305 TEST-RETEST REPRODUCIBILITY OF CEREBRAL AND SUBJECTIVE RESPONSES TO PAINFUL AND NON-PAINFUL CONTACT-HEAT EVOKED POTENTIAL STIMULATION (CHEPS)

Linking Pain Sensation to the Autonomic Nervous System: The Role of the Anterior Cingulate and Periaqueductal Gray Resting-State Networks

There are bi-directional interactions between the autonomic nervous system (ANS) and pain. This is likely underpinned by a substantial overlap between brain areas of the central autonomic network and areas involved in pain processing and modulation. To date, however, relatively little is known about the neuronal substrates of the ANS-pain association. Here, we acquired resting state fMRI scans in 21 healthy subjects at rest and during tonic noxious cold stimulation. As indicators of autonomic function, we examined how heart rate variability (HRV) frequency measures were influenced by tonic noxious stimulation and how these variables related to participants’ pain perception and to brain functional connectivity in regions known to play a role in both ANS regulation and pain perception, namely the right dorsal anterior cingulate cortex (dACC) and periaqueductal gray (PAG). Our findings support a role of the cardiac ANS in brain connectivity during pain, linking functional connections of the dACC and PAG with measurements of low frequency (LF)-HRV. In particular, we identified a three-way relationship between the ANS, cortical brain networks known to underpin pain processing, and participants’ subjectively reported pain experiences. LF-HRV both at rest and during pain correlated with functional connectivity between the seed regions and other cortical areas including the right dorsolateral prefrontal cortex (dlPFC), left anterior insula (AI), and the precuneus. Our findings link cardiovascular autonomic parameters to brain activity changes involved in the elaboration of nociceptive information, thus beginning to elucidate underlying brain mechanisms associated with the reciprocal relationship between autonomic and pain-related systems.

Contact heat-evoked potential stimulation for the evaluation of small nerve fiber function

BACKGROUND

Small fiber peripheral neuropathy (SFN) is emerging as a common complication in diabetes. Currently there are few, not easily available methods of determining the integrity of small nerve fibers. This study was designed to determine the utility of a noninvasive technique, contact heat-evoked potential stimulation (CHEPS), on the identification of SFN and compare it with standardized measures of diabetic peripheral neuropathy (DPN).

SUBJECTS AND METHODS

We evaluated 31 healthy controls and 30 participants with type 2 diabetes and DPN using neurologic examination, nerve conduction studies (NCS), autonomic function tests, quantitative sensory tests (QSTs), and CHEPS. Contact heat was administered to the thenar eminence, volar and dorsal forearms, lower back, and distal lower limb. Evoked potentials were recorded from the skull vertex. Latencies and amplitudes were determined.

RESULTS

Intrapeak amplitude (IA) values were significantly reduced in the DPN group at the lower back (44.93±6.5 vs. 23.87±3.36 μV; P<0.01), lower leg (15.87±1.99 vs. 11.68±1.21 μV; P<0.05), and dorsal forearm (29.89±8.86 vs. 14.96±1.61 μV; P<0.05). Pooled data from both groups showed that IA values at different sites significantly correlated with clinical neurologic scores, NCS, QSTs, and autonomic function. Receiver operator characteristic curve analysis, used to evaluate the performance of CHEPS in detecting nerve dysfunction, was most significant for IA at the lower back (area under the curve, 0.778;±SE, 0.06; 95% confidence interval, 0.654-0.875; P<0.0001).

CONCLUSIONS

This study suggests that CHEPS is a novel, noninvasive technique able to detect impairment of small nerve fiber function from skin to cerebral cortex, providing an objective measure of C and Aδ nerve dysfunction.

