Small nerve fiber selectivity of laser and intraepidermal electrical stimulation: A comparative study between glabrous and hairy skin

Eliciting the rubber hand illusion by the activation of nociceptive C and Aδ fibers

ABSTRACT

The coherent perceptual experience of one's own body depends on the processing and integration of signals from multiple sensory modalities, including vision, touch, and proprioception. Although nociception provides critical information about damage to the tissues of one's body, little is known about how nociception contributes to own-body perception. A classic experimental approach to investigate the perceptual and neural mechanisms involved in the multisensory experience of one's own body is the rubber hand illusion (RHI). During the RHI, people experience a rubber hand as part of their own body (sense of body ownership) caused by synchronized stroking of the rubber hand in the participant's view and the hidden participant's real hand. We examined whether the RHI can be elicited by visual and "pure" nociceptive stimulation, ie, without tactile costimulation, and if so, whether it follows the basic perceptual rules of the illusion. In 6 separate experiments involving a total of 180 healthy participants, we used a Nd:YAP laser stimulator to specifically target C and Aδ fibers in the skin and compared the illusion condition (congruent visuonociceptive stimulation) to control conditions of incongruent visuonociceptive, incongruent visuoproprioceptive, and no nociceptive stimulation. The illusion was quantified through direct (questionnaire) and indirect (proprioceptive drift) behavioral measures. We found that a nociceptive rubber hand illusion (N-RHI) could be elicited and that depended on the spatiotemporal congruence of visuonociceptive signals, consistent with basic principles of multisensory integration. Our results suggest that nociceptive information shapes multisensory bodily awareness and contributes to the sense of body ownership.

Characterizing Acute-Onset Small Fiber Neuropathy

BACKGROUND AND OBJECTIVES

Immune-mediated small fiber neuropathy (SFN) is increasingly recognized. Acute-onset SFN (AOSFN) remains poorly described. Herein, we report a series of AOSFN cases in which immune origins are debatable.

METHODS

We included consecutive patients with probable or definite AOSFN. Diagnosis of SFN was based on the NEURODIAB criteria. Acute onset was considered when the maximum intensity and extension of both symptoms and signs were reached within 28 days. We performed the following investigations: clinical examination, neurophysiologic assessment encompassing a nerve conduction study to rule out large fiber neuropathy, laser-evoked potentials (LEPs), warm detection thresholds (WDTs), electrochemical skin conductance (ESC), epidermal nerve fiber density (ENF), and patient serum reactivity against mouse sciatic nerve teased fibers, mouse dorsal root ganglion (DRG) sections, and cultured DRG. The serum reactivity of healthy subjects (n = 10) and diseased controls (n = 12) was also analyzed. Data on baseline characteristics, biological investigations, and disease course were collected.

RESULTS

Twenty patients presenting AOSFN were identified (60% women; median age: 44.2 years [interquartile range: 35.7-56.2]). SFN was definite in 18 patients (90%) and probable in 2 patients. A precipitating event was present in 16 patients (80%). The median duration of the progression phase was 14 days [5-28]. Pain was present in 17 patients (85%). Twelve patients (60%) reported autonomic involvement. The clinical pattern was predominantly non-length-dependent (85%). Diagnosis was confirmed by abnormal LEPs (60%), ENF (55%), WDT (39%), or ESC (31%). CSF analysis was normal in 5 of 5 patients. Antifibroblast growth factor 3 antibodies were positive in 4 of 18 patients (22%) and anticontactin-associated protein-2 antibodies in one patient. In vitro studies showed IgG immunoreactivity against nerve tissue in 14 patients (70%), but not in healthy subjects or diseased controls. Patient serum antibodies bound to unmyelinated fibers, Schwann cells, juxtaparanodes, paranodes, or DRG. Patients' condition improved after a short course of oral corticosteroids (3/3). Thirteen patients (65%) showed partial or complete recovery. Others displayed relapses or a chronic course.

DISCUSSION

AOSFN primarily presents as an acute, non-length-dependent, symmetric painful neuropathy with a variable disease course. An immune-mediated origin has been suggested based on in vitro immunohistochemical studies.

Influence of skin type and laser wavelength on laser-evoked potentials

BACKGROUND

Infrared laser stimulation is a valuable tool in pain research, its primary application being the recording of laser-evoked brain potentials (LEPs). Different types of laser stimulators, varying in their skin penetrance, are likely to have a large influence on the LEPs, when stimulating different skin types. The aim of this study was to investigate how LEPs depend on laser type and skin location.

METHODS

Two different laser stimulators (CO2 and Nd:YAP) were used to compare LEPs in healthy subjects. Stimuli were delivered to the hand dorsum and palm to investigate the effects of skin type on the evoked responses. Stimulus-evoked brain responses were recorded using EEG and perceived intensity ratings were recorded. Computational modelling was used to investigate the observed differences.

RESULTS

LEPs evoked by stimulation of the hairy skin were similar between CO2 and Nd:YAP stimulation. In contrast, LEPs elicited from the palm were markedly different and barely present for CO2 stimulation. There was a significant interaction between laser type and skin type (RM-ANOVA, p < 0.05) likely due to smaller CO2 LEPs in the palm. CO2 stimuli to the palm also elicited significantly lower perceived intensities. The computational model showed that the observed differences were explainable by the laser absorption characteristics and skin thickness affecting the temperature profile at the dermo-epidermal junction (DEJ).

