The conduction velocities of peripheral nerve fibres conveying sensations of warming and cooling

A computationally informed distinction of interoception and exteroception

While interoception is of major neuroscientific interest, its precise definition and delineation from exteroception continue to be debated. Here, we propose a functional distinction between interoception and exteroception based on computational concepts of sensor-effector loops. Under this view, the classification of sensory inputs as serving interoception or exteroception depends on the sensor-effector loop they feed into, for the control of either bodily (physiological and biochemical) or environmental states. We explain the utility of this perspective by examining the perception of skin temperature, one of the most challenging cases for distinguishing between interoception and exteroception. Specifically, we propose conceptualising thermoception as inference about the thermal state of the body (including the skin), which is directly coupled to thermoregulatory processes. This functional view emphasises the coupling to regulation (control) as a defining property of perception (inference) and connects the definition of interoception to contemporary computational theories of brain-body interactions.

Ocular surface information seen from the somatosensory thalamus and cortex

Ocular Surface (OS) somatosensory innervation detects external stimuli producing perceptions, such as pain or dryness, the most relevant symptoms in many OS pathologies. Nevertheless, little is known about the central nervous system circuits involved in these perceptions, and how they integrate multimodal inputs in general. Here, we aim to describe the thalamic and cortical activity in response to OS stimulation of different modalities. Electrophysiological extracellular recordings in anaesthetized rats were used to record neural activity, while saline drops at different temperatures were applied to stimulate the OS. Neurons were recorded in the ophthalmic branch of the trigeminal ganglion (TG, 49 units), the thalamic VPM-POm nuclei representing the face (Th, 69 units) and the primary somatosensory cortex (S1, 101 units). The precise locations for Th and S1 neurons receiving OS information are reported here for the first time. Interestingly, all recorded nuclei encode modality both at the single neuron and population levels, with noxious stimulation producing a qualitatively different activity profile from other modalities. Moreover, neurons responding to new combinations of stimulus modalities not present in the peripheral TG subsequently appear in Th and S1, being organized in space through the formation of clusters. Besides, neurons that present higher multimodality display higher spontaneous activity. These results constitute the first anatomical and functional characterization of the thalamocortical representation of the OS. Furthermore, they provide insight into how information from different modalities gets integrated from the peripheral nervous system into the complex cortical networks of the brain. KEY POINTS: Anatomical location of thalamic and cortical ocular surface representation. Thalamic and cortical neuronal responses to multimodal stimulation of the ocular surface. Increasing functional complexity along trigeminal neuroaxis. Proposal of a new perspective on how peripheral activity shapes central nervous system function.

Spatial summation of thermal sensitivity is limited to small areas: Comparisons of the forehead, forearm, abdomen, and foot

The purpose of the present study was to examine if spatial summation in thermal sensitivity exists when stimulating areas larger than about 1% of body surface area (BSA) (approximately 200 cm2). We hypothesized that spatial summation would exist within a limited area and the effect would be insignificant for over the 1%BSA. Fifteen young males participated in this study and we measured their warmth and hot sensation thresholds on the four body regions (the forehead, forearm, abdomen, and instep) using the three sizes of radiant film heaters (10 × 10, 15 × 15, and 20 × 20 cm2 heating film area). The heating panel was kept at a distance of 10 cm from the skin and the surface temperature of the heating panel increased by 1 °C·s-1. The results showed that warmth and hot sensation thresholds were higher for the 100 cm2 condition than the 225 or 400 cm2 conditions (P < 0.05), but no differences were found between the 225 and 400 cm2 conditions. Secondly, the instep was most insensitive to the gradual increase of radiant heat among the four body regions for all three stimulating film sizes, even though the hot threshold was lowest for the instep because the initial foot temperature was lower than other skin temperatures. In summary, spatial summation in thermal sensitivity was found for the 100 and 225 cm 2 conditions, but not for the 225 and 400 cm2 conditions. These results suggest that spatial summation exists but limited to small stimulating areas, smaller than approximately 1% BSA.

Evaluation of distal skin temperature and tissue oxygen saturation determined by near-infrared spectroscopy for predicting ultrasound-guided lateral infraclavicular block success

BACKGROUND

Changes in tissue oxygen saturation determined by near-infrared spectroscopy (NIRS) may help predict and determine the success of a lateral infraclavicular (LIC) block.We investigated whether evaluation of tissue oxygen saturation determined by NIRS couldbe an indicator of LIC block success.

METHODS

Forty patients scheduled for hand or forearm surgery under LIC block were studied. NIRS sensors were placed on the ventral aspect of both mid-forearms, and the contralateral hand was used as the control group. NIRS values were recorded before the block andat regular intervals during the following 30 min.

RESULTS

NIRS values were significantly higher in the successfully blocked patients whencompared to the complete failure, partial failure, and contralateral hand groups at the 10thmin. In the successfully blocked patients, NIRS values (mean ± SD [change in %]) increasedby 11.09 ± 4.86 (16.03%), 15.00 ± 4.53 (21.76%), 16.35 ± 5.14 (23.77%), 16.38 ± 4.88(23.85%), 16.67 ± 5.04 (24.29%), and 16.96 ± 5.71 (24.78%), respectively, from baselineto 5, 10, 15, 20, 25, and 30 min. ΔTs values were significantly higher in the successfullyblocked patients than in the complete failure patients and contralateral hand at the 30thmin. However, there was no statistically significant difference when comparing ΔTs values ofsuccessful block and partial failure block patients at the 30th min.

CONCLUSIONS

We conclude that measurement of tissue oxygen saturation by NIRS withinthe scope of evaluation of the lateral infraclavicular block is a rapid, effective, and applicabletechnique.