Contact Heat Evoked Potentials Using Simultaneous Eeg And Fmri And Their Correlation With Evoked Pain

BACKGROUND

The Contact Heat Evoked Potential Stimulator (CHEPS) utilises rapidly delivered heat pulses with adjustable peak temperatures to stimulate the differential warm/heat thresholds of receptors expressed by Adelta and C fibres. The resulting evoked potentials can be recorded and measured, providing a useful clinical tool for the study of thermal and nociceptive pathways. Concurrent recording of contact heat evoked potentials using electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI) has not previously been reported with CHEPS. Developing simultaneous EEG and fMRI with CHEPS is highly desirable, as it provides an opportunity to exploit the high temporal resolution of EEG and the high spatial resolution of fMRI to study the reaction of the human brain to thermal and nociceptive stimuli.

METHODS

In this study we have recorded evoked potentials stimulated by 51° C contact heat pulses from CHEPS using EEG, under normal conditions (baseline), and during continuous and simultaneous acquisition of fMRI images in ten healthy volunteers, during two sessions. The pain evoked by CHEPS was recorded on a Visual Analogue Scale (VAS).

RESULTS

Analysis of EEG data revealed that the latencies and amplitudes of evoked potentials recorded during continuous fMRI did not differ significantly from baseline recordings. fMRI results were consistent with previous thermal pain studies, and showed Blood Oxygen Level Dependent (BOLD) changes in the insula, post-central gyrus, supplementary motor area (SMA), middle cingulate cortex and pre-central gyrus. There was a significant positive correlation between the evoked potential amplitude (EEG) and the psychophysical perception of pain on the VAS.

CONCLUSION

The results of this study demonstrate the feasibility of recording contact heat evoked potentials with EEG during continuous and simultaneous fMRI. The combined use of the two methods can lead to identification of distinct patterns of brain activity indicative of pain and pro-nociceptive sensitisation in healthy subjects and chronic pain patients. Further studies are required for the technique to progress as a useful tool in clinical trials of novel analgesics.

Use of the novel Contact Heat Evoked Potential Stimulator (CHEPS) for the assessment of small fibre neuropathy: correlations with skin flare responses and intra-epidermal nerve fibre counts

Background

The Contact Heat Evoked Potential Stimulator (CHEPS) rapidly stimulates cutaneous small nerve fibres, and resulting evoked potentials can be recorded from the scalp. We have studied patients with symptoms of sensory neuropathy and controls using CHEPS, and validated the findings using other objective measures of small nerve fibres i.e. the histamine-induced skin flare response and intra-epidermal fibres (IEF), and also quantitative sensory testing (QST), a subjective measure.

Methods

In patients with symptoms of sensory neuropathy (n = 41) and healthy controls (n = 9) we performed clinical examination, QST (monofilament, vibration and thermal perception thresholds), nerve conduction studies, histamine-induced skin flares and CHEPS. Skin punch biopsies were immunostained using standard ABC immunoperoxidase for the nerve marker PGP 9.5 or the heat and capsaicin receptor TRPV1. Immunoreactive IEF were counted per length of tissue section and epidermal thickness recorded.

Results

Amplitudes of Aδ evoked potentials (μV) following face, arm or leg stimulation were reduced in patients (e.g. for the leg: mean ± SEM – controls 11.7 ± 1.95, patients 3.63 ± 0.85, p = 0.0032). Patients showed reduced leg skin flare responses, which correlated with Aδ amplitudes (r_s = 0.40, p = 0.010). In patient leg skin biopsies, PGP 9.5- and TRPV1-immunoreactive IEF were reduced and correlated with Aδ amplitudes (PGP 9.5, r_s = 0.51, p = 0.0006; TRPV1, r_s = 0.48, p = 0.0012).

Conclusion

CHEPS appears a sensitive measure, with abnormalities observed in some symptomatic patients who did not have significant IEF loss and/or QST abnormalities. Some of the latter patients may have early small fibre dysfunction or ion channelopathy. CHEPS provides a clinically practical, non-invasive and objective measure, and can be a useful additional tool for the assessment of sensory small fibre neuropathy. Although further evaluation is required, the technique shows potential clinical utility to differentiate neuropathy from other chronic pain states, and provide a biomarker for analgesic development.