CONCLUSIONS

This study shows that LEP elicitation depends on the combination of laser penetrance and skin type. Low penetrance stimuli, from a CO2 laser, elicited significantly lower LEPs and perceived intensities in the palm.

SIGNIFICANCE

This study showed that the elicitation of laser-evoked potentials in healthy humans greatly depends on the combination of laser stimulator type and skin type. It was shown that high penetrance laser stimuli are capable of eliciting responses in both hairy and glabrous skin, whereas low penetrance stimuli barely elicited responses from the glabrous skin. Computational modelling was used to demonstrate that the results could be fully explained by the combination of laser type and skin thickness.

Comparison of brain functional response to mechanical prickling stimuli to the glabrous and hairy skin

BACKGROUND

A kind of prickle sensation, which is a composite feeling of pain and itch, can be evoked by mechanical stimulation of fiber ends from fabric surface against to human hairy skin, rather than glabrous skin. Now, a functional magnetic resonance imaging (fMRI) study was conducted to investigate the cognitive differences in the brain for mechanical prickling stimuli to the two types of skin.

MATERIALS AND METHODS

A nylon filament with the diameter of 205 μm and the length of 8 mm was used to deliver mechanical prickling stimuli respectively to two skin sites, fingertip (glabrous skin) and volar forearm (hairy skin), of eight healthy male subjects. Simultaneously, the technology of fMRI was adopted to acquire BOLD (Blood Oxygen Level-Dependent) signals of brain functional response of the subjects.

RESULTS

Somatosensory areas, emotional areas, and the posterior parietal cortex (especially the precuneus) are important brain regions that distinguish between the two conditions. The representation of mechanical prickling stimulation to glabrous skin in the brain favors much more the tactile information of the stimulation and contains no itch, while the key brain area, precuneus, involved in itch was activated by the same mechanical prickling stimulation to hairy skin, and brain response for the condition of hairy skin contains more emotional information, which plays an important role in pain processing.

CONCLUSION

Therefore, it can be inferred that a kind of stronger prickle sensation, which contains both pain and itch, was evoked by mechanical stimulation to hairy skin than glabrous skin.

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.

Assessment of C Fibers Evoked Potentials in Healthy Subjects by Nd : YAP Laser

Introduction

Although laser stimuli activate both Ad- and C-fibres, the corresponding laser evoked potentials (LEPs) remain restricted to the Ad-fibers input, while the C-fibers related potential is hardly detectable.

Aims

To evaluate multichannel ultralate LEPs (U-LEPs) by using Nd : Yap laser pulses in healthy volunteers to stimulation of face and lower and upper limbs, in order to estimate the reliability of C-LEPs elicited from both trigeminal and somatic sites.

Methods

Twenty healthy volunteers participated in two stimulation sessions to record Aδ-LEPs and C-LEPs. We used a Nd : YAP Laser and 62 EEG recording electrodes. Stimuli parameters were set to activate either small myelinated (Aδ), eliciting purely warmth sensations, or unmyelinated (C) afferents, and eliciting pinprick sensations.

Results

At the trigeminal level, we obtained a negative-positive complex in a time interval compatible with the C fibers activation. In the somatic districts, the averaged responses consisted of an earlier negative-positive complex, followed by a later one. Single trials analysis of U-LEPs showed a maximal positive peak in a time interval in the range of C fibers. Topographical analysis of U-LEPs resembled that of LEPs. All subjects exhibited readable U-LEPs in at least 2 stimulated sites. Discussion. A purely warmth sensation seems to correspond to Aδ and C-fibers coactivation, at least in the somatic districts. While the related cortical waves seem hardly readable, their total absence could be a sign of systemic involvement of warm related C-fibers in specific clinical conditions.

Intra-epidermal evoked potentials: A promising tool for spinal disorders?

OBJECTIVES

To test the robustness and signal-to-noise ratio of pain-related evoked potentials following intra-epidermal electrical stimulation (IES) compared to contact heat stimulation in healthy controls, and to explore the feasibility and potential added value of IES in the diagnosis of spinal disorders.

METHODS

Pain-related evoked potentials induced by IES (custom-made, non-invasive, concentric triple pin electrode with steel pins protruding 1 mm from the anode, triangularly separated by 7-10 mm respectively) and contact heat stimulation were compared in 30 healthy subjects. Stimuli were applied to four different body sites. Two IES intensities, i.e., high (individually adapted to contact heat painfulness) and low (1.5 times pain threshold), were used. Additionally, a 40-year-old patient with unilateral dissociated sensory loss due to a multi-segmental syringohydromyelia was assessed comparing IES and contact heat stimulation.

RESULTS

Both IES and contact heat stimulation led to robust pain-related evoked potentials recorded in all healthy subjects. Low intensity IES evoked potentials (14.1-38.0 µV) had similar amplitudes as contact heat evoked potentials (11.8-32.3 µV), while pain ratings on the numeric rating scale were lower for IES (0.8-2.5, compared to 1.5-3.9 for contact heat stimulation). High intensity IES led to evoked potentials with higher signal-to-noise ratio than low intensity IES and contact heat stimulation. The patient case showed impaired pain-related evoked potentials in segments with hypoalgesia for both IES modes. IES evoked potentials were preserved, with delayed latencies, while contact heat evoked potentials were abolished.

CONCLUSION

IES evoked robust pain-related cortical potentials, while being less painful in healthy controls. The improved signal-to-noise ratio supports the use of IES for objective segmental testing of nociceptive processing. This was highlighted in a spinal syndrome case, where IES as well as contact heat stimulation reliably detected impaired segmental nociception.