Spatiotemporal Pinpoint Cooling Sensation Produced by Ultrasound-Driven Mist Vaporization on Skin

In this study, we achieved a noncontact tactile display that presents a pinpoint and instantaneous cooling sensation on the skin surface with no devices directly in contact with the user's body. We employed ultrasound phased arrays to generate a focused ultrasound, which locally and instantaneously expedites the vaporization of room-temperature water mist floating near the surface of the user's skin, offering a sudden pinpoint cooling sensation. In this article, we describe the physical configuration of the proposed method and show the measurement results, demonstrating how the user's skin surface was cooled. During the experiments, we discovered that a part of the skin exposed to a focused ultrasound within the floating mist was selectively cooled with negligible delay. Our prototype system offers a cooling spot of approximately 15 mm in diameter, which causes a temperature decrease of 4.6 K in 1 s and 3.3 K in the first 0.5 s on a hand situated 500 mm away from the device. Additionally, the ultrasound-driven cooling spot can be controlled on the skin surface, which is felt as a cool moving spot. Such a position-free cooling system with a high spatiotemporal resolution will open the door to unprecedented practical tactile applications.

The correlation of small fiber neuropathy with pain intensity and age in patients with Fabry's disease: A cross sectional study within a large Taiwanese family

Background

The relationships among small fiber neuropathy, age, sex and pain intensity in the context of Fabry's disease remain unclear. We aim to study the correlations of small fiber neuropathy, age, sex and pain intensity in Fabry patients.

Methods

We evaluated C-fiber function by recording the withdrawal latencies to painful heat stimulus (WLPHS) when each subject's right hand was immersed in a 50 °C hot water bath and correlated this parameter with the patient's perceived pain intensity and quality of life assessed by the short-form McGill Pain Questionnaire (SF-MPQ) in a large Taiwanese Fabry family and normal controls.

Results

Male Fabry patients showed a significantly increased WLPHS compared to that of normal controls. Furthermore, male Fabry patients showed a positive correlation of increased WLPHS with patient age. The SF-MPQ of male Fabry patients showed a bell distribution with age, and maximal pain scores were detected between the ages of the early 20s and late 40s. In contrast, the female Fabry patients had variable associations of WLPHS and SF-MPQ with age.

Conclusions

We proposed a probable mechanism by which globotriaosylceramide (Gb3) or globotriaosylsphingosine (lyso-Gb3) is gradually deposited into the small nerve bundles with increasing age, which induces continuous damage and produces injury discharges to sustain neuropathic pain in young male Fabry patients. However, once the small fibers are reduced to a certain degree, they no longer produce enough noxious discharges to sustain neuropathic pains in older male Fabry patients, which leads these patients to have lower SF-MPQ scores. In contrast, female Fabry patients had less and variable small fiber damage, pain intensity and clinical signs/symptoms.

Early neurophysiological biomarkers and spinal cord pathology in inherited prion disease

A common presentation of inherited prion disease is Gerstmann-Sträussler-Scheinker syndrome, typically presenting with gait ataxia and painful dysaesthesiae in the legs evolving over 2–5 years. The most frequent molecular genetic diagnosis is a P102L mutation of the prion protein gene (PRNP). There is no explanation for why this clinical syndrome is so distinct from Creutzfeldt-Jakob disease, and biomarkers of the early stages of disease have not been developed. Here we aimed, first, at determining if quantitative neurophysiological assessments could predict clinical diagnosis or disability and monitor progression and, second, to determine the neuropathological basis of the initial clinical and neurophysiological findings. We investigated subjects known to carry the P102L mutation in the longitudinal observational UK National Prion Monitoring Cohort study, with serial assessments of clinical features, peripheral nerve conduction, H and F components, threshold tracking and histamine flare and itch response and neuropathological examination in some of those who died. Twenty-three subjects were studied over a period of up to 12 years, including 65 neurophysiological assessments at the same department. Six were symptomatic throughout and six became symptomatic during the study. Neurophysiological abnormalities were restricted to the lower limbs. In symptomatic patients around the time of, or shortly after, symptom onset the H-reflex was lost. Lower limb thermal thresholds were at floor/ceiling in some at presentation, in others thresholds progressively deteriorated. Itch sensation to histamine injection was lost in most symptomatic patients. In six patients with initial assessments in the asymptomatic stage of the disease, a progressive deterioration in the ability to detect warm temperatures in the feet was observed prior to clinical diagnosis and the onset of disability. All of these six patients developed objective abnormalities of either warm or cold sensation prior to the onset of significant symptoms or clinical diagnosis. Autopsy examination in five patients (including two not followed clinically) showed prion protein in the substantia gelatinosa, spinothalamic tracts, posterior columns and nuclei and in the neuropil surrounding anterior horn cells. In conclusion, sensory symptoms and loss of reflexes in Gerstmann-Sträussler-Scheinker syndrome can be explained by neuropathological changes in the spinal cord. We conclude that the sensory symptoms and loss of lower limb reflexes in Gerstmann-Sträussler-Scheinker syndrome is due to pathology in the caudal spinal cord. Neuro-physiological measures become abnormal around the time of symptom onset, prior to diagnosis, and may be of value for improved early diagnosis and for recruitment and monitoring of progression in clinical trials.

Human experimental pain models

Pain is an unpleasant sensory experience, associated with existing or potential tissue damage. It has also strong cognitive and emotional components. Stimuli that causes pain goes through process of nociception, which includes transduction, transmission, modulation and perception of said stimuli. Depending on the type of stimuli, we can classify human experimental pain models into mechanical, electrical, thermal and chemical. Information about pain mechanisms can be obtained from the following: 1) in vitro studies, 2) animal experiments, 3) human experimental pain studies and 4) clinical studies. Chosing the appropriate method for pain evaluation is a key step in the design of pain studies. Combining it with different electro-physiological and imaging methods, it can provide better objectivity and quantification of pain mechanisms. Focus in experimental pain studies is slowly shifting from static parameters of pain, such as pain threshold and maximum tolerance, to dynamic parameters, which can give us valuable insight in function of endogenous analgesic systems. This can be done using conditioned pain modulation. Using experimental pain on healthy voulenteers is key step in switching from animal models to clinical studies, foremost for validization of data from animals, making them important in translational research. Results from experimental pain studies can help us in understanding nociceptive mechanisms of acute and chronic pain, alongside development of new therapeutic modalities.

