European Journal of Pain 13 (2009) 35–37

Contents lists available at ScienceDirect

European Journal of Pain

journal homepage: www.EuropeanJournalPain.com

Short Communication

Projected pain from noxious heat stimulation of an exposed peripheral nerve –A case report

T. Hoffmann a,*,1, S.K. Sauer a,1, R.E. Horch b, P.W. Reeh a

a Institute for Physiology and Pathophysiology, University of Erlangen-Nuremberg Universitaetsstrasse 17, D-91054 Erlangen, Germanyb Department of Plastic and Hand Surgery, University Hospital, University of Erlangen-Nuremberg, Krankenhausstrasse 12, 91054 Erlangen, Germany

a r t i c l e i n f o

Article history:Received 9 June 2008Received in revised form 22 August 2008Accepted 7 September 2008Available online 6 November 2008

Keywords:NociceptionSensory transductionNeuropathyNervi nervorum

1090-3801/$34.00 � 2008 European Federation of Chdoi:10.1016/j.ejpain.2008.09.014

Abbreviation: BK, bradykinin; CGRP, calcitonindorsal root ganglia; SR, superficial radial.

* Corresponding author. Tel.: +49 9131 8526730; faE-mail address: diskin@physiologie1.uni-erlangen.

1 Authors contributed equally.

a b s t r a c t

Distinct sensory properties of unmyelinated axons in the isolated rat sciatic nerve have previously beenrevealed by measuring stimulated neuropeptide (CGRP) release in response to noxious stimuli. Axonalsensitization to heat by inflammatory mediators has been demonstrated and shown to depend on theheat- and proton-activated ion channel TRPV1. Recently, we have demonstrated in vitro that heatstimulation of nociceptive axons generates ectopic action potential discharge which resembles the heatresponse of the corresponding cutaneous nerve endings. It remained however, to be established whetheradequate axonal stimulation could also generate projected sensations in a conscious human subject. In asingular human trial, the superficial radial nerve (SR) was exposed and stimulated mechanically as wellas with noxious cold (3 �C). These stimuli were unable to induce any conscious local or projected sensa-tions. However, controlled radiant heat applied to the nerve resulted in intense slowly adapting burningpain sensations projected into the center of the SR innervation area. No local sensation was reported.Thus, presumably activated nervi nevorum in the sheath of a healthy nerve do not cause conscious sen-sations, while axons of passage in mid-nerve exhibit a sensory transduction capacity for noxious heatthough not for mechanical and cold stimulation. Axonal heat transduction may therefore become asource of ectopic discharge and neuropathic pain when heat threshold drops to body temperature as isthe case with peripheral nerve endings in inflamed skin.� 2008 European Federation of Chapters of the International Association for the Study of Pain. Published

by Elsevier Ltd. All rights reserved.

1. Introduction

The understanding of underlying neuropathic pain mechanismsis the prime goal of studies dealing with pathologies such as carpaltunnel syndrome, sciatica and neural inflammation. Most of theattention hitherto was aimed at potentially sensitized peripheralnerve endings or at sensitized spinal nociceptive transmission(for review see (Campbell and Meyer, 2006)). Only a handful ofexperimental studies focused on sensory mechanisms in the nervetrunk itself, and in each case a pathological condition was inflictedon the nerve. In this respect, unmyelinated axons of peripheralnerves have previously been shown to develop various nociceptiveproperties at the proximal stump following transection (axotomy).These properties included the development of spontaneous dis-charge activity (Michaelis et al., 1995), axonal sensitivity to a vari-

apters of the International Associa

gene-related peptide; DRG,

x: +49 9131 8522497.de (T. Hoffmann).

ety of algesic agents such as bradykinin (BK), prostaglandin E2,acidic pH and capsaicin (Michaelis et al., 1997; Moalem et al.,2005) and the appearance of mechano- and thermosensitivityalong the axon itself at the developing neuroma site (Blenk et al.,1996; Koschorke et al., 1991; Michaelis et al., 1997, 1995). In addi-tion, experimental inflammation of an otherwise intact nerve wasalso shown to provoke local mechanosensitivity in nociceptive ax-ons of passage innervating deep structures (Bove et al., 2003). Thecommon hypothesis explaining these observations was that anter-ograde axonal transport of sensory transducer molecules such asstretch- or heat-activated ion channels causes an accumulation ofthese membrane proteins at the lesioned or affected site. Translo-cation of receptors and channels to the axonal membrane couldprovide the regenerating sprouts or axons with similar propertiestheir sensory nerve endings possessed prior to axotomy or inflam-mation (Blenk et al., 1996; Devor and Govrin-Lippmann, 1983;Koschorke et al., 1991, 1994; Michaelis et al., 1999).

