diabetes mellitus (DM). Recently, nociceptive nerve function was observed by nociceptive detection thresholds (NDTs) and brain evoked potentials (EPs) during intra-epidermal electrical stimulation (IES) targeting Aδ-fibers. In this proof of principle, we studied whether it is possible to measure NDTs combined with EPs in DM patients with and without neuropathic pain. Furthermore, we explored the sensitivity of NDTs and EPs for polyneuropathy in these patients. Five DM patients diagnosed with painful neuropathy (DMp), five DM patients without painful neuropathy (DM), and five healthy controls (HC) were analyzed. These preliminary results showed that we can accurately measure NDTs and EPs in patients with diabetes. Strikingly, increased NDTs were found in DM and DMp compared to HC, of which the DMp showed the largest NDTs. This suggests that NDTs during IES could be a powerful biomarker for monitoring peripheral dysfunctions. Current EEG data of patients did not show any significant differences. The population needs to be enlarged before we can investigate the sensitivity of these NDTs and EPs to diabetic polyneuropathy and associated changes in nociceptive processing in more detail.Clinical Relevance- This proof of principle in a small group of patients with diabetes mellitus potentially treats the observation of the loss of nociceptive function occurring with small fiber neuropathy. That helps the development of a diagnostic measure to monitor future (early-stage) nociceptive dysfunctions in a clinical environment.

Electrical Characterisation of Aδ-Fibres Based on Human in vivo Electrostimulation Threshold

Electrical stimulation of small fibres is gaining attention in the diagnosis of peripheral neuropathies, such as diabetes mellitus, and pain research. However, it is still challenging to characterise the electrical characteristics of axons in small fibres (Aδ and C fibres). In particular, in vitro measurement for human Aδ-fibre is difficult due to the presence of myelin and ethical reason. In this study, we investigate the in vivo electrical characteristics of the human Aδ-fibre to derive strength-duration (S-D) curves from the measurement. The Aδ-fibres are stimulated using coaxial planar electrodes with intraepidermal needle tip. For human volunteer experiments, the S-D curve of Aδ-fibre is obtained in terms of injected electrical current. With the computational analysis, the standard deviation of the S-D curve is mostly attributed to the thickness of the stratum corneum and depth of the needle tip, in addition to the fibre thickness. Then, we derive electrical parameters of the axon in the Aδ-fibre based on a conventional fibre model. The parameters derived here would be important in exploring the optimal stimulation condition of Aδ-fibres.

Increased Oxidative Stress Underlies Abnormal Pain Threshold in a Normoglycemic Japanese Population

Normal-high HbA1c levels are a risk factor for attenuated pain sensation in normoglycemic subjects. It is unclear, however, what mechanisms underlie the pathogenesis of attenuated pain sensation in such a population. We, therefore, explored the relationship between oxidative stress (OS) and pain sensation in a rural Japanese population. A population-based study of 894 individuals (average age 53.8 ± 0.5 years) and 55 subjects with impaired fasting glucose (IFG) were enrolled in this study. Individuals with diabetes were excluded. Relationships between pain threshold induced by intraepidermal electrical stimulation (PINT) and clinico-hematological parameters associated with OS were evaluated. Univariate linear regression analyses revealed age, BMI, HbA1c, the OS biomarker urine 8-hydroxy-2'-deoxyguanosine (8-OHdG), systolic blood pressure, and decreased Achilles tendon reflex on the PINT scores. Adjustments for age, gender, and multiple clinical measures confirmed a positive correlation between PINT scores and urine 8-OHdG (β = 0.09, p < 0.01). Urine 8-OHdG correlated positively with higher HbA1c levels and age in the normoglycemic population. Unlike in the normoglycemic population, both inflammation and OS were correlated with elevated PINT scores in IFG subjects. OS may be a major contributing factor to elevated PINT scores in a healthy Japanese population.

Lipopolysaccharide-binding protein is a distinctive biomarker of abnormal pain threshold in the general Japanese population

INTRODUCTION

Small fiber neuropathy (SFN) is an early manifestation in diabetic polyneuropathy (DPN); however, the mechanisms are not fully understood. In diabetes, SFN is presumed to be common in individuals with overt DPN, enhancing activation of polyol pathway, oxidative stress, advanced glycation end products (AGEs), and inflammation. We explored the relationship between clinicohematological factors related to DPN and pain sensation in the Japanese population.

RESEARCH DESIGN AND METHODS

We conducted a population-based study, recruiting 1030 individuals (average age 54.4±0.5 years), in 2017, to participate in our Iwaki project. After initial screening by fasting blood glucose and glycohemoglobin A1c (HbA1c) measurements, the subjects were categorized into control (n=894), type 2 diabetes (n=81), and impaired fasting glucose (n=55) groups. Clinical data were gathered, and relationships between pain threshold from intraepidermal electrical stimulation (PINT) and DPN were examined by analysis of variance, post hoc test, and χ² tests to study correlations among and between groups of the clinical data and DPN.