Effects of transcranial direct current stimulation on temperature and pain perception

Transcranial direct current stimulation modifies cortical excitability and in consequence some cerebral functions. In the present study we aimed to elucidate whether tDCS could affect temperature and pain perceptions in healthy subjects testing different stimulation parameters. A total of 20 healthy subjects were studied by means of quantitative sensory testing. Two different experiments were performed. First, we studied the effects of 15 minutes 2 mA anodal transcranial direct current stimulation applied over left M1 and parietal cortex in two separated sessions. Then, we tested the effects of 5 minutes tDCS over M1 by means of a sham controlled design to optimize the possibility to study minimal effects of tDCS using different polarities (cathodal and anodal) and intensities (1 and 2 mA). 2 mA anodal tDCS, when applied for both 15 and 5 minutes over the motor cortex, increased cold perception threshold. Conversely, motor cortex cathodal tDCS modulated cold perception threshold only when 1 mA intensity was used. M1-tDCS can modify the temperature perception; these effects are polarity and intensity dependent. As stimulation intensity seems critical to determine the effects, we suggest that for clinical application strong anodal tDCS (>1 mA) or weak cathodal tDCS (<2 mA) should be used for pain control.

Age changes in pain perception: A systematic-review and meta-analysis of age effects on pain and tolerance thresholds

Demographic changes, with substantial increase in life expectancy, ask for solid knowledge about how pain perception might be altered by aging. Although psychophysical studies on age-related changes in pain perception have been conducted over more than 70 years, meta-analyses are still missing. The present meta-analysis aimed to quantify evidence on age-related changes in pain perception, indexed by pain thresholds and pain tolerance thresholds in young and older healthy adults. After searching PubMed, Google Scholar and PsycINFO using state-of-art screening (PRISMA-criteria), 31 studies on pain threshold and 9 studies assessing pain tolerance threshold were identified. Pain threshold increases with age, which is indicated by a large effect size. This age-related change increases the wider the age-gap between groups; and is especially prominent when heat is used and when stimuli are applied to the head. In contrast, pain tolerance thresholds did not show substantial age-related changes. Thus, after many years of investigating age-related changes in pain perception, we only have firm evidence that aging reduces pain sensitivity for lower pain intensities.

Physical-Perceptual Correspondence for Dynamic Thermal Stimulation

Thermal displays have been applied in various haptic applications, from material simulation to interpersonal communication; however, there is insufficient knowledge about the temporal processing in human thermal sense to provide a　knowledge basis for thermal display design. In this study, we investigated the physical-perceptual correspondence for dynamic thermal stimulation to shed a light on the temporal processing of human thermal sense. In the experiments, participants reported subjective timings of the temperature onset and temperature peak of continuous temperature changes applied to the thenar eminence. We found that the physical-perceptual correspondence was not consistent for warm and cold stimulations. For warm stimulation, the subjective experience always came after the corresponding physical event. On the other hand, for cold stimulation, while the subjective onset always lagged the physical onset, the subjective temperature peak preceded the physical temperature peak. We analyzed these results in the framework of linear systems theory. The results suggest that the senses of warmth and cold have distinct temporal filtering properties, with the sense of cold being more transient than the sense of warmth. These findings advance our knowledge regarding temporal processing in human thermal sense and serve as a basis for thermal display design.

A Protocol of Manual Tests to Measure Sensation and Pain in Humans

Numerous qualitative and quantitative techniques can be used to test sensory nerves and pain in both research and clinical settings. The current study demonstrates a quantitative sensory testing protocol using techniques to measure tactile sensation and pain threshold for pressure and heat using portable and easily accessed equipment. These techniques and equipment are ideal for new laboratories and clinics where cost is a concern or a limiting factor. We demonstrate measurement techniques for the following: cutaneous mechanical sensitivity on the arms and legs (von-Frey filaments), radiant and contact heat sensitivity (with both threshold and qualitative assessments using the Visual Analog Scale (VAS)), and mechanical pressure sensitivity (algometer, with both threshold and the VAS). The techniques and equipment described and demonstrated here can be easily purchased, stored, and transported by most clinics and research laboratories around the world. A limitation of this approach is a lack of automation or computer control. Thus, these processes can be more labor intensive in terms of personnel training and data recording than the more sophisticated equipment. We provide a set of reliability data for the demonstrated techniques. From our description, a new laboratory should be able to set up and run these tests and to develop their own internal reliability data.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Human experimental pain models: A review of standardized methods in drug development

Human experimental pain models are essential in understanding the pain mechanisms and appear to be ideally suited to test analgesic compounds. The challenge that confronts both the clinician and the scientist is to match specific treatments to different pain-generating mechanisms and hence reach a pain treatment tailored to each individual patient. Experimental pain models offer the possibility to explore the pain system under controlled settings. Standardized stimuli of different modalities (i.e., mechanical, thermal, electrical, or chemical) can be applied to the skin, muscles, and viscera for a differentiated and comprehensive assessment of various pain pathways and mechanisms. Using a multimodel-multistructure testing, the nociception arising from different body structures can be explored and modulation of specific biomarkers by new and existing analgesic drugs can be profiled. The value of human experimental pain models is to link animal and clinical pain studies, providing new possibilities for designing successful clinical trials. Spontaneous pain, the main compliant of the neuropathic patients, but currently there is no human model available that would mimic chronic pain. Therefore, current human pain models cannot replace patient studies for studying efficacy of analgesic compounds, although being helpful for proof-of-concept studies and dose finding.