Using an isolated mouse skin-nerve preparation (Reeh, 1986),we could recently demonstrate that heat sensitivity, ectopic actionpotential generation and, to some extent, also bradykinin sensitiv-ity represent inherent sensory properties present in the axonal

tion for the Study of Pain. Published by Elsevier Ltd. All rights reserved.

Fig. 1. A human trial exploring stimulated pain projection. (A) Experimental set-up:the superficial radial nerve (1) of one healthy subject was surgically exposed andisolated with a rubber strip (2). The nerve was stimulated mechanically using aglass rod (3) and thermally by frozen sterile isotonic NaCl solution for coldstimulation (4) and by a feedback-controlled halogen lamp that was focused ontothe nerve for heat stimulation (5, 6, 7). Only heat stimulation produced a consciousprojected pain sensation (8) in the innervation territory of the nerve. (B) Illustrationof one heat stimulation. The graph exhibits the set temperature, the nerve surfacetemperature and a symbolic representation of the subject’s pain report; the painsensation commenced around 50 �C, peaked almost instantly with an estimatedintensity of 5 (on a 0–10 pain scale) and showed adaptation during the temperatureplateau.

36 T. Hoffmann et al. / European Journal of Pain 13 (2009) 35–37

membrane at all times rather than develop in pathological condi-tions. Single-fiber recordings enabled us to quantify and directlycompare, for the first time, the sensory properties of individual ax-ons in the peripheral nerve stem with their characterized cutane-ous terminals in the receptive field (Hoffmann et al., 2008).Electrical micro-stimulation of distinct fibers in human nerves gen-erates sensations subjectively projected into the receptive field ofthe unit inside the distal innervation territory of the nerve; in caseof stimulating nociceptive units, the sensations reported are pain-ful (Schady et al., 1983; Jorum et al., 1989).

The question arose, whether axonal sensory transduction acti-vated by noxious stimuli can also generate projected painsensations.

2. Methods

We have scrutinized the above question stimulating a surgicallyexposed skin nerve in a conscious human subject. For this purpose,the Ethical Committee of the University was addressed outliningthe planned experiment and approval was attained.

The superficial radial (SR) nerve of one healthy male subject(the senior author, 58 y.) was surgically exposed 6cm proximal tothe left wrist. The skin of the operation area was superficially infil-trated with 1% lidocaine (1 ml, i.c.) drawing a fine anesthetic stripbefore cutaneous incision. The nerve branch innervating digits 2and 3 was exposed over a length of 2 cm and isolated from the sur-rounding tissue by blunt dissection and a rubber strip placedunderneath (Fig. 1A). The subject reported no painful sensationduring the entire surgical procedure. The skin in the SR innervationarea retained its normal sensitivity to light touch, cooling andwarming throughout the whole experiment and thereafter. Thenerve was repeatedly stimulated during a three hours period usinga blunt glass rod (pressure and distension) and anatomical forceps(squeezing), sterile isotonic NaCl ice cubes, and a feedback-con-trolled halogen lamp that was placed in focus above the nerve(Fig. 1A). Glass rod pressure could not be precisely quantified butwas strong enough to cause an obvious indentation of about halfthe nerve diameter. The radiant heat ramps varied in rise time(10–30 s), plateau duration (10–20 s), and intensity (starting at32 �C and peaking at 50–60 �C on the very surface). During heatstimulation, the subject was blind to the progress of the heat rampthough he was aware of the peak temperature aimed at. The timeinterval between consecutive stimuli exceeded 5 min; the se-quence was as follows: mechanical – cold – heat 42 �C – heat49 �C – mechanical – heat 50 �C – heat 56 �C (3 times with differentrise times) – cold – heat 60 �C (6 times of which 5 times wereaborted) – mechanical. The exposed nerve was kept wet with sal-ine throughout the experiment.