RESULTS

Univariate linear regression analyses showed significant correlations between PINT and serum lipopolysaccharide-binding protein (LBP) level (ß=0.1025, p=0.001). Adjustments for the clinical measurements confirmed a positive correlation (ß=0.070, p=0.034). Logistic regression analysis revealed high LBP value (>6.7 mg/dL) as a significant risk factor toward abnormal PINT (≥0.35 mA). LBP significantly correlated with the high-sensitivity C reactive protein, inflammation marker, elevated similarly in both pre-diabetic and overt-diabetic groups, compared with controls, but it did not correlate with a decreased Achilles tendon reflex. In contrast, urine 8-hydroxy-2'-deoxyguanosine, oxidative stress marker, and pentosidine, AGEs, markedly increased in individuals with type 2 diabetes with high HbA1c.

CONCLUSIONS

Individuals with high LBP exhibited an elevated PINT in the Japanese population. Low level of inflammation evoked by metabolic endotoxemia is possibly implicated in the pathophysiology of SFN from pre-diabetic stage.

Pain thresholds are unaffected by age in a Japanese population

INTRODUCTION

Assessment of sensory impairment in diabetic patients by pain threshold test using intraepidermal electrical stimulation (IES) is a recently developed technique. However, there are no normative pain thresholds in healthy people.

METHODS

We examined pain, vibration, and pressure thresholds in 178 healthy subjects using IES, vibration perception testing (VPT), and Semmes-Weinstein monofilament testing (SWMT).

RESULTS

The mean values for each age group for pain threshold ranged from 0.07 to 0.12 mA. Pain thresholds were unaffected by age. As the age increased, VPT values decreased from 18.0 to 10.6 seconds and SWMT values increased from 21.4 to 45.3 g/mm² . There were no significant differences in pain threshold, VPT, and SWMT between men and women.

DISCUSSION

The pain threshold test appears to be useful for diabetic neuropathy screening because normative values are not affected by age.

IDE Degrades Nociceptin/Orphanin FQ through an Insulin Regulated Mechanism

Insulin-degrading enzyme (IDE) was applied to catalyze hydrolysis of Nociceptin/Orphanin 1-16 (OFQ/N) to show the involvement of the enzyme in degradation of neuropeptides engaged in pain transmission. Moreover, IDE degradative action towards insulin (Ins) was inhibited by the OFQ/N fragments, suggesting a possible regulatory mechanism in the central nervous system. It has been found that OFQ/N and Ins affect each other degradation by IDE, although in a different manner. Indeed, while the digestion of OFQ/N is significantly affected by the presence of Ins, the kinetic profile of the Ins hydrolysis is not affected by the presence of OFQ/N. However, the main hydrolytic fragments of OFQ/N produced by IDE exert inhibitory activity towards the IDE-mediated Ins degradation. Here, we present the results indicating that, besides Ins, IDE cleaves neuropeptides and their released fragments act as inhibitors of IDE activity toward Ins. Having in mind that IDE is present in the brain, which also contains Ins receptors, it cannot be excluded that this enzyme indirectly participates in neural communication of pain signals and that neuropeptides involved in pain transmission may contribute to the regulation of IDE activity. Finally, preliminary results on the metabolism of OFQ/N, carried out in the rat spinal cord homogenate in the presence of various inhibitors specific for different classes of proteases, show that OFQ/N proteolysis in rat spinal cord could be due, besides IDE, also to a cysteine protease not yet identified.

Clinical neurophysiology of pain

Clinical neurophysiologic investigation of pain pathways in humans is based on specific techniques and approaches, since conventional methods of nerve conduction studies and somatosensory evoked potentials do not explore these pathways. The proposed techniques use various types of painful stimuli (thermal, laser, mechanical, or electrical) and various types of assessments (measurement of sensory thresholds, study of nerve fiber excitability, or recording of electromyographic reflexes or cortical potentials). The two main tests used in clinical practice are quantitative sensory testing and pain-related evoked potentials (PREPs). In particular, PREPs offer the possibility of an objective assessment of nociceptive pathways. Three types of PREPs can be distinguished depending on the type of stimulation used to evoke pain: laser-evoked potentials, contact heat evoked potentials, and intraepidermal electrical stimulation evoked potentials (IEEPs). These three techniques investigate both small-diameter peripheral nociceptive afferents (mainly Aδ nerve fibers) and spinothalamic tracts without theoretically being able to differentiate the level of lesion in the case of abnormal results. In routine clinical practice, PREP recording is a reliable method of investigation for objectifying the existence of a peripheral or central lesion or loss of function concerning the nociceptive pathways, but not the existence of pain. Other methods, such as nerve fiber excitability studies using microneurography, more directly reflect the activities of nociceptive axons in response to provoked pain, but without detecting or quantifying the presence of spontaneous pain. These methods are more often used in research or experimental study design. Thus, it should be kept in mind that most of the results of neurophysiologic investigation performed in clinical practice assess small fiber or spinothalamic tract lesions rather than the neuronal mechanisms directly at the origin of pain and they do not provide objective quantification of pain.