An investigation into the hypoalgesic effects of transcutaneous piezoelectric current on experimentally induced thermal stimuli in healthy participants

OBJECTIVES

  To investigate the effects of transcutaneous piezoelectric currents on experimentally induced thermal pain in healthy human participants.

MATERIALS AND METHODS

  A repeated measure cross-over study recorded sensory detection and pain thresholds to contact thermal stimuli during active and placebo (no current) transcutaneous piezoelectric current in 15 pain-free healthy human volunteers. Active transcutaneous piezoelectric current (6 µA) was delivered as 35 high voltage single rectangular pulses (1 Hz) at the LI4 (Hegu) acupuncture point.

RESULTS

  Repeated measures ANOVA found that active and placebo transcutaneous piezoelectric current elevated thresholds for warm sensation, heat pain, and cold sensation. However, there were no statistically significant effects for active piezoelectric current compared with placebo for any outcome measure.

CONCLUSIONS

  Reductions in experimentally induced pain were not due to piezoelectric currents per se. These findings challenge claims about the efficacy of transcutaneous piezoelectric currents for pain relief. A clinical trial is needed.

Effects of local anesthetics on somatosensory function in the temporomandibular joint area

There is a need for systematic studies regarding the pathophysiology and pain mechanisms of somatosensory function in the temporomandibular joint (TMJ). So far, the effects on somatosensory functions of local anesthetics (LA) applied to the auriculotemporal (AT) nerve or intraarticularly (IA) into the TMJ have not been studied systemically. This study aimed to examine in a double-blinded, placebo-controlled manner the effects of LA on mechanical and thermal sensitivity in the TMJ area. Twenty-eight healthy subjects (27.4 +/- 6.2 years) without temporomandibular disorders (TMD) participated. The subjects received an AT nerve block (n = 14) or an IA injection (n = 14) with LA (Bupivacaine, 2.5 mg/ml) on one side, and a placebo injection (isotonic saline) on the contralateral side. Mechanical (tactile and pin-prick) and thermal sensitivity (40 and 5 degrees C) were assessed at 11 standardized points in the TMJ area before injections (baseline) as well as 30 min, 1 and 2 h after injections. All stimuli were rated by the subjects on a 0-100 numerical rating scale (NRS). TMJ pressure pain threshold (PPT) and pressure pain tolerance (PPTOL) were assessed laterally over both TMJs using an algometer. IA injections with LA were not associated with any changes in sensitivity of the TMJ or surrounding area. In contrast, AT nerve blocks with LA caused a decrease over time in the pin-prick sensitivity (P = 0.016), which however, did not differ significantly from saline, and an increase of the PPTs 30 min (P = 0.010) and PPTOLs 30 min, 1 h and 2 h (P < 0.05) after LA injections in comparison to saline. No other measures showed a significant change after the injections. Our results showed that IA bupivacaine injection in healthy subjects has no effect on the sensitivity of the TMJ or surrounding area, while AT nerve block has a more pronounced effect on deep mechanical, but not on superficial mechanical or thermal sensitivity. Further research to investigate the effect of LA on somatosensory functions in TMJ patients in comparison with this study results will give valuable information about the sensitivity in the TMJ area.

Thermographic temperature measurement compared with pinprick and cold sensation in predicting the effectiveness of regional blocks

We designed this study to evaluate the usefulness of thermographic temperature measurement with an infrared camera, compared with patient response to cold and pinprick, as a means of assessing the success or failure of axillary blockades. Axillary blocks were performed on 25 patients undergoing surgery on the hand or forearm using a nerve stimulator technique with mepivacaine 1.5%. Pinprick and cold sensation were assessed on the operative site at 5-min intervals for 30 min. A thermographic image of the operative limb was recorded at similar time intervals. Thermographic images of the unblocked limb were taken before block placement and at 30 min. Temperature values at the operative site and unblocked limb were calculated from the thermographic images. Results revealed that thermography had higher combined values for sensitivity, specificity, and positive and negative predictive values than both cold and pinprick at all time intervals, with statistically significant differences at 15 min (thermography versus cold, P = 0.006; thermography versus pinprick, P = 0.026) and 30 min (thermography versus cold, P = 0.038; thermography versus pinprick, P = 0.040). For thermography as a method of block assessment, an optimal time of 15 min after mepivacaine local anesthetic injection gives the highest combined values for predicting a successful block (P = 0.004). We conclude that thermography provides an early and objective assessment of the success and failure of axillary regional blockades.

ThermoTRP channels and cold sensing: what are they really up to?

Cooling is sensed by peripheral thermoreceptors, the main transduction mechanism of which is probably a cold- and menthol-activated ion channel, transient receptor potential (melastatin)-8 (TRPM8). Stronger cooling also activates another TRP channel, TRP (ankyrin-like)-1, (TRPA1), which has been suggested to underlie cold nociception. This review examines the roles of these two channels and other mechanisms in thermal transduction. TRPM8 is activated directly by gentle cooling and depolarises sensory neurones; its threshold temperature (normally approximately 26-31 degrees C in native neurones) is very flexible and it can adapt to long-term variations in baseline temperature to sensitively detect small temperature changes. This modulation is enabled by TRPM8's low intrinsic thermal sensitivity: it is sensitised to varying degrees by its cellular context. TRPM8 is not the only thermosensitive element in cold receptors and interacts with other ionic currents to shape cold receptor activity. Cold can also cause pain: the transduction mechanism is uncertain, possibly involving TRPM8 in some neurones, but another candidate is TRPA1 which is activated in expression systems by strong cooling. However, native neurones that appear to express TRPA1 respond very slowly to cold, and TRPA1 alone cannot account readily for cold nociceptor activity or cold pain in humans. Other, as yet unknown, mechanisms of cold nociception are likely.