At the end the skin was conventionally sutured and healedwithout complications.

3. Results

Repeated cold (3 �C) and mechanical (glass rod pressure andpinching of the epineurium) stimuli applied to the exposed SRnerve branch (�3 mm in diameter) were unable to induce anylocal or projected sensation. By that time, the local anesthesiahad worn off and the edges of the cut skin readily mediated coldand touch sensations. No sensory response was initially evokedalso when radiant heat (32 �C to 42–50 �C in 30 s) was repeat-edly applied. Only upon heating the nerve surface to 53.6 �C(20 s ramp), 55.7 �C and 53.9 �C (10 s ramps), intense burningpain sensations with thresholds of close to 50 �C (Fig. 1B),53.7 �C and 48.9 �C, respectively, occurred in the center of theSR innervation area (Fig 1A). On a 1–10 numerical pain scale,

the projected pain intensity was rated by the subject 4–5, 6–7and 6–7 for the three supra-threshold heat stimuli, respectively.During the 10–20 s heat stimulation plateau phase, adaptation ofthe pain intensity by 2–3 score points was reported. After theinitial suprathreshold heat stimulation, subsequent stimuli in-duced local burning pain sensations at the wound cavity. This lo-cal pain could be diminished by shielding the region withaluminium foil (leaving an oval hole above the nerve branch)but not completely prevented. Despite this local sensitizationand hyperalgesia to heat, probing and squeezing the nerve andsurrounding connective tissue still did not evoke any conscioussensation. Only pressure applied to the deeper lying periosteumof the radius bone caused some dull aching pain.

4. Discussion

Together with our previous data from in vitro experimentation,the results indicate that intact unmyelinated peripheral nerve ax-ons possess a specific heat sensing capacity that resembles thatof their individual cutaneous nociceptive terminals. Accordingly,

T. Hoffmann et al. / European Journal of Pain 13 (2009) 35–37 37

heat activation of such fibers of passage in a human skin nerve pro-duces a conscious projected burning pain sensation which remindsof a neuropathic pain attack. The fact that mechanical stimulationof the superficial radial nerve failed to produce conscious sensa-tions is in agreement with our previous results in vitro in whichmechanical stimulation evoked only a poor and hardly reproduc-ible axonal response in 7 of 51 tested fibers (Hoffmann et al.,2008). Thus, axons seem to lack specific mechanosensory transduc-tion; strong deformation of axons may still depolarize their mem-brane and result in sporadic action potentials firing, as was thecase in our previous experiments in vitro. In contrast, specific axo-nal cold transduction may still exist despite the fact that our re-sults in vitro were suggestive rather than conclusive. Only two ofthree mechano-cold-sensitive C-units responded to axonal coldstimulation with a prompt and rapidly adapting burst of spikes.(Hoffmann et al., 2008), and cold stimulation of the human nervefailed to evoke any conscious sensation. The apparent cold insensi-tivity of the superficial radial nerve in our human experiment mayhave resulted from a methodological problem; while the coolingprocedure used for the thin nerve in vitro was sufficient for induc-ing ectopic firing, cooling of the thick and blood-perfused nervein vivo may not have been strong and fast enough to produce thecold response which shows strong adaptation. In addition, heatand cold responsiveness require both sensory transduction and ac-tion potential generation, the latter enabling single-fiber recordingas well as conscious perception. In this respect, axons and nerveendings differ essentially in that C-fibers in mid-nerves, in contrastto terminals, do not express the slowly inactivating TTX-resistantsodium channel Nav1.8 (Pinto et al., 2008) which secures spikegeneration during tonic depolarization and at cold temperatures(Zimmermann et al., 2007). This difference may well explain theadapting character of the heat-induced projected pain as well asaccount for the lack of projected pain during noxious cooling ofthe human nerve as the axonal TTX-sensitive sodium channelsare completely inactivated at cold temperatures (Zimmermannet al., 2007).