Normal High HbA1c a Risk Factor for Abnormal Pain Threshold in the Japanese Population

Purpose: Small fiber dysfunction is common in subjects with diabetic polyneuropathy (DPN). It is unsettled, however, whether marginal glucose intolerance is implicated in the onset and progression of small fiber dysfunction. Herein, we explored the relationship between glycated hemoglobin levels (HbA1c) and pain sensation in the Japanese population. Methods: A population-based study of 894 individuals (352 men, 542 women; average age 53.8 ± 0.5 years) and 55 subjects with impaired fasting glucose (IFG) in the 2017 Iwaki project were enrolled in this study. Individuals with diabetes were excluded. Relationships between pain threshold for intraepidermal electrical stimulation (P-IES) and parameters associated with metabolic syndrome were examined. Results: P-IES was elevated with increasing of age in women but not in men. Average P-IES (mA) was increased in IFG subjects (n = 55, 0.20 ± 0.03) compared with normoglycemic/non-IFG individuals (n = 894, 0.15 ± 0.11) (p < 0.01). It was comparable between IFG and a group of normal high HbA1c (5.9-6.4%). Univariate linear regression analyses showed no influence of sex, triglyceride, or cholesterol on the value of P-IES. In contrast, there were significant correlations between P-IES and serum HbA1c level (ß = 0.120, p < 0.001) Adjustments for the multiple clinical measurements confirmed positive correlation of P-IES with HbA1c (ß = 0.077, p = 0.046). Conclusion: Individuals with normal high HbA1c exhibited an elevated P-IES in a healthy Japanese population which may be useful for the screening of subclinical DPN.

Impaired balance is related to the progression of diabetic complications in both young and older adults

AIMS

To investigate the balance ability in younger and older adults with diabetes and evaluate the associations between balance ability and microvascular complications.

METHODS

This cross-sectional observational study compared 162 participants and 177 controls with and without type 2 diabetes, respectively. Balance ability was assessed using two static (one-legged stance and postural sway area) and two dynamic (Timed Up and Go [TUG] and Functional Reach) tests. Diabetic microangiopathy was also evaluated.

RESULTS

Participants with diabetes, including both younger (<50years) and older (≥50years) participants, showed significantly worse balance ability in all four tests and were more likely to have a history of falls than the controls (all P<0.01). In all age groups, severe impairment of balance ability was associated with progression of diabetic microvascular complications. In all and older diabetic adults, a longer duration of diabetes (P=0.022) and higher TUG test score (P=0.004), and female sex (P=0.01) and higher TUG score (P=0.001), respectively, were related to a history of falls. On the other hand, among younger diabetic adults, only a non-significant association with longer duration of diabetes (P=0.066) was observed.

CONCLUSIONS

Impaired balance ability correlates with microvascular diabetic complications. Accurate assessment of balance ability in adults with diabetes could predict the risk of falls, particularly benefitting people with diabetic complications.

Conventional deep pressure algometry is not suitable for clinical assessment of nociception in painless diabetic neuropathy

Background

In diabetic persons with painless neuropathic foot ulceration, foot skin was found to be insensate to noxious pinprick stimulation (stimulation area less than 0.05 mm²), while compression of deep subcutaneous foot tissues by Algometer II^® (stimulation area 1 cm²) could evoke a deep dull aching. To elucidate this discrepancy, the Algometer II stimulation technique was critically reviewed by varying probe sizes and anatomical sites in the same study population 3 years later.

Methods

Ten control subjects without neuropathy and 11 persons with painless diabetic neuropathy (PLDN, seven of whom with diabetic foot syndrome, i.e., past painless foot ulcer, or inactive Charcot arthropathy) were re-examined using Algometer II. Deep pressure pain perception threshold (DPPPT) was measured in random sequence with stimulation areas of 0.5 cm², 1 cm², and 2 cm² (separated by 5 min intervals), at the plantar forefoot, the instep, and the hindfoot of both legs.

Results

In the control and PLDN groups, median DPPPTs differed significantly between stimulation areas (highest with 0.5 cm², intermediate with 1 cm², lowest with 2 cm²; p<0.001), and varied moderately by anatomical site. Between-group differences were relatively small. Results of the 1 cm² assessments repeated 3 years apart were similar.

Conclusions

Algometer II readings represent spatial summation of low-threshold pressure-receptor rather than of high-threshold nociceptor stimulation and are, thus, unhelpful for assessing PLDN. Reproducibility of the measurements is good.