Short-lasting impairment of temperature perception by high frequency rTMS of the sensorimotor cortex

OBJECTIVE

Repetitive transcranial magnetic stimulation (rTMS) has become a useful tool for investigating and even modulating human brain function. RTMS of the human motor cortex can produce changes in excitability that outlast the period of stimulation. To investigate the persistent effect of high-frequency rTMS of sensorimotor cortex (SM1) on somatosensory function.

METHODS

We evaluated the thermal thresholds (cold and warm sensation) in 14 normal subjects before and after a short train of 5Hz rTMS over the SM1 or occipital cortex (OC).

RESULTS

Threshold for cold perception was increased immediately after rTMS of the left SM1 and no effects at all were noticed after OC stimulation. There was a slight, not significant, increase of warm threshold immediately after the rTMS of the left SM1 and no effects at all were noticed after OC stimulation.

CONCLUSIONS

High frequency rTMS over primary sensorimotor cortex seems to modulate sensory function related to thermal (cold) perception.

SIGNIFICANCE

The method may be useful for both the study of normal human physiology of temperature perception and for rTMS based manipulation of brain plasticity in patients with sensory disturbances.

Technology literature review: quantitative sensory testing

The development of the personal computer has simplified the process of quantitating sensory thresholds using various testing algorithms. We reviewed the technical aspects and reproducibility of different methods to determine threshold for light touch-pressure, vibration, thermal, and pain stimuli. Clinical uses and limitations of quantitative sensory testing (QST) were also reviewed. QST is a reliable psychophysical test of large- and small-fiber sensory modalities. The results of QST are highly dependent on methodology and the full cooperation of the subject. QST has been shown to be reasonably reproducible over a period of days or weeks in normal subjects. The use of QST in research and patient care should be limited to instruments and their corresponding methodologies that have been shown to be reproducible. Literature data do not allow conclusions regarding the relative merits of individual QST instruments.

CNS Activation by Noxious Heat to the Hand or Foot: Site-Dependent Delay in Sensory But Not Emotion Circuitry

Recently, functional magnetic resonance imaging has been used as a novel method of evaluating the CNS response to noxious stimuli. In a previous study, a prolonged noxious thermal stimulus applied to the dorsum of the hand produced more than one hemodynamic response that was temporally segregated. The two major responses displayed activation in primary sensory regions (classic pain circuitry) and regions involved in emotion (reward/aversion circuitry), respectively. In the current study, we applied the same thermal stimulus separately to the dorsum of the left foot and the dorsum of the left hand in the same subjects and compared the hemodynamic responses to evaluate the effects of conduction distance on CNS activation within these two segregated systems. After stimulus delivery to the foot, the hemodynamic response in primary sensory networks occurs after a delay of 3.6 ± 1.3 s as compared with the response after hand stimulation. The relative delay of the hemodynamic response in reward/aversion regions is not significantly different between hand and foot stimulation (0.6 ± 2.1 s). These results within the primary sensory system are consistent with the greater conduction distance of the peripheral nerves from the hand versus the foot. The observation that the response within the reward/aversion pathways occurs with the same rapid temporal characteristics after either hand or foot stimulation supports the notion that the circuitry involved in the evaluation of aversive stimuli is rapid in onset and probably represents a major protective mechanism for survival.

Potential c-fiber damage in Wilson's disease

OBJECTIVES

To find out about potential involvement of the peripheral and autonomic nervous system in Wilson's disease (WD).

MATERIAL AND METHODS

Seventeen patients with laboratory proven WD were examined with quantitative sensory testing (QST) (thermal, pain and vibratory sensation), pupillometric evaluation and electrophysiological testing of basal ganglia motor function [frequency of most rapid alternating movements (MRAM), reaction time (RT), contraction time (CT)]. Results were compared with those obtained in 20 healthy controls.

RESULTS

After correction for multiple comparisons, patients with WD showed significantly higher thresholds for warm sensation [sural and peroneal nerve, thermal sensory limen (TSL), unpaired t-test]. Individual results were pathological in eight (peroneal) and nine (sural nerve) patients, respectively. Pupil function was not altered. Patients with WD showed significant slowing of MRAM and prolongation of RT and CT. There was no significant correlation between RT and QST results.

CONCLUSIONS

These findings are compatible with a potential involvement of unmyelinated warm-specific C fibers in WD, independent from predominant basal ganglia motor dysfunction.

The significance of A-delta and C fibres for the perception of synthetic heat

Synthetic heat is a perception of strong, but not painful, heat arising when skin is stimulated by an alternating pattern of adjacent cold and warmth. This study examines the contribution of different classes of nerve fibres to this perception. In 40 subjects changes in synthetic heat and thermal perceptions were studied during a 30-min ischaemic nerve block in one reaction time, and one threshold determination task. Synthetic heat stimuli were described as hot or warm, but not as painful, and were preceded by a transient cold. Reaction times for synthetic heat stimuli did not differ from those for cold stimuli. Thresholds for synthetic heat and thermal stimuli were similar. During A fibre nerve block the perception of synthetic heat lost the cold component whereas the frequency of hot and warm descriptors did not change. The perception of cold stimuli changed, such that pure cold was replaced by dysaesthetic descriptors. Reaction times and thresholds for thermal and synthetic heat stimuli increased equally during the nerve block. It is concluded that the perception of synthetic heat most likely arises from the fusion of signals dependent on unmyelinated low threshold cold and warm receptors. It is not dependent on A-delta cold fibres, and a contribution of nociceptors is quite unlikely. The possibility of a psychological contribution at the perceptual level is discussed.