Our findings are not in contradiction to the existence of epineu-rial nerve endings (‘‘nervi nervorum”) that in fact respond to electri-cal stimulation and superfusion of inflammatory mediators orcapsaicin with a robust release of CGRP (Sauer et al., 1999). Theseobviously nociceptive nerve fibers may also respond to noxiousheat with CGRP release. However, in the healthy human nerve theydid not mediate any conscious sensation, possibly due to a lowsynaptic impact in the spinal dorsal horn of these afferents – whichmay change in case of chronic nerve entrapment sensitizing thesenervi nervorum (Bove et al., 2003).

In conclusion, our findings establish encoding axonal heat sens-ing leading to conscious pain as a normal physiological capacity ofperipheral nerves. When exaggerated, this capacity may become asource of neuropathic pain if the heat threshold drops to body tem-perature as may be the case during inflammation-like processes inthe sick peripheral nerve (Reeh and Petho, 2000). Cooling the nerveor pharmacological blocking of the heat transduction would thenbe a therapeutic option.

References

Blenk KH, Michaelis M, Vogel C, Janig W. Thermosensitivity of acutely axotomizedsensory nerve fibers. J Neurophysiol 1996;76:743–52.

Bove GM, Ransil BJ, Lin HC, Leem JG. Inflammation induces ectopic mechanicalsensitivity in axons of nociceptors innervating deep tissues. J Neurophysiol2003;90:1949–55.

Campbell JN, Meyer RA. Mechanisms of neuropathic pain. Neuron 2006;52:77–92.Devor M, Govrin-Lippmann R. Axoplasmic transport block reduces ectopic impulse

generation in injured peripheral nerves. Pain 1983;16:73–85.Hoffmann T, Sauer SK, Horch RE, Reeh PW. Sensory transduction in peripheral nerve

axons generates ectopic action potentials and projected pain. J Neurosci2008;28(24):6281–4. 11.

Jorum E, Lundberg LE, Torebjork HE. Peripheral projections of nociceptiveunmyelinated axons in the human peroneal nerve. J Physiol 1989;416:291–301.

Koschorke GM, Meyer RA, Campbell JN. Cellular components necessary formechanoelectrical transduction are conveyed to primary afferent terminals byfast axonal transport. Brain Res 1994;641:99–104.

Koschorke GM, Meyer RA, Tillman DB, Campbell JN. Ectopic excitability of injurednerves in monkey: entrained responses to vibratory stimuli. J Neurophysiol1991;65:693–701.

Michaelis M, Blenk KH, Janig W, Vogel C. Development of spontaneous activity andmechanosensitivity in axotomized afferent nerve fibers during the first hoursafter nerve transection in rats. J Neurophysiol 1995;74:1020–7.

Michaelis M, Blenk KH, Vogel C, Janig W. Distribution of sensory properties amongaxotomized cutaneous C-fibres in adult rats. Neurosci 1999;94:7–10.

Michaelis M, Vogel C, Blenk KH, Janig W. Algesics excite axotomised afferent nervefibres within the first hours following nerve transection in rats. Pain1997;72:347–54.

Moalem G, Grafe P, Tracey DJ. Chemical mediators enhance the excitability ofunmyelinated sensory axons in normal and injured peripheral nerve of the rat.Neurosci 2005;134:1399–411.

Pinto V, Derkach VA, Safronov BV. Role of TTX-sensitive and TTX-resistant sodiumchannels in A{delta}- and C-fiber conduction and synaptic transmission. JNeurophysiol 2008;99(2):617–28.

Reeh PW. Sensory receptors in mammalian skin in an in vitro preparation. NeurosciLett 1986;66:141–7.

Reeh PW, Petho G. Nociceptor excitation by thermal sensitization – a hypothesis.Prog Brain Res 2000;129:39–50.

Sauer SK, Bove GM, Averbeck BA, Reeh PW. Rat peripheral nerve componentsrelease calcitonin gene-related peptide and prostaglandin E2 in response tonoxious stimuli: evidence that nervi nervorum are nociceptors. Neurosci1999;92(1):319–25.

Schady WJ, Torebjork HE, Ochoa JL. Peripheral projections of nerve fibres in thehuman median nerve. Brain Res 1983;277:249–61.

Zimmermann K, Leffler A, Babes A, Cendan CM, Carr RW, Kobayashi J, et al. Sensoryneuron sodium channel Nav1.8 is essential for pain at low temperatures. Nature2007;447:855–8.