Quantitative sensory testing

Quantitative sensory testing is a reliable way of assessing large and small sensory nerve fiber function. Sensory deficits may be quantified and the data used in parametric statistical analysis in research studies and drug trials. It is an important addition to the neurophysiologic armamentarium, because conventional sensory nerve conduction tests only the large fibers. QST is a psychophysical test and lacks the objectivity of NCS. The results are subject to changes owing to distraction, boredom, mental fatigue, drowsiness, or confusion. When patients are consciously or unconsciously biased toward an abnormal QST result, no psychophysical testing can reliably distinguish these patients from those with organic disease. QST tests the integrity of the entire sensory neuraxis and is of no localizing value. Dysfunction of the peripheral nerves or central nervous system may give rise to abnormalities in QST. As is true for other neurophysiologic tests, QST results should always be interpreted in light of the patient's clinical presentation. Quantitative sensory testing has been shown to be reasonably reproducible over a period of days or weeks in normal subjects. Because longitudinal QST studies of patients in drug trials are usually performed over a period of several months to a few years, reproducibility studies on the placebo-control group should be included. For individual patients, more studies are needed to determine the maximum allowable difference between two QSTs that can be attributed to experimental error. The reproducibility of thermal thresholds may not be as good as that of vibration threshold. Different commercially available QST instruments have different specifications (thermode size, stimulus characteristics), testing protocols, algorithms, and normal values. Only QST instruments and their corresponding methodologies that have been shown to be reproducible should be used for research and patient care. The data in the literature do not allow conclusions regarding the superiority of any QST instruments. The future of QST is promising; however, many factors can affect QST results. As is true for other neurophysiologic tests, QST is susceptible to many extraneous factors and to misuse when not properly interpreted by the clinician.

Botulinum toxin A and the cutaneous nociception in humans: a prospective, double-blind, placebo-controlled, randomized study

Aside from temporary chemodenervation of skeletal muscle and potential anti-inflammatory effects, a genuine peripheral antinociceptive effect of Botulinum Neurotoxin Type A (BoNT/A) has been suspected. To evaluate the effect of BoNT/A on cutaneous nociception in humans, 50 healthy volunteers received subcutaneous injections of 100 mouse units (MU) BoNT/A (Dysport) and placebo. Both forearms of each subject were treated in a double-blind fashion, one with verum, one with placebo. Heat and cold pain thresholds within the treated skin areas were measured with quantitative sensory testing (QST) and pain thresholds were evaluated with local electrical stimulation (ES). The tests were done before treatment, and after 4 and 8 weeks. No major side effects were noted. All participants completed the study. Heat and cold pain thresholds increased from baseline to week 4 by 1.4 degrees C for verum and by 1.1 degrees C for placebo. From baseline to week 8, the thresholds increased by 2.7 degrees C for verum and by 1.2 degrees C for placebo. Electrically induced pain thresholds shifted from baseline to week 4 by -0.07 mA for verum and by 0.01 mA for placebo. From baseline to week 8, the thresholds increased by 0.10 mA for verum and by 0.11 mA for placebo. None of these differences was statistically significant. The study shows that there is no direct peripheral antinociceptive effect of BoNT/A in humans. The efficacy of BoNT/A in various pain syndromes must be explained by other pathways such as chemodenervation or anti-inflammatory effects.

Slowly conducting afferents activated by innocuous low temperature in human skin

1. Microneurography was used to search for primary afferents responsive to innocuous low temperature in human nerves supplying the hairy skin of the hand or foot. Eighteen units were identified as cold-specific units: they displayed a steady-state discharge at skin temperatures in the range 28-30 degrees C, they were sensitive to small changes in temperature, and they responded vigorously when a cool metal probe touched their receptive fields (RFs). They were insensitive to mechanical stimuli and sympathetic activation. Their RFs comprised one, or at most two, spots less than 5 mm in diameter. 2. Nine units were characterised in detail by a series of 10 s cooling and warming pulses from a holding temperature of 35 degrees C. The threshold temperature for activation by cooling was 29.4 +/- 2.0 degrees C (mean +/- S.D.). Adaptation of the responses to supra-threshold cooling pulses was partial: mean peak and plateau firing rates were maximal on steps to 15 degrees C (35.9 and 19.9 impulses x s(-1), respectively). Three of these units also displayed a paradoxical response to warming, with a mean threshold of 42.3 degrees C. 3. Sixteen of the eighteen cold-specific units were also studied by electrical stimulation of their RFs. They conducted in the velocity range 0.8-3.0 m x s(-1). When stimulated at 2 Hz, their latency increased according to a characteristic time course, reaching a plateau within 3 min (mean slowing (+/- S.D.) 5.2 +/- 1.1 %) and recovering quickly (50 % recovery in 17.8 +/- 4.5 s). 4. To reconcile these findings with previous studies of reaction times and the effects of nerve compression on sensation, it is concluded that either human cold-specific afferent fibres are incompletely myelinated 'BC' fibres, or else there are C as well as A(delta) cold fibres, with the C fibre group contributing little to sensation.

Somatosensory evoked potentials associated with thermal activation of type II Adelta mechanoheat nociceptive afferents

We evaluated evoked potentials (EPs) to noxious contact heat pulses delivered to hairy skin of healthy adults. Heat pulses from an adapting temperature of 34 degrees C to a target temperature of 52 degrees C, produced two scalp positive waves. The first peaked at 44 degrees to 45 degrees C (approximately 500 ms following stimulus onset), while the second peaked approximately 300 ms following the 52 degrees C heat pulse (approximately 1 s after stimulus onset). The first positive wave was absent from an adapting temperature of 39 degrees C, suggesting loss of synchronized activation of warm and/or low threshold mechanothermal afferents. The second EP was observed following stimulation from both adapting temperatures and was associated with subjective report of first pain. Latency difference of the pain EP from arm and leg were consistent with conduction in Adelta nociceptive afferents (approximately 10/ms). EPs to painful contact thermal stimuli may be of value in the evaluation of small fiber peripheral neuropathies and assessment of altered pain states.

C- and A delta-fiber components of heat-evoked cerebral potentials in healthy human subjects

Feedback-controlled laser heat was used to stimulate the hairy skin of the hand dorsum and forearm, and heat-evoked cerebral potentials were recorded at midline (Fz, Cz, Pz) and temporal (T3, T4) scalp positions. Based on data from primary afferent electrophysiology a stimulus level (40 degrees C) was chosen, which is above C-fiber heat threshold, but clearly below A delta-nociceptor heat threshold in order to excite selectively C-fibers without concomitant excitation of A delta-fibers. Feedback-controlled stepped heat stimuli to 40 degrees C elicited ultralate laser evoked potentials (LEPs) at the vertex in a high proportion of experiments (90%). Estimates of conduction velocity calculated from latency shifts between the hand and forearm sites of ultralate LEPs (2.4 m/s) and of reaction times (2.8 m/s) confirmed mediation of ultralate potentials by unmyelinated nerve fibers (nociceptors and/or warm fibers). The ultralate LEP could be differentiated from resolution of contingent negative variation (CNV), an endogenous potential related to expectation and response preparation, by its scalp topography. Strong heat stimuli of 48 degrees C, which is suprathreshold for most A delta- and C-fiber nociceptors, elicited the well-known late LEPs mediated by nociceptive Adelta-fibers confirming previous studies. The LEP waveform to strong heat stimuli also contained an ultralate component reminiscent of an ultralate LEP following the late LEP. Ultralate and late LEP had identical scalp topography. In conclusion, the method of temperature-controlled laser heat stimuli allows the selective and reliable examination of A delta- and C-fiber-mediated afferent pathways and the related cortical processing without the complication of dissociating A-fiber nerve blocks.

Paradoxical heat sensation in healthy subjects: peripherally conducted by A delta or C fibres?

Paradoxical heat sensation upon cooling of the skin has been reported in central as well as in peripheral neurological conditions. In our study, we examined this phenomenon in 35 naive healthy test subjects, of whom 23 experienced paradoxical heat sensation under test conditions. We measured the peripheral conduction velocities of cold sensation, warm sensation and of paradoxical heat sensation by using a quantitative sensory testing model of indirect peripheral conduction velocity measurement. This was based on comparison of measurements at a proximal and a distal site using two measurement methods, one inclusive and the other exclusive of reaction time. We found that the conduction velocity of paradoxical heat sensation (0.70 m/s) was similar to that of warm sensation (0.68 m/s), and that the conduction velocity of cold sensation (7.74-8.01 m/s) was considerably faster. Thus, we conclude that paradoxical heat sensation in healthy subjects is conducted peripherally via slow unmyelinated C fibres and not via the faster A delta fibres. Consequently, we propose that paradoxical heat sensation is encoded via the heat sensing pathway, in accordance with the labelled-line code theory. The mechanisms proposed suggest a malfunctioning cold-sensing pathway disinhibiting the heat-sensing pathway, at peripheral, central or both levels, thus facilitating a paradoxical heat sensation.

CO2 laser activation of nociceptive and non-nociceptive thermal afferents from hairy and glabrous skin

In 8 healthy subjects we have recorded cerebral evoked potentials and reaction time (RT) to CO2 laser stimulation of the hairy and glabrous skin at low and high stimulus intensities, corresponding to subjective reporting of detection and pain, respectively. At each intensity we were able to identify an evoked potential; the latencies of the major vertex positive (VP) components fell into 2 distinct populations 320 +/- 30 (VP300) and 778 +/- 80 (VP800) which did not differ between stimulation sites. The frequency of the VP300 responses was greatest in the high stimulus conditions and lower in the low stimulus conditions whilst the opposite was true for the VP800 responses. BImodal distributions of RT were seen at both stimulus intensities. In a further group of of 10 subjects we recorded the latency shift of the vertex negativity following proximal and distal stimulation of hairy skin of the left upper limb and derived conduction velocities for the VP300 (13.21 +/- 2.8 m/sec) and VP800 (1.26 +/- 0.29 m/sec) responses. These results suggest that, following CO2 laser stimulation of both hairy and glabrous skin, two different fibre populations are activated. The VP300 responses appear to be related to A delta activation, while the characteristics of the VP800 responses are consistent with activation of thermoreceptors mediated by C fibres.

Quantitative measurement of cutaneous perception in diabetic neuropathy

To determine the diagnostic value of various cutaneous sensory modalities in diabetic neuropathy, we studied cutaneous perception at the dominant hallux of 113 subjects (32 normal healthy controls and 81 diabetic subjects). The cutaneous sensory perception tests included warm and cold thermal perception, vibration, touch-pressure sensation, and current perception testing (CPT). The sensitivity of each modality when specificity is held greater than 90% was as follows: warm = 78%, cold = 77%, vibration = 88%, tactile-pressure = 77%, 5-Hz CPT = 52%, 250-Hz CPT = 48%, and 2000-Hz CPT = 56%. Combination thermal and vibratory gave optimum sensitivity (92-95%) and specificity (77-86%). We conclude that vibratory and thermal testing should be the primary screening tests for diabetic peripheral neuropathy. Other modalities may be of use only in specific situations.

Quantitative assessment of thermal and pain sensitivity

Values for thermal specific and thermal pain thresholds were determined in 150 healthy volunteers, 67 women and 83 men, aged from 10 to 73 years. Warm-cold difference limen, heat pain and cold pain thresholds were assessed at the face, thenar, medial surface of the upper- and forearm, lateral mammary, lateral umbilical, anterior thigh and lateral leg regions, and lateral aspect of the dorsum of the foot. Temperature and pain sensitivity were assessed by the Marstock method. Temperature sensitivity was found obviously age-dependent. The correlation is linear. Women showed greater sensitivity for small temperature changes, reflected as warm-cold difference limen, and for heat pain and cold pain. Great variation of thermal and pain sensitivity of different body parts was significant in all volunteers, irrespective of age and sex. Interindividual variation was also considerable. Small intraindividual variability was found in measurements repeated in 4 consecutive days and after 4 weeks. Body length did not influence thermal and pain perception thresholds. There were no differences found in thermal and pain sensitivity between the left and the right side of the body.

Quantitative sensory examination of epidural anaesthesia and analgesia in man; dose-response effect of bupivacaine

Time and dose-response functions of 4 concentrations of epidural bupivacaine (0.075, 0.125, 0.25, and 0.5%; 20 ml) on somatosensory and motor functions were examined in 10 healthy volunteers. Every hour for 8 h the effect of epidural bupivacaine on perception of painful and non-painful stimuli was quantified with 12 psychophysical measures. In addition knee extension strength, reaction time and skin temperature were examined. The two lowest concentrations of bupivacaine induced hypoalgesia without impairment of motor function. Epidural bupivacaine produced differential blockade of non-painful somatosensory functions in the following order: warmth perception > cold perception > perception of electrical stimuli. Epidural bupivacaine induced more pronounced hypoalgesia for short-lasting than for prolonged stimuli. A modality-related blocking order was observed for prolonged painful stimuli: heat > mechanical > electrical. It is suggested that differential blockade of somatosensory functions is caused by differences both in conduction blockade of nerve fibres and in central integration of afferent nerve impulses.

Quantitative sensory examination in human epidural anaesthesia and analgesia: effects of lidocaine

To characterize the sensory effects of epidural lidocaine, 2 groups each of 10 human subjects received 25 ml of 2% lidocaine at the L1-L2/L2-L3 interspace (high lumbar group) or at the L3-L4/L4-L5 interspace (low lumbar group). Twelve quantitative sensory tests in the L5 and S1 dermatomes and leg extension strength were determined every 30 min for 3 h. Sensory and motor blockade were more pronounced in the low than in the high lumbar group. Sensory functions were blocked in the following order by epidural lidocaine: warm > cold > electrical stimuli. Perception of brief localized noxious stimuli was attenuated more than perception of noxious stimuli of longer duration and involving larger stimulus areas. It is suggested that both peripheral mechanisms (differential conduction blockade of afferent fibres) and central mechanisms (temporal and spatial summation) play a role in the sensory effects of epidural lidocaine.

Vibratory and thermal thresholds in diabetics with and without clinical neuropathy

Vibration and thermal detection threshold and heat pain threshold were determined in 34 diabetics scrutinized for clinical neuropathy using a standardized questionnaire and examination form. On the basis of the clinical grading patients were classified as having either no neuropathy or a neuropathy of increasing severity. As expected thermal and vibratory detection threshold increased with increasing severity of neuropathy. Comparison between diabetics without symptoms and signs of neuropathy and a corresponding non-diabetic control group showed that a warm sensibility index (WSI = the range in which non-noxious heat is perceived) was significantly lower on feet in diabetics than in their matched non-diabetic controls. The findings show that quantitative assessment of thermal sensitivity may be of value to detect early small nerve fiber dysfunction even in patients without symptoms or signs of a clinical neuropathy.

A comparison of two methods for measuring thermal thresholds in diabetic neuropathy

Thermal thresholds can be measured psychophysically using either the method of limits or a forced-choice method. We have compared the two methods in 367 diabetic patients, 128 with symptomatic neuropathy. The Sensortek method was chosen for the forced-choice device, the Somedic modification of the Marstock method for a method of limits. Cooling and heat pain thresholds were also measured using the Marstock method. Somedic thermal thresholds increase with age in normal subjects, but not to a clinically significant degree. In diabetics Marstock warm threshold increased by 0.8 degrees C/decade, Sensortek by 0.1 degrees C/decade. Both methods had a high coefficient of variation in normal subjects (Sensortek 29%, Marstock warm 14%, cool 42%). The prevalence of abnormal thresholds was similar for both methods (28-32%), though Marstock heat pain thresholds were less frequently abnormal (18%). Only 15-18% of patients had abnormal results in both tests. Sensortek thresholds were significantly lower on repeat testing, and all thresholds were higher in symptomatic patients. Both methods are suitable for clinical thermal testing, though the method of limits is quicker. In screening studies the choice of a suitable apparatus need not be determined by the psychophysical basis of the test.

The extent of small fibre sensory neuropathy in diabetics with plantar foot ulceration

Thresholds for cutaneous warming and cooling stimuli were measured in 20 diabetics with neuropathic foot ulcers. All patients had a profound disturbance of sensory perception in the ulcerated foot with complete loss of perception of warming; thresholds for vibration and cooling were highly abnormal in all but two patients. Measurements of thermal threshold were made on both feet in 10 patients: warming was lost bilaterally in all, and cooling was bilaterally absent in six. There was no clear pattern of sensory loss in those diabetics with unilateral foot ulceration to suggest that sensory impairment was the determining factor for the development of a plantar ulcer. Measurements of thermal thresholds were made at additional sites in 13 patients and although the most marked abnormalities of sensation were always found in the feet, in some severe neuropaths, abnormal thresholds on the hand and even the face were demonstrated. Thresholds for warming were invariably more abnormal than thresholds for cooling. The diabetics with neuropathic ulceration in this study all had severe generalised peripheral nerve disease involving large myelinated as well as both small myelinated and unmyelinated sensory fibres. The quantitative evidence on the distribution of sensory loss for thermal sensations supports the hypothesis that the neuropathic process affecting the small myelinated and unmyelinated fibres is length dependent.